Engineering Geology of tbe 

Government Hill Area, 

Anchorage, Alaska 

A cooperative research project between the U.S. 

Geological Survey and the State of Alaska Division 

of Geological and Geophysical Surveys 

Engineering Geology of the 

Government Hill Area, 

Anchorage, Alaska 

By RANDALL G. UPDIKE and BRUCE A. CARPENTER 

A cooperative research project between the U.S. 

Geological Survey and the State of Alaska Division 

of Geological and Geophysical Surveys 

A geotechnical parametric characterization of the 

late Quaternary sediments which underlie the 

transportation-industrial center for south-central 

Alaska, with particular reference to earthquake 

hazards 

U.S. GEOLOGICAl SURVEY BUllETIN 1588 
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1986 

For sale by the 

Books and Open-File Reports Section 

U.S. Geological Survey 

Federal Center 

Box 25425 

Denver, CO 80225 

Library of Congress Cataloging in Publication Data 

Updike, Randall G. 

Engineering geology of the Government Hill area, Anchorage, Alaska. 

(U.S. Geological Survey bulletin ; 1588) 

"A cooperative research project between the U.S. Geological Survey 

and the State of Alaska, Division of Geological and Geophysical 

Surveys." 

Bibliography: p. 

Supt. of Docs. No.: I 19.3:1588 

1. Engineering geology-Alaska-Anchorage 2. Government Hill 

(Anchorage, Alaska) 

I. Carpenter, Bruce A. II. Geological Survey (U.S.) Ill. Alaska. Division 

of Geological and Geophysical Surveys. IV. Title. V. Series. 

QE75.B9 no. 1588 557.3 s 85-600269 

[T A705.3.A4] [624.1 '51 '097983] 
CONTENTS 

Glossary of terms used v 

Abstract 1 

Introduction 1 

Scope of this investigation 2 

Acknowledgments 2 

General geologic setting 3 

Techniques of investigation 6 

Fieldwork 6 

Subsurface data compilation and assessment 7 

Description of engineering geologic units 8 

Introduction 8 

Bootlegger Cove Formation 8 

Distinction of facies 8 

F.I, clay with very minor amounts of silt and sand 8 

F.II, silty clay and (or) clayey silt 9 

F.III, silty clay and (or) clayey silt, sensitive 9 

F. IV, silty clay and clayey silt with silt and sand layers 9 

F. IV A, silty clay and clayey silt with silt and fine sand lenses 9 

F.IVB, silty clay with traces of coarse sand 10 

F.V, silty clay and (or) clayey silt with random stones 10 

F. VI, dense silty fine sand with silt and clay layers 10 

F. VII, fine to medium sand with traces of silt and gravel 10 

Textural characteristics of the facies 10 

Geotechnical characteristics of the facies 12 

Post-Bootlegger Cove Formation glaciofluvial deposits 16 

Tidal flat and tidal marsh deposits 17 

Landslide deposits 19 

Fluvial deposits associated with modem drainage 22 

Man-related fill deposits 22 

Surface and ground-water hydrology 23 

Behavior of the engineering geologic units 24 

Liquefaction 25 

Sensitive silty clays 29 

Other mechanisms 30 

Summary 30 

References cited 31 

Contents Ill 
FIGURES 

1. 

2. 

3. 

4. 

5. 

6--10. 

11. 

12. 

13. 

14. 

15. 

16--18. 

TABLES 

Photographs showing Government Hill area and Anchorage Port facilities 2 

Photograph of Government Hill area, looking east 4 

Sketch map showing location of study area 5 

Generalized map showing regional tectonic setting of Cook Inlet region 6 

Example of subsurface geotechnical data retrieval system card 7 

Graphs showing: 

6. Plot of coarsest 1 percentile versus median diameter for samples of 

the Bootlegger Cove Formation 12 

7. Correlation of shear strength and unconfined compressive strength for 

samples of cohesive facies of the Bootlegger Cove Formation 16 

8. Correlation of mean shear strength and mean compressive strength, for 

cohesive facies of the Bootlegger Cove Formation 16 

9. Correlation of mean undisturbed shear strength and mean sensitivity 

ratio for cohesive facies of the Bootlegger Cove Formation 17 

10. Correlation of mean sensitivity ratio and mean liquidity index for cohesive facies of the Bootlegger Cove Formation 17 

Photograph of exposure in a landslide block, showing glaciofluvial deposits 

of the Naptowne glaciation of Karlstrom (1964) overlying the Bootlegger 

Cove Formation 18 

Composite drawing of idealized stratigraphic sequence of tidal flat deposits 

19 

Map showing distribution of landslides in Government Hill area 20 

Photograph showing 1964 Government Hill School landslide 21 

Map showing inferred direction of unconfined ground-water flow into uplands 

of Government Hill area 24 

Graphs showing: 

16. Determination of CN as a function of effective OVerburden pressure 28 

17. Liquefaction potential for tidal flat sands and glaciofluvial sands 29 

18. Liquefaction potential for tidal flat silty sands and F. VI 29 

1. Comparison of mean textural variations of the seven facies of the Bootlegger 

Cove Formation, Government Hill area, Anchorage, Alaska 11 

2-8. Summaries of engineering-soils properties of facies of the Bootlegger Cove Formation, Government Hill area 

2. F.I 12 

3. F.II 13 

4. F.III 13 

5. F.IV 14 

6. F.V 14 

7. F.VI 15 

8. F.VII 15 

9. Qualitative assessment of liquefaction potential for engineering-geologic units, 

Bootlegger Cove Formation, Government Hill area, Anchorage, Alaska 26 

10. Tabulation of borehole data and computed values utilized to evaluate liquefaction potential 27 

IV Contents 
GLOSSARY OF TERMS USED 

active fault A fault is active if, because of its present tectonic 

setting, it can undergo movement from time to time in 

the immediate geologic future, or has shown evidence of 

movement during the Holocene (last 10,000 years). 

aseismic Herein the term refers to behavior of soils and (or) 

structures under non-earthquake conditions. 

attenuation A decrease in seismic amplitude during transmission as a result of geometrical spreading and variations 

of the physical properties of the transmitting medium; absorption and scattering of the energy waves. 

Atterberg limits Two tests, suggested by Atterberg, which 

determine the moisture content at the upper and lower 

limits of the moisture range within which a soil exhibits 

the properties of a plastic solid. 

cyclic stress The subjection of a rock or soil element to a series 

of alternating shear stresses which may vary in magnitude 

and orientation in a random fashion. The resultant forces 

being exerted on the element are termed "cyclic loading." 

diamicton A general term for a nonlithified clastic sedimentary deposit which is poorly sorted and composed of coarse 

clasts (sand to boulders) set in a clayey silt matrix. Typically, diamictons are not stratified but may have a clast 

fabric and crude large-scale bedding. 

dry density ( -y d) The weight of the dry solid particles in a unit 

volume of soil. 

effective stress (cr v) The total stress exerted on a plane of a 

soil element minus the pore-water pressure; this stress in 

tum controls certain aspects of soil behavior. 

facies, engineering geologic A sedimentary unit (soils unit) 

possessing the characteristics of a geologic facies, but in 

addition, defined on the basis of engineering soils criteria 

that are dictated by the geologic history and ambient conditions of the unit. 

facies, geologic Any observable set of attributes, such as overall appearance, composition, or conditions of formation, 

of one part of a stratigraphic unit as contrasted with other 

parts of the same unit. These attributes define a sedimentary mass that may have irregular lateral boundaries; its 

upper and lower limits may not necessarily correspond 

with boundaries of the stratigraphic unit that contains 

them or with the upper and lower limits of other facies. 

A geologic facies may occur singly or repeatedly in a 

vertical section. 

free face That part of the ground surface that stands more 

steeply than the general slope of the terrain above or 

below that slope. The lack of lateral buttress forces at 

the face allows the potential for displacement behind the 

face. 

frequency The number of cycles occurring in unit time. 

internal friction The resisting shear strength considered to be 

due to the interlocking of the soil grains and the resistance 

to sliding between the grains. 

least-squares linear regression A statistical method for finding the equation of a straight line which best fits a set 

of data points, thus providing a relationship between two 

variables. 

liquefaction A condition wherein a soil will undergo continued 

deformation at a constant low-residual stress or with no 

residual resistance, due to the buildup and maintenance 

of high pore-water pressures which reduce the effective 

grain-to-grain pressure to a very low value. 

liquid limit (W1) The minimum moisture content of a soil-water 

mixture at which the mixture will act as a viscous liquid. 

liquidity index (Ll) An index equal to the difference between 

the natural moisture content and the plastic limit divided 

by the plasticity index; the index indicates the value of 

the natural moisture content relative to the Atterberg 

limits. A value greater than 1.0 suggests that, upon remolding, the soil would be transformed into a viscous 

liquid. 

magnitude A logarithmic scale which is used to define an 

earthquake in terms of the motion that would be measured 

by a standard seismograph located 100 km from the epicenter of the earthquake. 

natural moisture content (W n) The ratio of the weight of the 

water to the dry weight of the soil solids in a given soil 

mass. 

Phi size ( <!>) The diameter of a soil particle as expressed by 

-log2 of diameter in millimeters. 

plastic limit (Wp) The minimum moisture content at which 

a soil mixture acts as a plastic solid. 

plasticity index (PI) The numerical difference between the 

plastic limit and liquid limit of a soil; it represents the 

range of moisture content within which the soil exhibits 

the properties of a plastic solid. 

pore-water pressure (U) The pressure or stress transmitted 

through the pore water filling the voids of a soil. 

sensitivity The loss of strength suffered by most natural clays 

when remolded at constant water content. The sensitivity 

ratio (Sr) equals the original shear strength divided by 

the remolded shear strength. 

shear strength (S) A measure of the ability of a soil to resist 

sliding along internal surfaces within the mass. 

soil Here used in an engineering sense, soil refers to all of 

the unconsolidated mineral material at or near the Earth's 

surface, plus the air, water, organic matter, and other 

substances that may be included therein. 

standard deviation A statistical measure of dispersion, the 

square root of the average of the squares of the deviations 

about the mean of a set of data. 

standard penetration test (SPT) A means of estimating relative density and (or) unconfined compressive strength, 

which consists of driving a standard split-spoon sampler 

by dropping a 63-kg mass from a height of 760 mm. 

Penetration resistance is reported in number of blows necessary to drive the spoon 305 mm. 

stratigraphy The order of succession of the different sedimentary rock units (or soils units) that occur in an area. 

unconf"med compressive strength (qu) The strength of a 

cylindrical soil sample in simple compression (no confining pressures), which is often used to describe the consistency of undisturbed cohesive soils. 

Glossary V 

Engineering Geology of the Government 

Hill Area, Anchorage, Alaska 

By Randall G. Updike 1 and Bruce A. Carpenter2 

Abstract 

The Government Hill area and immediate vicinity 

have been the site of much of the early and continuing 

economic development of the City of Anchorage, particularly as the principal marine port and railway terminal 

for the region. Because many of the geological engineering problems that confront construction throughout Anchorage are manifest in the Government Hill-Ship 

Creek-Anchorage Port area, a detailed geotechnical 

study has been conducted here. 

Late Quaternary glaciomarine and glaciofluvial 

sedimentation is primarily responsible for the sequence 

of sediments that directly underlie the Government Hill 

area. Most extensive of these sediments is the late Pleistocene Bootlegger Cove Formation. This formation has 

long been recognized as an engineering-soils unit likely 

to cause ground-stability problems when subjected to 

strong seismic-loading conditions. The static and dynamic geotechnical properties of these sediments have been 

evaluated by utilizing stratigraphic and geotechnical data 

from about 250 boreholes. The result of the analyses of 

these data has been the three-dimensional characterization of the engineering geology to 50 m below mean 

sealevel. 

Seven engineering geologic facies of the Bootlegger Cove Formation are distinguishable on the basis of 

our applying a series of sedimentologic and engineering 

parametric criteria, including particle size distribution, 

bedding structures and their interrelationships, Atterberg limits, natural-moisture content, natural density, 

shear strength, compressive strength, sensitivity, and 

standard penetration resistance. Five of the facies are 

cohesive silts and clays with varying amounts of sand 

and gravel as secondary constituents. The other facies 

are noncohesive and consist predominantly of coarse silt 

and sand. The cohesive facies accumulated in a re1 Alaska Division of Geological and Geophysical Surveys, Eagle 

River, AK 99577. 2

Present address Geraghty and Miller, Inc., Syosset, NY 11791. 

stricted marine to freshwater basin; their variations result from fluctuations in sediment source and transport 

energy. The noncohesive facies represent glacial-fan 

delta and emergent tidal-flat deposition. Mapping the 

subsurface extent and relationship of individual facies 

has provided a better understanding of both the 

geologic history and present-day geotechnical characteristics of the formation. 

One of the cohesive facies, F.lll, is a sensitive silty 

clay which, when subjected to seismic shear stresses, 

exhibits rapid strength diminution. It is this facies that 

was responsible for the catastrophic landslides during 

the 1964 earthquake. 

The liquefaction susceptibility of the sandy facies 

(F.VI and F.VII) has been examined using currently accepted techniques, and, based upon the existing data 

set, they do not appear to be liquefiable. A similar assessment of the liquefaction susceptibility of the Holocene tidal-flat sediments indicates that these soils are 

locally susceptible to seismically induced liquefaction. 

Numerous prehistoric landslide deposits appear on 

essentially every slope surrounding the Government Hill 

upland. An examination of the local surface and groundwater hydrologic regime indicates that these waters significantly enhance slope stability problems within the 

Bootlegger Cove Formation. The multiplicity of the 

mapped translatory, rotational, and flowage-type landslides strongly suggests that similar slope failures will 

occur in the future. The potential for sensitive clay collapse in the upland areas of Government Hill and for 

liquefaction of sands and silty sands on the tidal flats 

and in the lower flood plain of Ship Creek should be 

carefully evaluated prior to future development of these 

areas. 

INTRODUCTION 

From a rather recent birth in 1914, Anchorage has 

expanded to become the population center for the State 

of Alaska. The early social and economic growth of the 

city was focused in the vicinity of lower Ship Creek. SetIntroduction 1 
tlement soon spread from the flood-plain tidal flat area 

to the adjacent uplands north (Government Hill) and south 

(Anchorage townsite) of Ship Creek (figs. 1, 2). Development in the following 25 years was essentially limited 

to these areas. The years from the beginning of World 

War II into the mid-1950's saw even further emphasis 

on the development of Government Hill and the near sealevel areas to the west and south into what has now become the land and sea transportation hub for south-central 

Alaska. The area of Government Hill-Anchorage PortShip Creek remains central to the growth and well-being 

of the entire region. 

The natural setting of Anchorage has been quite 

conducive to its growth as a population and economic 

center. Topography in the Anchorage area is of low relief, 

gently sloping from the Chugach Mountains on the east 

toward Cook Inlet on the west and south (fig. 3). Much 

of the central part of Anchorage lies on a broad, gently 

sloping alluvial plain truncated by bluffs along Cook Inlet 

and along major streams flowing from the mountains to 

the inlet, the most important of which is Ship Creek. In 

addition to favorable topography, the region also offers 

a moderation of the more typical Alaskan climates, being 

warmer and wetter than the interior of the State and yet 

cooler and drier than most southern Alaska coastal areas. 

The climatic modification is a result of the surrounding 

mountain ranges and Cook Inlet. The climate and topography have provided the region with abundant water resources, through both surface runoff and ground-water 

aquifers, further stimulating socioeconomic growth. 

One aspect of the regional geology which has been 

far less favorable to inhabitants of Anchorage is the regional seismicity. Earthquakes are felt by residents on a 

rather regular basis, usually causing little or no damage 

and only moderate alarm. However, the Prince William 

Sound earthquake, magnitude 8.4-8.6, of March 27, 

1964, inflicted considerable destruction and loss of life 

in Anchorage, and its memory still lingers among longtime residents. Substantial damage occurred in the Government Hill-Anchorage Port area due to failure of structures, landslides, subsidence, and ground fissuring. 

Miraculously, no lives were lost. 

The years since the 1964 earthquake have seen a 

rebuilding in this part of Anchorage with an ever-increasing emphasis on industrialization: the port has been modernized for military and civilian purposes; military, railroad, and private agencies store fuel supplies here; the 

Alaska Raiiroad has expanded its facilities. Moreover, the 

population of Government Hill has increased to about 

5, 000 residents. The continued industrial and transportation growth and development coupled with a rather dense 

population have emphasized the need for detailed engineering geologic study of the Government Hill area, 

with particular attention placed on earthquake-related hazards. 

2 Engineering Geology, Government Hill Area, Alaska 

Figure 1. Oblique aerial photographs, Government Hill 

area. A, Juxtaposition of transportation, residential, and 

commercial uses in the area, which has had a history 

of ground failure. 

Scope of this Investigation 

The study of Government Hill and vicinity has two 

primary intents: (1) to accurately map the surface and subsurface geology at a scale that can be applied to sitespecific problems and (2) to evaluate this geology in terms 

of the area's geotechnical characteristics, particularly in 

light of the regional seismicity. The primary geologic hazard in the area is mass movement, which will be examined 

in relation to the mapping and parametric characterization 

of failure-susceptible units. The mapping phase has previously been completed under a separate report (Updike, 

1986), and the text of the present report will occasionally 

refer to that report. The reader is encouraged to have the 

map in hand while reading the present report, which deals 

primarily with the second phase of the study. 

Acknowledgments 

This project was supported by a cooperative agreement between the U.S. Geological Survey (USGS) and 
Figure 1-Continued. Oblique aerial photographs, Government Hill area. 8, View, looking north, of petroleum 

storage tank farms and Anchorage Port facilities. Both views by R. G. Updike, 1980. 

the Alaska Division of Geological and Geophysical Surveys, under the auspices of the USGS Earthquake Hazards 

Reduction Program. We wish to express special gratitude 

to D. J. V ames of the USGS, Denver, for his assistance 

in providing the base maps and historic aerial photography 

from which the mapping was accomplished, as well as 

his advice and encouragement throughout the project. H. 

E. Olsen (USGS, Denver) deserves special note, for it 

was through long conversations with him that the scope 

of this project evolved. We are also grateful for the advice 

and criticism of H. R. Schmoll (USGS, Denver), whose 

experience with upper Cook Inlet Quaternary stratigraphy 

proved very helpful. 

Numerous governmental agencies and private organizations provided geotechnical data that form the heart 

of this study. The following individuals have been particularly helpful, providing not only their data but also their 

personal experiences and interpretations: Harry Lee and 

David Cole, DOWL Engineers; Cliff Fugelstadt, Alaska 

Railroad; Larry Pennington, Municipality of Anchorage; 

and Patrick Galbraith, U.S. Army Corps of Engineers. 

Technical review of this report was provided by T. 

L. -Youd (USGS, Menlo Park), and by D. J. Varnes, H. 

R. Schmoll, and H. E. Olsen (all USGS, Denver). Their 

thorough efforts are most appreciated. 

GENERAL GEOLOGIC SETTING 

The active tectonic framework of the Anchorage region is evidenced both by the frequent occurrence of felt 

earthquakes and by the surface expression of active fault 

traces in nearby Quaternary deposits (fig. 4). Government 

Hill is located on the eastern shore of Knik Arm, itself 

a northeastern extension of Cook Inlet. During Quaternary 

time, this inlet has been influenced by fluctuations in sea 

level, advance and retreat of adjacent mountain glaciers, 

and tectonic subsidence and uplift. The geologic record 

produced by the interplay of these systems is indeed complex and certainly not fully understood by geologists, but 

some generalizations can be made. 

The Greater Anchorage area, of which Government 

Hill is a part, lies on the eastern margm of the Cook 

Inlet forearc basin. This tectonic basin is bounded to the 

west by the Bruin Bay-Castle Mountain fault system and 

General Geologic Setting 3 
to the east by the Border Ranges fault system (formerly 

called Knik fault) along the front of the Chugach Mountains, east of Anchorage. Seismicity appears to be occurring along both fault systems as well as along the underlying Benioff Zone (Aleutian megathrust), which resulted 

from the subduction of the Pacific tectonic plate beneath 

the North American tectonic plate. Although, during a 

given time period, most seismic events occur along this 

zone of underthrusting (Fogelman and others, 1978), the 

likelihood of near-surface events on or adjacent to the 

surface fault systems is significant. Seismic activity related to the Castle Mountain system generally comprises 

low-magnitude shallow events (M=3.0-4.5) even though 

the potential for larger magnitudes (greater than M = 6. 0) 

is quite real (John Lahr, oral commun., 1978). The unpublished work of investigators has shown evidence for 

low-magnitude seismicity along the mountain front, but 

uncertainty seems to prevail regarding the source of this 

seismicity. 

The bedrock beneath the Anchorage lowland consists of Tertiary clastic sedimentary rocks which form a 

wedge lapping up to the east against the dominantly Mesozoic metamorphic rocks of the Chugach Mountains. No 

bedrock is exposed in the Anchorage lowland area, as 

it is generally 100-300 m below the surface and has been 

only rarely encountered even in deep boreholes. 

A thick, nonlithified sedimentary sequence was deposited predominantly in Pleistocene time and modified 

by erosional processes during Holocene time. It is this 

surficial sequence that engineering and design problems 

focus on. The best summaries of the area's Quaternary 

history remain those of Karlstrom ( 1964), Miller and Dobrovolny (1959), and Schmoll and Dobrovolny (1972). 

Karlstrom proposed that at least five major glaciations occurred in upper Cook Inlet: the Mount Susitna, Caribou 

Hills, Eklutna, Knik, and Naptowne glaciations. The dates 

he established for some of these advances are now seriously questioned (for example, Schmoll and others, 1972; 

Reger and Updike, 1983), but the geomorphic and stratigraphic evidence still supports their use in the literature. 

The three earlier glaciations were presumably far more 

extensive than the later ones. Based upon stratigraphic 

Figure 2. Oblique aerial photograph of Government Hill area, looking east. Elmendorf Air Force Base visible 

at upper left corner. Although individual landslides are not discernible in photograph, many of the petroleum 

storage tanks of middle distance are constructed on old landslides. The 1964 Government Hill Elementary School 

landslide lies in middle distance at right edge of photograph. (Compare fig. 14.) Photograph by R. G. Updike, 

1979. 

4 Engineering Geology, Government Hill Area, Alaska 
and geomorphic criteria, Karlstrom (1964) envisioned that 

the ice of these earlier glaciations coalesced to fill the 

Cook Inlet trough. The two later glaciations were not extensive enough to fill the entire basin but did restrict or 

totally dam off lower Cook Inlet to form the so-called 

"Glacial Lake Cook." This lake could well have contained 

brackish and (or) fresh water during the many years of 

its existence (or multiple existences). Controversy continues regarding the time frame and extent of glaciers and 

lakes during middle and late Pleistocene time and, more 

specifically, the correlations that can be made between 

glaciofluvial, glaciodeltaic, glaciomarine, and true glacial 

tills within the basin. However, we do sense a general 

harmony among researchers that the Bootlegger Cove Formation, 3 a stratified sequence of clastic sediments ranging 

from clay to boulders, was laid down during late Pleistocene time. Radiometric dates obtained by Schmoll and 

others ( 1972) indicate a late Wisconsin age for the upper 

part of the formation (about 14,000 years B.P.), which 

they interpret as predating t_he glacier advance to the Elmendorf Moraine. Paleontologic evidence supporting the 

contention that the formation was primarily deposited in 

3

This stratigraphic unit was originally named the Bootlegger Cove 

Clay by Miller and Dobrovolny ( 1959) for typical sections exposed at 

Bootlegger Cove, in Anchorage. Because the unit varies greatly in composition, and clay commonly is a secondary constituent, it has been 

redefined (Updike and others, 1982; Schmoll and others, 1984) as the 

Bootlegger Cove Formation, which is more consistent with the North 

American Stratigraphic Code usage applied when two or more rock types 

are common in a unit. 

marine waters (Schmidt, 1963) through a glacially . restricted brackish environment cannot be disregarded. The 

stratigraphy of Government Hill indicates the following 

late Pleistocene events: (1) the deposition of stratified 

sands and gravels, now well below existing sea level, presumably from a glaciofluvial source to the north; (2) accumulation of the Bootlegger Cove Formation, presumably in a glaciomarine and (or) glaciolacustrine environment; and (3) deposition of late Wisconsin glaciofluvial 

sediments. The data from boreholes that penetrate the preBootlegger Cove deposits (about 10 m below the contact) 

show Iio evidence for till, though such till is believed 

to exist at greater depths. The Elmendorf Moraine (late 

Wisconsinan in age) occurs directly north of the project 

area, and the glaciofluvial deposits associated with it are 

both terraced and pitted (see description of mapped units 

following). 

Since the late Pleistocene deposits accumulated, 

erosion of the sequence has occurred along the axis of 

Knik Arm and by major streams draining from the 

Chugach Mountains into the arm. This erosion has been 

accomplished through the hydraulic removal of tide-level 

material, coupled with mass wasting of bluffs, and both 

fluvial downcutting and lateral planation. These 

mechanisms have been strongly influenced by (1) eustatic 

changes in sea level, (2) vertical crustal movements (see 

Brown and others, 1977; Plafker, 1969), and (3) long-term 

fluctuations in stream regimen and hydraulic geometry 

(for example, the present underfit conditions of lower 

Ship Creek). Associated with this erosion has been the 

deposition of Holocene tidal-flat and fluvial deposits in 

the downcut areas, and the accumulation of landslide deposits along the retreating upland scarp faces. 

Elmendorf Air Force Base 

10 KILOMETERS 

Figure 3. Location of the study area. 

General Geologic Setting 5 
60 

Figure 4. Generalized, regional tectonic setting of Cook Inlet region. 

Faults are approximately located; barbs show direction of movement; U, 

upthrown side; D, downthrown side. 

TECHNIQUES OF INVESTIGATION 

Fieldwork 

The field-mapping program was conducted during 

the summer of 1979. Initially, a helicopter and groundbased reconnaissance of land bordering upper Cook Inlet 

was conducted to evaluate the variability of the Bootlegger Cove Formation. Particular note was made of compositional and stratigraphic variations, different modes of failure due to slides and flows, and the relation of this formation to other stratigraphic units. This work strongly 

suggested that the behavior of the Bootlegger Cove Formation is, in part, a function of local composition of the 

formation and also due to local hydrologic conditions. 

Fieldwork within the mapped area involved detailed 

6 Engineering Geology, Government Hill Area, Alaska 

mapping of surficial geologic units using a topographic 

base map at a scale of 1:1 ,200, contour interval 2 feet. 

Concurrently, several sets of aerial photography were employed at various scales including USGS-M373-

8AUG50, 1:40,000; Mohawk ARD-U.S. Army M-64-

76--4APR64, 1:1,826; Alaska RP-66-1002-100CT66, 

1:4,265; DOT-AUG66, 1:2,823; Air Photo Tech ANC2MAY67, 1:24,000; U2-CIR-78-119-25AUG78. The 

mapping was accomplished using field exposures, handdug trenches, hand-augered drill holes, and geomorphology (both field observations and aerial photographs). Observations made at construction and excavation sites from 

July 1978 until July 1980 were also incorporated into the 

mapping. When field mapping was complete, the results 

were transcribed from the 1: 1 ,200-scale maps and aerial 

photographs onto a 1 :2 ,400-scale base map used for final 

publication (Updike, 1986). 
Station No. __ 2==2----------------------------

Quadrangle Anch. A-8 NW U. T.M. Coor. 2645250 N 520225 E 

Given Coord. Sketch map Date of Hole 1964 Date of tests __,;,. 8/64 _______ _ Agency/Firm Shannon-Wilson (PD-206) Project Earthquake research for Corps Eng. 

Application: Centerline/Material site/Foundation study/Other: Bootlegger Cove Clay research 

Location (verbal) Port dock area (city) 

STRATDAT r---

Total depth of hole: 1601 (elev. +20) ntent • 

Stratigraphy: Detailed --(Acceptab"ij? -- Poor imi ts • 

Top of Boot 1 egger: c:re5;J I NOk Base: ~I No** 

ANCHDAT 

Moisture co 

Atterberg L 

Penetration 

Shear stren 

Compressive 

Sensitivity 

Shear wave 

Field tests 

Lab tests o 

Lab and fie 

Other data 

data 

Sampling: None /C[poradi~/ Intervals /Continuous 

Bedrock encountered: Yes /(NO) Depth: 

gth 

strength 

• 

Hydrologic data: Yes /(NO) WT Depth: • 

Sediment descrip.: Poor ;<A£ceptabf'e.> I Detailed 

Specific data: (e.g., ash, fossils, etc.) 

velocities 

only 

* -52 ** -130 nly 

ld • 

( ) 

U.T.M. Coordinates: 264525QN 52QZ2SE Station No. 22 

Figure 5. Example of subsurface geotechnical data retrieval system card utilized to catalog information used 

in present study. Triangle indicates that the listed geotechnical data (ANCHDAT) were reported for the specific 

borehole site. Verbal comments under STRATDAT are qualitative assessments of relative quality or quantity of 

data compared to other sites. Footnoted numbers under "specific data" are elevations in feet of top and base 

of the Bootlegger Cove Formation as reported in borehole log. 

Subsurface Data Compilation and Assessment 

Beginning in fall 1978, borehole data from geotechnical and hydrologic wells were collected by the authors 

from numerous Federal, State, municipal, and private 

sources. As of publication time (1985), 246 logs and 

associated test data have been accumulated and compiled 

into catalog form using a standardized format (fig. 5). 

This format allows for recording of data available for each 

drill hole similar to a library retrieval card system, for 

both geologic (STRATDA T) and engineering 

(ANCHDAT) information. 

Based on the field mapping and subsurface data 

bank, compatible stratigraphic map units were identified 

including the Bootlegger Cove Formation, post-Bootlegger Cove glaciofluvial sand and gravel units (Pleistocene), 

fluvial sediments associated with modem drainages (Holocene), tidal flat and tidal marsh sediments associated with 

modem Knik Arm (Holocene), landslide deposits (Holocene), and man-related fill deposits. 

Specific attention was directed to the sedimentary 

successions of the Bootlegger Cove Formation. Seven 

facies were differentiated based on visual geologic and 

geotechnical characteristics, lateral extent and correlation, 

and transition probabilities. The facies states were then 

characterized by quantitative sedimentologic textural 

criteria, and this delineation of facies facilitated interpretation of stratigraphic successions and paleoenvironmental 

changes. 

We have hypothesized that the nature of the facies 

variations, both vertical and lateral, could dictate the 

intraformational characteristics under both static and dynamic conditions. With this premise in mind, the engineering parameters for each facies were compiled using 

existing test data. Standard soils-engineering criteria were 

used including natural moisture content (Wn), plastic limit 

(Wp), liquid limit (W1) plasticity index (PI), liquidity index 

(Li), shear strength (S), unconfined compressive strength 

(qu), sensitivity (Sr), standard penetration (SPT), and dry 

density ( 'Y d). Statistical tests were applied to the bulk data 

to reduce them to meaningful values. 

The result of the above exercise has been the derivation of engineering geologic facies of the Bootlegger Cove 

Formation which are distinguished both by their particular 

geologic signature and by their geotechnical characteristics. These facies can be traced throughout the mapped 

Techniques of Investigation 7 
area, providing a basis for interpreting paleoenvironmental 

changes and the resultant stratigraphic successions, and 

also for understanding and predicting the regional distribution of subsurface geotechnical conditions. 

DESCRIPTION OF ENGINEERING GEOLOGIC 

UNITS 

Introduction 

Upon even casual examination of upper Cook Inlet 

and the glacially fed streams draining therein, one is impressed by the buff-gray, silt-laden waters which seem 

wholly uniform and ubiquitous throughout the region. The 

subsequent deposition, erosion, countless episodes of redistribution, and, ultimately, the burial of these fine sediments constitute the rule rather than the exception for 

Cook Inlet. The tidal flat muds of Turnagain and Knik 

Arms are the typical result of such cycles. In the bluffs 

bounding the inlet, a variety of Pleistocene clastic sedimentary units are exposed, but the most predictable constituents are stratified clays and silts which are essentially 

identical to analogs being laid down in the modem tidal 

inlet. We are led to conclude that the types of sedimentation have not altered dramatically in terms of texture. 

Bootlegger Cove Formation 

The older sediments of dominantly silt-clay texture 

which are exposed discontinuously about the perimeter of 

upper Cook Inlet have been referred to as the "Bootlegger 

Cove Clay" (Miller and Dobrovolny, 1959). Although earlier studies suggest that all such exposures mark the same 

stratigraphic unit, in recent years authors more commonly 

use phrases such as "Bootlegger Cove equivalent" and 

"Bootlegger Cove-type material," suggesting the unsettled 

question of regional correlation and contemporaneity. In 

visualizing, as Karlstrom (1964) did, a grand glacially 

dammed "Lake Cook," the regionality of a singular Bootlegger Cove unit becomes more plausible. Paleontologic 

studies (Schmidt, 1963; Smith, 1964) have indicated entirely marine microfauna, and geochemical studies by Kerr 

and Drew (1965) support that interpretation. In our opinion, insufficient evidence exists to make broad regional 

correlations of Bootlegger Cove correlatives, though such 

efforts have been successful for the Anchorage vicinity 

(for example, Barnwell and others, 1972). 

The formation was recognized by Miller and Dobrovolny (1959) as having the potential to produce serious 

landslides, and, 5 years after their publication, this recognition was borne out by the massive failures resultant from 

the 1964 earthquake. Several publications soon followed, 

each focusing on the landslide hazards of the formation 

8 Engineering Geology, Government Hill Area, Alaska 

(for example, Shannon and Wilson, Inc., 1964; Hansen, 

1965; Kerr and Drew, 1965, 1968; Long and George, 

1966a, b; Seed and Wilson, 1967). In addition, the important role that the formation plays as a confining layer in 

the ground-water hydrologic system of the region was 

identified (for example, Cederstrom and others, 1964; 

Trainer and Waller, 1965; Barnwell and others, 1972) and 

continues to influence the viability of municipal water 

supplies. 

Distinction of Facies 

Our studies have identified seven facies, which usefully characterize the intraformational variation of the 

composition and geotechnical parameters. The examination of these facies in a spatial-stratigraphic framework 

provides a sharper image of how the Bootlegger Cove 

Formation relates to late Pleistocene events in upper Cook 

Inlet. Assuredly, facies are intricately intercalated so that 

the scale of units being defined as mappable facies is a 

concern. Generally, units less than 1 m thick will be considered as lenses or layers within a facies. As a function 

of depositional environment and postdepositional modifications-for example, normal consolidation and groundwater leaching, the engineering soils characteristics for a 

given facies can be summarized and contrasted with those 

of other facies of the formation. The facies (F) differentiated in the Bootlegger Cove Formation are: 

F.I Clay with very minor amounts of silt and sand 

F.II Silty clay and (or) clayey silt 

F.III Silty clay and (or) clayey silt, sensitive 

F .IV A Silty clay and clayey silt with silt and fine 

sand lenses 

F .IVB Silty clay with traces of coarse sand 

F. V Silty clay and (or) clayey silt with random 

stones 

F.VI 

F.VII 

Dense silty fine sand with silt and clay layers 

Fine to medium sand with traces of silt and 

gravel 

In the following description of the facies, grain 

sizes are discussed in terms of phi units. The millimeter 

equivalents are as follows: 

-gravel-I--sand--1--- silt --1-- clayphi(<f>) -3 -2 -I 0 I 2 4 6 

mm 8 4 2 I 0.5 0.25 0.125 0.062 0.031 0.016 0.008 0.004 0.002 

F.l, Clay with Very Minor Amounts of Silt and Sand 

Although the Bootlegger Cove Formation is often 

referred to as a clay unit, most of the stratigraphic sequence is recognized to consist instead of abundant silt. 

Facies F .I is a notable exception in containing very small 

amounts of sediment coarser than 8<f>. The silt and fine-
sand fractions usually occur as scattered grains and less 

commonly as thin discontinuous lenses a few millimeters 

thick, in a massive blue-gray to olive-blue-gray clay 

(Munsell Color Chart, 2.5Y 4/0). Sedimentary structures 

(for example, bedding, partings, color changes) are exceedingly rare. Thickness of F.I varies from less than 1 

m to more than 13 m, but in this latter case some coarser 

lenses can be expected. Although the facies can locally 

be several meters thick, lateral continuity is lacking so 

that its occurrence in boreholes and in exposures is limited. Two general stratigraphic positions are suggested by 

elevations and succession; one is about present-day sea 

level (that is, about midpoint in the stratigraphic sequence) where it is bounded above and below by either 

F .III or F .IV; the other is in the upper part of the sequence where it is associated with F .II or F .IV below 

and F. VII or post-Bootlegger Cove sediments above. 

F.ll, Silty Clay and (or) Clayey Silt 

This facies is one of the less extensive within the 

formation. Most of its occurrences are in the upper half 

of the Bootlegger Cove section, where it is generally 

thicker than 3 m but rarely thicker than 10 m. Where 

it occurs lower in the section, bedding of the facies is 

more massive, that is, on the scale of several centimeters 

between horizons; higher in the section the tendency is 

toward thinner beds a few centimeters in thickness, each 

being distinguished by slightly different textures than 

those layers above and below. Contacts between horizons 

are rather abrupt, sometimes slightly inclined, but generally planar. Colors are blue gray to olive gray (2.5Y 4/0 

to 5Y 311). Although the textural bedding is quite visible, 

textural analyses are significantly uniform and clasts 

coarser than 4<f> are exceedingly rare. 

F.lll, Silty Clay and (or) Clayey Silt, Sensitive 

The distinction of this facies from F .IT and F .IV 

is based primarily on physical characteristics other than 

the traditional visual sedimentologic criteria. Generally, 

it is visually identical to the thicker bed examples of F.II 

but is distinguished by being much softer and more easily 

molded; on remolding, it exhibits a greatly diminished 

strength compared to its F .II counterpart. It nearly always 

occurs in the upper half of the section between about 

mean sea level (MSL) and + 15 m elevation. In almost 

all instances, it is bounded above and below by either 

F.II or F.IVA. In most places, either F.VI or F.VII is 

in very close stratigraphic proximity, a factor which may 

play a key role in interpreting the geologic history and 

behavior of this facies. F.III is often thinner than the other 

facies, usually ranging from less than 1 m to about 12 

m, but commonly less than 6 m. For the purposes of 

stratigraphic description and correlation, the internal variability of the facies in terms of texture and sensitivity is 

generalized, but it should be noted that discrete sand 

lenses as well as layers of stiffer silts and clays are present 

within F. III. In cores, several instances are recorded of 

fractures, near-vertical sand seams, silty partings at oblique angles, and what seem to be sheared bedding planes. 

These structures may have been produced in sampling, 

but we strongly believe that some may be products of 

the in-place strain history of the facies. 

F.IV, Silty Clay and Clayey Silt with Silt and Sand Layers 

We have chosen to subdivide this facies into F.IVA 

and F .IVB, based on textural distinctions visible in core 

samples and natural exposures. However, this distinction 

has not been mapped or distinguished under physical 

properties because the two variations of the F .IV have 

strong engineering similarities. This facies is the most 

abundant in the formation, and the discrimination of the 

subtle variations of F .IV A and F .IVB may become more 

critical in future studies. 

F.IVA, Silty Clay and Clayey Silt with Silt and Fine Sand 

Lenses 

This facies is the most common in the formation. 

It occurs throughout the project area in virtually every 

logged borehole, and likewise at nearly any vertical location in the section. Whereas most of the other facies have 

low probability of repeating in a given section, F.IVA 

can virtually be expected to reappear at different elevations within a given profile. Thicknesses of individual occurrences of F.IVA generally exceed 3 m and in some 

places approach 30 m. Thin F.IVA horizons have been 

recorded above and (or) below F.III, and the authors believe these may sometimes represent prehistoric postdepositional strain events associated with the shearing, 

soil structure collapse, and subsequent consolidation of 

F.III, the sensitive facies. Bedding is prominent in F.IVA 

with thin seams and pockets of silt and sand interbedded 

with the finer clayey silt layers. Silt and fine sand grains 

are scattered throughout most of the facies, and thin clay 

seams occur here and there amidst the coarser layers. 

F .IV A bedding is more prominent in the upper part of 

the section where layers are a few centimeters thick. In 

some instances, the textural variation and bedding thickness are regular and appear cyclic, suggesting varves, but 

the connotations of varved sediments (that is, annual, 

lacustrine) are not suggested by the authors. Particular 

note should be taken of the fact that throughout most of 

the recorded profiles, F.IVA seems often to alternate with 

other facies states; and whereas the other states may only 

appear once or, at most, twice, in a given profile, F.IVA 

reappears to account for the major part of the profile. 

This would, it seems, suggest that this facies state may 

Description of Engineering Geologic Units 9 
represent the overall regional norm for a depositional environment during the span of time represented by the 

Bootlegger Cove Formation, and that the other facies may 

represent subtle divergences from that norm. 

F.IVB, Silty Clay with Traces of Coarse Sand 

Another facies of considerable abundance is F. IVB. 

Here, the bedding is more massive than that of F .IV A 

and the clay-size fraction is dominant. The state is further 

distinguished by the often random scattering of coarse 

sand grains throughout the unit. Less commonly, the 

coarse sand occurs as very thinly laminated seams 

oriented nearly horizontally through the silty clay. In contrast to the facies heretofore described, F .IVB is usually 

found in the lower half of the Bootlegger Cove Formation, 

and in fact, generally either this facies or F. V (and rarely 

F.II) is the lowermost facies of the formation. This is 

interesting in that both are crudely bedded silty clays with 

randomly scattered coarser clasts. F .IVB can occur at or 

near the top of the Bootlegger Cove section, but such 

instances are rare. Although thicknesses may exceed 30 

m, seldom are they greater than 15 m or less than 3m. 

F.V, Silty Clay and (or) Clayey Silt with Random Stones 

Perhaps the single most distinctive facies of the formation is F. V. It has been noted to occur in the lower 

part of the Bootlegger Cove Formation throughout the region (Schmoll and others, 1972, p. 1107); at Government 

Hill, it primarily exists at elevations near or below MSL, 

so that no exposures of the facies are recorded. This facies 

does appear much higher in the section farther north at 

Cairn Point (Miller and Dobrovolny, 1959, p. 39), but 

no continuity can be drawn between these exposures and 

the project area due to poor intervening exposures and 

lack of subsurface data to the north. The facies is characterized by having (1) very weakly developed stratification, 

(2) poor sorting ranging from clay to boulders, with peak 

size abundance in the silt-clay range, (3) occasional disturbed laminations, and (4) irregular contacts with units 

above and below. F. V may well be classified as a diamicton. 

F. VI, Dense Silty Fine Sand with Silt and Clay Layers 

Although thin, discontinuous lenses of silty sand 

occur in F.I, F.m, F.IVA, and F.IVB, the presence of 

independently mappable sand layers is limited. However, 

at an elevation of between + 12 and +18m above MSL, 

an unusually uniform and predictable F. VI layer can be 

traced in the subsurface of the eastern half of the mapped 

area. This layer appears to be a clastic wedge which has 

a maximum thickness of about 20 m in the northeast, 

splits into layers separated by other facies in the east-ceo10 Engineering Geology, Government Hill Area, Alaska 

tral area, and ultimately thins to about 3 m in the southeast. The unit is confined above and below by F .III and 

F.IV. F.VI is normally described as dense, indicating a 

well-packed structure with significant amounts of silt. 

Locally, however, borehole logs record this facies as 

"running," or "quicksand," suggesting loose packing and 

saturated conditions. Although thin lenses a few millimeters thick of silt and clay may be present, the facies 

is dominantly coarse silt to fine sand (2-5 <f>). Other representatives of this facies occur at lower positions in the 

stratigraphic sequence but lack the remarkable lateral continuity exhibited by the aforementioned layer. In this 

sense, the F. VI unit appears to be a continuous stratigraphic horizon throughout the study area and to have 

been deposited at the same time in the same depositional 

environment over a broad area. 

F.VII, Fine to Medium Sand with Traces of Silt and Gravel 

In the uppermost part of the Bootlegger Cove stratigraphic sequence, a facies notably coarser than those previously described commonly appears. The facies consists 

predominantly of olive-gray, fine to medium sand ( l-4<f>) 

with lenses of coarser sand and scattered pebbles. Generally, it is bounded above by post-Bootlegger Cove sediments (glaciofluvial sandy gravels). Although the sand 

normally is massive, faint bedding can sometimes be 

noted by subtle variations in the textural and color 

homogeneity of the sand. Silt is a typical minor constituent throughout, but only rarely assumes prominence 

as distinct silt lenses. The gravel is randomly dispersed 

and rarely approaches cobble size, being primarily 

rounded to subrounded pebbles less than -34> diameter. 

Contacts with other facies are generally abrupt but, for 

the most part, conformable. An alternation between thin 

layers of F.II, F.IV, and F.VII will in many places occur 

before the F. VII facies assumes predominance in the uppermost part of the section. If this facies proves to have 

regional extent in the same stratigraphic position beyond 

Government Hill, it may well merit member status within 

the formation. 

Textural Characteristics of the Facies 

The original field descriptions used by a variety of 

geologists and engineers logging the geotechnical 

boreholes were reduced by the authors; of an original 32 

different descriptive units, 7 facies are presently being defined. The premise of using these facies is that each represents a slightly different sedimentologic environment during the history of deposition of the Bootlegger Cove Formation. The products of these varied depositional regimes 

should then be distinct from each other, and one basic 

criterion for this distinction should be texture. To test this 

reasoning, the textural analyses of 66 samples were as-
Table 1. Comparison of the mean textural variations of the seven facies of the Bootlegger Cove Formation, Government Hill area, Anchorage, Alaska 

[4> equals negative log to the base 2 of diameter in millimeters] 

Facies F.l F.ll 

Mean diameter (mm) ...... 0.0014 0.0040 

Mean diameter ( <1>) . . . . . . . 9.5 7.9 

Coarsest 

1 percentile (mm) •••• 0. .035 .07 

Coarsest 

1 percentile (<I>) . . . . . . . . 4.84 3.84 

Percent gravel 0 0 •••• 0. 0 0 0 0 

Percent sand • 0. 0 ••••••• 1.0 2.6 

Percent silt and clay . . . . . . 99.0 97.4 

sembled and for each analysis four characteristics computed: (1) size frequency distribution, (2) mean grain size 

of sample, (3) size of coarsest 1 percentile of sample, 

and (4) size class distribution in percent of total by weight 

(table 1). Comparison of the sample description (upon 

which facies states were first defined) and the textural 

name based on the mechanical and statistical analyses 

demonstrated that the two (visual and quantitative) coincided in all but three cases. This significantly increased 

the credibility of the logging of the geotechnical holes 

and of the field geologic descriptions. 

To further distinguish the textural variation of the 

facies, the C-M diagram of Passega (1957) is used, in 

which the mean grain size of the sample (M) is plotted 

against the grain size of the coarsest 1 percentile (C). 

This graphic representation has proven effective in defining transport regimes and depositional environments in 

mobile fluid environments such as flood plains (Passega, 

1957), beach and marine systems (Passega, 1964), and 

fluidized flow of volcanic tephra (Sheridan and Updike, 

1975). Each analysis was plotted by a reference number 

to avoid bias. Each data point was then identified to facies 

as defined by us. With only one exception, the data points 

were plotted into distinct facies domains (fig. 6). Mechanical analyses of post-Bootlegger Cove glaciofluvial deposits were also plotted describing an additional independent domain. 

This figure shows two things: first, that the facies, 

as defined verbally, appear to exhibit real textural differences; second, that the depositional fluid regimes can be 

quantified. Domain pbg represents a group of sediment 

samples from post-Bootlegger Cove deposits that have 

both median and maximum grain sizes in the gravel class, 

typical of the well-sorted glaciofluvial deposits that one 

would observe in outwash fans and plains. In contrast, 

F. VII domain contains maximum clast sizes in the gravel 

range, but is dominantly sand, such as would occur along 

F. Ill F.IV F.V F.VI F.VII 

0.0044 0.015 0.006 0.15 0.37 

7.83 6.06 7.38 2.7 1.43 

.10 .15 12.90 .63 5.0 

3.32 2.74 -3.69 .67 -3.91 

0 0 7.6 0 12.1 

3.6 4.3 9.4 80.3 67.1 

96.4 95.6 83.0 19.5 19.5 

high-energy beach and tidal flats where sediment too 

coarse for continuous suspension transportation was being 

moved by tractive forces. Two of the data points in this 

domain are, in fact, modem tidal flat sand samples. The 

domain of the diamicton phase (F. V) is consistent with 

field descriptions, that is, scattered gravel set in a siltyclay matrix. As Passega (1957) pointed out, such divergency from the C = M line often indicates two independent depositional functions in the same aqueous system. 

Under such conditions, traditional sedimentologic criteria 

such as the coefficient of sorting cannot be rigorously applied to express the character of deposition. The authors 

believe this domain indicates a quiet glaciomarine environment where the dominant mode of deposition is from 

a clayey-silt suspended load, with ice-rafted drop-stones 

sporadically introduced. F. VI domain closely parallels the 

C=M line and consists of well-sorted fine sand. This sediment represents a boundary condition between tractive and 

suspension transport, as occurs in the intertidal to very 

shallow offshore regime. This boundary environment was 

probably strongly influenced by tidal conditions and renewed introduction of silty sand from subaerial sources. 

F. VI commonly contains thin lenses of silt and clay but 

very rarely clasts larger than medium sand, further supporting the suspension-traction threshold concept. The remaining domains of F.l, F.II, and F.IV are distinct from 

each other but also form a divergent pattern, suggesting 

the depositional variability that occurs in a large, quiet, 

perhaps tidally influenced, glaciomarine-glaciolacustrine 

environment. The extreme case of F .I would seem to require exceedingly quiet conditions, such as one might find 

in tidal flat depressions, isolated sloughs, and back-eddy 

basins that are periodically recharged with water and suspended sediment. F.III does not describe a distinct zone 

on the graph but instead is superimposed on F. II and 

F .IV. The reasons for this exception are elaborated later 

in this report. 

Description: of Engineering Geologic Units 11 
Geotechnical Characteristics of the Facies 

The data bank of geotechnical boreholes in the project area was carefully screened by us, to acquire a file 

of engineering soils test data. The data were cataloged 

according to the defined geologic facies. Some data were 

rejected because of lack of stratigraphic control, questionable sampling techniques, or poorly defined laboratory 

procedures. Once the data had been processed, values for 

each parameter of each facies were ranked in ascending 

numerical order and the standard statistical values were 

computed: (1) the arithmetic mean, X, (2) the median, 

Md, (3) the maximum value of the array, Xmax• (4) the 

minimum value of the array, Xmin• and (5) the standard 

deviation ax· Each of the standard geotechnical soils parameters for each facies was thus characterized, and the 

results of our calculations are presented in tables 2-8. 

The geotechnical parameters used for facies discrimination 

are defined in the glossary. For comparative purposes, 

the mean values of each physical property for each facies, 

as provided in tables 2-8, are used in the graphic discriminations discussed later in this section. 

The Atterberg limits, plasticity index, and liquidity 

index proved to be helpful, though inconclusive, tools in 

differentiating the cohesive soils facies (F .I to F. V). The 

mean natural moisture contents and plastic limits do not 

vary appreciably within the formation; however, the true 

clay material which predominates in F.I and the sensitive 

silt/clay material of F.III have the highest mean Wn. The 

plasticity indices of predominant constituents of F.I, F.II, 

F.IV, and F.V differ very little, whereas the mean plasticen 

1-

z -4 ::> 

-e.. 

,v-" 

I •\ 

I. I 

I • I 

\ • I \ I 

\ I 

\ • I ",; 

_...-IV---

//. --.............. 

~ '· ... · ...... w ,_ -11 • • •} 

en __,...,... • •' "'/ ~ 4 --.... fi ( ~~ .~ ....... _.., ...... 0 ==! • \.. ...... u en, --r 

I /1 

6 f e/ ('1\e I/ ~'--

4 

MEDIAN DIAMETER, IN f/> UNITS 

-4 

Figure 6. The C-M diagram of Passega (1964) used to 

plot coarsest 1 percentile (C) versus median diameter 

(M) for samples of the Bootlegger Cove Formation; diameters in phi (<j>) notation. Facies domains discussed 

in text; pbg, post-Bootlegger Cove glaciofluvial deposits. 

ity index for F.III is less than half of the value typical 

for the other facies. It appears that F .III differs from the 

other facies in that the liquid limit has shifted to such 

a low mean value that the moisture content of F .III is 

above the liquid limit. The liquidity index, Li, measures 

Table 2. Summary of engineering-soils properties of F.l, Bootlegger Cove Formation, Government Hill area 

[nd, not determined] 

Maxi- MiniSample mum mum 

array Mean value value Median Standard 

size value of of value deviation 

n X array array Md CTx 

X max X min 

Thickness (meters) ................ 5 7.9 14.3 2.6 6.6 e) 

Moisture content (percent) ........... 13 29.8 37 23 30 3.9 

Plastic limit (percent). . . . . . . . . . . . . . 5 23.0 23 23 23 0 

Liquid limit (percent). . . . . . . . . . . . . . 9 39.0 45 35 40 3.3 

Plasticity index . . . . . . . . . . . . . . . . . . 7 16 20 13 16 2.3 

Standard penetration. . . . . . . . . . . . . . . 23 10.8 17 5 nd nd 

Shear strength (kN/m2

) . . . • . • . . . • • . • 8 46.9 65.1 28.7 47.9 12.4 

Compressive strength (kN/m2

) ••.••••. 5 161.8 241.3 134.1 143.6 40.2 

Sensitivity ratio ..... . . . . . . . . . . . . 3 8.7 10 8 8 .94 

1

Not applicable. 

12 Engineering Geology, Government Hill Area, Alaska 
Table 3. Summary of engineering-soils properties of F.ll, Bootlegger Cove Formation, Government Hill area 

[nd, not determined] 

Maxi- MiniSample mum mum 

array Mean value value Median Standard 

size value of of value deviation 

n X array array Md <Tx 

X max X min 

Thickness (meters) ................ 12 7.8 >16.7 3.3 6.6 e> 

Moisture content (percent) ........... 77 28.4 40.0 16 27 4.4 

Plastic limit (percent) .............. 44 22.1 25 17 23 2.3 

Liquid limit (percent) .............. 44 37.2 45 30 37 4.2 

Plasticity index . . . . . . . . . . . . . . . . . . 44 15.1 24 5 15 3.9 

Standard penetration. . . . . . . . . . . . . . . 72 10.2 35 3 nd nd 

Shear strength (kN/m2

) ••..•..••..•• 52 75.7 140.0 44.0 71.8 22.0 

Compressive strength (kN/m2

) .•.••..• 65 142.7 308.3 50.8 120.7 67.0 

Sensitivity ratio ••••••••• 0 0 ••• 0. 0 14 2.8 5 2 2.5 1.0 

1

Not applicable. 

Table 4. Summary of engineering-soils properties of F.lll, Bootlegger Cove Formation, Government Hill area 

[nd, not determined] 

Sample 

array 

size 

n 

Thickness (meters) ................ 16 

Moisture content (percent) ........... 57 

Plastic limit (percent) .............. 37 

Liquid limit (percent) .............. 38 

Plasticity index . . . . . . . . . . . . . . . . . . 36 

Standard penetration ............... 32 

Shear strength (kN/m2). . . . . . . . . . . . . 54 

Compressive strength (kN/m2) . . . . . . . . 24 

Sensitivity ratio ••••• 0. 0 •• 0 0 •••• 0 63 

1

Not applicable. 

the relation between natural moisture content and the liquid limit, as calculated by the equation 

Li=(Wn-Wp)I(Wl-Wp) 

where Wm WP, and W1 are averages of all test data for 

a given unit. F.III alone has a mean value of Li greater 

than 1.0. 

Shear strength values (S) were obtained from unconMaxi- Minimum mum 

Mean value value Median Standard 

value of of value deviation 

X array array Md <Tx 

X max X min 

6.6 20.7 0.7 5.9 e> 

29.8 45 22 30 5.0 

22.0 25 17 22 1.9 

28.3 37 22 27 3.8 

7.0 13 3 7 2.5 

42 80 1 nd nd 

49.8 95.8 23.9 47.9 12.4 

88.1 134.1 68.0 86.2 15.3 

19.7 40 9 18 6.6 

solidated, undrained triaxial compression and from laboratory vane-shear tests. Considerable caution was exercised 

in the selection of shear strength values for this study 

to minimize variability in the data due to possible problems in laboratory procedures, sample handling, and the 

accuracy of the laboratory apparatus used. The authors 

have attempted to include only values for test samples 

retaining natural moisture content. 

Description of Engineering Geologic Units 13 
Table 5. Summary of engineering-soils properties of F. IV, Bootlegger Cove Formation, Government Hill area 

[nd, not determined] 

Maxi- MiniSample mum mum 

array Mean value value Median Standard 

size value of of value deviation 

n X array array Md fTx 

X max X min 

Thickness (meters) ................ 53 8.2 >56.1 1.6 7.2 e> 

Moisture content (percent) ........... 61 28.0 36.9 18 28 3.8 

Plastic limit (percent) .............. 26 20.5 26 13 22 4.0 

Liquid limit (percent). . . . . . . . . . . . . . 37 34.8 45 20 35 6.7 

Plasticity index . . . . . . . . . . . . . . . . . . 36 16 27 6 24 6.2 

Standard penetration. . . . . . . . . . . . . . . 71 22 100 3 nd nd 

Shear strength (kN/m2

). . . . . . . . . . . . . 57 66.1 114.9 19.2 68.9 21.1 

Compressive strength (kN/m2

) ........ 26 111.1 210.7 40.2 114.0 33.5 

Sensitivity ratio ................. 37 6.1 21 5 4.3 

1

Not applicable . .. 

Table 6. Summary of engineering-soils properties of F.V, Bootlegger Cove Formation, Government Hill area 

[Leaders ( --) indicate that in three of the five cases, F. V occurred at bottom of hole; nd, not determined] 

Sample 

array 

size 

n 

Thickness . . . . . . . . . . . . . . . . . . . . . 5 

Moisture content (percent). . . . . . . . . . . 16 

Plastic limit (percent). . . . . . . . . . . . . . 10 

Liquid limit (percent). . . . . . . . . . . . . . 10 

Plasticity index . . . . . . . . . . . . . . . . . . 10 

Standard penetration. . . . . . . . . . . . . . . 12 

Shear strength (kN/m2). . . . . . . . . . . . . 6 

Compressive strength (kN/m2) . • . • • • . • 11 

Sensitivity ratio . . . . . . . . . . . . . . . . . 3 

1

Not applicable. 

In the ideal case, the unconfined compressive 

strength, qu, should equal twice the shear strength of the 

same sample under undrained conditions. However, due 

to differences in testing conditions as well as associated 

errors and approximations consequential to the testing procedures, the qu values do not generally agree with the 

ideal case (fig. 7), and are therefore reported separately 

in tables 2-8. For this study, only values of qu obtained 

14 Engineering Geology, Government Hill Area, Alaska 

Maxi- Minimum mum 

Mean value value Median Standard 

value of of value deviation 

X array array Md fTx 

X max X min 

>9.5 2.6 e) 

28.4 37 8 28 6.4 

22.2 25 28 23 2.4 

36.9 41 29 37 3.8 

14.7 18 9 15 2.7 

18 50 4 nd nd 

104.4 124.5 80.4 95.8 17.2 

182.9 268.1 80.4 183.9 54.6 

4.1 4.4 3.7 4.2 .5 

by laboratory testing in an undrained and unconfined compression apparatus are reported. We have refrained from 

using pocket penetrometer values in this report because 

those data lack reliability control for specific values, although such values may have considerable merit as indicators of general engineering stratigraphic trends. 

The mean values of shear strength for the predominant materials of F. I and F. III are nearly equal at about 
Table 7. Summary of engineering-soils properties of F.VI, Bootlegger Cove Formation, Government Hill area 

[nd, not determined; nc, noncohesive. F.VI is dominantly sand; for plastic limit, liquid limit, plasticity index, Atterberg 

limits obtained on interbedded silts and clays] 

Maxi- MiniSample mum mum 

array Mean value value Median Standard 

size value of of value deviation 

n X array array Md CJ"x 

X max X min 

Thickness (meters) ................ 7 2.3 4.9 1.3 2.0 e) 

Moisture content (percent) ........... 22 25.0 37 18 25 3.0 

Plastic limit (percent) .............. 5 22.4 25 18 23 2.4 

Liquid limit (percent). . . . . . . . . . . . . . 5 32.8 45 27 30 6.6 

Plasticity index . . . . . . . . . . . . . . . . . . 5 10.4 21 5 9 5.7 

Standard penetration. . . . . . . . . . . . . . . 5 27.6 50 8 nd nd 

Shear strength (kN/m2

) ............. 6 58.4 62.2 47.9 59.4 4.8 

Compressive strength (kN/m2) •• 0 0 0 ••• 6 126.4 164.7 101.5 108.2 26.8 

Sensitivity ratio •• 0. 0 ••• 0. 0 0. 0 0 •• nc nc nc nc nc nc 

1

Not applicable. 

Table 8. Summary of engineering-soils properties of F.VII, Bootlegger Cove Formation, Government Hill area 

[nd, not determined; np, nonplastic; nc, noncohesive; nv, nonviscous] 

Sample 

array 

size 

n 

Thickness (meters) . . . . . . . . . . . . . . . . 11 

Moisture content (percent). . . . . . . . . . . 8 

Plastic limit (percent). . . . . . . . . . . . . . np 

Liquid limit (percent). . . . . . . . . . . . . . nv 

Plasticity index . . . . . . . . . . . . . . . . . . np 

Standard penetration. . . . . . . . . . . . . . . 17 

Shear strength (kN/m2). . . . . . . . . . . . . nd 

Compressive strength (kN/m2

) . . . . . . . . nd 

Sensitivity ratio . . . . . . . . . . . . . . . . . nc 

1

Not applicable. 

50 kN/m2 , but differ markedly in unconfined compressive 

strength: F.I has a mean qu= 162 kN/m2 , and F.III has 

a mean qu=88 kN/m2 (tables 2 and 4, fig. 8). This may 

reflect a difference in depositional history and postdepositional consolidation dependent on the original soil structure and subsequent loading. 

The predominant materials of each of the five cohe-

. sive facies are essentially the same in mineralogy, clast 

Maxi- Minimum mum 

Mean value value Median Standard 

value of of value deviation 

X array array Md CJ"x 

X max X min 

5.6 19.0 0.7 4.3 e) 

20.4 29 14 20 4.3 

np np np np np 

nv nv nv nv nv 

np np np np np 

34 60 12 nd nd 

nd nd nd nd nd 

nd nd nd nd nd 

nc nc nc nc nc 

shape, and size distribution, with the exception of F.l, 

which is primarily composed of clay-size particles. We 

believe that the variations in shear- and compressivestrength characteristics of each facies are significantly influenced by primary microstructures in the soil fabric, and 

by amount and distribution of secondary constituents, such 

as sand. Table 6 shows that the unconfined compressive 

strength of F. V is significantly higher than that of the 

Description· of Engineering Geologic Units 15 
N 125tE 

z 

.::.! 

g1oor-

-

-

-

I I I 1 

2 5 50 7 5 1 00 1 2 5 1 50 1 7 5 

UNCONFINED COMPRESSIVE STRENGTH (qu)/2, IN kN/m2 

Figure 7. Correlation of shear strength and unconfined 

compressive strength for samples of cohesive facies (F.I 

to F.V) of the Bootlegger Cove Formation, Government 

Hill area. Long-dashed line, ideal case; short-dashed line 

based upon a least-square linear-regression analysis of 

plotted data points. 

other facies. This strength is believed to be, in large part, 

due to (1) a compact texture which is normally consolidated by the effective overburden pressure, (2) a wellgraded size distribution, and (3) coarse clasts acting as 

fabric binders. The uniform fine-grained texture and better 

packing probably account for the slightly improved 

strength of F. II as compared to F. IV. 

Although F .III alone shows pronounced sensitivity, 

all the cohesive facies show some degree of sensitivity. 

Generally, the data indicate that those soils with higher 

initial shear strength also have lower sensitivity ratios. 

In figure 9, the mean undisturbed shear strength, S, is 

plotted against the mean sensitivity ratio, Sr, for each 

facies. A general trend of diminishing strength retention 

emerges, with F.V having the highest value, whereas the 

clay facies (F .I) and sensitive facies (F .III) plot divergently at the base of the graph, again suggesting a significant difference in soil fabric between these two facies. 

Finally, a comparison is made of mean liquidity indices and mean sensitivity ratios for the predominant material of the facies (fig. 10). The mean curve for data 

points given by Shannon and Wilson, Inc. (1964) for the 

Fourth A venue and Turnagain Heights landslides of 1964 

is plotted for comparison. Sensitivities generally increase 

dramatically with liquidity indices greater than one. F.III 

plots high on the Shannon and Wilson line, and the other 

facies cluster about the base of the curve. 

In summary, although the five cohesive soil facies 

are primarily distinguished by textural differences, they 

can also be distinguished by subtle differences in strength. 

As expected, F .III is quite distinct from the other four 

facies. The strength of F. V is slightly higher than that 

16 Engineering Geology, Government Hill Area, Alaska 

125.----,~------~-----,~----~~-----~~--~ 

NE 

z 

.::.! 

~ 

I 

1-

{!) 

z 

U.J 

100f-

~ 75l-- (f) 

a: 

<t 

U.J 

I 

(f) 

z 

<t 

U.J 

~ 

501-

-

• F.ll -

• F.IV 

• F.lll -

25 I I I I 

50 100 150 200 

MEAN UNCONFINED COMPRESSIVE STRENGTH, IN kN/m2 

Figure 8. Correlation of mean shear strength and 

mean compressive strength, for cohesive facies (F.I 

to F.V) of the Bootlegger Cove Formation, Government Hill area. 

of any of the other facies, both initially and after remolding, probably due to the presence of sand and gravel, 

as well as to a higher degree of consolidation. F.II and 

F .IV are very similar to each other based upon laboratory 

testing, and the distinction between these facies has 

primarily geologic rather than engineering significance. If 

the standard geotechnical measurements reported here can 

be assumed to give approximations of relative static and 

dynamic behavior, then F .II and F .IV should both behave 

in a similar manner. Even though the initial shear strength 

of the clay of F .I is nearly identical to that of the clayey 

silt of F.III, mean sensitivity of F.I is less than half that 

of F.III. This may signify a fundamental difference in 

soil structure between the two facies. It is believed that, 

should in-place remolding of F .III occur, ultimately this 

facies would have close affinity to F. II, and the two may 

basically differ only in their strain histories. 

Post-Bootlegger Cove Formation Glaciofluvial 

Deposits 

In the eastern half of the project area where the 

Bootlegger Cove Formation has not been subjected to 

Holocene erosion, the formation is overlain by well-stratified sands and gravels. Where exposed, the contact be-
1 25,_----,----- 1--,-l--,--1------rl--,---1-

'E e F.V 

I z 100 - I - ~ 

~ I 

I- I f- I <.9 

z I UJ 

a: I f-

(/) I a: I <X: 

UJ 75 1-- ~F II - I 

(/) 

0 \ UJ 

ill \ a: \ eF.IV ::::> 

f- \ (/) 

0 \ z \ ::::> \ z '\. <X: 50 -

"' •F.III - UJ 

~ " F.t•-..........._ ____ -""' ./ 

~~--~l~---~~---1~---.1~6----2~~~~24 

MEAN SENSITIVITY RATIO, Sr 

Figure 9. Correlation of mean undisturbed 

shear strength and mean sensitivity ratio for 

cohesive facies of the Bootlegger Cove Formation, Government Hill area. Note trend of line 

from overconsolidated dense facies (F .V) to 

normally consolidated low-strength facies 

(F.III). 

tween these two units is normally defined by an abrupt 

transition from the Bootlegger Cove Formation to finely 

stratified beds of sand and gravel which exhibit a variety 

of sedimentary features characteristic of fluvial deposition, 

including crossbedding, cut -and-fill structures, graded 

bedding, and well-sorted planar beds of gravel (fig. 11). 

The sequence varies in thickness from less than 1 0 m 

in the southwest part of Government Hill to 26 m at the 

northern and eastern limits of our study area (Elmendorf 

Air Force Base). A series of four terraces constitute the 

geomorphic forms composed of this sand and gravel. 

These terraces lie directly outside the limit of the Elmendorf Moraine. We conclude that these deposits are indeed 

glaciofluvial deposits laid down directly in front of the 

Elmendorf Glacier, although the actual terrace morphology seems more likely erosional than depositional. 

The clasts are subrounded to well rounded and moderately to loosely packed, and both porosity and permeability are high. As a result, ground water accumulates 

at the base of the unit forming a perched aquifer above 

I eli 

0 

f= 

<X: 

a: 

>-

f20~------~~-------,--1------~~------~ 

•F.III 

15- -

> 10tf= iii eF.I 

-

z 

UJ 

(/) 

z 

<X: 

UJ 

~ 5teF.V 

eF.II 

eF.IV 

-

o~------~------ ------~~------~ 0.4 0.6 0.8 1 .0 1.2 

MEAN LIQUIDITY INDEX, Li 

Figure 10. Correlation of mean sensitivity ratio and 

mean liquidity index for the five cohesive facies of the 

Bootlegger Cove Formation. 

the Bootlegger Cove Formation which, in turn, behaves 

as a low-permeability bed. This perched-aquifer relation 

generally assures that intermediate sand, if present, is 

saturated, which is significant in evaluating liquefaction 

potential. Due to the topography on the upper surface of 

the Bootlegger Cove Formation (Updike, 1986), the drainage within this perched aquifer is directed toward topographic lows on the Bootlegger Cove superface. This 

drainage pattern strongly affects the slope stability of the 

upper part of the Bootlegger Cove Formation by assuring 

saturation throughout the sand facies (F. VI and F. VII), 

a prime factor in liquefaction susceptibility. The drainage 

pattern also causes the natural moisture content to exceed 

the liquid limit in F. III. 

Tidal Flat and Tidal Marsh Deposits 

Seaward from the base of bluffs and associated 

landslides are sediments which have been deposited subsequent to sea level planation of the Bootlegger Cove Formation. The distinction between tidal flat and tidal marsh 

deposits is made on the basis of respective dominance 

of inorganic clastic sediments versus organic deposits. Although the two units are intercalated in the subsurface, 

the tidal marsh deposits generally overlie the tidal flat 

units landward from the high tide strandline. The borehole 

data indicate that the combined units form a seawardthickening wedge, laid down on the beveled superface of 

the Bootlegger Cove Formation. The surface of these deposits ranges in elevations from a maximum of about 9. 7 

Description of Engineering Geologic Units 17 
m above sea level to below sea level, sloping uniformly 

from the landward limit to high tide strandline at less 

than 0. 5° and then increasing in surface slope to greater 

than 2. 0° to beyond mean low sea level. 

The tidal flat unit comprises sediments with distinct 

bedding which vary in stratigraphic thickness and elevation from one borehole to the next but when examined 

collectively exhibit a sequential record as shown in figure 

12. If one examines modem tidal flat clastic deposition 

in upper Cook Inlet, some generalized analogies can be 

drawn for environments of deposition. The coarse clastic 

deposits (gravels, sandy gravels) are emergent units, that 

is, deposited at or above high strandline. Silty clay, 

clayey silt, silty fine sand, and sandy silt are more typical 

of the intertidal zone and seaward from this zone, with 

the minor amounts of coarser sand and gravel being introduced by slope wash from behind strandline and by 

tidal ice rafting. The rather limited deposits of clays containing minor amounts of silt and fine sand represent restricted depositional depressions, either within the high 

intertidal zone or as backwater depressions of the coastal 

flats. Although it is tempting to purport that the composite 

sequence represents finite tectonic or eustatic sea level 

changes of Cook Inlet, the map area is far too limited 

to make such deductions. At this point we conclude that, 

subsequent to planation of the Bootlegger Cove Formation, tidal inlet clastic deposits several tens of meters thick 

have accumulated, that this sequence has been subjected 

to incremental burial by similar deposits, and that, because the older sediments of the section are several tens 

of meters below modem MSL, either the depositional 

basin has been subsiding or sea level has been rising, 

or both. 

Several logs indicate qualitatively the degree of consolidation by terms such as loose, compact, dense, and 

very dense. Again, by assimilating the data from various 

holes, one can conclude that the sequence appears to be 

normally consolidated from top to bottom (fig. 12). This 

undoubtedly reflects the progressive loading of the older 

sediments but may also denote tidal energies imparted to 

Figure 11. Exposure in landslide block, in gully west of Elmendorf Air Force Base, showing glaciofluvial deposits 

of the Naptowne glaciation of Karlstrom (1964) overlying the Bootlegger Cove Formation (lower left corner). Note 

apparent dip of units to lower right (east) due to slide dislocation. Photograph by R. G. Updike, 1980; bar scale 

(lower left) is approximately 1 m long. 

18 Engineering Geology, Government Hill Area, Alaska 
Q) 

<I) 

0 

0 

....J 

Q) 

<I) 

c 

Q) 

""0 

2 

.... 

u 

ro 

c. 

E 

0 

u 

Q) 

<I) 

c 

Q) 

""0 

> 

w 

> 

2 

Q) 

<I) 

c 

Q) 

0 

~~~~__: ~.~ -__:_.~~- ·~~·,..:_ ~·.~~ ~~~ ~~ ~~-~---~

~~~~~~~ ~~~~~~~~~~~ 

'·:· :·: :. ::,}:~d::d t; ~ ~ ',;~1 

Formation 

(") 

Q) 

u 

> 

u 

N 

Q) 

u 

> 

u 

Q) 

u 

> 

u 

Figure 12. Idealized stratigraphic sequence of 

tidal flat deposits based upon a composite of 

borehole logs at several sites, Government Hill 

area. Relative thicknesses of individual layers are 

accurate, although variation in thickness will 

occur at different locations, from a few to several 

tens of meters. Three cycles of deposition of clastic tidal marine sediments are recorded. Each 

cycle is initiated with deposition of coarse sediment (gravel) and ends with silt and fine sand. 

Each cycle is less compacted than the cycle below 

it, which we believe is due to overburden pressure and increasing age (late Holocene) from 

cycle 3 to cycle 1. 

the older sediments different from modem energy regimes, causing a denser packing and, in the case of finer 

sediments, better sorting factors. The increase in consolidation with increasing depth reduces concerns for tidal 

flat liquefaction but certainly does not remove the potential. For example, at approximately modem low-tide elevation there exists, discontinuously, a gravelly sand and 

sandy gravel zone which is moderately loose, saturated, 

and overlain by as much as 10-20 m of finer tidal flat 

deposits, tidal marsh deposits, and fill, upon which buildings have been built. During the 1964 earthquake, numerous ground fissures, some slumping, and lateral spreading 

occurred in the tidal flat deposits. Although no serious 

damage resulted, the potential for more serious failures 

was demonstrated. The cracking may well have been the 

function of vertical acceleration causing consolidation, lateral shear along stratigraphic horizons, or, in the extreme 

case, limited liquefaction. No evidence (for example, sand 

boils, sand-filled fissures) has been recorded to document 

that liquefaction was responsible . 

The tidal marsh deposits, as much as 4 m thick, 

are composed primarily of organic silts interbedded with 

lenses and beds of peat and fine sand. These deposits 

have formed landward from high tide line. Early photography of the coastal flats and 1940-1960 vertical aerial 

photography indicate that many of the areas now covered 

by fill were originally mantled by tidal marsh deposits. 

In some instances, the organic silt and peat zone was 

stripped away before the fill aggregate was spread; however, more often the fill was laid down directly on the 

organic silt layer. This can result in problems to structures 

placed over peat zones, such as pronounced amplification 

of some frequencies of seismic accelerations, development 

of permafrost, and ground settlement. 

Landslide Deposits 

Historic and prehistoric slope failures have been discussed at some length by Varnes (1969), who was particularly interested in the mechanics of slides associated with 

the Bootlegger Cove Formation. He recognized the regional extent of such failures, but actual mapping was 

outside the focus of his study (D. J. V ames, oral commun., 1979). Our present project has found that numerous, presently inactive slides encompass most of the bluff 

line of Government Hill. Three types of failures have been 

distinguished: translatory slides, rotational slumps, and 

flows. In some instances, failure was initiated as one type 

and progressed into another type; for example, the Government Hill Elementary School slide ( 1964) consisted of 

rotational blocks, which progressed into successive slurry 

flows at the toe of the failing mass. Elsewhere, the 

mechanism is simple; for example, a series of active, 

Description of Engineering Geologic Units 19 
slow-moving rotational blocks occur along the bluffs west 

of Elmendorf Air Force Base. In a prominent gully in 

that area, excellent exposures have been stream-cut normal to the long axis of large slide blocks, again showing 

simple rotational failure in coherent blocks (fig. 11). An 

apparently unique failure occurred in the area of what is 

now the Chevron Petroleum storage-tank farm. Varnes 

(1969) recognized and described this unusual (and now 

totally obscured) landform, which exhibited a low-gradient, lobate form extending from the base of the thenexisting bluff line (herein referred to as the Chevron Tank 

Farm slide, fig. 13). We intuitively favor a flow origin, 

but because of the antiquity of the mass movement and 

Tidal flats 

200 

subsequent cultural modification, we cannot know for 

sure. A prehistoric slide near the 1964 Government Hill 

School slide appears to have been a translatory slide similar to the 1964 "L" Street slide in downtown Anchorage, 

but having an additional 5-m vertical component of movement. 

We have mapped individual landslide deposits (fig. 

13) because, first, the deposits are distinctly different in 

composition from the parent soils units due to oftenchaotic mixing of sediments and disruption of stratigraphy. Second, distinguishing areal extent, type, and relative age of slides aids in potential hazard assessment. 

Finally, we can better evaluate seismically induced failure 

Elmendorf Air Force Base 

SHIP CREEK VALLEY 

400 600 METERS 

EXPLANATION D Older landslides 

- Younger landslides 

1964 landslides 

Figure 13. Distribution of landslides in Government Hill area, as discussed 

in text. A, Elmendorf; B, Government Hill Elementary School; C, Curling 

Club; D, Chevron Tank Farm; E, P.O.L. Tank Farm. Approximate spot elevations shown, in meters. 

20 Engineering Geology, Government Hill Area, Alaska 
potential of engineering geologic facies of the Bootlegger 

Cove Formation if we examine past behavior. The mapping units have been distinguished on the basis of relative 

age of the individual slide masses, qualitatively referred 

to in order of decreasing age as "older," younger," and 

"1964 earthquake." This classification takes into account 

modification of slide morphology (for example, erosion 

of scarps, rounding of ridges, filling of troughs, erosion 

or burial of the toe), vegetation and soil development, 

crosscutting and superimposed relations between slides, 

and historic written and photographic records. 

"Older" slides are characterized by ( 1) highly modified to gently hummocky surfaces, (2) obscure slide toes 

often gradational to tidal flat or tidal marsh units, (3) 

heavy vegetation and continuous soil cover, and, often, 

( 4) being overlain by "younger" slides. In contrast, the 

"younger" slides still exhibit ridge-and-trough topography, 

ponded surface water, pioneer vegetation, and discontinuous soils with breaks exposing sand gravels and (or) the 

Bootlegger Cove Formation; and they generally occur directly downslope from the identifiable slide scarp. In particularly slide-prone areas, multiple "younger" slide units 

can be superimposed on each other. The two 1964 slides 

(herein named Elmendorf and Government Hill Elementary School (figs. 13, 14)) are well documented in the 

literature (Shannon and Wilson, Inc., 1964; Varnes, 

Figure 14. Government Hill Elementary School landslide, immediately after the March 27, 1964, earthquake. View looking west; slide moved from right to left in photograph view. Note graben-like structures in foreground which vertically 

displaced south wing of the school. An older slide (Curling Club slide) visible between school and houses in background. 

Photograph provided with permission of the Anchorage Historical and Fine Arts Museum (No. 877.118.121) from negative 

398 taken by U.S. Corps of Engineers. 

Description of Engineering Geologic Units 21 
1969). In the subsequent 20 years, these slides have begun 

to assume many of the features used as criteria for the 

"younger" slides. In addition to the two primary slides 

resulting from the 1964 earthquake, a considerable amount 

of ground cracking and high-angle slip occurred in the 

preexistent landslides. Although partially obscured by 

vegetation on these older units, cracks and scarps can 

readily be distinguished on the Mohawk aerial photograph 

flown on April 4, 1964. Although cracks were noted at 

numerous locations (see Varnes, 1969, fig. 15), among 

the most noteworthy were east of the Government Hill 

Elementary School slide, west of the Cherry Hill Apartments, north of the buried P. 0. L. tanks, and east of 

Ocean Dock Road behind the Independent Lumber Company warehouse. In each instance, vertical slip occurred 

along scarps parallel to the topographic contours of the 

original slide surface and, in most instances, developed 

along preexisting lineaments within the slide mass. The 

Elmendorf landslide was similar to the Native Hospital 

slide (see Shannon and Wilson, Inc., 1964) in that both 

consisted in part of an old landslide, and in part of a 

newly activated block. At the Elmendorf slide, the newly 

detached block moved in compression against the older 

slide, resulting in compression ridges, again parallel to 

topographic contours. Some small post -1964 slides have 

been identified, presumably resultant from a seismic mass 

wasting. Adjacent ground cracks suggest that slow, progressive, small-scale slump failures are developing along 

bluffs north from West Bluff Road. 

Fluvial Deposits Associated with Modern 

Drainage 

The southern segment of the study area is bounded 

by the flood plain of Ship Creek, a stream that drains 

westward from the Chugach Mountains. After the stream 

issues from the mountains, only its lower 4--5 km has 

a well-defined flood plain. Along its route, the stream 

erodes and retransports silt, sand, and gravel associated 

with late Quaternary sedimentary units, as well as bedrock 

in the headwaters region. The valley geometry indicates 

that the hydrologic regime of the stream was far more 

dynamic in the past than at present. Anchorage and Government Hill have average upland elevations of about 40 

m above MSL. The broad valley of Ship Creek has eroded 

to an elevation of 3-5 m above modern MSL. This morphology may reflect the drainageway of an ancestral Eagle 

River that flowed south of the Elmendorf Moraine. Subsequent to this period of downcutting, the stream has aggraded 5-7 m of well-stratified silt, sand, and gravel over 

the erosion surface developed on the Bootlegger Cove 

Formation. Boreholes indicate that these Holocene sediments thin north and south toward the bluffs at the margins of the valley. In general, silty sand and silt units, 

with lenses of coarser sand and gravel, occur near the 

22 Engineering Geology, Government Hill Area, Alaska 

surface, whereas with increasing depth stratified sand and 

gravel become more abundant. Most of the flood plain 

has been covered with man-related fill and (or) reworked 

fluvial sediments so that the original flood-plain morphology is obscure. Farther upstream, outside the project area, 

the stream shows evidence of active incision into the adjacent bluffs; however, in the mapped area such lateral planation is not exhibited. Adjacent to the present channel 

of Ship Creek, a subtle terrace surface is exposed at elevations of 3-5 m above MSL. This is interpreted as being 

an erosion surface on the Bootlegger Cove Formation 

where the Holocene fluvial sediments have been stripped 

away, exposing the older, more resistant formation. The 

depth of the water table varies from 1 to 7 m, although 

the sediments are consistently near saturation throughout. 

During the 1964 earthquake, ground fissures developed 

along the Ship Creek drainage and in the intergradational 

zone between the tidal flat deposits and the fluvial sediments. The proximity to open bodies of water indicates 

that saturation and a subdued free face were prerequisites 

for most of the fissures that occurred in the fluvial sediments. 

Farther north, at the mouth of the drainage flowing 

through a major gully from the uplands of Elmendorf Air 

Force Base, an alluvial fan has accumulated, presumably 

in Holocene time. The silt, sand, and gravel of this fan 

grade out into the tidal flat and tidal marsh deposits to 

the west. The three units are probably intercalated in the 

transition zone. The fan showed no evidence of disturbance during the 1964 earthquake. 

All the fluvial deposits postdate both the Bootlegger 

Cove Formation and the glaciofluvial sediments but 

should be considered approximately contemporaneous 

with the tidal and landslide deposits. 

Man-related Fill Deposits 

Much of the flood plain of Ship Creek and the tidal 

flats from Ship Creek north to the municipal port are covered with 1-3 m of fill. This fill has been introduced 

into the area for at least the past 50 years. Generally, 

the fill is composed of aggregate excavated from nearby 

borrow pits in glaciofluvial deposits or in flood-plain deposits. Subsequent to the 1964 earthquake, larger quantities of landslide debris were hauled to the flood plain 

and tidal flats as part of the reconstruction and municipal 

development plan. This material (in many cases consisting 

exclusively of slide debris derived from the Bootlegger 

Cove Formation) was typically laid down in 0.5- to 1.0-m 

layers, graded, artificially compacted, and subsequently 

covered with NFS (non-frost susceptible) aggregate, paving, or structural foundation materials. On the Ship Creek 

flood plain, the underlying fluvial sands and gravels have 

in many places been excavated, sometimes sorted or 

mixed with binding additives, and redistributed as site fill. 
Some of these borrow pits still remain on the flood plain. 

The fills that were in place at the time of the 1964 earthquake performed well, although ground fissures, spreading, or subsidence commonly developed along some 

buried topographic and geologic features, or as a result 

of underlying unit failure being expressed through the fill. 

SURFACE AND GROUND-WATER 

HYDROLOGY 

The static and dynamic properties of the soils at 

a site are directly influenced by water conditions at and 

adjacent to that site. Surface water may be operating independently, or as an extension of, the ground-water regime. The potential for surface water-ground water interaction is particularly obvious where aquifers are shallow 

and unconfined. Surface water occurs as open streams 

(Ship Creek and other minor spring-fed streams), ponds 

and marshes, and the tidal inlet (Knik Arm). Unconfined 

aquifers consist of the glaciofluvial deposits, tidal flat and 

tidal marsh deposits, and deposits associated with modern 

drainages. In each of these instances, the subsurface flow 

is expressed locally by surface-water bodies, but ultimately all flow is trending toward Ship Creek and Knik 

Arm. Within the areas mapped as landslide deposits, unconfined aquifers also occur but are often discontinuous, 

so that subsurface drainage often emerges as springs or 

seeps within, or at the toe of, slide masses. The unconfined ground-water table reasonably reflects the topography of the superface of the disrupted Bootlegger Cove 

Formation and, as a result, has a steep hydraulic gradient 

through the slide. Within the slide zone, the unconfined 

aquifer is thin, and saturated conditions will approximate 

the ground surface. 

The connection of confined aquifers with surface 

drainages is less clear, although the ultimate flow direction is presumed to be toward Knik Arm. Two primary 

confined systems are in operation, aquifers beneath the 

Bootlegger Cove Formation, and F. VI and F. VII within 

the Bootlegger Cove Formation. The principal water-reservoir aquifers for Anchorage are confined beneath the 

Bootlegger Cove Formation; potentiometric surfaces commonly lie within 5 to 10 m of those of the unconfined 

aquifers above the formation (Barnwell and others, 1972). 

However, the characterization of vertical ground-water 

movement through the formation, which serves as an 

aquitard, is as yet unclear. Indeed, F.VI and F.VII, as 

well as adjacent cohesive soils of other facies, are saturated; but the gradient-flow directions are uncertain. We 

can argue strongly that the existence of the sensitive F .III 

is in part a function of in-place leaching of marine salts, 

which fill soil pores and act as connector assemblages 

in the open flocculated fabric of the undisturbed sediment 

(see Lambe and Whitman, 1979). It is not necessary for 

the fresh water to circulate through F. III, but the mere 

presence of percolating water in adjacent silt and sand 

lenses (such as F. VI and F. VII) can accomplish the leaching by diffusion (Torrance, 1974; Mitchell, 1976). That 

most of the F. II facies occurs at or above sea level lends 

further support to a leaching process, and may partly account for the mutual proximity of extensiv~ landslide deposits and major surface streams and spring lines. The 

water-table data for 70 boreholes in the Government Hill 

area have been examined. Of these, only eight wells can 

confidently be said to penetrate into the confined aquifer 

beneath the Bootlegger Cove Formation. Although these 

wells indicate significant hydrostatic head, the data are 

too diverse in time, location, and method for recording 

to prove meaningful. Synoptic readings of screened wells 

fully penetrating the confined aquifer are necessary to give 

insight into ground-water movement, and these types of 

data have not been available. 

The saturation of near-surface silts, sands, and 

gravelly sands in the Government Hill area raises concerns 

for their performance during seismic events. Particularly, 

the tidal flat deposits, some of the glaciofluvial deposits, 

and F. VI and F. VII of the Bootlegger Cove Formation 

are saturated, noncohesive, near-surface soils which could 

have potential for seismically induced liquefaction (see 

"Behavior of the engineering geologic units"). 

In addition to liquefaction, compaction of cohesionless soils can be a problem. The loose to moderately dense 

saturated silt and sand bodies in the tidal flat and lower 

Ship Creek flood-plain deposits are vulnerable to compaction during earthquakes. The fissures that occurred during 

the 1964 earthquake in tank farms, near port facilities, 

and in other areas of lower Ship Creek were likely caused 

by compaction in conjunction with minor lateral shifting. 

The potential for differential settlement due to compaction 

could also be a problem with sandy fill deposits. 

The structure contour map of the superface of the 

Bootlegger Cove Formation (Updike, 1986) suggests that 

unconfined aquifer water in the glaciofluvial deposits 

above the formation would tend to drain southwest along 

a line between the military tank farms and the old Government Hill Elementary School. We suggest that the flow 

then divides, and we believe its water emerges along the 

free faces near the tank farms and Government Hill 

Elementary School (fig. 15). Springs do, in fact, emerge 

in the sands and gravels behind the Army P.O.L. tanks; 

in 1979 the authors observed new drains being installed 

to transport this significant discharge beneath the rehabilitated storage tanks at P.O.L. Further, notable spring discharge is presently occurring over the Government Hill 

School slide and has, since the 1964 event, eroded channels into the slide debris. The buried topography on the 

upper surface of the Bootlegger Cove Formation causes 

supraformational water to flow toward the Army tank 

farm (P. 0. L.) and Government Hill Elementary School 

Surface and Ground-Water Hydrology 23 
area, thereby increasing stability problems in these areas. 

This resultant ground-water flow causes shallow soils 

farther west to have below-saturation conditions, a situation that greatly improves the free-face stability near the 

point of Government Hill. The site improvement that has 

resulted from the Corps of Engineers' work with underdrains at the P.O.L. tanks should markedly improve stability in that area. 

BEHAVIOR OF THE ENGINEERING GEOLOGIC 

UNITS 

The intrinsic characteristics of the soils coupled with 

external factors such as seismicity, climate, and local hydrology can yield soil responses which site-specific investigators should consider and anticipate. These potential responses could cause superficial damage to a facility, render it unusable, or destroy it altogether. Historic performance of soils in the Government Hill area has provided 

evidence for a variety of soil-foundation failures, from 

instantaneous (for example, the 1964 earthquake-induced 

Government Hill Elementary School landslide (fig. 14)) 

to imperceptibly slow but progressive (for example, the 

floor deformation of certain petroleum storage tanks). Although the extent of foundation problems is substantial 

and the evaluation of conditions at specific locations is 

beyond the scope of any single report, some general empirical assessments can be made based upon case histories 

in the vicinity as well as analogies drawn from outside 

Anchorage. 

Elmendorf Air Force Base 

Tidal flats 

SHIP CREEK VALLEY 

200 400 600 METERS 

Figure 15. Inferred direction of unconfined ground-water flow (large arrows) beneath uplands of Government Hill area. Hachured line, present-day 

scarps along upland perimeter. Compare with figure 13 showing distribution 

of landslides. 

24 Engineering Geology, Government Hill Area, Alaska 
Our primary concern is soil stability in response to 

seismic events. However, the performance of certain 

geologic units under aseismic conditions is also significant. The following types of failure should be considered: 

1. Liquefaction and consequent failure of cohesionless 

soils, including flow slides, translational block slides, 

settlement, and lateral spreading; 

2. Strength loss in sensitive clays and consequent failure, including flow slides, translational block slides, 

and rotational slumps; 

3. Failures involving both liquefaction and sensitive 

clays; and 

4. Failures independent of liquefaction and sensitive 

clays (for example, aseismic gravity free-fall, subsidence, and piping). 

The ground failures of the Bootlegger Cove Formation in 1964 prompted considerable research into the cause 

and mechanics of these slides. A literature search was 

conducted on two causes, liquefaction of silts and sands 

(Shannon and Wilson, Inc., 1964; Seed, 1968, 1976; Seed 

and Wilson, 1967) and failure of sensitive silty clays 

(Hansen, 1965; Long and George, 1966a; Kerr and Drew, 

1965, 1968). The area has, in fact, become a case-history 

model for both types of landslide mechanisms, but to 

1985, investigators have not agreed on which mechanism 

was primarily responsible for the slides, nor decided 

which is of preeminent concern for potential future failure. Seed (1968) drew the conclusion-based upon cyclic 

testing data under simulated earthquake conditions, combined with eyewitness accounts-that liquefaction of sand 

lenses was primarily responsible for initiation of the 

slides. Once inertial forces were sufficient to overcome 

the strength of the adjacent sensitive clays, translatory 

block failures occurred. Seed's work has proven exceedingly important in the recognition of the significant role 

that liquefaction plays in seismically induced ground failure. 

Liquefaction 

The reader is encouraged to review recent literature 

on the mechanism of seismically induced liquefaction and 

on how liquefaction potential in noncohesive soils is currently evaluated (Seed, 1976, 1979; Bennett and others, 

1981 ; Seed and Idriss, 1981). Generally, investigators 

agree that if a saturated, noncohesive soil (ideally a wellsorted sand, but also some clayey sands, silty sands, and 

gravelly sands) is subjected to seismically induced cyclic 

shear stresses, soil structure tends to become more compact, resulting in transfer of stress to the pore water and, 

therefore, diminished stress on the soil particles. If cyclic 

stresses are sustained, a peak cyclic pore-pressure ratio 

of 100 percent (Seed, 1979) can be attained; and if cyclic 

stress is continued, resulting strain can cause "cyclic mobility" (Casagrande, 1976). Depending upon the density 

of the sand, confining pressures, and magnitude of the 

stress cycles, the soil may be mobilized to unlimited deformation or may experience only limited strain before 

dilating to a stable condition. 

Liquefaction potential of the engineering geologic 

units described in this report can be assessed by three 

general approaches: 

1. Qualitative assessment, based on geologic criteria 

commonly recognized as indicators of liquefaction 

potential; 

2. Quantitative assessment, based on field and laboratory soils data that have been used as indicators of performance of noncohesive soils in previous earthquakes; 

3. Evaluation of cyclic stress conditions necessary for 

liquefaction, based on field and laboratory determinations of stress conditions for a given site. 

At least 70 of the 246 borehole logs assembled for 

the present study provide the level of information needed 

to make a qualitative assessment of liquefaction potential 

and, when coupled with detailed mapping (Updike, 1986), 

can give a preliminary evaluation of the liquefaction potential. Sherif and Ishibashi (1978) provided a flow chart 

for evaluation which we have modified and applied to 

the engineering geologic units of Government Hill (table 

9). If a cohesionless soil unit is at a depth of confinement 

of 30 m or more, the soil is usually old enough, and 

therefore dense enough, not to have sufficient pore-water 

pressure buildup for liquefaction. In column 2, table 9, 

a conservative maximum depth of occurrence of 20 m 

is used, although some units also occur at greater depths. 

In order for liquefaction to occur, the soil voids must 

be saturated with water, which usually means that the soil 

is at or below the water table (column 3, table 9). Several 

of the engineering geologic units contain significant quantities of sand, which is a primary consideration for 

liquefaction. However, the total textural framework of the 

soil interval must be considered, including sorting, percentages of non-sand-size particles, and packing. If clay, 

for example, occurs within the sand framework, liquefaction is not likely (Seed and Idriss, 1981). Among the engineering geologic units of Government Hill, few meet 

these sedimentologic criteria (column 4, table 9). Those 

that do include Holocene alluvium, tidal flat deposits, 

Pleistocene glaciofluvial deposits, and F. VI and F. VII of 

the Bootlegger Cove Formation. The final step (column 

5, table 9) in the preliminary evaluation utilizes the Standard Penetration Test (SPT), wherein a 63-kg mass, 

dropped from a height of 760 mm, drives a 50.8-mmdiameter split-spoon sampler into the ground. The penetration resistance (N) is reported in number of blows of the 

weight to drive the sampler 305 mm. Although some recorded values can vary significantly, reflecting technique 

Behavior of the Engineering Geologic Units 25 
and testing conditions (see Gibbs and Holtz, 1957; McLean and others, 1975), the SPT remains the most universally applied test for relative density. Although several 

correlations have been drawn between SPT and liquefaction potential, the conclusions of Nishiyama and others 

( 1977) are here applied, specifically that critical N-values 

of 0 to 15 should be considered liquefiable. Based on 

this final parameter, those units which have met the previous liquefaction-potential criteria are more critically defined. For example, SPT values for most layers within 

the Holocene fluvial sediments of Ship Creek have blow 

counts generally above 25 and only rarely are at values 

less than 15, so that capacity for liquefaction is questionable. Conversely, SPT numbers for tidal flat deposits are 

commonly below 15 to 20, and those sediments are liquefiable. F.VI and F.VII have N values greater than 20, 

so that liquefaction potential is questionable to none. 

A second method of evaluating liquefaction potential determines specific field and laboratory properties of 

noncohesive soils, as described by Seed (1976). The 

method requires some specific data for the particular stratigraphic sequence as well as some assumptions regarding 

the seismic input. Careful examination of the available 

geotechnical data for the 246 boreholes in the study area 

yielded 41 having sufficient information. Although other 

site investigations may have actually amassed the necessary data for the following analyses, the data did not appear in the reports associated with those investigations and 

we surmise that they have been lost. 

Table 10 summarizes the determinations calculated 

from the 41 boreholes for sand and silty sand units, respectively. Inch-pound measurement units are used on this 

table because these units are exclusively reported in the 

original geotechnical documents from which the data were 

acquired. For ease of future reference by engineers, inchpound units are likewise used in the following analyses, 

with metric equivalents in parentheses. Depth to the potentially liquefiable sand, in feet, and depth to groundwater table, also in feet, were given in the borehole logs. 

Generally, natural-moisture content (Wn) was also given, 

but where values were not provided, approximations have 

been used based on numbers from nearby correlative 

sands. Dry densities ( 'Y d) of the various soil layers including the sand units of interest are generally not provided 

in borehole logs. Therefore, estimated mean densities (;yd) 

have been used for the calculations, assuming that for 

sand layers less than 15 ft (5 m) below ground surface 

the mean dry-soil density is 100 lb/ft3 (1.6 g/cm3). Similarly, for sands between 15 and 30 ft, ;yd= 110 lb/ft3 (1. 76 

g/cm3

), and for sands greater than 30 ft below ground 

surface ;yd= 120 lb/ft3 (1.92 g/cm3). These divisions are 

somewhat arbitrary but are supported by available density 

Table 9. Qualitative assessment of the liquefaction potential for engineering-geologic map units, 

Bootlegger Cove Formation, Government Hill area 

Water 

Map At 20m or table Grain-size Penetration Liquefaction 

unit less below wrt distribution resistance potential 

surface unit1 

F.I ............ Yes Above Clay N onliquefiable. 

F.II ........... No Nonliquefiable. 

F.III ........... No N onliquefiable. 

F.IV ........... Yes Above Silt and clay Nonliquefiable. 

F.V ........... Rare Above Silt and clay Nonliquefiable. 

F.VI ........... Yes Above Silt and sand >20__. Questionable. 

F.VII .......... Yes Above Sand with gravel >20----+ Questionable. 

Glaciofluvial Yes In or above Silt/ sand/ gravel 8-65---+ Questionable. 

deposits. 

Tidal flat deposits. . . Yes In or above Silt/sand/gravel 4-55--+ Liquefiable. 

Tidal marsh Yes Above/below Silt/sand organic 1-12----+ Questionable. 

deposits. 

Landslide deposits Yes Discontinuous Clay to gravel 2-18-+ Questionable. 

Holocene alluvium · · Yes Within top Silt to gravel 5-70~ Questionable. 

3m. 

Man-related fill · · · · Yes Below Nonliquefiable. 

1

wrt, with respect to. 

26 Engineering Geology, Government Hill Area, Alaska 
Table 10. Tabulation of borehole data and computed values utilized to evaluate liquefaction potential 

Depth to 

top of 

Sample interval 

No. (feet) 

20 

2E 

3 

4A 

48 

4C 

5 

6 

42A 

428 

42C 

420 

43A 

438 

43C 

430 

44 

62A 

628 

73A 

738 

73C 

36 

37 

202 

lA 

18 

IC 

2A 

28 

2C 

?A 

78 

7C 

70 

7E 

12A 

128 

l2C 

120 

17 

39 

45 

37 

40 

44 

50 

46 

45 

7 

12 

17 

21 

9 

12 

16 

19 

15 

6 

II 

II 

15 

10 

26 

26 

61 

10 

15 

22 

15 

20 

25 

10 

15 

21 

47 

53 

5 

13 

30 

43 

35 

Water 

table 

depth 

(feet) 

I 

7 

6 

6 

6 

2 

0 

6 

6 

6 

6 

6 

6 

6 

6 

6 

0 

0 

2 

2 

2 

3 

6 

39 

0 

0 

0 

I 

0 

0 

0 

0 

0 

I 

0 

Natural 

moisture 

content 

(pet) 

Wn 

24 

20 

16 

23 

23 

19 

21 

21 

25 

25 

25 

25 

25 

25 

25 

25 

25 

25 

25 

25 

25 

25 

24 

21 

30 

25 

29 

30 

30 

27 

28 

19 

32 

18 

26 

18 

31 

25 

24 

20 

28 

Total 

vertical 

stress 

(lblff) 

5,811 

6,480 

5,143 

5,920 

6,512 

7,150 

6,670 

6,525 

875 

1,500 

2,346 

2,898 

1,125 

1,500 

2,208 

2,622 

1,875 

750 

1,375 

1,375 

1,875 

1,250 

3,536 

3,458 

9,516 

1,250 

1,935 

3,146 

1,950 

2,800 

3,525 

1,190 

1,980 

2,730 

7,097 

7,526 

655 

1,625 

4,080 

6,192 

5,390 

Effective 

overburden 

pressure 

(lb/ff) 

4,709 

5,424 

4,573 

4,968 

5,448 

6,138 

5,570 

5,400 

850 

1,350 

2,038 

2,478 

1,050 

1,350 

1,928 

2,258 

1,650 

600 

1,100 

1,_150 

1,550 

1,050 

2,938 

2,998 

8,364 

Static 

stress 

ratio 

CTvfCTv 

Tidal flat sands 

1.23 

1.19 

1.12 

l.l9 

1.20 

l.l6 

1.20 

1.21 

Stress 

reduc- Cyclic 

tion stress 

factor ratio 

fd TavfCTv 

0.87 

.85 

.88 

.87 

.85 

.83 

.85 

.85 

0.21 

.20 

.19 

.20 

.20 

.19 

.20 

.20 

Glaciofluvial sands 

1.03 

I. II 

1.15 

1.17 

1.07 

!.II 

1.15 

1.16 

l.l4 

1.25 

1.25 

1.20 

1.21 

1.19 

0.95 

.95 

.94 

.93 

.97 

.96 

.95 

.93 

.95 

.98 

.96 

.96 

.95 

.97 

Tidal flat silty sands 

1.20 

1.15 

1.14 

0.91 

.91 

.80 

0.19 

.21 

.21 

.21 

.20 

.21 

.21 

.21 

.21 

.19 

.23 

.22 

.22 

.25 

0.21 

.20 

.18 

F. VI of the Bootlegger Cove Formation 

100 

1,500 

2,420 

1,530 

2,230 

2,781 

1,000 

1,500 

2,310 

5,640 

6,360 

531 

1,325 

3,326 

5,184 

4,200 

1.25 

1.29 

1.30 

1.27 

1.26 

1.27 

1.19 

1.32 

l.l8 

1.26 

1.18 

1.23 

1.23 

1.23 

1.19 

1.28 

0.97 

.95 

.93 

.95 

.93 

.91 

.97 

.95 

.93 

.84 

.82 

.99 

.95 

.90 

.85 

.88 

0.23 

.24 

.24 

.24 

.23 

.23 

.23 

.24 

.21 

.21 

.19 

.24 

.23 

.22 

.20 

.22 

Penetration 

resistance 

(blows/ft) 

N 

26 

10 

78 

38 

44 

78 

54 

52 

8 

13 

20 

8 

12 

13 

22 

21 

12 

30 

28 

17 

10 

13 

55 

40 

50 

24 

13 

13 

7 

14 

9 

29 

II 

14 

46 

78 

23 

23 

29 

92 

64 

Dimensionless 

conversion 

factor 

eN 

1.06 

1.02 

1.00 

.99 

.96 

.92 

.98 

1.02 

1.60 

1.44 

1.26 

.92 

1.60 

1.44 

1.28 

1.24 

1.30 

1.60 

1.60 

1.60 

1.50 

1.60 

1.12 

1.06 

.60 

1.60 

1.60 

1.42 

1.51 

1.44 

1.31 

1.60 

1.60 

1.42 

1.00 

.96 

1.60 

1.60 

1.20 

1.06 

1.13 

Corrected 

penetration 

resistance 

(blows/ft) 

N, 

28 

10 

78 

38 

42 

72 

53 

53 

13 

20 

25 

7 

19 

19 

28 

26 

16 

48 

45 

27 

15 

21 

69 

50 

38 

46 

33 

26 

18 

28 

19 

54 

25 

27 

54 

83 

44 

44 

42 

105 

80 

Behavior of the Engineering Geologic Units 27 
data, and the modest discrepancies that may actually exist 

will not significantly affect the following calculations. The 

total vertical stress, O'v, can be calculated from these data 

(the reader may wish to consult a soils-engineering text 

for the required calculations, for example, Dunn and 

others, 1980). Because the layers under consideration 

exist below the water table, that is, are under saturated 

conditions, it is necessary to compute the pore-water pressure, U, and subtract this value from O'v to obtain the 

effective overburden pressure, <Tv, or effective stress. The 

results of these calculations also appear in table 10. 

The cyclic stress ratio, Tajfiv, as proposed by Seed 

(197 6, 1979), is a meaningful parameter for expressing 

cyclic liquefaction potential as a ratio of the average cyclic shear stress acting on horizontal surfaces within the 

soil as a result of seismic loading, T av• to the initial effective vertical stress acting on the soil layer before cyclic 

loading, C:T v· This ratio can be computed from 

where amaxl g is the maximum estimated earthquake acceleration at the ground surface with respect to gravitational 

acceleration and r d is a stress-reduction factor which varies from a value of 1 at the ground surface to 0. 9 at 

a depth of 30 ft (Seed, 1979). Table 10 gives the calculated values of the cyclic stress ratio using the interpolated 

values of rd and assuming amaxlg=0.3. 

The SPT blow-count (blows per foot) information 

(N values) is utilized as a measure of relative density. 

Based on the work of Gibbs and Holtz (1957), a corrected 

penetration resistance, N~> is proposed by Seed (1976) 

as 

where CN is a dimensionless conversion factor. The appropriate values of CN have been determined from the 

curves published by Seed and Idriss (1981) (this report, 

fig. 16). The potential for liquefaction of silty sands is 

substantially less than for clean sands. Seed ( 1979) has 

shown that a factor of 7. 5 should be added to the N 1 

values for silty sands; where applicable this is reflected 

in the N 1 values of table 10. Following the method of 

Seed (1979), the corrected penetration, N~> is plotted 

against the cyclic stress ratio, Tavl ITv, in figures 17 and 

18. Seed and others ( 1977) were able to draw a liquefaction-potential boundary line on this graph based on historical data from liquefaction sites. A suite of curves for different magnitude earthquakes have been subsequently developed by Seed and ldriss ( 1981) and are plotted on the 

28 Engineering Geology, Government Hill Area, Alaska 

graphs (figs. 17, 18). Data points falling to the left of 

a particular curve should be considered as representing 

sands or silty sands which are liquefiable under earthquake conditions equivalent to the magnitude of the curve. 

Several glaciofluvial sand data points fall within or 

to the left of the suite of curves (fig. 17). These are saturated, moderate to loosely packed sands overlying .the 

Bootlegger Cove Formation. The data points for tidal flat 

sands (fig. 17) and tidal flat silty sands (fig. 18) are generally in the nonliquefiable field, although enough points 

fall within the liquefiable domain to give some concern 

for tidal flat stability in earthquakes of magnitudes greater 

than 6. Three values for F. VI silty sands are also given 

in figure 18, all decidedly non1iquefiable. This is supported by the work of Idriss and Moriwaki (1982) for 

sands within the Bootlegger Cove Formation elsewhere 

in Anchorage, which they also concluded are nonliquefiable. Estimating liquefaction potential by this series of 

measurements and calculations is well within the scope 

and capabilities of most site-foundation studies, and this 

increment of additional work is strongly recommended for 

future construction in the Anchorage area. 

The third technique for evaluating liquefaction potential, utilizing the determinations of in-place stress conui 

a: 

~ 4 en 

w 

a: 

Q_ 

z 5 w 

0 

a: 

:::::> 

~ 6 w 

> 

0 

~ 7 

f= 

u 

w 

u.. 

tt 8 

9 

D,: 40 to 60 percent 

D,: 60 to 80 percent 

1.6 

CN FUNCTION 

Figure 16. Graph for determining eN as a function of 

effective overburden pressure, modified from Seed and 

ldriss (1981). Relative density (Dr) should be considered 

at effective overburden pressures greater than 2,000 

pounds per square foot (95.8 kN/m2). 
ditions and relating these data to laboratory cyclic stress 

determinations, is far more rigorous than the first two 

methods (Castro and Poulos, 1976; Cho and others, 

1976). To date of this writing, this type of study has 

not been performed in the Government Hill area. 

Based on the foregoing analyses, the role that 

liquefaction played in the 1964 earthquake ground failures 

is in doubt. Some of the ground cracking and minor displacements that occurred on the tidal flats and Ship Creek 

flood plain were most probably a result of liquefaction. 

The Government Hill Elementary School and Elmendorf 

slides were due to failures within the Bootlegger Cove 

Formation. Typically, the very soft silty clay units are 

directly above or below the sand facies, which may have 

confused the failure plane analyses. The work of Shannon 

and Wilson, Inc. (1964), and Seed (1967) indicates that 

under cyclic loading the sands would fail before the sensitive clays. Some question remains about the 1964 method 

of testing using reconstituted sands as well as about inherent errors in the in-place strength measurements of F.III. 

Based upon the calculations summarized in this report, 

we must conclude that the Government Hill Elementary 

School and Elmendorf slides are not the result of sand 

liquefaction, as previously proposed. 

Sensitive Silty Clays 

The origin of the silty clays of the Bootlegger Cove 

Formation in quiet brackish to saline water is compatible 

0.5 

~}C?f ~/ I.Q,'I.Q/ I.Q/(()/ (()/ "-/QJ/ :EJ~I :E.' :E.I~I 0 I I I I 

i= I I I 

<( I I 

c:: / / 

/ 

(f) 0.3 (f) 

U.J 

c:: 

1-

(f) 

u 0.2 :::::i 

u 

>-

u 

0.1 

o~--~1o~--~20~---3~o~---4~o-----5~o-----6Lo ____ ~7o 

MODIFIED PENETRATION RESISTANCE, N1 , IN BLOWS/FT 

EXPLANATION 

• Tidal flat sand 

o Glaciofluvial sand 

Figure 17. Liquefaction potential for tidal flat sands and 

glaciofluvial sands. Earthquake magnitude boundary 

curves from ldriss and Moriwaki (1982), dashed where inferred; amax (maximum seismic acceleration), 0.30 g. 

Points falling within or to left of curves are potentially 

liquefiable. 

with the formation of an open flocculated framework of 

soil particles. This, in itself, may not fully account for 

the sensitivity of certain layers within the formation (that 

is, F.III). Six causes for clay sensitivity have been proposed by Mitchell (1976), including (1) metastable fabric, 

(2) cementation, (3) weathering, (4) thixotropic hardening, (5) leaching and ion exchange, and (6) formation 

or addition of dispersing agents. Seemingly, the most apparent cause of increased local sensitivity for the Bootlegger Cove Formation is the prehistoric fluctuations of sea 

level with associated salt leaching within the flocculated 

structure. 

In addition, some evidence suggests that soils such 

as F.III can attain moderate sensitivity by way of 

thixotropic hardening (Mitchell, 1960). The process requires remolding after the initial sedimentation, followed 

by relaxation of the externally applied stresses. This process allows for a reorganization of the interparticle force 

balance and a tendency toward flocculation of particles 

in a newly established water-cation structure. We visualize 

this process as not being responsible for the initial sensitivity of F .III (which relates to the original undisturbed 

framework of the soil), but being instead a supplementary 

mechanism to enhance sensitivity. Thixotropic hardening 

would thus play an important role in areas adjacent to 

those which have failed massively. The effect of limited 

shear strain during prior seismic events serves as the 

mechanism for remolding the silty clays along failure 

planes and, in fact, expands the sensitive zone vertically 

0.5 

/.QIQI I I I 

"!' -~~J/.Qii.Q/ 

0.4 I.Qf((), (()/ "-/QJ/ 

0 :Et':EI :E.' :E.I~J I I I I 

i= I I I I I <( / I c:: / 

(f) 0.3 (f) 

U.J 

c:: 

1-

(f) 

u 

:::::i 

u 

>-

u 

. .. 0.2 I> 

I> 

0.1 

oL_--~10~--~20~---3~0~---4~0~--~5~0~--~6~0----~70 

MODIFIED PENETRATION RESISTANCE, N1 , IN BLOWS/FT 

EXPLANATION 

• Tidal flat silty sand 

1> Bootlegger Cove 

Formation, F. VI 

Figure 18. Liquefaction potential for tidal flat silty sands 

and F.VI (Bootlegger Cove Formation). Earthquake-magnitude boundary curves from ldriss and Moriwaki (1982), 

dashed where inferred; amax (maximum seismic acceleration), 0.30 g. 

Behavior of the Engineering Geologic Units 29 
during successive earthquakes over a period of hundreds 

of years. 

Under shear stresses, such as those induced by an 

earthquake, the flocculated structure of F.III collapses 

with an attendant decrease in volume. F.III is normally 

confined by low-permeability facies (for example, F .II 

and F. IV), and if the drainage of pore water is too slow, 

effective stresses in the zone of collapse would approach 

zero, resulting in lateral flow and low-angle planar gliding. 

The translatory failure affects the overlying units, 

typically resulting in block movement along numerous 

secondary shear planes. Although the initial movement 

may closely resemble liquefaction along deep cohesionless 

horizons, the sensitive clay failure would be more likely 

to continue after the cyclic shear stresses cease. At Turnagain Heights during the 1964 earthquake, the apparent 

continuation of ground failure after the ground shaking 

had ended (Hansen, 1965) was probably due to sensitive 

clay mobilization. The two major landslides that occurred 

at Government Hill were related to shear planes developed 

above MSL within the Bootlegger Cove Formation. At 

both locations, F. VI and the sensitive F. III occur at the 

probable depth of initial failure. Both areas have higher ' 

than normal amounts of ground water and surface runoff, 

which combine with the factor of a steep free-face bluff 

line to increase slope instability. 

Other Mechanisms 

In addition to failures resulting from liquefaction 

and sensitive clays, soils within the study area have shown 

potential for vertical and (or) lateral displacement due to 

a variety of other stress-field conditions. As indicated earlier in this report, numerous landslides have happened 

along the bluff scarps, often independent of seismic 

events. These slides may have various natural or man-related causes, which include over-steepened slopes; undercutting of the toe of the slope; ground-water piping, which 

removes cohesionless soils from within or below slopes; 

induced loading at the head of, or directly on,. the freeface slope; and lateral spreading or slumping due to removal of buttressing material. Separation of the moving 

blocks typically occurs along high-angle shear planes at 

shallow depths (less than 10 m) behind the free face, with 

vertical displacements from 1.0 to 15 m. 

Surface fissures, heaving, and subsidence of fill and 

regraded soils are occasionally reported in those areas of 

near-horizontal slopes. These failures are limited in displacement to a few centimeters but can cause disruptions 

of foundations, utilities, pipelines, and roadways. In most 

instances, failure can be attributed to unstable soil conditions below the fill grade. These include soils that are 

poorly drained and have high organic content, are poorly 

consolidated, have low shear strength due to previous fail30 Engineering Geology, Government Hill Area, Alaska 

ure, are overconsolidated subgrade cohesive soils, have 

lateral variation of subgrade soil types resulting in differential settlement, or contain artificially induced permafrost 

beneath fluid storage tanks. In most instances, site foundation engineering has been inadequate. Subgrade excavation, soil improvement using admixtures, subgrade drains, 

compaction, and modification of fill thickness and composition provide the necessary remedy for most of these 

problems. 

SUMMARY 

The Government Hill-Anchorage Port-Ship Creek 

area is a critical transportation center for Alaska. The potential landslide hazard throughout the area could seriously 

impact upon its operation. Certain facies within the Bootlegger Cove Formation are here of particular concern, 

especially during possible future seismic events. Previous 

slope failures exist nearly continuously along the entire 

bluff line around Government Hill and north to beyond 

Cherry Hill, where slides then become discontinuous. The 

primary mode of failure is by slump blocks, which may 

have rotational or inclined glide-plane displacement. 

Larger blocks tend to break up upon movement into smaller entities, which may move either in retrograde or normal rotational dislocations. The 1964 Government Hill 

Elementary School slide was typical of the majority of 

these slides along the bluffs. Major slides show a rather 

consistent range of vertical displacement (6-10 m), but 

vary distinctly in volume according to the horizontal area 

of the moving block, or blocks. Two areas have particularly marked earlier histories of failures: the area north 

from the military P.O.L. tank farm approximately 1 km 

and the southeastern segment of Government Hill near 

the old Government Hill Elementary School. The 1964 

slides were located in these same areas, and both slides 

appear to have partially involved soil masses which had 

already failed tens to hundreds of years prior to 1964. 

Little evidence for landslides was found in the northernmost part of the study area: a change in the dominant 

facies of the Bootlegger Cove Formation (F .III being replaced by F.II and F.IV) increased effective overburden, 

and we also see evidence for late Pleistocene glacial loading of the sedimentary sequence, which caused overconsolidation. In all instances, landslides along the Government Hill bluffs can be attributed to failures within the 

Bootlegger Cove Formation. We conclude that most of 

this mass movement, including the 1964 slides, is due 

to sensitive clay failure; sand liquefaction played no significant role. The clay failure problem is further enhanced 

by ground-water and surface-runoff conditions; that is, 

where ground water is discharging from above or within 

the Bootlegger Cove Formation to the surface, recent 

slope failures are far more extensive. 
Based on an examination of the distribution and 

character of Holocene landslides along the bluffs and on 

present activity (ground fissures, slope deformation, and 

surfacing waters), we are particularly concerned for the 

slope stability of the area extending from West Bluff 

Drive, north through the P.O.L. tank farm, and thence 

north to the bluffs just south of Cherry Hill. This area 

includes both former slides below the present bluff line 

and upland surfaces within 700 m of the existing bluffs. 

We have tried here to develop a method for examining the three-dimensional characteristics of the Quaternary 

deposits that occur in the Government Hill area of Anchorage, with particular emphasis on the variability of the 

Bootlegger Cove Formation. The relations among the 

geologic history, present ambient geologic conditions, and 

engineering characteristics of each mappable geologic unit 

dictate the three-dimensional behavior of the soils in a 

given site. Although our available data provide an adequate base for characterizing the static parameters of each 

unit, the data do not provide sufficient information for 

an adequate assessment under dynamic conditions. In this 

sense, based upon the high seismic exposure of the area 

and the observed and recorded performance of certain of 

these soils, a strong plea is made for a more unified approach to site foundation testing and assessment throughout Anchorage. 

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