Engineering Geology and 

Design of Slopes for 

Cretaceous Potomac 

Deposits in Fairfax 

County, Virginia, and 

Vicinity 

GEOLOGICAL SURVEY BULLETIN 1556 

Engineering Geology and Design 

of Slopes for Cretaceous 

Potomac Deposits in Fairfax 

County, Virginia, and Vicinity 

Engineering Geology of Potomac Formation 

Deposits in Fairfax County, Virginia, and Vicinity, with 

Emphasis on Landslides 

By STEPHEN F. OBERMEIER 

Design and Construction of Slopes in 

Potomac Formation Deposits 

By PAUL G. SWANSON and JOHNS. JONES, JR. 

Evaluation of Permanent Cut Slope 

Stability in Potomac Formation Deposits 

By JAMES J. SCHNABEL 

A Summation and Critique: A Review of Procedures for 

Design of Permanent Slopes on the Potomac Formation 

and Some Relevant Field Observations 

By STEPHEN F. OBERMEIER 

Edz"ted by STEPHEN F. OBERMEIER 

GEOLOGICAL SURVEY BULLETIN 1556 

A collectz'on of papers discussz'ng the 

relatz'ons between the geologz'c set#ng and 

the desz'gn of slopes z'n the Potomac 

Formatz'on z'n Faz'rfax County, Va., and 

vlcz'nz'ty 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON:l984 
UNITED STATES DEPARTMENT OF THE INTERIOR 

WILLIAM P. CLARK, Secretary 

GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Library of Congress Cataloging in Publication Data 

Main entry under title: 

Engineering geology and design of slopes for Cretaceous 

Potomac deposits in Fairfax County, Virginia, and vicinity. 

(Geological Survey bulletin ; 1556) 

Bibliography: p. 46, 62, 77, 88 

Supt. of Docs. no.: I 19.3: 1556 

1. Engineering geology-Washington Metropolitan Region. 

2. Slopes (Soil mechanics)-Washington Metropolitan Region. 

3. Landslides-Washington Metropolitan Region. 

4. Geology, Stratigraphic-Cretaceous. I. Obermeier, 

Stephen F. II. Series. 

QE75.B9 no. 1556 557.3s [623.1 '51755 '291] 

[TA705.3.W3] 

For sale by the Distribution Branch, Text Products Section 

U.S. Geological Survey, 604 South Pickett Street, Alexandria, VA 22S04 

83-600248 
CONTENTS 

Page 

Preface------------------------------------------------------- 1 

Symbols, terms, and definitions----------------------------------- 3 

Conversion factors---------------------------------------------- 3 

Qualitative and quantitative expressions of consistency of fine-grained 

soils and compactness of coarse-grained soils -------------------- 4 

Engineering geology of Potomac Formation deposits in Fairfax 

County, Virginia, and vicinity, with emphasis on landslides, by 

Stephen F. Obermeier--------------------------------------- 5 

Design and construction of slopes in Potomac Formation deposits, 

by Paul G. Swanson and JohnS. Jones, Jr. ----------------------49 

Evaluation of permanent cut slope stability in Potomac Formation 

deposits, by Jamesj. Schnabel--------------------------------63 

A summation and critique: A review of procedures for design of 

permanent slopes on the Potomac Formation and some relevant 

field observations, by Stephen F. Obermeier --------------------79 

III 

PREFACE 

Landslides are a commonplace and widespread problem in sediments of the Potomac Formation, which crops out in a narrow belt 

from southern Virginia to New Jersey. Although the belt is only a few 

miles wide, many cities and towns are located along the Fall Line 

where the Potomac Formation crops out. Serious land-use and development problems often result from the slope instability and the 

shrink-swell properties of the fine-grained Potomac sediments. The 

following papers address engineering geology and the design of slopes 

of the Potomac Formation in Fairfax County, Va., and vicinity (a part 

of the Washington, D.C., metropolitan area), but the principal conclusions are valid wherever Potomac sediments are present. 

The Potomac Formation of Early Cretaceous age is clay rich and 

highly plastic in many places and is usually highly overconsolidated 

owing to the weight of formerly overlying formations, which have 

been eroded away. Engineering design of slopes is difficult and often 

somewhat subjective because of the lack of well-developed engineering analysis and design methods for highly plastic, highly overconsolidated materials. As a result, realistic engineering design must rely 

extensively on an empirical evaluation of previous experience with 

the Potomac sediments. The papers in this report are intended to 

present an overview of the nature of the problem and some different 

design methodologies that have been used. 

The first paper (by Obermeier) discusses the geologic setting of the 

Potomac Formation, the physical and chemical properties of its sediments, and the nature of its slope stability problems. The second and 

third papers (by Swanson and Jones and by Schnabel) have been written by geotechnical engineers associated with private consulting 

firms that have accumulated much experience, especially in the 

Washington, D.C., metropolitan area. These authors were invited to 

submit papers about slope design because they have design philosophies that differ somewhat from one another and thus offer a comprehensive perspective on the problems presented by the Potomac Formation. Inclusion of their papers does not represent an endorsement 

of their designs by the U.S. Geological Survey but is intended only to 

disseminate information and to provide a forum for discussion. 

The scopes of the two papers by the geotechnical engineers are 

somewhat different. Swanson and Jones discuss stability of cut, fill, 

and natural slopes for both the short term (during construction) and 

1 
2 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

the long term. Schnabel concentrates on design for the long-term 

stability of cut slopes in clay-rich sediments. 

The final paper (by Obermeier) is a critique and summary of the 

design engineers' papers and also points out some research needs. 

These papers are intended mainly for engineering geologists and 

geotechnical engineers. Some redundancies are inevitable because 

each paper is intended to be intelligible individually, except for the 

final critique and summary. 

Technical terms such as ((compactness," ~~consolidation," and 

others are used in the engineering sense in all four papers. usoil" is 

used interchangeably with usediment" because Potomac sediments 

behave as soils in the geotechnical sense. 

English units, followed by metric units in parentheses, are used 

throughout the text and in the tables. Conversion factors from 

English to metric units and semiquantitative terms for consistency 

and compactness follow the preface. Terms such as usoft," ustiff," 

uhard," ucompact," and uloose" are used in accordance with their 

semiquantitative definitions. 
SYMBOLS, TERMS, AND DEFINITIONS 

SYMBOLS, TERMS, AND DEFINITIONS 

c 1 Cohesion intercept of Mohr-Coulomb strength envelope, in 

terms of effective stress 

~ Shear displacement 

~Us Shear-generated pore pressure 

T Shear stress 

T~ Residual effective shear stress applied to specimen in direct 

shear apparatus 

a Normal stress 

a 1 Normal effective stress 

a~ Normal effective stress applied to specimen in direct shear 

apparatus 

cp~ Residual friction angle, defined by the residual coefficient of 

f . . ( change of T

1 

) r1cbon --. ----=--=---J.-- unit change of a~ 

Ko Ratio of horizontal to vertical effective normal stress 

U Unconfined compression shear strength 

UU Unconsolidated undrained shear strength, measured in 

triaxial test apparatus 

FS F to f £ t _ resisting forces (cohesion +friction forces ) 

ac r 0 sa e Y- driving forces driving forces 

CONVERSION FACTORS 

Metric unit English unit 

1m 3.28 ft 

1 em----------------------------------------- .39 in. 

1 rn/s ---------------------------------------- 3.28 ft/s 

1 kilogram force (kgf) --------------------------- 2.20 lb 

1 kilogram force per square centimeter (kgf/cm2

) -----2048 lb/ft2 

1 kilogram force per cubic meter (kgf/ m3) ---------- .062 lb/ft3 

3 
4 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

QUALITATIVE AND QUANTITATIVE EXPRESSIONS OF 

CONSISTENCY OF FINE-GRAINED SOILS AND COMPACTNESS 

OF COARSE-GRAINED SOILS 

Consistency 

Field 

description 

Fine-grained soils 

Very soft --------Easily penetrated several 

inches by fist. 

Soft ------------Easily penetrated several 

inches by thumb. 

Medium ---------Can be penetrated several 

inches by thumb with 

moderate effort. 

Stiff ------------Readily indented by thumb 

but penetrated only with 

great effort. 

Very stiff --------Readily indented by thumbnail. 

Hard -----------Indented with difficulty by 

thumbnail. 

Unconfined 

compressive 

strength, in 

kgf/cm2 

<1.0 

1.0-2.0 

2.0-4.0 

4.0-8.0 

8.0-16.0 

>16.0 

Standard 

Penetration 

Test 

resistance, in 

blows!ft 

<2 

2-4 

4-8 

8-15 

15-30 

>30 

Compactness 

Standard Penetration Test 

resistance, in blows/ft 

Coarse-grained soils 

Very loose--------------------------------------

Loose ----------------------------------------- Medium compact --------------------------------

Compact---------------------------------------

Very compact-----------------------------------

<4 

4-10 

10-30 

30-50 

>50 
ENGINEERING GEOLOGY AND DESIGN OF SLOPES 

FOR CRETACEOUS POTOMAC DEPOSITS IN 

FAIRFAX COUNTY VIRGINIA, AND VICINITY 

ENGINEERING GEOLOGY OF 

POTOMAC FORMATION DEPOSITS IN 

FAIRFAX COUNTY, VIRGINIA, 

AND VICINITY, 

WITH EMPHASIS ON LANDSLIDES 

By STEPHEN F. OBERMEIER 

CONTENTS 

Page 

Abstract ------------------------- 7 

Acknowledgments ----------------- 7 

Introduction---------------------- 7 

General setting-------------------- 9 

Geologic history ----------------12 

Landslide characteristics ---------16 

Overview of soil engineering 

properties -----------------20 

Penetration resistance, 

plasticity, strength, and 

consolidation properties -----20 

Shrink-swell properties -------22 

Regional significance of data ---24 

Potomac Formation characteristics 

relevant to slope stability -------24 

Sedimentation characteristics -----24 

Mineralogical characteristics -----29 

Properties of montmorillonite 

facies --------------------29 

Properties of kaolinite-illite 

facies --------------------32 

Page 

Potomac Formation characteristics 

relevant to slope stability-Continued 

Faults and other fractures --------32 

Properties of sheared zones, 

joints, and bedding-plane 

partings: Montmorillonite 

facies --------------------33 

Properties of joints and 

bedding-plane partings: 

Kaolinite-illite facies--------37 

Shear strength along joints 

and bedding-plane partings---37 

Properties of surface 

weathering fractures: 

Montmorillonite facies ------39 

Properties of surface 

weathering fractures: 

Kaolinite-illite facies--------39 

Shear strength along surface 

weathering fractures --------43 

Ground-water conditions ---------43 

Slope stability engineering -------45 

Selected references -----------------46 

5 
6 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

ILLUSTRATIONS 

Page 

FIGURE 1. Index map showing Potomac Formation outcrop belt from 

Virginia to New Jersey------------------------------------ 8 

2. Map showing Coastal Plain deposits in the principal study area ----10 

3. Block diagram showing geologic-topographic setting in 

Fairfax County, Va., and vicinity----------------------------13 

4. Photograph showing fault cutting Potomac Formation clay 

in Alexandria, V a ----------------------------------------15 

5, 6. Block diagrams showing: 

5. Typical landslide types in Potomac Formation sediments-----17 

6. Relationship of weak zones to local geologic-hydrologic 

features-------------------------------------------18 

7. Schematic cross section of excavation at Greenbelt, Md., 

showing part of a wall that failed ----------------------------19 

8. Diagram and graph showing typical strength relationships for 

drained direct shear tests on Potomac Formation sediments ------23 

9-17. Photographs showing: 

9. Typical interbedded strata of Potomac Formation sand 

and silty clay ----------------~----------------25 

10. Closeup of typical Potomac Formation sand and 

graveldeposit --------------------------------------26 

11. Typical outcrop of Potomac Formation thick sand deposit-----26 

12. Typical outcrop of thick Potomac Formation clay-rich 

deposit unconformably overlain by gravel----------------27 

13. Weathered, softened bedding surfaces in Potomac 

Formation clay-rich deposits --------------------------28 

14. Sheared zones in clay-rich Potomac deposits ---------------34 

15. Slickensides along a sheared zone in the Potomac Formation --35 

16. Well-developed vertical and subhorizontal joints in the 

montmorillonite facies -------------------------------36 

17. Typical softened zones in unweathered clay of the 

Potomac Formation ----------------------------------38 

18. Diagrams showing locations of softened zones along weathered 

and infilled partings and fractures of the Potomac Formation -----40 

19-21. Photographs showing: 

19. Surface fractures in clay-rich Potomac Formation sediments 

of montmorillonite facies -----------------------------42 

20. Surface fractures in clay-rich Potomac Formation sediments 

of kaolinite-illite facies -------------------------------44 

21. Water seeping from a sand-filled channel cut into 

Potomac Formation clay-rich sediments -----------------45 

TABLES 

Page 

TABLE 1. Geologic time chart and relation to major units of the Coastal Plain 

of Fairfax County, Va., and vicinity ------------------------------11 

2. Areal extent and dip of discrete thin layers in the Potomac Formation ----29 

3. Textural and chemical data and Atterberg limits for some unweathered 

clay-rich Potomac Formation samples-----------------------------31 
ENGINEERING GEOLOGY, POTOMAC FORMATION 7 

ABSTRACT 

Potomac Formation deposits crop out or are near the ground surface in a belt that 

includes parts oflarge metropolitan areas such as Washington, D.C., and Baltimore, 

Md. Both naturally occurring and construction-related landslides are commonplace 

and widespread in this belt and cause serious land-use problems. Landslides often take 

place many years after slopes are excavated in clay-rich sediments of the Potomac 

Formation, and the results can be disastrous to unsuspecting home owners. 

The Potomac Formation has two regional facies. In one, the clay minerals contain 

much montmorillonite; in the other, the clay minerals are dominantly kaolinite and 

illite. Unweathered clay-rich sediments of the Potomac Formation are typically highly 

overconsolidated and stiff to hard in consistency. The consolidation was imparted by 

the weight of younger deposits that have since been eroded away. Within the uppermost 20 ft (6 m) of the Potomac Formation, however, the sediments are generally 

weakened by physical and chemical weathering. Fractures, joints, and parting planes 

are also preferentially weakened along the breaks within this zone of weathering. 

Landslides generally take place in the zone of weathering. Engineering design of 

slopes in the clay-rich sediments is difficult and is often very subjective, especially 

where the Potomac Formation sediments are highly plastic. As a result, realistic 

engineering design must rely mainly on empirical evaluation of previous experience 

with the Potomac Formation. 

Relevant geologic factors that affect slope stability include the geologic history of 

Potomac Formation sediments, the nature of sedimentation, the properties of clays in 

the montmorillonite and kaolinite-illite facies, the properties of sheared zones, joints, 

surface weathering fractures and bedding-plane partings, and the ground-water setting. 

ACKNOWLEDGMENTS 

This paper is based largely on information collected by the author in preparation for 

a presentation at the 1977. American Society of Civil Engineers, National Capital 

Section, Geotechnical Engineering Seminar, Washington, D.C. The following people 

kindly consented to discuss their experiences and make their data available: James D. 

Hainley, Ackenheil & Associates, Baltimore, Md.; Carl G. Bock and Robert N. Evans, 

Bechtel Associates, Coral Hill, Md.; E. 0. Gooch, E. 0. Gooch & Associates, Charlottesville, Va.; Joseph B. Klein III, Law Engineering Testing Co., McLean, Va.; James J. 

Schnabel, Schnabel Engineering Associates, Bethesda, Md.; Ronald E. Smith, 

Woodward-Clyde Consultants, Rockville, Md.; Clarence S. Coleman, William C. Smith, 

and Stuart T. Terrett, County of Fairfax, Va.; John D. Glaser, Maryland Geological 

Survey, Baltimore, Md.; Ernest G. Gray, Maryland State Highway Commission, 

Baltimore, Md.; and Richard D. McConnell, Veterans' Administration, Washington, 

D.C. 

INTRODUCTION 

Both naturally occurring and construction-related landslides are 

commonplace and widespread in the belt of Lower Cretaceous 

Potomac Formation1 deposits extending from Virginia to New Jersey 

(fig. 1). The landslides usually occur in generally strong, massiveappearing silt and clay and in sand-clay strata. Slopes underlain by 

'The Potomac Formation is raised in rank to Potomac Group and divided into three formations in areas of 

Maryland and in Washington, D.C. In Virginia, it is considered to be of formation rank only. 
8 ENGINEERING GEOLOGY, CRETACEOUS OF FAffiFAX COUNTY, VA. 

PENNSYLVANIA 

l'J·~ 

If 

EXPLANATION 

Potomac Formation outcrop belt 

Kl LOM ETERS: 

Predominantly kaolinite and illite (northern clay facies) 

Predominantly montmorillonite (southern clay facies) 

Transition between montmorillonite facies and kaolinite-illite facies 

FIGURE 1.-Potomac Formation outcrop belt from Virginia to New Jersey 

(after Force and Moncure, 1978). 
ENGINEERING GEOLOGY, POTOMAC FORMATION 9 

the Potomac Formation are a classical example of areas where slope 

stability cannot be evaluated indiscriminately by conventional 

design techniques. Conventional slope stability analysis methods 

using peak shear strength parameters often result in factors of safety 

that are much too high, on the basis of field experience. Residual shear 

strength parameters are appropriate for some geologic settings but 

too conservative for others. A major problem is selection of strength 

parameters that will ensure a safe yet economical design. 

In order to document the mechanisms that cause the landslides and 

to determine the factors involved in selecting strength parameters, 

this author met with engineers and geologists from numerous consulting firms and government agencies in the greater Washington, 

D.C.-Baltimore, Md., area. He also studied about 20 landslide case 

histories documented by consulting engineers. Field investigations 

were then undertaken to evaluate additional factors that could contribute to the landslides. These investigations were made primarily 

in outcrops of the Potomac Formation in the Coastal Plain of Fairfax 

County northward to Arlington County, Va., and the city of Alexandria, Va., and in Washington, D.C. (fig. 2). This area is the uFairfax 

County, Virginia, and vicinity" of the title. These studies were mostly 

restricted to the montmorillonite clay facies shown in figure 1, but 

limited investigations were made in the kaolinite-illite clay facies as 

far north as the northern limits of the Chesapeake Bay. 

This report summarizes the results of the meetings with engineering practitioners, studies of landslide case histories, and the author's 

own field investigations. Specific recommendations for slope design 

are generally not made in this paper; rather, an attempt is made to 

identify relevant factors that cause the landslides. Although the 

report is applicable specifically to the greater Fairfax County erea, 

most comments are thought to be valid for Potomac sediments 

throughout the region shown in figure 1. 

GENERAL SETTING 

The portion of the Atlantic Coastal Plain physiographic province 

shown in figure 1 is bounded by the Piedmont province on the west 

and the Continental Shelf on the east (not shown in fig. 1). Much of 

the Coastal Plain is separated from the Piedmont province by the Fall 

Line (or Fall Line Zone), the topographic feature where major streams 

rapidly descend from the crystalline uplands into tidal rivers, estuaries, and embayments. The Coastal Plain is a wedge of continental 

and marine sediments that thicken from a few feet on the western 

boundary to many thousands _()f feet at the coastline of the Atlantic 

Ocean. Potomac Formation deposits of Early Cretaceous age are the 

basal sediments throughout much of the Atlantic Coastal Plain. 
10 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

/'-·. 

/ 

/ 

/ 

N 

1/ 

/ 

/ 

/ 

LOUDOUN // 

COUNTY/ 

/ 

/ 

/ 

/ 

/ 

I 

/ 

// 

/ 

/ 

/ 

/ 

/ 

/ 

FAIRFAX 

COUNTY 

(_.--;; 

\ PIEDMONT PROVINCE 

\ . ..._ 

l_·,.-~r' 

0 

L------~------~ 

MONTGOMERY 

COUNTY 

DISTRICT OF 

EXPLANATION 

Coastal Plain sediments 

Potomac Formation outcrops 

FIGURE 2.-Coastal Plain deposits in the principal study area. 

Potomac sediments are commonly overlain in part by Upper Cretaceous and marine strata, but, in some places, they are overlain by 

Tertiary and Quaternary deposits and are separated from them by 

major unconformities. Principal geologic events and geologic units in 

Fairfax County and vicinity are shown in table 1. 

The length of the Potomac outcrop belt in figure 1 exceeds 200 mi 

(320 km), but the width rarely exceeds 15 mi (25 km). Erosional outliers too small to be shown on the map occur along the western edge of 

the belt and south of Fredericksburg, V a. The maximum thickness of 

the Potomac Formation in the vicinity of the outcrop belt is perhaps 

1,200 ft (370m). 

In Maryland and in Washington, D.C., the Lower Cretaceous strata 

are subdivided into three nonmarine formations that comprise the 

Potomac Group: (1) the basal Patuxent Formation, (2) the lenticular 

middle unit, the Arundel Clay, and (3) the upper unit, the Patapsco 

Formation. These formations are not separable by field mapping 
TABLE !.-Geologic time chart and relation to major units of the Coastal Plain of Fairfax County, Va., and vicinity 

Absolute age, Map symbol Range of thickChronostratigraphic units in years Geologic units (fig. 3) Origin ness, in feet 

Holocene Alluvium, dominantly sand, Q 0-20 (avg., 10) 

{ Pleistocene 

silt, or clay, highly variable. tz:j 

z Quaternary system 10,000 _Unconformity Nonmarine Q 

H 

Terrace deposits, various Q (fluviatile). 0-150 (avg., 35) z 

levels, dominantly gravel or tz:j 

tz:j 

sand in silt or clay matrix. :::0 

H 

2,000,000 _Unconformity z 

Q 

Pliocene Upland gravel deposits, Tug 0-50 (avg., 35) 

various levels, dominantly ~ 0 

sand and gravel. t"-4 

0 Miocene _Unconformity Q 

Tertiary system < Dominantly clayey and silty -- Marine 0-400 (avg., 200) ~~ 

sand deposits of great '"d 

0 

Oligocene lateral extent, separated 8 

0 by multiple unconformities. lS: 

Eocene > 0 

Paleocene "%j 

63,000,000 0 

{ 

:::0 

Upper lS: 

Cretaceous system _Unconformity ~ Lower Potomac Formation, Kp Nonmarine 0-1,200 (avg., 500) H 

0 

dominantly sand, silt, or (fluviatile z 

clay layers, highly variable. and deltaic?). 

138,000,000 _Unconformity 

Precambrian Crystalline rocks Metamorphic 

and igneous. 

~ 

~ 
12 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

south of the Potomac River (Drake and others, 1979), and the sequence 

is called the Potomac Formation. In this report, the Potomac sequence 

is considered as the Potomac Formation, irrespective of geographic 

locale. 

The Potomac deposits have at least two regional sedimentary facies 

that show no sharp lateral demarcation; one is orthoquartzite sand 

interbedded with clay-rich beds of kaolinite and illite north of the 

Potomac River (the kaolinite-illite facies), and the other is arkosic 

sand interbedded with montmorillonite-rich clay layers (the montmorillonite facies) south of the river. Glaser (1969) has hypothesized 

that these facies result from different source provenances, and Force 

and Moncure (1978) have presented supporting data. Irrespective of 

the facies, however, the upper part of the Potomac sequence generally 

has more clay than the lower part. 

GEOLOGIC HISTORY 

Potomac Formation sediments were deposited on subsiding crystalline rocks by freshwater rivers meandering along the inner margin of 

the Coastal Plain and are characterized by crossbedding, channels 

filled with clays or sands, rapid pinching and thickening of beds, and 

lateral and vertical size grading. Continued subsidence and tilting of 

the region resulted in a gentle eastward dipping of the sediments. 

Possibly, some of the deposits are deltaic in character, formed where 

rivers debouched into brackish or marine water. Small- and largescale erosion surfaces caused by ancient meandering rivers are common within this group of sediments. 

Potomac Formation deposits were intermittently eroded and were 

overlapped by primarily fine grained sediments, which were deposited 

in an oscillating marine environment during Late Cretaceous and 

Tertiary times. A major regional unconformity, during which hundreds of feet of Potomac strata were eroded away in many places, 

separates the Potomac Formation from the overlying marine 

deposits. These marine deposits are presently preserved east of the 

Potomac River (fig. 3). In many places, a gravel layer a few feet thick 

is present immediately above this unconformity. Above the basal 

gravel layer, the marine sands and clays tend to be in thin beds, commonly of great lateral extent. 

The seas next withdrew and subjected the area to subaerial erosion. 

Fluvial sands and gravels that were spread over the eroded surface 

now unconformably overlie the marine strata and the Potomac 

sequence and locally extend to the Piedmont rocks. During the late 

Tertiary, an ancestral Potomac River system (Hack, 1955; Schlee, 

1957) apparently deposited broad alluvial aprons of sand and quartz 
~ 

~ 

Potomac 

Formation 

Saprolite 

Crystalline 

rocks 

EXPLANATION 

Alluvium, terrace deposits (QUATERNARY) 

Upland gravels (TERTIARY) 

Potomac Formation (CRETACEOUS) 

FIGURE 3.-Geologic-topographic setting in Fairfax County, Va., and vicinity. 

trl 

z 

0 

~ 

z 

trl 

trl 

:::tl 

~ 

z 

0 

0 

t%j 

0 

t-t 

0 

0 

~~ 

""0 

0 

t-3 

0 

~ 

> 0 

"%j 

0 

:::tl 

~ 

> t-3 

~ 

0 

z - ---····-··--

Potomac 

Formation 

I-" 

c..:l 
14 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

gravel at various steplike levels across low relief plains. These 

deposits are commonly referred to as the Hupland gravels." 

In Fairfax County, the upland gravels are believed to be a succession of at least four terraces (Force, 1975; Drake and others, 1979) 

that are 0.5 to 2 mi (0.8-3.2 km) wide and locally up to 60 ft (18 m) 

thick (average about 20ft (6 m)). The terraces descend stepwise to the 

tidal Potomac River. The upland gravel terraces have been dissected 

extensively during periods of erosion as the streams cut to lower 

levels. The older deposits are found only as caps of isolated hilltops. 

The relationship of terraces and other geologic units to topography in 

the area along the Potomac River immediately south of the Nation's 

Capital is shown in figure 3. 

-During the Pleistocene, continental glaciers did not reach Washington, D.C., but glaciation had a profound effect on erosion and deposition in the area. Sea level was hundreds of feet lower during times of 

maximum glaciation than it is at present and caused major downcutting of Coastal Plain valleys (Hack, 1957). During recession of the 

glaciers, stream flow increased, sea level rose, and comparatively 

coarse grained materials were deposited. As a warming trend continued, valleys were inundated and fine-grained sediments laid down. 

As a consequence of the multiple glacial advances and retreats, a 

series of Pleistocene river terrace deposits was left behind. The terraces are a mixture of silty and sandy clay and sand and gravelly 

sand, complexly interlayered and lensed. Most of downtown 

Washington, D.C., is underlain by these terrace deposits, which have 

an average thickness on the order of 30 ft (9 m), although, locally, 

they may exceed 60 ft (18 m). In the ancient channels of the Potomac 

River in Fairfax County, the deposits are as much as 160ft (50 m) 

thick. 

During the last 5,000 or 6,000 years, sea level has risen 25 to 35 ft 

(8-11 m) with respect to land along the northern Atlantic coast. This 

rise submerged the Potomac and Anacostia Rivers to the Fall Line 

and deposited fine-grained alluvium in river channels. 

Clay, silt, and clay-bearing sand of the Potomac Formation have 

been consolidated to a generally stiff to hard consistency, principally 

by the weight of sediments subsequently stripped away. In many 

places near the ground surface and along the unconformity separating the Potomac Formation from younger sediments, however, intense chemical and physical weathering have greatly softened the 

Potomac sediments. Numerous joints and fractures are present in 

many localities, especially in near-surface deposits of weathered finegrained sediments. 

Post-Cretaceous parallel faults at the Fall Line and through the 

Potomac Formation extend at least from the southern part of Fairfax 

County to Fredericksburg, Va. (fig. 1) (Mixon and Newell, 1977). 
ENGINEERING GEOLOGY, POTOMAC FORMATION 15 

These faults are within the Stafford fault system, a zone at least 

thousands of feet wide. Within the fault system, the faults having the 

largest offsets trend northeast, but smaller faults are present at 

various strikes (Prowell, 1983) and at unpredictable locations. The 

major zones of shearing are commonly steeply dipping reverse faults 

thought to have a strike-slip component (D. C. Prowell, unpub. data). 

Cumulative vertical displacement exceeds 300ft (90 m) near Fredericksburg (Mixon and Newell, 1977). Other fault systems in the Coastal 

Plain have been documented about 20 mi (33 km) east of Fairfax 

County (Jacobeen, 1972) and in Washington, D.C. (Darton, 1951). 

In Fairfax County, several minor faults and associated swarms of 

fractures near the faults have been documented (Langer and 

Obermeier, 1978; Froelich, 1978) in the Potomac Formation deposits. 

In most places in Fairfax County, the fault displacements are probably less than 1 to 2ft (0.3-0.6 m). Figure 4 shows one of these small 

faults exposed in a stream valley. The orientation and mode of occurrence of these faults strongly suggest that they are associated with 

the tectonic activity south of the county. Many geologists suspect that 

this faulting extends north of Washington, D.C., possibly through the 

Potomac Formation, and the author has observed fractures thought 

to be faults through the Potomac Formation near the northern end of 

Chesapeake Bay. 

FIGURE 4.-Fault cutting Potomac Formation clay in Alexandria, Va. Fault passes 

along line indicated. 
16 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

LANDSLIDE CHARACTERISTICS 

Some typical landslide types are shown in figure 5. Slumps and 

planar glide blocks are predominant in occurrence and economic significance. Minor sliding, usually of small economic significance, is 

also caused by sloughing of very steep exposures of clay and silt on 

slopes exceeding about 50 percent, although sloughs can occur on 

slopes as low as 30 percent. Many slopes showing no evidence of landslides display evidence of creep in the form of trees bent at the base. 

Slumps (fig. 5A) are the most common type of landslide and are normally about 50 to 200ft (16-63 m) from crown to toe, although saturated clayey soil at the toe can be remolded to the consistency of a 

viscous fluid and flow hundreds of feet. Slumping is commonly retrogressive (that is, new portions of the surface of rupture form progressively upslope in previously unfailed soils). Some natural landslides 

have pressure-induced ridges and hummocks near their toes. The 

length of the slide along the slope generally equals or exceeds the 

length of rupture down the slope. The maximum depth of the surface 

of rupture ranges from about 15 to 20ft (4.6-6 m) for most slumps, the 

deepest recorded being about 40 ft (12 m). 

Local depositional characteristics and physical properties of the 

Potomac Formation are very important with respect to locations of 

sliding. Ancient channels filled with sand provide conduits for subsurface water migration. Relative positions of coarser and finer 

grained materials can serve to improve or reduce stability. These 

stratigraphic details, together with the severity of weathering and 

the presence of structural features (such as sheared zones, ancient 

landslide slip surfaces, and partings and fractures of different types), 

control the types and locations of the slope failures. These landslides 

can occur on natural or construction-altered slopes, either during construction or many years later. 

Figure 6 shows the relationship of geologic-hydrologic features to 

weakened zones commonly critical to landsliding. For many slumps, 

the surface of rupture dominantly traverses soils greatly softened by 

near-surface weathering and cut with fractures or goes in part along 

a particularly weak, weathered stratum or bedding-plane parting or a 

fracture. The crown of the slump also heads in a very permeable sand 

and gravel cap in many places. The unconformity separating the 

Potomac Formation from younger sediments is a locus of concentration for both slumps and planar glide blocks, especially where the 

Potomac Formation is overlain by water-bearing, relatively permeable sand-and-gravel-filled channels. Planar glide blocks (fig. 5B) 

develop most often where the channel dips in the direction of the 

slope. Planar glide blocks are often not recognized in the field, possi-
B 

ENGINEERING GEOLOGY, POTOMAC FORMATION 

I 

/ Potomac 

Formation 

sediments 

FIGURE 5.-Typical landslide types in Potomac Formation sediments. A, Slump. B, Planar glide block. 

17 
18 ENGINEERING GEOLOGY, CRETACEOUS OF FAffiFAX COUNTY, VA. 

Physically and chemically weathered clay 

and sand, 0 to 5 ft 

(Q-1.7 m) thick---._ 

Ancient landslide surface of 

rupture, 1 in. (2.5 em) 

thick or more 

Sand and gravel cap 

High-angle joints 

Highly plastic clay 

Subhorizontal joint 

High-permeability 

sand layer 

Inclined surface on 

clay stratum 

Low-permeability 

clay 

Chemically weathered sand 

and clay, 0.1 in. (0.25 em) 

thick or more 

Tectonic shear zone, 1 in. (2.5 

em) thick or more 

FIGURE 6.-Relationship of weak zones to local geologic-hydrologic features. Arrows 

indicate locations of concentrated water flow. 

bly because soils at the toe have a tendency to flow when they are 

remolded. 

Slides are rather commonplace on natural terrain underlain by 

either massive clay or by interbedded sand and clay where long slopes 

exceed 15 percent, but slides can also begin on flatter slopes. Field 

mapping by the author has shown that landslides are so common on 

the steep slopes that they must be a major factor in landform development on a geologic time scale (periods on the order of thousands of 

years). 

Catastrophic movements of topples are encountered in construction 

excavations having steep walls. Opening of relict joints is thought to 

have been responsible for a major construction disaster that killed 

five men in Greenbelt, Md., a suburb of Washington, D.C. (Withington, 1964). A schematic cross section showing the setting of the accident is given in figure 7. According to Withington, relict joints in the 

clay opened slightly during excavation because of dessication and 

lack of lateral support. Surface water probably flowed into the joints 

and toppled an unsupported clay slab about 40ft (12 m) long, 15 ft (4.6 

m) high, and 1ft (0.3 m) thick. 

Construction-period problems also have been reported for trenches 

excavated in uncemented sands (Cleaves, 1968). Where flowing water 

is present, uncemented sands are susceptible to caving in and also 
ENGINEERING GEOLOGY, POTOMAC FORMATION 19 

/Original ground surface 

.......--,--...,..--,--.....V··. . . . . . . . . . . . . . . . . . . . • .........•..............•.•.•......•...•............•...•....... 

> Ctl 

(3 15ft (4.6 m) t: 

0 (i) 

-~ Q.l 

'(3 E E Ctl 

0 -Q.l ~ "" -~ .::::: 

cU co Failed clay slab -~ t: ~ 

0 

Ctl 

:::. 

E 

L!) Concrete ~ form .::::: 

co 

I; tt (2.1 rr:)l 

Relict fractures 

FIGURE 7.-Schematic cross section of excavation at Greenbelt, Md., showing part of 

a wall that failed (after Withington, 1964). 

have the potential for undermining clay beds. This problem is most 

common in very weakly cemented, clean sands associated with the 

kaolinite-illite facies and, to a much lesser extent, in sands associated 

with the montmorillonite facies, because these sands are commonly 

bound with clay. 

Landslide initiation is nearly always closely associated with 

ground-water-related mechanical forces. The water may be in the 

more permeable sand-rich strata, under confined conditions, or in 

joints or near-surface fractures. The slides normally begin (or at least 

move enough to be easily noticed) after a prolonged rainy season or 

after an intense rainstorm, which commonly causes increased porewater pressure. Years may elapse before movement starts again in 

response to another prolonged rainy season or intense rainstorm, but 

the next small rain may be sufficient to reactivate sliding. Casual 
20 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

observations by many investigators suggest that the rate of movement is highly variable during the early stages of development; 

observed scarps of about 0.5 in. (1 em) to 3 ft (1 m) or more can develop 

at the crown within a day. The rate of movement tends to increase as 

a landslide develops until the slide finally moves rapidly (up to 10 

ft/d) and returns to equilibrium. Once a landslide has begun to move, 

corrective measures are often undertaken as soon as practicable 

because, at many places, the slides fail progressively and continue to 

enlarge upslope. 

OVERVIEW OF SOIL ENGINEERING PROPERTIES 

The generalizations on engineering properties that follow apply to 

the montmorillonite facies of the greater Fairfax County area and are 

based on hundreds of borings and many laboratory test data. Properties described are penetration resistance, plasticity, strength, and 

consolidation characteristics for both the unweathered and the 

weathered condition, swelling properties, and the regional significance of the data. 

PENETRATION RESISTANCE, PLASTICITY, STRENGTH, AND 

CONSOLIDATION PROPERTIES 

Standard Penetration Test (American Society for Testing and Materials, 1978) blow counts on unweathered clay typically vary between 

30 and 100 blows/ft. On weathered clay, the blow counts generally 

range between 10 and 35, although lower values are occasionally 

observed. The blow counts for unweathered arkosic sand typically 

range between 30 and 60, but counts are higher at many places and 

can exceed 100. Where the sand is weathered, within about the upper 

10ft (3 m), the blow counts commonly range between 10 and 40; scattered lower values also occur. 

The liquid limits of the clays in the thick massive clay bodies average about 60 to 70 but can exceed 100, and the plastic limits average 

in the high 20's and low 30's. At many places in these thick bodies, 

the clays are classified as CH. 2 Some unweathered clay samples do 

not break down easily during remolding for Atterberg limit testing 

but form very small, gritty clasts. As remolding continues, the clasts 

are further broken and the clay fraction increased; thus, the liquid 

limit, the plastic limit, and the plasticity index increase. 

The natural water content is normally less than the plastic limit at 

depths greater than 20 to 30 ft (6-9 m) beneath the surface where 

Potomac clays crop out. At shallower depths, the natural water con-

•Capitalized letters refer to the Unified Soil Classification System (Wagner, 1957). 
ENGINEERING GEOLOGY, POTOMAC FORMATION 21 

tent tends to be higher and often exceeds the plastic limit by as much 

as 10 percent. The relation between natural water content and plastic 

limit is rather erratic as a function of depth at many places, however, 

even in the same borehole. This erratic variation probably results 

from the multiple water entries in sand lenses, joints, fractures, and 

sheared zones. 

Weathering effects are normally most intense on hilltops, as the 

higher water contents of intact clays and the oxidization of feldspathic 

sands indicate. On slopes, slumping and erosion have apparently 

removed much of the intensely weathered soil. 

Consolidometer tests on unweathered samples commonly have preconsolidation values 20 to 40 kips per square foot (ksf) (10-20 kfg/cm2

) 

in excess of existing overburden, even on hilltops. Preconsolidation 

values tend to increase with depth by an amount approximately 

equal to the increased overburden stress. Therefore, it appears that 

the high preconsolidation has been imparted primarily by the weight 

of overlying sediments, which were later removed by erosion. 

Weathered and softened samples commonly have much lower preconsolidation values; in many places, near-horizontal thin zones along 

softened fractures have preconsolidation values less than or equal to 

the present overburden stress. 

For intact unweathered samples, the unconfined compression (U) 

and unconsolidated undrained (UU) shear strengths of clay-rich sediments commonly range between 4 and 6 ksf (2-3 kgf/cm2

). Shear 

strengths of unweathered clay-rich sediments are commonly reduced 

by the presence of fractures and slickensides. Peak UU strengths on 

fractured and slickensided samples are very erratic and average 

about one-half the values for intact samples. Axial strains at peak 

strengths on the unweathered samples generally range between 2 

and 8 percent. 

The undrained shear strength of intact weathered clay averages 

about 2 ksf (1 kgf/cm2) near the ground surface and along the unconformity at the top of the Potomac Formation. In many localities, however, the soil is weakened only a little along the unconformity. The 

natural water content of the softer soils is significantly higher than 

that of soils in the unweathered condition. 

Clays and clay-cemented sands above a landslide surface of rupture 

typically have peak undrained shear strengths between 0.5 and 2.0 

ksf (0.25-1.0 kgf/cm2), but, occasionally, they are so hard that they 

cannot be sampled with thin-wplled tubes. Landslide materials are 

usually highly fractured and broken or remolded by the movement. 

For normally consolidated, highly plastic clays, the effective friction angle of the peak shear strength in drained direct shear tests 

averages about 25 o and increases to about 35 o for the clayey sands. 

The residual drained direct shear strength of unweathered soils is 
22 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

much lower than the peak shear strength, as figure 8 shows; the ratio 

of residual strength to peak strength is commonly quite large for 

overburden stresses equivalent to about 10 to 20 ft (3-6 m) of soil. 

Angles of 1"2 o to 15 o are common for clays, and residual friction 

angles as low as 6 o have been reported for highly plastic clays. Figure 

8 also shows that there can be a significant difference between peak 

and residual shear strengths of intensely weathered and softened 

clay. 

SHRINK-SWELL PROPERTIES 

The highly plastic montmorillonite clay has a high potential for 

shrinking and swelling and can cause problems for houses and roads 

wherever it occurs. Large vertical movements of a house foundation 

can be caused by alternate wetting and drying of clay beneath the 

foundation. Some houses have been destroyed, and economic losses 

caused by this problem in Fairfax County may exceed those caused by 

landslides. To reduce these destructive effects (Dallaire, 1976), Fairfax County has enacted ordinances requiring that foundations on 

highly plastic clays be at least 4 ft (1.2 m) beneath final exterior 

grade, that foundations be drained, and that trees be kept away from 

foundations. 

Other swelling-related problems include collapse of basement walls 

where highly plastic clays have been used as backfill material. 

Damage to roads occurs in the form of distorted pavements and curbs 

and weakened subgrades. 

Swelling is also a factor in causing landsliding in that it must be an 

effective mechanism for the destruction of diagenetic bonds and the 

consequential weakening of the soil, in the sense discussed by 

Bjerrum (1967). It seems likely that diagenetic bonds are broken to 

depths of at least 10 to 20ft (3-6m) in the highly weathered plastic 

clays, where the natural water content approaches or exceeds the 

plastic limit. Within a few years, at construction sites where water 

can enter into permeable sand or fractures in unweathered deposits, 

the bonds in clay may be largely destroyed in a thin, swollen zone 

adjacent to the path of flowing water. The softening and swelling produce stresses in surrounding materials. These stresses, of course, lead 

to further softening and weakening along the fractures and more 

fracturing. Mter many years, at places where creep and slope movements are very small, the shear strength is probably further reduced 

over a large area to a value approaching that retained by consolidation under existing vertical stresses. Where there is much creep, the 

strength may be much lower. 
ENGINEERING GEOLOGY, POTOMAC FORMATION 

Shear _j 

displacement I a (normal stress) 

••••• • -· •• • • : • t • • • • 

T ------+~· ••• ·.·.?<?II· .....•. 

23 

. . . . . . . ~?----- -r (shear stress) 

A 

en 

en 

UJ 

0:: 

~ 

en 

0:: 

<( 

UJ 

I 

en 

B 

Peak shear 

strength 

. . . 

a 

Very stiff to hard 

unweathered soils 

Residual drained direct 

shear strength 

Intensely weathered and softened soils 

d (SHEAR DISPLACEMENT) 

FIGURE 8.-Typical strength relationships for drained direct shear tests on Potomac 

Formation sediments. A, Direct shear test stress and displacement conditions. B, 

Plot of shear stress versus shear displacement constant normal stress. 
24 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

REGIONAL SIGNIFICANCE OF DATA 

The preceding generalizations apply in the main not only to montmorillonite-facies deposits outside Fairfax County and vicinity but 

also to clay- and silt-rich deposits of the kaolinite-illite facies. Many 

thick clay beds in the kaolinite-illite facies have sufficient plasticity 

to be designated as CH soils, but clays having a large shrink-swell 

potential are probably not as commonplace as they are in the montmorillonite facies. 

Sand-rich deposits of both the montmorillonite and the kaoliniteillite facies are generally compact; a major difference is that sands in 

the kaolinite-illite facies are commonly clean quartz and only weakly 

cemented, although, at some places, lenses are strongly cemented. 

POTOMAC FORMATION CHARACTERISTICS RELEVANT 

TO SLOPE STABILITY 

Specific geologic and engineering geologic characteristics that bear 

on investigations of slope stability include sedimentation and mineralogical characteristics, faults and other fractures, and ground-water 

conditions. Each characteristic is discussed separately, by facies if the 

difference between facies is important, and related to slope stability. 

SEDIMENTATION CHARACTERISTICS 

The nature of the bedding is commonly closely involved with landsliding in that thick clay-rich deposits are more prone to sliding than 

thin-layered sand and clay strata are. In layered strata, the surface of 

rupture can follow a single thin lens for a long distance. 

Sedimentation characteristics of the Potomac Formation (Glaser, 

1969) include large and abrupt variations in lithology, both vertically 

and laterally, and variable combinations of interbedded and intertongued layers of gravel, sand, silt, and clay (see fig. 9 for example). 

As a result, the composition and grain size of the soils in a borehole 

may differ greatly from those in a location only 10 feet (3 m) a way. 

Gravel-bearing sands (fig. 10) are present at some places within the 

lowermost 100 feet (30 m) of the Potomac Formation but are much 

less common higher in the section. The section generally fines upward, a notable exception being some extensive, continuous, sandfilled channels at different levels. Sands are generally crossbedded, of 

medium texture, and uniformly graded (fig. 11). 

Fine-grained sediments of the Potomac Formation were deposited 

as overbank flood-plain deposits or as fillings in abandoned meanders. 

These sediments occur in irregular or lenticular bodies interbedded 

wfth-thesands and gravels at all levels. Remnants of bedding are 
FIGURE 9.-Typical interbedded strata of Potomac Formation sand (light colored) and silty clay (dark colored). 

t;r.:j 

z 

0 

z 

tzj 

tzj 

~ 

z 

0 

0 

tzj 

0 

t"' 

0 

0 

~~ 

~ 

0 

t-3 

0 

~ 

~ (":) 

~ 

0 

~ ~ 

~ 0 

z 

t:-.:) 

01 
26 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

FIGURE 10.-Closeup of typical Potomac Formation sand and gravel deposit. Entrenching 

shovel shown for scale. 

FIGURE H.-Typical outcrop of Potomac Formation thick sand deposit. 
ENGINEERING GEOLOGY, POTOMAC FORMATION 27 

variably preserved in the clay-rich strata (figs. 12, 13), but the thicker 

bodies are commonly devoid of any trace of internal stratification. 

The areal extent and dip of an individual stratum are generally 

functions of bed thickness and texture. Fine-grained and thick beds 

tend to persist for greater distances. Although individual thin beds of 

sand, silt, or clay commonly change lithology or end laterally over a 

short distance, thick beds can be continuous for thousands of feet. 

Characteristic areal extents and dips of the thinner strata are shown 

in table 2. All dips shown in table 2 can continue in a clay bed for 100 

feet (30 m) or more downslope, a distance long enough to be important 

to the types of landslides commonly occurring in the Potomac 

Formation. 

The regional tilt of the formation (that is, the dip of the base of the 

formation) is generally toward the southeast and varies between 15' 

and 50' (25-75 ft/mi) (Glaser, 1969). The steepest regional tilts are 

usually in the westernmost sediments. 

Deposition-related slump features are commonplace and are 

evidenced by crumpled beds and thoroughly mixed materials. These 

ancient slumps were subsequently consolidated by the weight of the 

overburden, and they presently have no inherent weaknesses that 

are particularly important to modern landsliding. Irregular undulatory bedding is common and undoubtedly the result of differential 

compaction in most cases. 

FIGURE 12.-Typical outcrop of thick Potomac Formation clay-rich deposit unconformably overlain by gravel. 
28 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

FIGURE 13.-Weathered, softened bedding surfaces (thin, light-colored zones) in 

Potomac Formation clay-rich deposits. Entrenching shovel shown for scale. 
ENGINEERING GEOLOGY, POTOMAC FORMATION 29 

TABLE 2.-Areal extent and dip of discrete thin layers in the Potomac Formation 

[Based on detailed field observations throughout the Baltimore, Md., area and supplementary observations 

throughout the greater Fairfax County, Va., area. Data by Juergen Reinhardt (written commun., 1978)] 

Areal extent,2 in feet (meters) Dip,3 in percent 

Material1 Bed thickness Maximum4 Average ·Maximum Average 

Clay ---------- 1 in. (2.5 em) 100 (30) 5 (1.5) 25 ::5;2 

Do.---------- 6 in. (15 em) 500 (150) 25 (8) 20 ::5;2 

Do.---------- 3ft (1 m) 1,000 (300) 100 (30) 10 ::5;2 

Silt ----------- 1 in. (2.5 em) 20 (6) 3 (1) 35 ::5;3.5 

Do.------·---- 6 in. (15 em) 100 (30) 10 (3) 20 ::5;3.5 

Do.---------- 3ft (1 m) Not observed 

Sand---------- 1 in. (2.5 em) 10x5 1 (0.3) 55 ::5; 10 

(3xl.5) 

Do.---------- 6 in. (15 em) 100 X 50 5 (1.5) 35 ::5; 10 

(30x15) 

Do.---------- 3ft (1m) 500x100 25 (8) 20 ::5;5 

(150x30) 

'Materials approximate end members of clay, silt, or sand beds; that is, they are comprised almost entirely of clay, 

silt, or sand. 

'Maximum areal extent values are probably rare. 

•Observed dip of beds is a result of initial (depositional) dip plus tectonic dip owing to regional tilt. 

•Bodies approximately equidimensional except where two dimensions are listed. 

Another common deposition-related feature is clay clasts in sand or 

gravel beds, particularly along the base of the beds. These clasts are 

roughly spherical balls ranging from 0.1 in. to 3 ft (0.3 cm-1 m) or 

more in diameter, the smaller sizes typically being most abundant. In 

some places, the clasts comprise the major proportion of the sand- or 

gravel-sized materials, and the shear strength of the mass can be controlled by properties of the clasts. 

MINERALOGICAL CHARACTERISTICS 

Force and Moncure (1978) have described primary and weatheringrelated clay minerals of the Potomac Formation. Their work, in conjunction with Glaser's (1969) treatise, comprises the most thorough 

extant mineralogical studies. 

PROPERTIES OF MONTMORILLONITE FACIES 

The clay-rich beds in the montmorillonite facies are dominated by 

montmorillonite-illite mixed-layer clay and relatively pure montmorillonite; these beds ~lso have some layers of almost pure montmorillonite or illite or of clay mixtures of montmorillonite and illite 

containing small amounts of vermiculite or kaolinite. Clay beds in 

the southern third of Virginia have much more mixed-layer clay and 
30 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

illite. Exchangeable cations in unweathered clay-rich Potomac Formation sediments in Fairfax County are dominantly Ca++ and Mg++; 

greatly subordinate N a+ and K + are also present (table 3). Dominant 

cations are probably the same or very similar throughout the montmorillonite facies. 

The unweathered clay-rich layers in the montmorillonite facies are 

waxy and commonly contain a notable proportion of silt. Their colors 

vary considerably. These layers are commonly gray or greenish 

brown but, in many places (usually the overbank deposits), are variegated yellow and red owing to brightly colored iron oxides precipitated during or very shortly after deposition. In some places (usually 

the meander fillings), the clay-rich layers have been colored dark 

gray or black by carbonaceous matter and contain pieces of plants 

and woody material. 

Unweathered dark-colored clay-rich sediments (that is, gray, 

greenish-brown, black, and some reddish-tinted sediments) have inclusions of pyrite or marcasite in some places. Water introduced into 

or flowing over these sediments is usually highly acidic (pH 2-3) 

(Wagner and others, in press). The color can change to a yellow brown 

within a few hours after exposure to air and water, probably because 

iron oxidization is accelerated by acidic water. 

Force and Moncure (1978) have shown that the montmorillonite is 

currently weathering to other clay minerals (dominantly kaolinite) 

where it has been softened and leached by freely flowing ground 

water. Tests by the author and by Wagner and others (in press) show 

that, in many places, the Ca++ cations have been largely removed 

from the soft clays. Highly acidic water probably greatly accelerates 

the rate of extraction of Ca ++ and the alteration of montmorillonite to 

kaolinite. The rate of alteration of montmorillonite to kaolinite is 

still probably slow (say, 100 years) in relation to man's actions, but it 

is possible that, in some places, the cations can be extracted quite 

easily and quickly. Alteration of montmorillonite to kaolinite almost 

certainly increases the residual shear strength of the clay (Mitchell, 

1976); the removal of cations has an unpredictable influence (Mitchell, 

1976) but probably decreases the residual shear strength. 

With few exceptions, the sand-sized fraction of sand layers in the 

montmorillonite facies consists of potash and plagioclase feldspars 

and quartz, which are typically cemented with kaolinite, clasts of 

montmorillonite-rich soil, and possibly some iron oxides and silica. 

The feldspars typically make up a large percentage of the sample and 

are intensely weathered. The kaolinite cement was probably derived 

from weathering of the feldspar soon after deposition, and this claymineral matrix was subsequently hardened largely by the weight of 

the overburden. Typical light-gray unweathered sand bodies have 

unconfined compression strengths approaching those of the clay 
TABLE 3.-Textural and chemical data and Atterberg limits for some unweathered clay-rich Potomac Formation samples 

[Tests performed by the U.S. Soil Conservation Service, National Soil Survey Laboratory, Lincoln, Nebr. Test procedures described in a report 

by the staff of the U.S. Soil Conservation Service (1972). tr, trace amount] 

Cation exchange Ammonium acetate A t.terberg limits, 

capacity extractable bases, in percent 

Texture by sum of cations, in milli- water content 

Clay ( < 2 mm), Silt (2-5 rom), Sand ( > 5 mm), in milli- equivalents/100 g Liquid Plasticity 

Sample in percent in percent in percent equivalents/100 g Ca++ Mg·· Na· K• limit index 

Montmorillonite facies 

(Fairfax County, Va.): 

Site A --------------------- 69.0 29.3 1.7 62.8 33.1 21.7 0.4 2.4 104 61 

Site B --------------------- 52.8 46.7 .5 55.4 25.2 18.7 .1 1.4 82 46 

Kaolinite-illite facies 

(Washington, D.C.)--------- 35.0 57.0 8.0 5.4 .5 .7 tr tr 38 18 

tz:j 

z 

8 

z 

tz:j 

tz:j 

~ 

z 

0 

0 

tz:j 

0 

t""l 

0 

~~ ~ 

0 

1-3 

0 

~ 0 

"%j 

0 

~ 

~ 0 

z 

CJj 

1--' 
32 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

layers. Selective chemical weathering in the sand layers has produced very thin reddish-tinted weak zones along the contact with an 

underlying clay-rich layer. 

Iron-oxide-cemented sands are widespread and commonplace. Very 

strong cemented layers 6 ft (2 m) thick or more are not rare. Normally, 

the iron-oxide-cemented zones are in layered crusts concentrated at 

unconformities where ground water has been flowing and are present 

in association with the more permeable sands. 

PROPERTIES OF KAOLINITE-ILLITE FACIES 

Clay-rich beds in the kaolinite-illite facies are typically dull white, 

yellow, or red, except locally, where they are dark gray or black when 

carbonaceous material is present. Unweathered clay-rich beds of this 

facies are also waxy, and silt is abundant in many of the clay-rich 

beds. The clay minerals are primarily kaolinite and illite; rarely, a 

small amount of montmorillonite is present. A small percentage of 

vermiculite occurs in the upper 6 ft (2 m) of the formation in many 

places, probably from ongoing, very slow weathering. The presence of 

vermiculite has been associated with regions of most intense oxidization and probably results from alteration of illite and other minerals 

(Schluger and Roberson, 1975). Thus, there is a good possibility that 

chemical factors are involved in weakening of the clays. 

Large masses and concretions of rocklike siderite are randomly 

distributed in the more massive clay deposits of the kaolinite-illite 

facies. 

Sand layers of this facies are normally almost pure quartz and contain only minor amounts of feldspar. These sands are usually very 

weak and friable and exhibit little or no cohesion; locally, however, 

the sands are so strongly cemented with siderite or silica that they 

cannot be penetrated with tools routinely used for soil investigations. 

Where sands are at the ground surface, they are commonly weakly 

cemented, probably with silica. 

Iron-oxide-cemented sands are as common in this facies as they are 

in the montmorillonite-clay facies. Typical thicknesses, strengths, 

and locations of the cemented sands are about the same for both 

facies. 

FAULTS AND OTHER FRACTURES 

The faults that cut the Potomac Formation in Fairfax County and 

southward (that is, in the montmorillonite facies) are not commonplace, but they are widespread. Numerous sheared zones (or very 

small faults) and relict joints3 are present in many places near the 

larger faults at diverse orientations dipping from nearly vertical to 
ENGINEERING GEOLOGY, POTOMAC FORMATION 33 

horizontal. Near the faults, the joints are commonly continuous for 

long distances and have strongly preferential strikes (Langer and 

Obermeier, 1978). No faults have been documented in the kaoliniteillite facies, possibly because there have been no significant studies of 

tectonic effects in the Potomac Formation north of Washington, D.C. 

A feature suspected to be a fault, observed by the author near the 

northern end of Chesapeake Bay, had many smaller ancillary shear 

zones and joints, which appeared to be similar in form and occurrence 

to those in Fairfax County. 

Faults, sheared zones, and joints form zones of weakness in otherwise stronger sediments. Faults and shear zones have similar shear 

strength characteristics that are important to landslide considerations; they typically differ from characteristics along bedding-plane 

partings or joints and surface weathering fractures. 

PROPERTIES OF SHEARED ZONES, JOINTS, AND BEDDING-PLANE PARTINGS: 

MONTMORILLONITE FACIES 

The properties discussed reflect observations made primarily in 

Fairfax County, although the author believes that, in the main, they 

apply throughout the montmorillonite facies. Sheared zones (and 

faults) have gouge that ranges in thickness from 0.25 in. (0.6 em) or 

less (fig. 14) to 6 in. (15 em) or more. They occur as a single shear or as 

a set of smaller shears spaced tenths of inches (0.2 em) to 6ft (2 m) 

apart. Shear surfaces are commonly planar to gently undulating and 

slickensided (fig. 15). The sheared zones are commonly softened 

because of remolding during shearing or because of weathering or 

soaking with water. Fragments of hard, unaltered, original material 

surrounded by soft soil are present in some of the sheared zones. 

Sheared zones are rarely coated with strong black material (probably 

a manganese oxide). The author believes that the shear strength of 

the soil in sheared zones (except those coated with this black material) should be considered as being at the residual state, with only a 

very small or no apparent cohesion on the Mohr strength envelope. 

Even far removed from faults, joints are rather common in the 

uppermost 20 ft (6 m) of the Potomac Formation in thick, highly 

plastic clays. In Fairfax County and vicinity, both vertical and subhorizontal joints in the thick, highly plastic clays are commonly 

throughgoing for long distances and are generally planar (fig. 16) 

(Langer and Obermeier, 1978); some of these ttjoints" are beddingplane partings, and it is impossible to distinguish between the two in 

3

Joints are defined as those fractures, excluding faults, sheared zones, and bedding-plane partings, that are present beneath the zone of intense weathering and wetting and drying near the ground surface (that is, joints are the 

breaks present at depths greater than about 5 ft (1.5 rn)). Fractures include all breaks, irrespective of location, form, 

or origin. 
34 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

FIGURE 14.-Sheared zones in clay-rich Potomac deposits. Six-inch ruler for scale. 
ENGINEERING GEOLOGY, POTOMAC FORMATION 35 

FIGURE 15.-Slickensides along a sheared zone in the Potomac Formation. Nine-inch 

ruler for scale. 

many places. Although they are generally planar for long distances, 

the subhorizontal joints and bedding-plane partings typically have 

many small, closely spaced undulations; their wavelengths are on the 

order of a few inches to several feet (5 cm-1 m), and their amplitudes 

range from 0.25 in. to a few inches (0.5-5 em). These subhorizontal 

fractures generally parallel the land surface and tend to be much 

more closely spaced in the uppermost 3 ft (1 m) of the Potomac Formation. Weathering and softening along joints and partings are more intense near the ground surface and at the erosional unconformity at 

the top of the Potomac Formation than they are at greater depth. The 

subhorizontal fractures are commonly offset horizontally a fraction of 

an inch (a few millimeters), even on nearly level ground. These subhorizontal fractures are probably formed by an exfoliation type of 

phenomenon caused by rebound from erosional unloading and by 

swelling of the montmorillonite. 

In Fairfax County, joints are commonly well developed in massive, 

highly plastic (CH) clay beds thicker than 3 to 5 ft (1-1.5 m). Thinner 

layers of highly plastic clay have either poorly developed joints or no 

joints, and alternating beds of clay and low-plasticity silt or sand 

thinner than 6 in. (15 em) rarely have joints. Thick, massive clay of 

low plasticity (CL) and silt of high plasticity (MH) have well-developed, 

closely spaced (1-2ft) (0.3-0.6 m) vertical to horizontal joints at some 

localities, but joints are rare or absent at other sites. Thinly bedded 
36 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

FIGURE 16.-Well-developed vertical and subhorizontal joints in the montmorillonite 

facies. Ruler is divided in inches. 
ENGINEERING GEOLOGY, POTOMAC FORMATION 37 

layers of silt and sand intercalated with low-plasticity sediments commonly have no recognizable joint development. Massive sand units 

(SC or of lesser plasticity) and massive silts of low plasticity. (ML) 

rarely have joints. Rare occurrences of jointing in sand and silt of low 

plasticity are probably related to nearby shears of tectonic origin and 

are normally strongly cemented with a black manganese oxide. 

Almost all sites in Fairfax County that have abundant high-angle 

joints also have low-angle joints that are slightly undulating and continuous for at least several tens of feet. Water seeps from these joints 

at many localities, and the soil along the joint generally has a soft to 

medium consistency. The softened soil is present in a zone having a 

thickness of between 0.1 and 0.5 in. (0.3-1.2 em). The transition from 

the softened soil to the hard, unweathered soil is commonly very 

abrupt (fig. 17). The weakest material is generally light colored. Subhorizontal zones containing soft soil are probably commonplace in 

clay and highly plastic silt within approximately the upper 20ft (6 m) 

throughout the montmorillonite facies. 

PROPERTIES OF JOINTS AND BEDDING-PLANE PARTINGS: 

KAOLINITE-ILLITE FACIES 

Joints are also common in the upper 20ft (6 m) of thick bodies of 

highly plastic clays in the kaolinite-illite facies. Relict vertical, randomly oriented, generally irregular throughgoing joints spaced 1 to 2 

ft (0.3-0.6 m) apart commonly extend much farther than 20ft (6 m) 

beneath outcrops and beneath the erosional unconformity at the top 

of the Potomac Formation in the thick clays of the kaolinite-illite 

facies in Washington, D.C. In the Baltimore area, near-vertical, very 

irregular throughgoing joints commonly extend at least 20 ft (6 m) in 

the most highly weathered massive clays; these fractures are typically 

about 2 to 10 ft (0.6 to 3 m) apart at random strikes (Juergen 

Reinhardt, oral commun., 1978). Subhorizontal joints appear to be 

much less common in the kaolinite-illite facies than they are in the 

montmorillonite facies, possibly because clays in the kaulinite-illite 

facies typically have lower swell potential. 

SHEAR STRENGTH ALONG JOINTS AND BEDDING-PLANE PARTINGS 

The softened zone along a subhorizontal fracture commonly contains soil in highly variable preconsolidation states (determined by a 

consolidometer); the state can vary from being less than existing 

overburden vertical stress to being greater than that stress within a 

distance of less than 1ft (0.3 m) along the joint. Figure 18 illustrates 

how the small, slightly weathered projections in point-to-point contact can carry most of the overburden load. Between these points, the 

fractures are open in many places; in other places, they are filled with 
38 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

FIGURE 17.-Typical softened zones (light colored) in unweathered clay (dark colored) 

of the Potomac Formation. Hammer is approximately 12 in. long. 
ENGINEERING GEOLOGY, POTOMAC FORMATION 39 

washed-in or swollen clay. The lowest possible average vertical preconsolidation state of stress along the fracture surface equals the 

present average vertical overburden stress. 

The shear strength along a subhorizontal fracture that resists landslide movement depends on (1) the consolidation state along the fracture, (2) the state of drainage along the fracture during shearing, and 

(3) the force required to move the hard points over one another or the 

force required to shear through the hard points. For slope design, in 

many places, it is realistic and prudent to assume that the consolidation state along the fracture equals the existing overburden stress. 

Many fractures are so clay choked that the soils must fail in undrained 

shear along a thin, restricted zone; this failure can be 'closely simulated by the direct shear test method. The force required to move hard 

points over one another or to shear through the hard points obviously 

imparts an apparent cohesion to the shear strength but cannot be 

realistically appraised in the laboratory by using conventional equipment. Only a lower bound shear strength that accounts for the three 

factors listed above can be determined, and that strength may be 

much lower than the true shear strength. 

PROPERTIES OF SURFACE WEATHERING FRACTURES: 

MONTMORILLONITE FACIES 

Surface weathering fractures, defined as breaks at or near the 

ground surface, make up another class of breaks that are important 

to slope stability. Surface fractures are numerous and very commonplace in the clay-rich deposits of the montmorillonite facies. In Fairfax County (Langer and Obermeier, 1978), surface fractures commonly 

extend about 5 ft (1.5 m) beneath the ground surface at various orientations between vertical and subhorizontal (fig. 19); the fractures 

commonly form irregular chunks, and subhorizontal breaks can be 

well developed and throughgoing. Movement on some subhorizontal 

breaks has been sufficient to make slickensides in the highly plastic 

clays, although the relative movement along the two sides of the 

break probably rarely exceeds a fraction of an inch. 

Surface-fracture density decreases with diminishing clay content. 

In sand-rich deposits, surface fractures are present only where 

weathering has been intense and probably prolonged. Here, the fractures are typically short (0.25-3 in.) (0.6-7.5 em), discontinuous 

planar breaks, either vertical or horizontal and usually widely 

spaced. 

PROPERTIES OF SURFACE WEATHERING FRACTURES: 

KAOLINITE-ILLITE FACIES 

Surface fractures are also numerous and commonplace in clay-rich 

sediments of the kaolinite-illite facies. Here, however, the surface 
40 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

.. ', .. ·.· ... 

:! 

: ·. 

A 

FIGURE 18.-Locations of softened zones along weathered and infilled partings and fractures of the Potomac Formation. A, Along bedding partings. 

B, Along fractures. 
ENGINEERING GEOLOGY, POTOMAC FORMATION 41 

FIGURE lB.-Continued. 
42 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

FIGURE 19.-Surface fractures in clay-rich Potomac Formation sediments of the 

montmorillonite facies. Shovel handle approximately 21 in. long. 
ENGINEERING GEOLOGY, POTOMAC FORMATION 43 

FIGURE 19.-Continued. 

fractures are oriented more nearly vertically and make large blocks 

(fig. 20). Many of these fractures extend at least 5 ft (1.5 m) beneath 

the ground surface. The fractures in the sands are usually only short, 

discontinuous breaks. 

SHEAR STRENGTH ALONG SURFACE WEATHERING FRACTURES 

Surface fractures probably develop within a few years after exposure of unweathered clay-rich sediments. Because of the intense 

development and movement along fractures, the shear strength of 

highly plastic soils must be much less than that of adjacent intact 

sediments. The movement along fractures, however, is probably not 

large enough to reduce the shear strength to the residual strength 

condition. The presence of stronger projections along the fractures 

and infilled soils also contributes to strengths greather than residual. 

GROUND-WATER CONDITIONS 

The water table is generally a subdued replica of local topography 

on outcrops of the Potomac Formation and in places where the 

Potomac Formation is near the ground surface. The water table is 

commonly very close to or at the ground surface, beneath clay-rich 

slopes and at the base of slopes; on hilltops, it is rarely deeper than a 

few tens of feet. During wet seasons, the water table is very close to or 
44 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

FIGURE 20.-Surface fractures in clay-rich Potomac Formation sediments of the 

kaolinite-illite facies. Entrenching shovel approximately 18 in. long. 

at the ground surface, and perched water tables and artesian conditions are very common. 

The local geology exerts an important influence on ground-water 

conditions because of the highly differing permeabilities of sands and 

clays and because fractures form passageways for water. 

The thin feldspathic sands of the montmorillonite facies are typically 

so clay rich that, except where there are coarse-grained channel 

fillings, they are usually not good aquifers. The sands are still much 

more permeable than the montmorillonite-rich layers, however. The 

quartz sands associated with the kaolinite-illite facies are generally 

very permeable. As a result, in both facies, perched and confined 

water tables are common in the layered clay and sand sequences. 

Water in some confined sand layers is under high artesian pressure. 

The water table in thick sand deposits on hilltops and slopes is commonly deeper than that in adjoining clay and silt bodies. Both layered 

and massive clays and silts are typically so impermeable that they 

greatly restrict the rate of flow and help to prevent drainage of water 

from these sites. It is possible (and indeed probable, given the remarkable lack of weathering of montmorillonite in most places) that some 

massive nonfractured clays are impermeable to laminar flow at gradients commonly present. 

Most massive clays and silts have many fractures near the land surface that serve as ready entrances for surface water. The fractures are 
ENGINEERING GEOLOGY, POTOMAC FORMATION 45 

usually slightly opened by offsetting or have root holes that persist to 

a depth of about 10ft (3m) beneath the ground surface. Water can fill 

these fractures during wet seasons. 

In Fairfax County and vicinity, the unconformity separating the 

upland gravels from the Potomac Formation marks th~ site ·where 

numerous small channels were cut into the Potomac Formation. 

These sand- or gravel-filled channels are currently sites for concentrated flow of water, as figures 6 and 21 show. Freely flowing water 

normally persists in these channels even during dry seasons. 

In many places, near the ground surface, the upland gravels have a 

nearly impermeable hardpan, less than 3 ft (1 m) thick, that ponds 

water during wet seasons. It is possible that breaking this hardpan 

permits significantly increased water flow into the underlying permeable sand and gravel and, consequently, enhances the potential for 

landslides. 

SLOPE STABILITY ENGINEERING 

Residual shear strength parameters should be used to evaluate 

slope stability along any preexisting surface of rupture, which is a 

permanent zone of weakness. A practical problem often encountered 

by the designer is the recognition of former landslides. Many of 

the old slides are so subtly expressed that recognition in the field is 

FIGURE 21.-Water seeping from a sand-filled channel cut into Potomac Formation 

clay-rich sediments. Outcrop is approximately 15ft (4.6 m) high. 
46 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

extremely difficult (Obermeier and Hollocher, 1976). Although tilted 

and bent trees can sometimes be an identification aid, they must be 

used with caution because mechanisms unrelated to landsliding also 

can tilt and bend trees. Slides that are in the early stages of development and that have very small scarps at the crown are virtually 

impossible to detect in most instances. 

There are geologic settings in which it is impossible to recognize old 

slides from morphologic evidence because they are buried beneath 

younger sediments or have been modified. One such setting is along 

some major rivers, such as the Anacostia River (see fig. 3), where 

downcutting during glacial times at low sea level caused sliding. Subsequent readvance of the seas caused deposition of sediments over the 

slides. Other locations suspected of being old slide sites are at steeply 

sloping unconformities separating the Potomac Formation from 

younger sediments. 

Where there is not an existing surface of rupture, the selection of 

appropriate shear strength parameters is much more complicated. 

Some of the factors known to be important to slope stability of 

Potomac Formation sediments are the presence of surface fractures, 

the presence of joints and partings, the presence of tectonic shear 

zones, the breakdown of diagenetic bonds by swelling and by creep, 

water penetration into joints, and the presence of physically and 

chemically softened zones. Obviously, there can be no completely 

theoretical basis for selecting shear strength design parameters when 

long-term slope stability is being evaluated. 

A time-honored method of slope design in highly overconsolidated 

swelling sediments is to look for similar geologic settings in existing 

slopes. This method can work well for deposits that are not highly 

variable, but Potomac Formation sediments do not lend themselves to 

this sort of analysis unless a large factor of safety is used to account 

for uncertainties. Accumulated experience, summarized in the following papers, is the only realistic basis. 

SELECTED REFERENCES 

American Society for Testing and Materials, 1978, Annual book of ASTM standards, 

pt. 19, Designation D 1586-67 (Reapproved 197 4), Standard method for penetration 

test and split barrel sampling of soils: Philadelphia, Pa., p. 235-237. 

Bjerrum, L., 1967, Progressive failure in slopes of overconsolidated plastic clay and 

clay shales: American Society of Civil Engineers Proceedings, Journal of the Soil 

Mechanics and Foundations Division, v. 93, no. SM 5, p. 3-49. 

Chowdhury, R.N., 1980, Landslides as natural hazards-Mechanisms and uncertainties: 

Geotechnical Engineering, Journal of the Southeast Asian Society of Soil Engineering, v. 11, p. 135-176. 

Cleaves, E. T., 1968, Piedmont and Coastal Plain geology along the Susquehanna 

Aqueduct, Baltimore to Aberdeen, Maryland: Maryland Geological Survey Report 

of Investigations no. 8, 45 p. 
ENGINEERING GEOLOGY, POTOMAC FORMATION 47 

Dallaire, Gene, 1976, Consultants reviewing plans of other consultants in Fairfax 

County, Va.; la.ndslides greatly reduced: Civil Engineering, v. 46, no. 9, p. 77-79. 

Darton, N.H., 1951, Structural relations of Cretaceous and Tertiary formations in part 

of Maryland and Virginia: Geological Society of America Bulletin, v. 62, p. 

743-779. 

Drake, A. A., ,Jr., Nelson, A. E., Force, L. M., Froelich, A. J., and Lyttle, P. T., 1979, 

Preliminary geologic map of Fairfax County and vicinity, Virginia: U.S. Geological 

Survey Open-File Report 79-398, 2 pls., scale 1:48,000. 

Force, L. M., 1975, Preliminary geologic map of the Coastal Plain in Fairfax County, 

Virginia: U.S. Geological Survey Open-File Report 75-415, 21 p., 2 pls., scale 

1:48,000. 

Force, L. M., and Moncure, G. K., 1978, Origin of two clay mineral facies of the 

Potomac Group (Cretaceous) in the Middle Atlantic States: U.S. Geological Survey 

Journal of Research, v. 6, no. 2, p. 203-214. 

Froelich, A. J., 1978, Map showing planar and linear features of Fairfax County, 

Virginia: U.S. Geological Survey Open-File Report 78-443, 1 pl., scale 1:48,000. 

Froelich, A. J., Johnston, R. H., and Langer, W. H., 1978, Preliminary report on the 

ancestral Potomac River deposits in Fairfax County, Virginia, and their potential 

hydrogeologic significance: U.S. Geological Survey Open-File Report 78-544, 37 p., 

1 pl., 3 figs. 

Glaser, J. D., 1969, Petrology and origin of Potomac and Magothy (Cretaceous) sediments, Middle Atlantic Coastal Plain: Maryland Geological Survey Report of 

Investigations no. 11, 101 p. 

Hack, J. T., 1955, Geology of the Brandywine area and origin of the upland of Southern 

Maryland: U.S. Geological Survey Professional Paper 267-A, 43 p. 

__ 1957, Submerged river system of Chesapeake Bay: Geological Society of America 

Bulletin, v. 68, p. 817-830. 

Jacobeen, F. H., Jr., 1972, Seismic evidence for high angle reverse faulting in the 

Coastal Plain of Prince Georges and Charles County, Maryland: Maryland 

Geological Survey Information Circular 13, 21 p. 

Langer, W. H., and Obermeier, S. F., 1978, Relationship of landslides to fractures in 

Potomac Group deposits, Fairfax County, Virginia: U.S. Geological Survey OpenFile Report 78-779, 37 p., 4 figs. 

Lupini, J. F., Skinner, A. E., and Vaughn, P.R., 1981, The drained residual strength 

of cohesive soils: C'xeotechnique, v. 31, no. 2, p. 181-213. 

Mitchell, J. K., 1976, Fundamentals of soil behavior: New York, John Wiley, 422 p. 

Mixon, R. B., and Newell, W. L., 1977, Stafford fault system: Structures documenting 

Cretaceous and Tertiary deformation along the Fall Line in northeastern Virginia: 

Geology, v. 5, no. 7, p. 437-440. 

Mueser, Rutledge, Wentworth & Johnston, 1971, Final report-Subsurface investigation, Huntington Route, Sta. 269+00(C008) to 556+30(C010), April, 1971: U.S. 

Department of Commerce National Technical Information Service Report PB 

200-847, 24 p. 

__ 1973, Final report-Subsurface investigation, Addison Route, Sta. 290+00(G001) 

to 488+00(0003), November, 1973: U.S. Department of Commerce, National 

Technical Information Service Report PB 225-853, 21 p. 

__ 1973, Final report-Subsurface investigation, Branch Route, Sta. 108+60(F003) 

to 454+00(F008), June, 1973: U.S. Department of Commerce, National Technical 

Information Service Report PB 216-974, 26 p. 

Nelson, J.D., and Thompson, E. G., 1977, A theory of creep failure in overconsolidated 

clay: American Society of Civil Engineers Proceedings, Journal of the Geotechnical 

Engineering Division, v. 103, no. GT 11, p. 1281-1294. 

Obermeier, S. F., 1979, Slope stability map of Fairfax County, Virginia: U.S. Geological 

Survey Miscellaneous Field Studies Map MF -1072, 1 pl., scale 1:48,000. 
48 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

Obermeier, S. F., and Hollocher, Kurt, 1976, Map of landslides in Coastal Plain 

deposits of the Franconia area, Fairfax County, Virginia: U.S. Geological Survey 

Open-File Report 76-589, 15 p., 1 pl. (scale 1:24,000), 2 figs. 

Prowell, D. C., 1983, Index of Cretaceous- and Cenozoic-age faults in the eastern 

United States: U.S. Geological Survey Miscellaneous Field Studies Map MF-1269, 

1 pl., scale 1:2,500,000. 

Schlee, J., 1957, Upland gravels of Southern Maryland: Geological Society of America 

Bulletin, v. 68, p. 1371-1410. 

Schluger, P.R., and Roberson, H. E., 1975, Mineralogy and chemistry of the Patapsco 

Formation, Maryland, related to the ground-water geochemistry and flow system: 

A contribution to the origin of red beds: Geological Society of America Bulletin, v. 

86, p. 153-158. 

Skempton, A. W., 1970, First-time slides in overconsolidated clays: Geotechnique, 

v. 20,no. 3,p. 320-324. 

__ 1977, Slope stability of cuttings in brown London Clay, in International Conference on Soil Mechanics and Foundation Engineering, 9th, Tokyo 1977, Proceedings: v. 3, p. 261-270. 

Townsend, F. C., and Gilbert, P. A., 1976, Effects of specimen type on the residual 

strength of clays and clay shales, in Soil specimen preparation for laboratory 

testing: American Society for Testing and Materials Special Technical Publication 

599, p. 43-63. 

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for collecting soil samples: U.S. Soil Conservation Survey Soil Survey Investigations Report 1, 63 p. 

Wagner, A. A., 1957, The use of the Unified Classification System by the Bureau of 

Reclamation, in International Conference on Soil Mechanics and Foundation Engineering, 4th, London 1957, Proceedings: vol. I, p. 125-127. 

Wagner, D. P., Fanning, D. S., Foss, J. E., Patterson, M. S., and Snow, P. A., 1982, 

Morphological and mineralogical features related to sulfide oxidization under 

natural and disturbed land surfaces in Maryland, in Acid sulfate weathering: Soil 

Science Society of America Special Publication 10, 234 p. 

Withington, C. F., 1964, Joints in clay and their relation to slope failure at Greenbelt, 

Maryland, December 28, 1962: U.S. Geological Survey Open-File Report 706, 7 p. 
ENGINEERING GEOLOGY AND DESIGN OF SLOPES FOR 

CRETACEOUS POTOMAC DEPOSITS IN 

FAIRFAX COUNTY, VIRGINIA, AND VICINITY 

DESIGN AND CONSTRUCTION OF SLOPES 

IN POTOMAC FORMATION DEPOSITS 

By PAUL G. SWANSON1 and JOHNS. JONES, JR. 2 

CONTENTS 

Page 

Abstract-------------------------------------------------------------50 

lntroduction-----------------------------------------------------------50 

Field investigation -----------------------------------------------------51 

Selection of design parameters --------------------------------------------64 

Design conditions ---------------------------------------------------64 

Design shear strength------------------------------------------------57 

Intactsoil-------------------------------------------------------58 

Failed soil ------------------------------------------------------68 

Ground-water pressures ----------------------------------------------69 

Analysis and selection of factors of safety ----------------------------------60 

Construction procedures -------------------------------------------------60 

Selected references -----------------------------------------------------62 

ILLUSTRATIONS 

Page 

FIGURES 1-4. Diagrams showing: 

1. Typical relationship between slope indicator and soils data -----53 

2. Shear stress response as a function of consolidation and 

drainagestates ---------------------------------------65 

3. Relative drained shear strengths for different states of 

consolidation -----------------------------------------66 

4. Flow paths for selection of design parameters for slopes on 

Potomac Formation sediments ---------------------------57 

5. Summary plot of laboratory residual strength data for Potomac 

Formation clays-------------------------------------------69 

1Senior geotechnical engineer, Law Engineering Testing Co., McLean, Va. 2Chief engineer, Law Engineering Testing Co., McLean, Va. 

49 
50 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

ABSTRACT 

On the basis of experience with Potomac Formation sediments, specific guidelines 

have been developed for investigation, design, and construction of slopes. These guidelines cover four phases: (1) field investigation, (2) design parameter selection, (3) 

analysis and selection of factors of safety, and (4) construction. All phases are used for 

evaluation of construction-period and permanent slope stability. 

The field investigation includes a surface reconnaissance, during which indicators of 

previous landslides may be noted. Observations are made of bowed tree trunks, hummocky and disturbed ground, wetland vegetation, springs, and drainage pathways. 

This surface reconnaissance is followed by a subsurface investigation to determine subsurface stratification and the consistency of the layers, the locations of water-bearing 

zones and anticipated potentiometric levels, and the locations of fractures or shear 

surfaces. 

Design shear strength parameters for normally consolidated Potomac Formation 

sediments are based on conventional test procedures. For first-time slides in overconsolidated sediments, however, design parameters are based largely on fracture intensity and laboratory shear strength data. Lower strength parameters are used for more 

highly fractured sediments. Either for highly fractured soils or along previously 

sheared surfaces, residual strength parameters are used for design of constructionperiod and permanent slopes. For soils having no fractures or widely spaced fractures, 

"fully softened" (normally consolidated, drained) shear strength parameters are used 

for design of permanent slopes; peak undrained triaxial shear strength data are used 

for design of construction-period slopes. 

Selection of a safety factor for design depends on the uncertainty of slope conditions, 

the cost of construction or repair, and the potential hazards of slope failure. The critical 

design condition for the completed slope requires, in addition, that the slope be 

marginally stable, in case a subsurface drainage system fails completely. 

It is important that a geotechnical engineer familiar with the design be at the site 

during construction to ensure that assumptions used for design are valid. Problems can 

arise when clay-rich soils are exposed to water for prolonged construction periods; consequent softening can cause localized failure. 

INTRODUCTION 

Rational design for fill and cut slopes in Potomac Formation sediments requires detailed knowledge of the material properties and 

geologic setting of the site. Clay- and silt-bearing sediments of the 

Potomac Formation are highly overconsolidated, stiff to hard soils, 

except where they have been softened locally by mechanisms such as 

weathering or by previous shearing. The highly overconsolidated 

soils have fractures in which softened soils are restricted to a thin 

zone along the fracture surfaces at many localities. Because slope 

design parameters commonly depend on the method of softening 

(previous shearing or weathering), a thorough field examination of 

the site is usually required. If a thorough investigation is not made, 

conservative design parameters must be used. As for permanent 
SLOPE DESIGN AND CONSTRUCTION, POTOMAC FORMATION 51 

slopes, design parameters for construction-period (short-term) cut 

slopes and fill sections are strongly affected by the presence of 

softened soils. 

Typical of highly overconsolidated soils, the stiff to hard unweathered Potomac Group sediments tend to dilate (that is, expand in 

volume) and thus develop negative pore pressures when they are 

stressed during construction of a cut or fill slope. The overconsolidated soils in the potential failure zone weaken and soften (that is, 

strain soften) during landslide development; the amount of strength 

reduction caused by this softening depends on the plasticity characteristics and shear displacement. In contrast to the highly overconsolidated soils, intensely weathered soils that are approximately 

normally consolidated can develop positive pore pressure during 

shearing, the result being either strain hardening at small strains or 

strain weakening at large strains. Previously sheared zones in landslides and faults can develop either positive or negative pore pressures during renewed shearing, depending on loading conditions, and 

the lower bound strength in these sheared zones will be equal to the 

residual shear strength. 

Specific guidelines have been developed for investigation, design, 

and construction of slopes on the basis of experience with Potomac 

Formation sediments. These guidelines are for four phases: (1) field 

investigation, (2) design parameter selection, (3) analysis and selection of factors of safety, and ( 4) construction. All phases are used for 

evaluating short- or long-term slope stability. 

FIELD INVESTIGATION 

Slope design requires that a field investigation carefully document 

information ranging from an overview of the geologic setting to local 

geotechnical details exposed in trenches or revealed by borings. A 

field investigation begins with a surface reconnaissance, during 

which indicators of previous landslides may be noted. Observations 

are made of bowed tree trunks, hummocky and disturbed ground, and 

wetland vegetation. Also noted are springs, swales, and drainage 

pathways. Aerial photographs are used whenever it is possible. The 

surface reconnaissance is followed by a detailed topographic survey 

and then by a subsurface investigation. 

The amount and detail of the subsurface data collected depend to a 

large extent on the anticipated consequences of any slope movement. 

Consideration is given to both potential economic losses and the 

possibility of endangering people. At a minimum, the subsurface investigation must determine the subsurface stratification and appro xi· 

mate consistency of the layers, the location of water-bearing zones, 

and the location of fractures or shear surfaces. 
52 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

Test pits, soil borings, static cone probes, slope inclinometers, and 

observation wells are all commonly used to define subsurface conditions. In general, test pits and static cone probes are the best methods 

for locating water-bearing zones and fractured or sheared zones. Thin 

sand seams capable of transmitting water to the failure surface and 

thin softened zones are commonly not detected by using tube samples 

from borings. In contrast to borings, both test pits and static cone 

probes provide a continuous, detailed record of subsurface materials 

and conditions. A test pit is preferable for locating shallow shear surfaces, whereas cone probing is more suitable for detecting deeper 

shear surfaces. 

Relatively undisturbed specimens of highly overconsolidated sediments are often difficult to obtain. The best specimens for laboratory 

evaluation of in-situ shear strength are from block samples excavated 

in test pits or from piston samples. For depths greater than about 15 

ft (4.6 m), samples are obtained from borings advanced either by using hollow-stem augers or by continuous use of Shelby tubes. Where a 

shear zone is present, the shear strength is often determined by using 

remolded samples; this method eliminates the need for undisturbed 

samples. 

It is imperative that sheared zones in previously failed slopes be 

located. Often, a change in drilling resistance, an abrupt change in 

water content, or a loss of drilling fluid (in the sheared zone) indicates 

a sheared zone. The best method for accurately locating the shear surface of an actively moving slide is by using slope inclinometer data. 

Figure 1 is a plot of slope inclinometer readings taken during investigation of a slide. Data on the figure illustrate that there are two shear 

surfaces and that the lower shear surface is about 16ft (4.9 m) deep, 

in the weathered portion of the Potomac Formation. 

The shear surface in an active slide can be located approximately by 

using a simple inclinometer consisting of a small-diameter plastic 

pipe or a stiff rubber hose (into which a steel rod has been inserted to 

maintain initial straightness) embedded in the borehole. The steel 

rod is withdrawn after backfilling around the hose. Probes are periodically inserted in the pipe or hose to detect any squeezing or shearing of the casing, which indicates the upper boundary of the failure 

zone. To define the lower boundary, a short steel rod is placed in the 

bottom of the tubing; a thin wire attached to the rod is used to pull the 

rod up the tube periodically until displacement resulting from slope 

movement restricts movement. 

Slotted sections of slope inclinometer casing wrapped in filter cloth 

can be used to monitor slope movements and potentiometric levels 

simultaneously. Although this system is relatively inexpensive, it 

does have several disadvantages. It can monitor only one potentiometric level, and its response time may be greatly delayed because of 
SLOPE DESIGN AND CONSTRUCTION, POTOMAC FORMATION 53 

0.-----------------~=== 

51-

10 IIUJ 

UJ 

LL 

~15f-­ J:-

1-

a.. 

UJ 

0 

20 I25-

Fill 

soils 

Potomac 

Formation 

clay (CH) 

--- 10 

~--=-=- ---

1--- -=-=-

f----_ 

~--==== 30 f----_ 

!-_-_- ~--=-=-

1---

~-----=- 46 

~--=-:_=­ !-_-_-

--

f-~-_-_ 

f-:---_ 64 

MAXIMUM CASING DEFLECTION, 

IN INCHES 

0 

0 

0 

I * 

+ 

+ 

+ 

+ 

+ 

+ 

1.0 

N 

0 

* 

1.0 

N 

0 

* 

* 

* 

* 

* 

* 

EXPLANATION 

I; ZONE A 

~ZONE B 

0 

1.0 

ci 

FIGURE 1.-Typical relationship between slope indicator and soils data. Numbers to the 

right of the soil symbols are Standard Penetration Resistances (in blows per foot). 

Inclinometer reading dates are I (initial), May 26, 1978; *, June 2, 1978. Soil properties are as follows: zone A, liquid limit, 60 percent; plastic limit, 30 percent; natural 

water content, 38 percent. Zone B, liquid limit, 69 percent; plastic limit, 33 percent; 

natural water content, 39 percent. 

the large volume of water in the casing required to equalize pore 

pressure within the casing and pore pressure in the soil. 

One of the more accurate methods for determining the potentiometric level at the failure surface or within confined layers uses a 

system of pressure seals and low-volume-change potentiometric collars. The system can be installed in a single borehole. The potentiometric collars have small pneumatic diaphragms that require only a 

small change in water volume to equalize pressures within the casing 

and existing ground-water pressures. Readings of the potentiometric 

levels can be made periodically by using a pneumatic probe. This type 

of installation has the advantage of monitoring several layers from a 

single boring. Although this type of system is expensive, it is recom-
54 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

mended for investigations in the Potomac Formation where accurate 

knowledge of piezometric levels is needed. 

SELECTION OF DESIGN PARAMETERS 

Slope design parameters depend on the intended permanence of the 

slope, the geologic setting, and anticipated ground-water conditions. 

DESIGN CONDITIONS 

Design parameters for cut slopes or embankments or for correction 

of existing slope failures depend on the critical design situation. 

Upper and lower bound strength characteristics depend on the drainage status of pore pressures induced by a change in stress conditions. 

An undrained condition implies that an excess pore-water pressure 

has been generated in the soil by a change in stress conditions. A 

drained condition implies that enough time has elapsed for the excess 

pore-water pressure to dissipate. For most typical design situations, 

the construction period of an embankment or cut slope is considered 

to be the undrained or short-term condition. Figure 2 illustrates that 

excavation or filling causes an initial loss of strength, but, if excess 

pore-water pressure is drained, normally consolidated soils can 

strengthen. Highly overconsolidated soils are initially strengthened 

by negative pore-water pressures but can weaken over the long term 

as negative pressures dissipate. Both consolidation states are commonly present in the Potomac Formation. Thus, for the long-term 

drained condition, the Potomac Formation can be either stronger or 

weaker than it is for the short-term condition. 

Design shear strength parameters for normally consolidated 

Potomac Formation sediments are evaluated by using conventional 

test procedures outlined in basic soil mechanics texts. For first-time 

slides in overconsolidated sediments, however, the design procedure 

is based on concepts proven approximately valid for overconsolidated 

clays elsewhere (Skempton, 1964, 1970, 1977) modified by local experience with the Potomac Formation; specifically, the concept of the 

critical state (Roscoe and others, 1958) is used in combination with 

the anticipated magnitude of shear deformation at failure to select 

the design shear strength. 

A saturated clay in a drained test is at its critical state if any further increment in shear distortion does not cause any change in water 

content (Skempton, 1970). In the ideal case, a clay will expand or contract during drained shear until the critical state is reached, irrespective of the original state of consolidation of the clay. When the critical 

state is reached, the clay continues to deform under constant shear 

stress at constant volume. The critical state corresponds to the 
SLOPE DESIGN AND CONSTRUCTION, POTOMAC FORMATION 55 

(") 

b 

I ('I 

b 

(J) 

(J) 

LlJ 

a: 

1-

(J) 

a: 

<! 

Shear-induced negative, 

pore-water pressure V\ .

1 

W llncrease in / 

shear strengt 

Failure envelope Drained stress path 

w ·--·Stress conditions at 1 

V5 before construction 

<r 1 +a3 > A AVERAGE NORMAL STRESS 2 

(") 

b 

IN 

b 

(J) 

{/) 

LlJ 

a: 

1-

(J) 

a: 

<! 

LlJ 

I 

(J) 

Shear-induced positive 

pore-water pressure 

Decrease 

strength 

in shear ~ 

~-r-- ---L--Drained 

__L_ Jt stress ~ath 

___;--Undramed 

Failure envelope ~ ljl stress path I 

I 

11 _...-' 

/ ...--<Ko line 

_..,;...-

~~s---- \ d.· 1 ...-~ tress con 1ttons at 

...-...-...- before construction 

AVERAGE NORMAL STRESS 

B 

Embankment 

stability /----~, 

/ ' 

o Sample site 1 

Cut slope stability 

C -----~Sample site 1 

FIGURE 2.-Shear stress response as a function of consolidation and drainage states. A, Shear stress response for 

highly overconsolidated soils. B, Shear stress response 

for normally consolidated soils. o-10 major principal 

stress; o-3 , minor principal stress; K0 , ratio of horizontal 

to vertical effective stress on level ground. C, Sample 

locations for embankments and cut sections. 
56 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

strength of a normally consolidated, idealized clay; by definition, the 

water content at this state is equal to that ultimately achieved by the 

overconsolidated clay owing to dilatency with subsequent increase in 

water content during shear. This strength is designated the ufully 

softened" shear strength. Figure 3 is a schematic depiction of the 

relationship of the fully softened shear strength to the peak shear 

strength for real, highly overconsolidated clay-rich soils. For real 

soils, the fully softened shear strength is affected in a complex way by 

sensitivity, bonding, and particle orientation, but, for practical purposes, it can be considered as the peak strength associated with the 

remolded, normally consolidated, drained condition (Skempton, 

1970). The peak strength of a remolded, normally consolidated clay is 

also the theoretical upper strength limit of an overconsolidated clay 

that has undergone weathering intense enough to obliterate evidence 

of previous overconsolidation (excluding the possible influence of 

changes in clay mineralogy and in absorbed and adsorbed ions). 

For practical purposes, the fully softened shear strength is the 

lower bound strength of an overconsolidated clay-rich soil that has 

had particles reoriented and bonds disrupted by swelling or weathering but that has not experienced enough shear displacements to 

develop a discreet, smooth, continuous shear surface during the 

weakening process. Where there are large shear displacements during weakening and development of a continuous shear surface, real 

soils can weaken further to the residual strength, as figure 3 illustrates. The residual strength is characterized by an alining of the 

clay particles from continued deformation along a failure plane 

(Bjerrum, 1967; Chandler and Skempton, 1974; Lupini and others, 

1981). 

(/) 

(/) 

w 

a: 

1-

(/) 

a: 

<X: 

w 

I 

(/) 

Overconsolidated peak strength 

Normally consolidated peak strength 

or fully softened strength 

~------- Residual strength 

SHEAR DISPLACEMENT 

FIGURE 3.-Relative drained shear strengths for different states of consolidation. 
SLOPE DESIGN AND CONSTRUCTION, POTOMAC FORMATION 57 

The displacement required to reduce an overconsolidated clay to the 

fully softened condition is several times that required to mobilize the 

peak strength but is nevertheless not large and is considerably less 

than that corresponding to the residual strength. Typically, clay-rich 

soils develop residual strengths in laboratory direct shear tests after 

accumulating about 2 to 3 in. (5-7 .5 em) of displacement along the 

failure surface. 

For first-time slides in highly overconsolidated Potomac Formation 

soils, the shear strength deemed appropriate for design is based on an 

evaluation of the soil's tendency to expand. This tendency, in turn, is 

determined by the fracture intensity, as the following section explains. 

DESIGN SHEAR STRENGTH 

Figure 4 is a flow diagram showing the procedure used for selecting 

design parameters for short- and long-term slope stability of Potomac 

Formation sediments. This procedure is used for design of cut slopes 

and embankments and for correction of existing landslides. Figure 4 

shows that, after the subsurface investigation, a decision is made to 

design for ''intact soil" or ~~failed soil." Criteria are based on the local 

experience of engineers and represent 30 to 40 slope stability studies 

conducted by the authors over the past 10 years. 

l SUBSURFACE INVESTIGATION I 

I 

DEFINITION OF SUBSURFACE STRUCTURE, I 

STRATIGRAPHY, AND GROUND-WATER CONDITIONS 

I I 

INTACT SOIL FAILED SOIL 

NO EVIDENCE OF EVIDENCE OF PREVIOUSLY 

PREVIOUS FAILURE SLIDING OR HIGHLY 

FRACTURED SOILS 

/ ~ I 

IOVERCONSOLIDATED I 

SOILS 

I NORMALLY CONSOLIDATED I 

SOILS 

I DEFINE FAILUREJ SURFACES 

I I I I 1 

LONG-TERM SHORT-TERM LONG-TERM SHORT-TERM STABILITY 

STABILITY STABILITY STABILITY STABILITY ALONG 

(CRITICAL) (CRITICAL) FAILURE 

SURFACES 

FULLY PEAK PEAK PEAK BASED ON 

SOFTENED UNDRAINED DRAINED UNDRAINED DRAINED 

DRAINED STfiENGTH STRENGTH STRENGTH RESIDUAL 

STRENGTH STRENGTH 

FIGURE 4.-Flow paths for selection of design parameters for slopes on Potomac Formation sediments. 
58 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

INTACT SOIL 

~~Intact soil" applies to slopes where there is no evidence of previous 

failures or creep and the soils are not intensely fractured. An intensely fractured soil has many cracks that are observable when it is 

extracted from the ground. An intensely fractured clay-rich soil is extremely difficult to sample, and excavated portions of the soil are 

fragmented and can break into pieces 2 in. (5 em) in diameter or 

smaller during gentle handling. Soils within the top 2 to 4ft (0.7-1.2 

m) of the soil profile are often intensely weathered and contain many 

fractures caused by the weathering process. Highly fractured soils 

below this depth may be more indicative of slide movement. 

The critical design situation for permanent slopes in highly overconsolidated Potomac Formation soils is the drained condition. The 

stress changes described earlier that were caused by embankment 

construction or regrading induce excess negative pore-water pressures, which, in turn, temporarily strengthen the soil (fig. 2). A 

drained condition corresponding to long-term stability fails at a lower 

value of shear stress. The drained, fully softened shear strength is 

used for design of permanent slopes in massive to somewhat but not 

highly fractured, highly overconsolidated clay- and silt-bearing soils 

where no evidence of previous sliding has been observed. The fully 

softened shear strength may be obtained from the peak strength of 

the same clay when it is remolded and tested to determine the normally consolidated shear strength (Skempton, 1970). 

The critical design situation for softer, normally consolidated soils 

is reversed. An increased shear stress temporarily induces positive 

excess pore-water pressure, which causes the soil to fail at a lower 

shear stress. Therefore, for these soils, short-term stability is critical. 

For short-term stability, the peak undrained shear strength as determined from laboratory triaxial shear tests or direct shear tests is used 

for intact soils. The direct shear test is often used to determine 

strength properties along a bedding plane or a zone of weakness. 

FAILED SOIL 

Residual strength parameters are appropriate for design of slopes 

having previously sheared or highly fractured soils. The residual 

parameters are generally determined by using direct shear tests with 

large amounts of accumulated deformation along the shear surface 

(Chandler and Skempton, 1974). For preexisting landslides where the 

geometry of the failure surface and ground-water conditions are 

known, a back calculation of strength parameters that assumes a factor of safety of 1.0 can be used as a check on laboratory test data. 
SLOPE DESIGN AND CONSTRUCTION, POTOMAC FORMATION 59 

On small projects that do not justify the cost of shear strength testing, shear strength parameters are estimated on the basis of a correlation between plasticity characteristics and residual shear strength. 

Laboratory residual strength data versus liquid limit for Potomac 

Formation clays are shown in figure 5. The rather large scatter probably results from the wide variations in clay content and clay mineralogy in the test specimens (Lupini and others, 1981). 

GROUND-WATER. PRESSURES 

The potentiometric level used for design is generally the highest 

recorded during the investigation. If levels are not measured during 

an unusually wet weason or if some alteration of the observed potentiometric level is expected as a result of construction, then some conservative estimate must be made. Where fractured sediments are 

present, it is assumed that surface water can fill the fractures 

throughout their depths and develop hydrostatic pressures. 

The potentiometric level used to back calculate strength parameters 

is the lowest level recorded during observed movement. This assumption permits calculating the lowest residual strength for continued 

failure. Design of corrective measures is based on using the minimum 

shear strength parameters and the highest potentiometric levels 

anticipated after construction. 

28 

UJ 

....J 26 

(.9 

z 24 

<(en 22 

zUJ 

0 _a: UJ 20 0 

t; ~ 18 0 

0: 0 16 u.z 

~: 14 0 

:::>- 12 o3-

Ci5 10 

UJ 

a: 8 0 

6 0 

40 45 50 55 60 65 70 75 80 85 

LIQUID LIMIT, IN PERCENT 

FIGURE 5.-Laboratory residual strength data for Potomac Formation clays. 
60 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

ANALYSIS AND SELECTION OF FACTORS OF SAFETY 

The accuracy of any slope stability analysis is highly dependent on 

the capability of the analytical method to model the field situation 

accurately. The analysis should account for factors such as changes in 

ground-water levels, seepage forces, tension cracks, the geometry of 

the failure surface, and the influence of weak planes. The authors 

generally use Bishop's (1960) Modified Method for analysis of circular 

arc failures and the Morgenstern and Price (1965) or Janbu (1973) 

method for noncircular surfaces. All of these methods evaluate the influence of the factors previously mentioned on the analysis. The 

wedge method can be used when a planar shear surface exists; this 

method, however, is generally conservative and yields a factor of 

safety on the order of 15 percent higher than that of the previously 

mentioned analyses (Law and Lumb, 1978). 

To analyze a cut slope or embankment in the clay-rich Potomac 

soils (which have a history of retrogressive-type failures), the authors 

use a failure analysis similar to the one developed by the National 

Research Council of Canada (Law and Lumb, 1978). This type of limit 

equilibrium analysis divides the soil mass into a number of slices in 

which the conditions for localized failure are considered. Strengths of 

the various slices are chosen according to the procedure outlined in 

figure 4. If localized failure is possible, a postpeak strength equal to 

the residual shear strength is assumed; thus, redistribution of interslice forces and the possibility of further localized failure are introduced. This procedure is continued until an overall factor of safety, 

expressed in terms of reserve strength, can be assessed. 

The selection of an appropriate factor of safety for design is dependent on several factors. Among these are the degree of uncertainty of 

the slope conditions, the cost of construction or repair, and the hazards 

of slope failure to human life. 

Generally, a safety factor of 1.25 is considered adequate for the 

design of small projects in which the probability of loss of life and 

property is small. A factor of safety of 1.5 is generally required for 

projects potentially involving loss of life or large property loss; this 

factor may be increased to 2.0 in high-risk situations. 

CONSTRUCTION PROCEDURES 

Various remedial construction methods are used for correcting 

slides in Potomac Formation soils. These methods include excavationreplacement schemes, various retention systems, berm counterbalances, and others. Irrespective of the construction method used, it is 

very important for the design to incorporate procedures that isolate 

the slope from ground water and surface water as much as possible. 
SLOPE DESIGN AND CONSTRUCTION, POTOMAC FORMATION 61 

It has been the authors' experience that subsurface drainage systems have limited applicability in Potomac Formation clays and silts. 

The residual shear strength of these soils is generally too low to 

stabilize failed slopes economically by using improved subsurface 

drainage only. Drainage systems are quite useful, however, for 

stabilizing permanent slopes or portions of slopes where failure has 

not yet reduced the strength to the low residual values. 

Relatively few guidelines have been published for the design of subsurface drainage systems to stabilize slopes. A recent model study has 

provided some generalized design charts for establishing the spacing 

and length of subsurface drains to obtain an incremental increase of 

the safety factor (Kenney and others, 1977). However, because of the 

highly fractured nature of many Potomac Formation clay-bearing 

soils and their low intact permeabilities, generalized design charts 

may not be applicable. It has been the authors' experience that the 

locations of springs or seeps encountered during construction often 

change as the landslide moves. Drainage systems constructed into 

seeps often yield little or no water. 

The authors feel that slopes having observed drainage problems 

require some kind of drainage correction even though the apparent 

success of such measures may be questionable. The authors typically 

use trench drains placed 30 to 50ft (11-15 m) apart and constructed 

as deep as practical. In many places, the drains are connected to cutoff drains parallel to the slope at the top and bottom of the slope. 

The critical design conditions for the completed slope require the 

slope to be marginally stable (a factor of safety equal to about 1.1) in 

case the subsurface drainage system should fail completely. This 

requirement is considered prudent because existing ground-water 

conditions, as well as the ground-water conditions that cause failure, 

often cannot be defined accurately during the investigative phase. 

Furthermore, clay- and silt-rich Potomac Formation sediments are 

easily eroded in many places and can clog drainage systems. To minimize the clogging, subsurface drainage systems are normally used in 

combination with filter fabric. 

It is important that a geotechnical engineer familiar with the 

design be at the site during construction to ensure that assumptions 

used for design are valid. Problems can arise when clay-rich soils are 

exposed to water for prolonged construction periods and consequently 

soften and cause localized failure. Other important items that require 

careful field inspection include proper alinement and installation of 

piling or tieback systems, proper compaction of embankment soils, 

and proper placement of subsurface drains to intercept surface and 

subsurface water. 
62 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

SELECTED REFERENCES 

American Society for Testing and Materials, 1978, Annual book of ASTM standards, 

pt. 19, Designation D 1586-67 (Reapproved 197 4), Standard method for penetration 

testing and split barrel sampling of soils: Philadelphia, Pa., p. 235-237. 

Bishop, A. W., 1960, The use of the slip circle in the stability analysis of slopes: 

Geotechnique, v. 10, no. 4, p. 129-150. 

Bjerrum, L., 1967, Progressive failure in slopes of overconsolidated plastic clay and 

clay shales: American Society of Civil Engineers Proceedings, Journal of Soil 

Mechanics and Foundation Engineering Division, v. 95, no. SM 5, p. 1-49. 

Chandler, R. J., and Skempton, A. W., 1974, The design of permanent cut slopes in stiff 

fissured clays: Geotechnique, v. 24, no. 4, p. 457-466. 

Janbu, N., 1973, Slope stability computations, in Hirschfeld, R. C., and Poulos, S. J., 

eds., Embankment-dam engineering: New York, John Wiley, p. 47-86. 

Kenney, T. C., Pazin, M., and Choi, W. S., 1977, Design of horizontal drains for soil 

slopes: American Society of Civil Engineers Proceedings, Journal of Geotechnical 

Engineering Division, v. 103, no. GT 11, p. 1311-1323. 

Law, K. T., and Lumb, P., 1978, A limit equilibrium analysis of progressive failure in 

the stability of slopes: Canadian Geotechnical Journal, v. 15, no. 1, p. 113-122. 

Lupini, J. F., Skinner, A. E., and Vaughan, P.R., 1981, The drained residual strength 

of cohesive soils: Geotechnique, v. 31, no. 2, p. 181-213. 

Morgenstern, N. R., and Price, M. A., 1965, The analysis of the stability of general 

slip surface: Geotechnique, v. 15, no. 1, p. 79-93. 

Roscoe, K. H., Schofield, A. N., and Wroth, C. P., 1958, On the yielding of soils: 

Geotechnique, v. 8, no. 1, p. 22-53. 

Skempton, A. W., 1964, Long-term stability of clay slopes: Geotechnique, v. 14, no. 2, 

p. 77-101. 

__ 1970, First-time slides in overconsolidated clays: Geotechnique, v. 20, no. 3, 

p. 320-324. 

__ 1977, Slope stability of cuttings in brown London Clay: International Conference on Soil Mechanics and Foundation Engineering, 9th, Tokyo 1977, Proceedings: v. 3, p. 261-270. 
ENGINEERING GEOLOGY AND DESIGN OF SLOPES FOR 

CRETACEOUS POTOMAC DEPOSITS IN 

FAIRFAX COUNTY, VIRGINIA, AND VICINITY 

EVALUATION OF PERMANENT CUT 

SLOPE STABILITY IN 

POTOMAC FORMATION DEPOSITS 

By JAMES J. SCHNABEL 1 

CONTENTS 

Page 

Abstract--------------------------------------------------------------64 

Introduction-----------------------------------------------------------64 

Geotechnical investigation -----------------------------------------------64 

Geotechnical evaluation -------------------------------------------------67 

Selection of soil strength parameters ---------------------------------------67 

Factorsofsafety--------------------------------------------------------72 

Slope protection measures -----------------------------------------------7 3 

Referencescited--------------------------------------------------------77 

ILLUSTRATIONS 

Page 

FIGURE 1. Graphs showing typical consolidated drained direct shear test results 

for a clay-rich Potomac Formation sediment having precut shear 

surfaces--------------------------------------------------69 

2. Diagram showing typical subdrain-----------------------------74 

3, 4. Cross sections showing: 

3. Typical surface and subdrainage detail showing drainage 

parallel to slope---------------------------------------7 5 

4. Typical subdrainage detail showing drainage perpendicular 

to slope----------------------------------------------75 

5. Diagram showing typical subdrainage for previously failed slope-----76 

'Principal and professional civil engineer, Schnabel Engineering Associates, Consulting Geotechnical Engineers, 

Bethesda, Md. 

63 
64 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

ABSTRACT 

Evaluation of slope stability in Potomac Formation deposits is complicated because of 

variable geologic settings and soil strength parameters, the irregularity of the shear 

surface, and other factors not fully understood at this time. Stability conditions are 

evaluated by traditional methods of investigation and analysis, coupled with soil 

mechanics laboratory testing to estimate the in-situ strength of potential failure 

planes. Selection of soil strength parameters is based in part on geologic factors such as 

the presence of water-bearing layers within potential slide zones, the intensity and 

orientation of fractures and bedding-plane partings, the presence of slickensides on 

fractures, clay consistency, the thickness of clay-rich beds, the plasticity characteristics of clays, and the stability of natural slopes at the site. Evaluating the importance 

of each of these factors requires a careful evaluation of the geologic setting at each site. 

Surface and subsurface drainage systems are used to improve the stability of most 

cut slopes. Factors of safety used in design are based on the consequences of land slippage, as well as on the amount and sophistication of field studies and laboratory testing, and on the geotechnical engineer's confidence in his evaluation of stability 

conditions. 

INTRODUCTION 

The geotechnical engineering evaluation of slope stability conditions in Potomac Formation deposits is particularly difficult because 

of variable geologic settings and soil strength parameters, the irregularity of the shear surface, and other factors not fully understood at 

this time. Engineers are required to evaluate stability conditions by 

traditional methods of investigation and analysis, coupled with soil 

mechanics laboratory testing to estimate the in-situ strength of 

potential failure planes. Selection of soil strength parameters must 

take into account geologic factors such as shear zones and fractures, 

previous landsliding, and previous experience with slope instability 

in a similar geologic setting. Surface and subsurface drainage 

systems are used to improve the stability of most cut slopes. Factors 

of safety used in design are based on the human and economic consequences of land slippage, as well as on the amount and sophistication 

of field studies and laboratory testing, and on the geotechnical engineer's confidence in his evaluation of stability conditions. Design procedures for permanent cut slopes discussed in this paper have been 

developed over the years by Schnabel Engineering Associates of 

Bethesda, Md. 

GEOTECHNICAL INVESTIGATION 

Geotechnical investigation for slope stability in Potomac Formation 

sediments requires careful evaluation of site conditions. An investigation for a cut slope in a hillside that has no apparent actively moving slides should meet the following minimum requirements: 
PERMANENT CUT SLOPE STABILITY 65 

1. The pattern of subsurface layering and intertonguing of sediments 

having different engineering properties must be clearly established. Texture and strength are the principal properties used to 

define this pattern, in combination with observations of sheared 

or fractured zones. Field tests used to define this pattern are primarily soil test borings made with the Standard Penetration 

Test (SPT) (American Society for Testing and Materials, 1978). 

SPT tests are made in the borehole at vertical intervals less than 

5 ft (1.5 m) apart, except in zones suspected to have been sheared 

by previous landsliding, where tests are usually continuous. Borings should be at critical locations on a slope, especially where 

slopes are steepest or where preexisting landslides are suspected. Cuts greater than about 30 ft (9 m) high should have borings located along the slope at the top, at the midpoint, and at 

the base to define subsurface conditions adequately. For cuts less 

than 30 ft (9 m) high, one boring at the top and one at the base 

are usually sufficient to provide necessary data. It is important 

to delineate thin layers during this investigation. SPT spoon 

samples can detect sand layers that are only 2 to 3 in. (5-7 .6 em) 

thick; although these sand layers are quite thin, they may be 

water bearing during wet seasons. 

2. SPT blow count values are used to evaluate the in-situ compactness of granular soils and the consistency of cohesive soils and to 

guide the undisturbed sampling program. SPT values are also 

helpful for locating weak shear zones within otherwise strong 

deposits; abruptly lower penetration resistances are commonly 

encountered in these shear zones. SPT blow count values of 15 

per foot or less are often recorded in old slide debris, whereas the 

underlying undisturbed materials commonly have values ranging from 20 to 70. Relatively low SPT values of 10 or less in 

cohesive soils may also indicate the presence of water-bearing 

sand seams interlayed with weaker clay layers and weathered 

materials. 

3. Water observation wells should be installed in borings in the upper 

regions of a slope to allow long-term observations of groundwater levels. Perforated plastic pipe surrounded by sand is normally suitable for completing these wells. 

4. Undisturbed samples 3 in. (7.6 em) in diameter are taken in finegrained soils, where sampling is possible. Sampling normally 

can be done by pushing a Shelby tube into clays and silts that 

have SPT values of less than 20 blows/ft. The tubes are sealed 

and delivered to a soil mechanics laboratory for testing. 

5. Test pits should be dug to supplement data obtained from the soil 

test borings and to allow undisturbed sampling of soils that cannot be sampled by normal sampling methods. Test pits excavated 
66 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

as deep as 12ft (3.7 m) with a small backhoe are usually sufficient. Pits should be logged by an experienced engineering geologist who will note geologic details that cannot be detected by 

examining samples from test borings. These details include the 

jointing pattern, the presence of fractures and slickensides, and 

the thickness and orientation of strata. Any water flowing into 

the pit also should be noted. Block specimens of soils also may be 

removed from the pit for laboratory testing. 

6. Experienced geotechnical engineering personnel must make at 

least one trip to a site to check for evidence of previous instability. This site visit is of paramount importance and should be conducted with the aid of a topographic map and other available 

geologic aids, such as detailed maps or aerial photographs. The 

geotechnical engineer must investigate variations in the normal 

topography to ascertain if they are caused by previous landslides, erosion, or the activity of man. This investigation should 

be conducted after field investigation data are available, when 

there is a clear understanding of subsurface conditions. 

Additional borings and sampling may be required if the site investigation reveals possible previous landsliding, jointing, or tectonically 

induced shearing. Such previously failed areas must be delineated 

accurately enough to determine localities that may need special 

treatment. 

During site visits, particular attention should be paid to tilted trees 

and to tree trunks bent at the base, which can be indications of creep 

or landsliding. Seasonal streams and permanent and seasonal springs 

must be located and mapped because they are potential discharge 

sites of a locally high ground-water table. Wet-ground vegetation 

should be noted because it is an indicator of springs or a high groundwater table. 

Where active landslides are present, slope inclinometer systems are 

installed; piezometers are also placed in some slides. Slope inclinometer data permit the recognition and identification of shear zones in 

active landslides. Readings taken on rapidly moving landslides 2 to 3 

days after installation are commonly sufficient to determine the zone 

of shearing. Longer term readings are required for locating shear 

zones in more slowly moving soil masses. 

Piezometers are used to determine artesian pressures in confined 

granular layers. These piezometers can be made from well points 

packed with sand at the tip and then packed with bentonite above the 

sand. Readings should be made on piezometers during wet and dry 

seasons to determine the relationship between precipitation and 

piezometric levels within confined layers. 
PERMANENT CUT SLOPE STABILITY 67 

GEOTECHNICAL EVALUATION 

Geotechnical evaluation requires determining the critical potential 

failure surface and the shear strength of the soil along this failure 

surface and applying various limiting equilibrium techniques to 

assess the overall stability of the soil mass. Slope stability evaluation 

proceeds in one of two modes, depending on the presence or absence of 

either landslides or sheared zones. The same methodology is used for 

assessing ancient or recent slides. 

For an existing landslide, the analysis of a cut slope should use the 

geometry of the existing failure surface. For previously stable, unfailed slopes, investigation for a proposed cut slope commonly uses a 

trial-and-error procedure to determine the critical failure surface. 

The Simplified Bishop (1960) Method is used to evaluate slope 

stability where circular slip surfaces can be used in the analysis. This 

method is readily adapted for computer analysis, and various trial 

circles are calculated until a minimum factor of safety is determined. 

The Simplified Bishop Method assumes that the forces acting on the 

sides of any slice of soil have zero resultant in the vertical direction, 

and static equilibrium therefore is not satisfied. Lateral and shear 

forces on the sides of slices thus are eliminated from consideration, 

but the method still permits inclusion of seepage forces. The Simplified Bishop Method yields a factor of safety within 2 to 7 percent of 

the more accurate, statically satisfied Morgenstern and Price (1965) 

Method. The Simplified Bishop Method is preferred over the statically 

correct methods because it is simple. 

Stability calculations for surfaces other than circular surfaces of 

sliding are evaluated by the Modified Swedish Method (U.S. Army 

Corps of Engineers, 1970). This method is used for less than 20 percent of stability calculations because most calculations are for unfailed slopes. 

SELECTION OF SOIL STRENGTH PARAMETERS 

It is of major importance to select soil strength parameters that fit 

the conditions anticipated at failure. Different strength parameters 

are normally used for stability calculations, depending on the presence or absence of a shear surface (such as a landslide rupture surface 

or a fault shear zone) in the proposed cut. 

For the test program, consolidated drained direct shear tests using 

three normal loads are conducted on specimens ofundisturbed soil2.5 

in. (6.35 em) in diameter. Strength testing is done at a very low strain 

rate, and the following sequence is used: 
68 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

1. A specimen is carefully extruded from a thin-walled sampling tube 

3 in. (7 .62 em) in diameter and trimmed to fit a direct shear box 

ring 2.5 in. (6.35 em) in diameter. 

2. The specimen is cut horizontally in the 2.5-in. (6.35 em) shear box 

by using a wire saw. 

3. A vertical pressure (normal load) is applied to the specimen, which 

is then allowed to equilibrate (consolidate). The vertical pressure 

used is normally arbitrarily taken as 1.00 kip/ft2 (ksf) (4.88 

kgf/cm2

) for the minimum pressure; increments of 1.00 to 1.50 

ksf (4.88-7 .32 kgf/cm2) are used for the second and third normal 

stresses. 

4. After consolidation is essentially complete, the specimen is 

sheared while the normal load is still being applied; a controlled 

strain rate of about 2 percent per hour, intended to allow complete drainage during shearing, is used. The shearing is stopped 

at a horizontal displacement of 0.50 in. (1.27 em). 

5. While the normal load is still being applied, the specimen is 

reversed in the testing machine, and the top and bottom segments are returned to their original positions. 

6. The sample is again sheared along the failed surface, as described 

above, in the same direction as the original shearing. This procedure is repeated until the total cumulative shear movement 

along the failure surface is at least 1.50 in. (3.81 em). A plot is 

made of the cumulative displacement of the specimen versus 

applied shear stress for all three normal loads, as figure 1A 

illustrates. 

7. Two more specimens of similar material are tested as described 

above under different normal stresses. 

8. Normal stress versus shear stress data are plotted to determine 

the angle of internal friction and the cohesion intercept of the 

Mohr-Coulomb envelope, as figure 1B shows. The minimum 

shear strength after 1.50 in. (3.81 em) of displacement is plotted 

versus normal stress, and a line is fitted between the points. 

The method described above will usually result in a shear strength 

envelope that passes through or slightly above the origin. The test 

method permits determination of the ((residual shear strength," 

which is defined as the lower limit of the drained strength envelope 

when cohesion is reduced to zero. 

The slope of the shear strength envelope (cf>r ~obtained by using this 

method gives a measure of the residual friction angle that can be used 

for slope design, but values of cf>r' obtained from laboratory testing are 

adjusted to conform to experience with similar slopes in similar geologic settings. The measured cohesion also must usually be adjusted 

downward for the evaluation of slopes where landslide surfaces of 

rupture or fault shear zones are absent. This adjusted cohesion, the 
fi: 

en 

a. 

~ 2.0 

z 

u) 

(/) 

UJ 

a: 

1-

(/) 1.0 a: 

<( 

UJ 

I 

(/) 

A 

PERMANENT CUT SLOPE STABILITY 69 

-----First forward run for peak strength 

---Second forward run 

;&---...., 0 1.0 kips/ft' normal stress 

/ , o 2.5 kips/ft' normal stress 

/ 6 '-- t::,. 6 4.0 kips/ft' normal stress 

,t::. ------e:,. __ -----6-------------6 

I 

I -a- =a-~-----------A I o I I t/ 

---o---0------0 

0.1 0.2 0.3 0.4 0.5 0.6 

SHEAR DISPLACEMENT, IN INCHES 

3.0 ,------,------,--,-------,--,-----, y 

fi: /A·p=25° en 

a. 

~ 2.0 

z /0 ~~ 

C' 

B 

0 ~ 0 1.0 kips/ft' normal stress 

0~0 o 2.5 kips/ft' normal stress 

6 4.0 kips/ft' normal stress 

FIGURE 1.-Typical consolidated drained direct shear test results for a clay-rich 

Potomac Formation sediment having precut shear surfaces. A, Shear stress versus 

displacement for different normal stresses. B, Peak and residual shear strength 

envelopes. ¢p', peak friction angle along precut surface; ¢/, residual friction angle; 

c p', cohesion for peak strength curve; c', cohesion for 1.5-in. displacement curve. 

Points used for determining maximum shear strength (upper curve) were taken at 

the peak of their corresponding stress displacement curves. Points used for determining residual shear strength (lower curve) were taken after 1.5 in. of total 

displacement. 
70 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

~~available cohesion," is reduced to a minimum to account for the 

presence of joints and other features that allow penetration of water 

into the soil and thus cause further long-term weakening of clay-rich 

soils. This weakening is believed to be associated with stress release 

caused by cutting, which allows joints and fractures to open. The 

mechanisms whereby strong clay-rich sediments weaken after a cut 

is made are not fully understood, and the strength parameters 

selected must be reduced to reflect the uncertainty in the amount of 

anticipated softening. 

The effective cohesion (c? (see fig. 1B) recorded in the testing normally varies from 0 to 0.175 ksf (0-0.855 kgf/cm2

). For the design of 

slopes cut in previously stable slopes, these cohesion values are arbitrarily reduced to 0.025 to 0.050 ksf (0.122-0.244 kgf/cm2); the reductions are based on experience gained from engineering geologic 

studies of Potomac Formation landslides. For the design of slopes in 

which no previous movement has occurred, such as first-time slides in 

cuts in London Clay, Skempton (1977) advocated the theory that some 

cohesion is present ip first-time slides; this hypothesis seems to be 

confirmed by experience with Potomac Formation sediments (Schnabel 

and Grefsheim, 1982). It is recognized that direct comparisons of soil 

behavior between London Clay and Potomac Formation clay-rich 

deposits cannot be made. It is the author's opinion, however, that 

enough similarities exist between these clays to allow consideration 

of Skempton's hypothesis; that is, displacements preceding firsttime slides are sufficient to cause some progressive failure, which 

reduces the available shear strength toward a Hfully softened" value, 

but movements are not sufficient to reduce strength to residual 

values. This strength lies between the lower residual strength and 

the peak strength and possesses a small cohesion. The author's observations of slope failures in the Potomac Formation seem to confirm 

that the clay structure does not break down completely and behave as 

a purely frictional material. 

The angle of internal friction for first-time slides in clay-rich 

Potomac Formation sediments is usually taken as the measured 

residual friction angle, ¢/, determined by using the test method 

described, although some adjustment to this value may be necessary. 

Laboratory testing of precut samples provides an estimate of the 

minimum or residual strength value applicable after initial movements. Adjustments to the test values can be made depending on 

whether a first-time slide condition is being analyzed, the degree of 

confidence in a first-time slide condition, experience with similar clay 

soils, and evaluation of the local geologic factors. For this adjustment 

and for the final selection of design strength parameters, it is useful 

to utilize back-analysis calculations for full-scale existing landslides 

in soils of similar classification and geologic conditions (Schnabel and 
PERMANENT CUT SLOPE STABILITY 71 

Grefsheim, 1982). Judgments concerning shear strength parameters 

can best be formed by careful study of the geology and the ground 

water of a site, back-analysis calculations of previous slides, and test 

data. The influence of various geologic factors on the selection of 

appropriate strength parameters is as follows: 

Geologic conditions 

Presence of water-bearing layers within ---

potential slide zones, possibly only 

seasonal flow. 

Presence of fractures -------------------

Orientation of fractures and bedding------

planes. 

Presence of hard, thick, unfractured or-----

weakly fractured clay beds having no 

granular layers or lenses. 

Presence of lower plasticity clayey silt-----

soils or high percentage of granular 

grain sizes. 

Existing natural slopes relatively steep. ---

Absence of measurable ground water------

within potential slide zone. 

Overall consistency of clay-rich soils ------

relatively low as measured by SPT 

blow counts. 

Presence of troughs filled with -----------

permeable soils or a high groundwater table. 

Effect on strength 

parameters(</>', C') 

selected for design 

Use slightly lower 

strength parameters. 

Use lower strength 

parameters with 

higher intensity of 

fracturing. 

Use lower strength 

parameters where 

fractures and bedding 

parallel potential 

slide surfaces. 

Use higher strength 

parameters. 

Use higher strength 

parameters with 

either factor. 

Use higher strength 

parameters. 

Use higher strength 

parameters. 

Use lower strength 

parameters. 

Use lower strength 

parameters. 

Design shear strength parameters developed by using the test procedure outlined herein vary considerably, apparently owing to the 

plasticity of the specimen tested, clay mineralogy, sand content and 

gradation, details of testing, and interpretation of test results. The 

range of parameters experienced for the soil types noted is as follows: 

Highly plastic clay,-------------¢/= 8 o to 19 o 

CH (liquid limit 70 percent), c' = 0 to 0.050 ksf 

small percentage of sand. (0-0.244 kgf/cm2) 
72 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

Plastic clayey silt --------------c/Jr' = 14 ° to 22 ° 

MH (liquid limit 55-65 c' = 0 to 0.050 ksf 

percent) (0-0.244 kgf/cm2

) 

Testing of lower plasticity clay (CL) appears to yield strength parameters similar to those of CH specimens. 

Normally, minimum residual friction angles (c/Jr ~of 12 o to 15 o are 

used for Potomac Formation clays or clayey silts, cohesion being zero 

if previous movements are suspected. A nominal cohesion of 0.025 to 

0.050 ksf (0.122-0.244 kgf/cm2) may be used in some cases if there is 

no evidence of previous sliding. 

For evaluating failure along existing shear surfaces or where existing slide surfaces are suspected, it is customary to assume that cohesion is zero on the failure surface. The friction angle is taken as the 

laboratory-measured residual friction angle. Considerable judgment 

should be exercised in selecting the parameters used for design, however; factors such as back analysis of slides at the site should be taken 

into account because the laboratory test method often results in 

values that are unrealistically high or low. 

The Potomac deposits in Fairfax County and vicinity contain many 

extensive, relatively compact silty and clay-bearing sands. These 

soils are generally quite stable on natural slopes and do not weaken 

as severely as the more plastic sediments. The following effective 

strength parameters are commonly used for design: 

SM, silty sand,------------ cp' = 30° 

compact. c' = 0 

SC, clayey fine sand,------- cp' = 30 o 

compact. c' = 0 to 0.050 ksf 

(0-0.244 kgf/cm2) 

Laboratory testing should always be performed to evaluate parameters at sites that are marginally stable and at sites where slopes are 

relatively high or where life or property may be endangered. 

FACTORS OF SAFETY 

Uncertainties that are commonly present in slope evaluation may 

be categorized as follows: 

1. Geologic features such as the dip of strata, water-bearing strata, 

and the presence of fissures, slickensides, and old slip surfaces or 

fault planes. 

2. Soil strength parameters along the surface of sliding. 
PERMANENT CUT SLOPE STABILITY 73 

3. Piezometric levels within the slope and variations in piezometric 

conditions with time. 

4. Destabilizing forces that may act on the slope in the future. 

5. Locations of critical surfaces of failure. 

6. Simplifying assumptions used for analysis. 

The geotechnical engineer must consider these uncertainties when he 

is selecting a reasonable factor of safety for design. A factor of safety 

of 1.5 is normally required for design of a permanent cut slope where 

failure would endanger life or property or would have serious adverse 

effects. For analysis of stability of existing slopes, a value of 1.2 is 

usually acceptable, if available information is adequate and if conservative strength parameters are used. Low factor-of-safety values may 

be used for areas where there is only a possibility of minor slippage or 

where adjacent natural or cut slopes have remained stable. Higher 

factors should be used where structures are located on the slope or 

near the crest of the slope (that is, less than half the slope height from 

the crest). Considerable judgment must be exercised in selecting a 

factor of safety consistent with the various uncertainties noted above. 

It should be realized that present methods of investigation, testing, 

and analysis may fail to disclose geologic details that can lead to 

failure of a slope. Slopes in clay and silt materials of the Potomac 

Group may prove unstable regardless of the numerical factor of safety 

that may be developed from analysis. 

SLOPE PROTECTION MEASURES 

Surface water and ground water should be drained from all major 

cut slopes. Stability is enhanced by reducing the possibility that 

water will soften the soil and by increasing the available shear 

strength for resisting sliding on the critical sliding surface. Groundwater pressures can have a major influence on the effective shear 

strength along the surface of failure because a major portion of that 

strength is derived from the normal intergranular forces in the soil. 

Strength reductions caused by a high water table and high pore pressures can be decreased by using subdrainage and surface drainage to 

control the potentiometric levels in the cut slope. Subdrainage is also 

used to drain seasonal aquifers within the slope. 

Often, surface water infiltrates into weathering fractures to depths 

of 2 to 5 ft (0.6-1.5 m) below the surface of Potomac Formation materials. This perched water is caused by precipitation and can best be 

drained by a subdrainage system placed about 6 to 8 ft (2-2.5 m) 

below final grade. This subdrainage should be located immediately 

above the crest of the cut slope, particularly in low areas where surface water can accumulate. 
74 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

Subdrainage is usually accomplished by placing perforated or 

slotted plastic pipe 4 to 6 in. (10-15 em) in diameter and having a protective filter in trenches spaced vertically at intervals of about 10ft (3 

m) down the slope. A typical subdrain is shown in figure 2. Except in 

the upper 2ft (0.6 m), the trench is filled with pervious sand or stone 

to intercept water flowing in fractures, joints, and more permeable 

layers. Pipes are placed at a minimum slope of 0.05 in./ft and should 

lead to catch basins for disposal. Cleanouts should be placed to allow 

periodic flushing of subdrains. Drains should be constructed to avoid 

freezing and to allow monitoring of flow. 

Drains in previously unfailed slopes are placed parallel to the final 

grade contours and are installed in benches 5 to 6 ft (1.5-2 m) wide to 

permit maintenance and access. Drains should always be placed near 

E 

cv:i 

J 

~ 

.:t:::: 

0 

0 

+"' 

00 

~ 

co 

C,) 

·c.. 

> I-

. 

Impervious 

clayey soil 

~Pervious backfill 

..A-in. diameter perforated pipe 

~ encased in filter fabric 

·Minimum of 6 in. (15 em) of granular 

filter material around pipe 

FIGURE 2.-Typical subdrain. 
PERMANENT CUT SLOPE STABILITY 75 

the crest to intercept seasonal perched ground water. In many places, 

a shallow drain is placed at the toe of the slope to minimize the possibility that ground water will come to the surface. A typical drainage 

cross section is shown in figure 3. 

Where spring activity is localized or the ground-water table is deep, 

it may be desirable to install a perpendicular drainage arrangement 

into the slope, as figure 4 illustrates. This drainage system can be installed from the toe of the slope where seepage is observed. Normally, 

the depth into the slope is controlled by the excavation equipment 

available and seldom exceeds about 20 ft (6 m). The drainage pipe and 

porous material are placed in such a way that workmen do not have 

to enter the excavation. 

Subdrains in previously failed slopes are oriented in a general 

downslope configuration, as figure 5 shows. Where additional movement can be anticipated, the subdrains are often positioned at an 

5- to 6-ft (1.5-1.9 m) 

widebenchN 

Surface drainage ditch\ 

EXPLANATION 

!

Impervious clayey soil 

· Pervious backfill 

· Perforated pipe enclosed 

in granular filter material 

EJ Undisturbed surface 

FIGURE 3.-Typical surface and subdrainage detail showing drainage parallel to 

slope. Details of subdrain shown in figure 2. 

Open end (exposed) 

Closed end 11 ~111~11~ ,t--3;1~-:..::::j 1

1':::::..=1 11 20ft±(6.1m)----J ' 1

··-11-, 

FIGURE 4.-Typical subdrainage detail showing drainage perpendicular to slope. 
76 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA . 

... ""~,.,--------r--/-----

Perforated 4- to 6-in subdrainage 

pipes in~renches backfilled 

with porous material 

50 100 FEET 

CONTOUR INTERVAL 2 FEET 

FIGURE 5.-Typical subdrainage for previously failed slope. Contour elevations 

in feet. 
PERMANENT CUT SLOPE STABIITY 77 

angle of about 45 o to the slope in a herringbone pattern. On actively 

moving slides, the subdrains are located perpendicular to the contours. Horizontal spacing of drainage pipes is usually about 25 to 40 ft 

(7.5-12 m), depending on the permeability of the soil, the slope angle, 

existing slide movements, and the amount of water present. Subdrains in failed slopes are usually 8 to 15 ft (2.5-4.5 m) deep and 

should extend below the shear surface to be most effective. The piezometric level within the slope is assumed to be represented by a 

straight line extending through the inverts of the subdrainage pipes. 

Surface water is also collected and removed from the slopes, usually 

by means of paved concrete ditches located at the crest of cut slopes 

where there may be extensive runoff and on benches on the slopes. 

Benches are constructed on high slopes to allow access by maintenance equipment in case of local instability. Slopes are seeded or 

sodded as soon as possible after final grading to avoid erosion of soil 

and possible clogging of drainage facilities. 

REFERENCES CITED 

American Society for Testing and Materials, 1978, Annual book of ASTM standards, 

pt. 19, Designation D 1586-67 (Reapproved 197 4), Standard method for penetration 

test and split barrel sampling of soils: Philadelphia, Pa., p. 235-237. 

Bishop, A. W., 1960, The use of the slip circle in the stability analysis of slopes: 

Geotechnique, v. 10, no. 4, p. 129-150. 

Morgenstern, N. R., and Price, M. A., 1965, The analysis of the stability of general slip 

surfaces: Geotechnique, v. 15, no. 1, p. 79-93. 

Schnabel, J. J., and Grefsheim, F. D., 1982, Slope stability in the Washington, D.C., 

area: Cretaceous clays: Transportation Research Record 873, p. 1-8. 

Skempton, A. W., 1977, Slope stability of cuttings in brown London Clay: International Conference on Soil Mechanics and Foundation Engineering, 9th, Tokyo 

1977, Proceedings: v. 3, p. 261-270. 

U.S. Army Corps of Engineers, 1970, Engineering and design of earth and rock-fill 

dams: U.S. Army Corps of Engineers Engineer Manual EM 1110-2-1902, 110 p. 

ENGINEERING GEOLOGY AND DESIGN OF SLOPES 

FOR CRETA CEO US POTOMAC DEPOSITS IN 

FAIRFAX COUNTY, VIRGINIA, AND VICINITY 

A SUMMATION AND CRITIQUE: 

A REVIEW OF PROCEDURES FOR DESIGN 

OF PERMANENT SLOPES ON THE 

POTOMAC FORMATION AND SOME 

RELEVANT FIELD OBSERVATIONS 

By STEPHEN F. OBERMEIER 

CONTENTS 

Page 

Abstract--------------------------------------------------------------79 

Introduction-----------------------------------------------------------80 

Slope stability evaluation of permanent cut and natural slopes ------------------80 

Parameters for existing landslides and for intensely fractured and (or) 

shearedsediments -------------------------------------------------81 

Parameters for first-time failures in "intact" sediments --------------------82 

Ground-water table location -------------------------------------------82 

Some relevant geologic observations ---------------------------------------83 

Naturalslopes------------------------------------------------------83 

Cutslopes----------------------------------------------------------86 

Sand and permeable strata --------------------------------------------87 

Hillside fills --------------------------------------------------------87 

Design problem --------------------------------------------------------87 

Suggestions for future studies --------------------------------------------88 

Referencescited--------------------------------------------------------88 

ABSTRACT 

The design engineers who authored the papers in this volume all agree that residual 

shear strength parameters should be used to evaluate the possibility that failure will 

be reinitiated along the surfaces of rupture for preexisting landslides. These residual 

parameters may be determined by laboratory testing, but laboratory data should be 

verified by back analysis of the landslide if recurrence would have serious conse79 
80 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

quences. The engineers also agree generally that low shear strength parameters 

should be used for slope design on intensely fractured clay-rich sediments. Swanson 

and Jones advocate using residual strength parameters, whereas Schnabel may also 

include a "reduced cohesion" component of strength. 

The designers also agree that effective shear strength parameters higher than 

residual shear strength parameters should be used to evaluate the stability of slopes 

where there are no preexisting shear zones and in places where the soils are intact and 

not intensely fractured. Failure is assumed to take place in drained shear. Swanson 

and Jones advocate using "fully softened" parameters. The strength parameters used 

by Schnabel seem to be about the same, although direct comparison is not possible. 

This author believes that the geotechnical properties of natural slopes are fundamentally different from those of cut slopes and that these differences are important to 

understanding slope instability in the Potomac Formation. Natural slopes commonly 

have many weathered, weakened zones and fracture surfaces in the upPermost 20 ft (6 

m), the main region traversed by the surface of rupture. The shear strength is probably 

controlled by soil in highly varying states of drainage and consolidation, and determination of exact strength parameters is almost impossible. However, the "fully 

softened" strength is probably a realistic value for the great majority of slopes; slopes 

that have unusually adversely oriented fractures, that have experienced significant 

creep, or that have clays whose tendency to swell is strong are exceptions. Unweathered, less intensely fractured clay-rich sediments are probably stronger than those that 

are more weathered. 

Much additional research is needed to better evaluate the slope behavior of Potomac 

Formation sediments. The highly variable nature of the sediments makes it necessary 

to pay great attention to geologic details. 

INTRODUCTION 

This paper summarizes the preceding two papers' most salient 

points regarding permanent slope design in materials of the Potomac 

Formation. As those papers state in many ways, the behavior of many 

slopes cannot be evaluated with any great degree of precision where 

clay-rich layers of Potomac Formation sediments are present. As a 

consequence, the design engineer must try to determine realistic 

bounds for the shear strength parameters and ground-water pressures. This author attempts to compare the methodologies that the 

consultants who have contributed to this volume use to determine 

those bounds and then discusses the relationship of their design techniques to the geologic setting. 

In addition, some fundamental problems of the mechanics of slope 

failure are discussed, as well as how these problems bear on slope 

design. Finally, some research needs are suggested. 

SLOPE STABILITY EVALUATION OF PERMANENT CUT 

AND NATURAL SLOPES 

Even though Schnabel specifically addresses the problem of cut 

slope stability, he notes that the same basic procedure is used to evaluate natural slopes. Neither paper makes any fundamental distinc-
SUMMATION AND CRITIQUE 81 

tion between the methodology used to evaluate a cut and that used to 

evaluate a natural slope. Both papers place major emphasis on estimates of fracture intensity and slickenside development; in addition, 

Schnabel uses the overall consistency of the sediments in the slope to 

guide his selection, lower parameters being used for weaker materials. Both papers agree that design shear strength parameters and 

ground-water pressures are major influences on slope stability 

evaluation. 

PARAMETERS FOR EXISTING LANDSLIDES AND FOR INTENSELY 

FRACTURED AND (OR) SHEARED SEDIMENTS 

For existing landslides, both papers agree that residual shear 

strength parameters should be used to evaluate the possibility that 

failure will reinitiate along the surface of rupture. These residual 

parameters may be determined by laboratory testing, but laboratory 

data should be verified by back analysis of the landslide if recurrence 

would have serious consequences. 

There is also general agreement that low shear strength parameters should be used for intensely fractured clay-rich sediments. 

Swanson and Jones advocate using residual strength parameters, 

whereas Schnabel may also include a ~~reduced cohesion" component 

of strength. It should be noted, however, that Schnabel's laboratorydetermined ~~residual" strength parameters may be somewhat higher 

than the residual shear strengths conventionally reported in the geotechnical engineering literature. Schnabel defines the ~~residual" 

strength as the shear strength after 1.5 in. (3.8 em) of displacement 

along a precut shear plane. Data reported in the most recent definitive study of residual shear strength (Lupini and others, 1981) show 

that a minimum shear displacement of about 2.5 in. (6.3 em) and 

typically on the order of 10 in. (25 em) is required to determine 

residual shear strength for plastic clays. Limited test data obtained 

by the author on remolded specimens of Potomac Formation clays by 

using a repeated direct shear technique showed no cohesion intercepts on the strength envelopes. 

Whether there is a cohesion intercept on the laboratory-determined 

strength envelope is probably irrelevant to the analysis of first-time 

slides in highly fractured and (or) sheared sediments because the 

laboratory test method does not simulate field conditions. Clay-rich 

Potomac Formation sediments are typically highly fractured in the 

uppermost 5 ft (1.5 m); these fractures commonly have rather rough 

surfaces, even where they are slickensided, and these surfaces are 

coated with washed-in clay in many places. These washed-in clays 

have usually been strengthened somewhat by drying. Thus, 

Schnabel's inclusion of a cohesion component of strength seems con-
82 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

sistent with field observations. The only possible method for deter· 

mining reasonable cohesion values uses back analyses of landslides. 

This author believes, however, that extreme caution must be used 

in extrapolating information from back analyses. If no large body of 

data from previous studies of the Potomac Formation is available, it 

would seem judicious to use Swanson and Jones' method. The 

approach used by Schnabel, which considers many types of data (in· 

eluding laboratory strength data, Atterberg limits, landslide 

geometry, fracture intensity and orientation, and other geologic factors) obviously can be best used whenever the designer has accumulated much experience. 

PARAMETERS FOR FIRST-TIME FAILURES IN "INTACT" SEDIMENTS 

Both papers agree that effective shear strength parameters higher 

than residual should be used to evaluate the stability of slopes where 

there are no preexisting shear zones and in places where the soils are 

intact and not intensely fractured or jointed; failure is assumed tc 

take place in drained shear. Swanson and Jones advocate using fully 

softened parameters. To justify using these parameters, much emphasis is placed on Skempton's (1970, 1977) research on the Londo11 

Clay. In a similar vein, Schnabel advocates use of his ((residual" 

strength parameters and inclusion of a cohesion component of the: 

shear strength. For any given sample, it is not possible to compare: 

directly the strength parameters that would be obtained by using 

Swanson and Jones' test method and those that would be obtained by 

using Schnabel's method. 

Both methods described above are, in this author's opinion, empirical. The technique using fully softened strength parameters is the: 

only one that approaches having a well-formulated theoretical basis: 

however, the method as developed and advocated by Skempton (1970, 

1977) applies only to nonswelling soils, to soils having closed jointf 

Goints so tight that ground water or surface water does not readily 

penetrate through them), and to slopes that do not fail because of 

seasonal variations in rainfall. Many of these conditions are not met 

in natural or cut slopes of the Potomac Formation. Discussion of the: 

applicability of the fully softened method to Potomac Formation sediments is deferred to a later section, in which this author comparef 

what he.believes are relevant geologic details of the London Clay anc1 

of the clay-rich Potomac Formation sediments. 

GROUND-WATER TABLE LOCATION 

Well points are generally used to monitor the ground-water table ir 

thick, permeable soils. In critical locations where thin, confinecl 
SUMMATION AND CRITIQUE 83 

strata are suspected to have artesian pressure, it seems wise to use 

the method suggested by Swanson and Jones, which uses a closed lowvolume-change system for measuring pressure. 

The papers seem to disagree about the effectiveness of an underground drainage system in increasing the stability of a failed slope. 

Swanson and Jones think that, for design, it is realistic and reasonable to assume complete failure of the system, whereas Schnabel 

assumes that the water table is lowered to the invert of the subdrainage pipes. 

Materials in the landslide mass above the surface of rupture are 

commonly highly fractured and quite permeable in the upslope part. 

Buried drains unquestionably lower the water table there and aid significantly in stabilization. However, there is some doubt about the 

effectiveness of the system, especially during prolonged wet seasons. 

For that reason, this author believes that Swanson and Jones' suggestion that the safety factor not be less than about 1.1, assuming failure 

of the drains, is reasonable. 

SOME RELEVANT GEOLOGIC OBSERVATIONS 

This author believes that the properties of natural slopes are fundamentally different from those of cut slopes and that these differences 

are important to understanding the stability of slopes in the Potomac 

Formation. For the great majority of design problems, however, he 

emphatically agrees with the consultants that the same methodology 

can safely be used to design both natural and cut slopes. This author 

distinguishes cut slopes from natural slopes because he believes that 

both vertical and subhorizontal fractures are important to stability; 

he also feels that the number of fractures is not necessarily as important as orientation, continuity, degree of softening, and amount of infilling of clay along the breaks. 

It should also be emphasized that most landslides in cut and 

natural slopes of the Potomac Formation take place during intense 

rainstorms or after prolonged rainy seasons. Much, if not all, of the 

surface of rupture commonly traverses the zone of seasonal weathering, where surface water readily flows into many of the fractures. 

NATURAL SLOPES 

Within approximately the upper 5 ft (1.5 m) of the ground surface 

are many fractures (here called surface fractures). They typically 

form an anastomosing network of closely spaced breaks that make 

numerous, more or less continuous ruptures both vertically and subhorizontally. These fractures are best developed in highly plastic 
84 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

clays, where they may be slickensided, and are at random inclinations varying from vertical to subhorizontal. 

Beneath these surface fractures and beneath other deposits such as 

colluvium and upland gravels that generally cover the Potomac sediments, there may be Hjoints," which typically are vertical or subhorizontal (approximately paralleling existing topography) breaks. These 

joints are best developed and common and can be continuous for large 

distances laterally in the massive and thick strata of clay-rich and 

plastic, silt-rich sediments. Both vertical and subhorizontal joints are 

more widely spaced and more poorly developed at increasing depths. 

The maximum depth for subhorizontal joints is usually less than 20 ft 

(6 m) beneath the unconformity separating the Potomac from younger 

materials, but the vertical joints extend much deeper in some places. 

Figure 18 in the first paper in this volume (p. 40) is a schematic depiction of these joints. Roots grow in and along joints in some places, 

water seeps from some joints, infilled and chemically softened soil is 

present along many of the joint surfaces, and there are some Hpointto-point" contacts (localized points where hard soil is in contact along 

the joint). In some places, the joints are open as much as 0.1 in. (0.25 

em). 

On the basis of numerous field observations, this author believes 

that the infilled and chemically weathered material along subhorizontal joints is commonly less consolidated than normal under the 

present overburden and that much of the overburden load is carried 

by the point-to-point contacts. He also believes that there has been 

enough small horizontal movement along many of the subhorizontal 

joints to accommodate wedging apart by tree roots and infilling by 

soil along vertical joints. The good match between opposite-facing 

irregular joint surfaces indicates that this horizontal movement is 

typically less than 0.25 in. (0.6 em). 

This physical setting is obviously so complex that no conceptual 

model can simulate precisely the mechanics of landsliding in nature. 

What, then, are the shear strength parameters that should be used 

for design problems, and what kinds of tests should be used to determine these parameters? 

Some insight can be gained by using various assumptions to bound 

the problem. To obtain realistic lower bound shear strength parameters for first-time slides, let it be assumed that there are no point-topoint contacts along the subhorizontal joints and that the weight of 

the overburden is carried by the softened soil along a subhorizontal 

joint that is a critically oriented potential surface of rupture. If the 

entire surface of rupture is considered, the softened soil would be 

approximately normally consolidated under the total unit weight of 

existing overburden; an exception would be where the permanent 

ground-water table is at or very near the ground surface (an uncom-
SUMMATION AND CRITIQUE 85 

mon occurrence), in which case the softened soil would be approximately normally consolidated under the effective unit weight of existing overburden. Obviously, a landslide surface of rupture would pass 

along this joint with normally consolidated soil for much of its length, 

but would the drainage state be drained or undrained? This author 

believes it probable that much of the soil would fail in undrained 

shear, for several reasons: (1) landslides almost always take place 

during or soon after spring thaw, severe rainstorms, or prolonged wet 

seasons, (2) landslide movement is commonly rapid in the sense that 

the scarp at the head may move perceptibly in a day, and (3) the permeability of clays and plastic silts is very low, and the rate of dissipation of shear-generated pore pressure (~ U J along subhorizontal joints 

must be extremely slow, especially for the deeper subhorizontal 

joints, where there are fewer vertical joints to relieve ~Us. Near the 

ground surface, however, the vertical surface fractures commonly 

intersect the subhorizontal joints at close intervals; these breaks 

would serve as urelief wells" and allow ~Us to dissipate. To summarize, at depths greater than about 5 ft (1.5 m), where surface fractures 

are rare, the lower bound shear strength would approximate the normally consolidated undrained shear strength; closer to the ground 

surface, the lower bound shear strength would be the normally consolidated drained shear strength. Where the shear surface follows the 

softened rupture, failure would be in direct simple shear; this shear 

strength is probably 20 to 30 percent lower than triaxial compression 

shear strength (Johnson, 1975). If factors such as the state of sheargenerated pore pressure and the type of shear are considered, this 

lower bound shear strength through the deep joints is probably only 

slightly higher than the residual shear strength. This author, however, emphasizes his belief that, rather than being a common occurrence, the physical setting assumed in this lower bound problem is 

uncommon or rare and is most likely only on natural slopes where 

soils are intensely weathered. Furthermore, the irregularities of the 

subhorizontal joint surface would cause the strength to be somewhat 

higher. 

Consider now a different model. Assume that the entire weight of 

the overburden is carried by the point-to-point contacts, which can be 

in one of two conditions-either crushed by the weight of the overburden or very strong and uncrushed. Because the contact area of the 

points is very small in either condition, any shear failure would occur 

primarily in drained shear. Thus, the average shear strength along 

any surface of rupture would approach the normally consolidated 

drained strength or would be higher (for uncrushed points); if the 

irregularities of the subhorizontal joint are considered, the average 

shear strength along any surface of rupture that followed the joint 

would be increased further and would probably become much higher. 
86 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

To summarize this model, the shear strength must at least approach 

the normally consolidated drained shear strength. 

For the great majority of slopes, however, other factors to be considered are that part of the overburden weight is carried by the softened 

soil and that the surface of rupture does not follow exactly a preexisting subhorizontal joint. The first factor would probably cause the 

shear strength to be less than the normally consolidated drained 

strength because of the tendency to fail in undrained shear. The second factor would cause the shear strength to exceed the normally consolidated drained shear strength and probably to greatly exceed that 

strength for most slopes. 

It is virtually impossible, or at least impractical, to determine 

precisely the true shear strength parameters for a given slope. Thus, 

the designer is forced to make a decision based largely on judgment 

and previous experience. It is this author's opinion that, for the great 

majority of natural slopes on clay-rich Potomac Formation deposits 

that have never failed or have not experienced significant creep, firsttime failure shear strengths approximately equal or exceed the normally consolidated drained shear strength; in rare instances, the 

first-time failure shear strengths approach being as low as the normally consolidated undrained shear strength, at depths greater than 

about 5 ft (1.5 m). 

CUT SLOPES 

For discussion purposes, cut slopes are excavations so deep that the 

possible surfaces of rupture go beneath the existing chemically 

weathered and infilledjoints and fractures. Obviously, there is some 

overlap between natural and cut slopes, especially in excavations less 

than 10 to 20 ft (3-6 m) deep. 

Using this definition for cut slopes in clays of the Potomac Formation, this author believes that the Potomac's physical setting closely 

resembles Skempton's (1970, 1977) research sites for cuts in the London Clay, where joints are tight and unweathered and surface water 

does not readily penetrate along them. Highly plastic clays of the 

Potomac Formation having liquid limits exceeding about 80 to 90 are 

an exception because they can swell and fracture and create slickensides in the material, probably within a few years after excavation in 

many places. For all clays except such highly plastic ones, a realistic 

lower bound shear strength appears to be the normally consolidated 

drained shear strength (normally consolidated under the overburden 

weight of the cut section after excavation). For highly plastic clays, 

this author believes that somewhat lower parameters should be used. 

Field observations indicate that highly plastic clays normally become 

intensely fractured and slickensided in the upper 5 ft (1.5 m) or more 
SUMMATION AND CRITIQUE 87 

during the useful lifetime of an excavation and probably are somewhat fractured to much gi-eater depths. Thus, the design methods 

used by the consultants represented in this volume seem generally 

consistent with geologic conditions. 

SAND AND PERMEABLE STRATA 

Instances have been reported where the surface of rupture passes 

through permeable sand-bearing strata in the Potomac Formation, 

where artesian pressures are high. This type of failure appears to 

take place almost exclusively in the area of the kaolinite-illite facies 

(although most surfaces of rupture in that facies certainly are in the 

clays and silts). Except in very unusual circumstances where there is 

a rather large cohesion component of strength, this author believes 

that failure of permanent slopes would be through the fine-grained 

materials. 

HILLSIDE FILLS 

A serious problem not addressed previously is failure of hillside fills 

placed over Potomac sediments. A construction practice sometimes 

used to combat this problem is to place better quality, stronger fill 

material directly on the natural Potomac Formation materials. Land-

, slides may take place later because the fill has stopped drainage from 

permeable strata, fractures, and joints in the underlying Potomac 

Formation materials. This type of problem can be eliminated by using 

subdrains at the contact of the Potomac Formation and the fill material and by excavating very weak, weathered Potomac Formation 

materials before placing the fill. 

DESIGN PROBLEM 

For sites where expensive structures may be affected by landsliding 

or where there is a danger to people, all designers agree that very conservative strength parameters should be used for design. But the 

designer often faces a dilemma in planning a project such as a housing subdivision, where very conservative parameters would require 

so much earthwork that the project would not be economically feasible. This author believes that there are always uncertainties attendant on the use of any but the most conservative shear strength 

parameters in the clay-rich sediments and that these uncertainties 

change the design process from a matter of engineering judgment to a 

social or legal problem. This belief holds true especially for shallow 

excavations, the type most commonly used in housing subdivisions, 

because the most probable landslide sites would be in the weathered 

zones where evaluation of strength parameters is uncertain. 
88 ENGINEERING GEOLOGY, CRETACEOUS OF FAIRFAX COUNTY, VA. 

SUGGESTIONS FOR FUTURE STUDIES 

Obviously, additional research is needed to determine appropriate 

design shear strength parameters. Because the Potomac Formation is 

so variable, both vertically and laterally, and because there are so 

many factors involved, such as joints, fractures, shear zones, artesian 

pressures, and swelling forces, extreme care would be required in 

selecting meaningful research sites, and extrapolating the data to 

other sites would be questionable in many cases. This author does not 

mean to discourage future studies but rather to urge that a very careful evaluation of the various factors be made before definitive conclusions are reached. 

An especially fruitful area for research would seem to the effectiveness of subdrainage systems for different geologic settings. Another 

area would be correlations between the residual friction angle and 

the liquid limit for clays of the montmorillonite and kaolinite-illite 

facies. Such research could result in correlations much better than 

the one shown by Swanson and Jones (see their fig. 5), because the 

friction angle is very much dependent on clay mineralogy (Lupini and 

others, 1981). 

Other items that clearly warrant research include the monitoring 

of long-term slope movements and potentiometric levels for both 

natural and cut slopes, detailed studies of the actual geometries of 

failure, a systematic laboratory evaluation of material strengths 

under the different stratigraphic and weathering conditions commonly present in the field (the evaluation should be done in terms of 

clay mineralogy, gradation, Atterberg limits, and consolidation 

state), and reconnaissance of active failures to determine how many 

are on man-modified slopes and how many are on natural slopes and 

how many are shallow and how many are deep. Also needed are better guidelines for designing slopes on small projects, where sophisticated exploration and testing costs are prohibitively expensive. 

REFERENCES CITED 

Johnson, S. J., 1975, Analysis and design relating to embankments: A state of the art 

review: U.S. Army Corps of Engineers Waterways Experiment Station Miscellaneous Paper S-75-3, 48 p. 

Lupini, J. F., Skinner, A. E., and Vaughn, P.R., 1981, The drained residual strength 

of cohesive soils: Geotechnique, v. 31, no. 2, p. 181-213. 

Skempton, A. W., 1970, First-time slides in overconsolidated clays: Geotechnique, 

v.20,no. 3,p. 320-324. 

__ 1977, Slope stability of cuttings in brown London Clay: International Conference on Soil Mechanics and Foundation Engineering, 9th, Tokyo 1977, Proceedings: 

v. 3, p. 261-270.