Geological, Geophysical, and 

Engineering Investigations of 

the Loveland Basin Landslide, 

Clear Creek County, Colorado, 

1963-65

Geological Investigations

Electrical Resistivity Investigations

Seismic Refraction Studies

Engineering Investigations

Compilation of Results of Geological, Geophysical,

and Engineering Investigations

Stability Analysis

Summary and Recommendations

GEOLOGICAL SURVEY PROFESSIONAL PAPER 673-A, B, C, D, E, F, G
Geological, Geophysical, and

Engineering Investigations of the

Loveland Basin Landslide,

Clear Creek County, Colorado, 1963 65
FRONTISPIECE. Loveland Basin landslide, view northwestward (1965).
Geological, Geophysical, and 

Engineering Investigations of 

the Loveland Basin Landslide, 

Clear Greek County, Colorado, 

1963-65

Geological Investigations

By CHARLES S. ROBINSON and FITZHUGH T. LEE

Electrical Resistivity Investigations 

By R. WOODWARD MOORE

Seismic Refraction Studies

By RODERICK D. CARROLL, JAMES H. SCOTT, and FITZHUGH T. LEE

Engineering Investigations

By JOHN D. POST and CHARLES S. ROBINSON

Compilation of Results of Geological, Geophysical,

and Engineering Investigations

By CHARLES S. ROBINSON and FITZHUGH T. LEE

Stability Analysis

By ROBERT A. BOHMAN

Summary and Recommendations

By CHARLES S. ROBINSON and FITZHUGH T. LEE

GEOLOGICAL SURVEY PROFESSIONAL PAPER 673-A, B, C, D, E, F, G

A study using several methods of investigation 

to define the geometry and behavior of a 

landslide

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1972
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary

GEOLOGICAL SURVEY 

V. E. McKelvey, Director

Library of Congress catalog-card No. 72-600035

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, B.C. 20402 - Price 75 cents (paper cover)

Stock Number 2401-2134
CONTENTS

[Letters in parentheses preceding titles designate the chapters]

Page

Introduction __________________________________________________________________ 1

(A) Geological investigations, by Charles S. Robinson and Fitzhugh T. Lee ________________________ 3

(B) Electrical resistivity investigations, by R. Woodward Moore_____________.__________________- 11

(C) Seismic refraction studies, by Roderick D. Carroll, James H. Scott, and Fitzhugh T. Lee _____________ 17

(D) Engineering investigations, by John D. Post and Charles S. Robinson ________________ 21 

(E) Compilation of results of geological, geophysical, and engineering investigations, by Charles S. Robinson and

Fitzhugh T. Lee _________________________________________________________'.____ 27

(F) Stability analysis, by Robert A. Bohman ___________________________________________ 35

(G) Summary and recommendations, by Charles S. Robinson and Fitzhugh T. Lee ___________ 41

References cited _______________________________________________________________ 43

ILLUSTRATIONS

Page

FRONTISPIECE. Loveland Basin landslide ____________._______________-________ II

FIGURE 1. Index maps of the Loveland Basin landslide area, Clear Creek County, Colo. ______ 1 

2. Geologic map of Loveland Basin landslide _____________________________ 4 

3-6. Photograph of 

3. Unfinished cut near east portal of the Straight Creek Tunnel prior to development of landslide 7

4. Cut near east portal after start of landslide _____________________-_ _____ 8

5. Landslide fault scarp at the northwest border of the landslide. Note striations 9

6. Landslide thrust fault at southeast edge of landslide. Note destruction of plant life as landslide

advanced ___________________________________________________- 10

7. Map of the Loveland Basin landslide showing location of geophysical traverses ____ 12

8. Schematic diagram showing principle of earth-resistivity measurement using the Wenner configuration 13

9. Graph showing resistivity-depth curve for talus overlying weathered bedrock and cumulative resistivity

curve used to empirically determine depth to subsurface layers __ _ 13

10. Graph showing resistivity-depth curves typical of data obtained on Loveland Basin landslide area 14

11. Section through east part of sliding mass _______________________________________ 15

12. Photograph of a part of seismic record obtained on seismic line 2V______________ 18

13. Time-distance graphs obtained on seismic lines K and D _____________________________ 18

14. Longitudinal and transverse sections of the Loveland Basin landslide showing thickness of low-velocity

layer ________________________ _______________.___________________ 19

15. Generalized map of the Loveland Basin landslide showing the location of the measurement points at the

margins of the landslide and the approximate locations of the surface and subsurface drainage pipes 22

16. Graphs of records of the relative vertical and horizontal changes in distance between pairs of points

outside and within the landslide mass ____ ____________________________________- 23

17. Maps showing the location and initial and final dimensions of surveyed grid established in the plaza

area southeast of the landslide mass _____ __________________________ __-__ 25

18. Isopach map of the Loveland Basin landslide to the top of the slip zone ___________ 29

19. Isopach map of the Loveland Basin landslide to the base of the slip zone __________ 30

20. Structure contour map drawn on the top of the slip zone of the Loveland Basin landslide ________ 31

21. Structure contour map drawn on the base of the slip zone of the Loveland Basin landslide ________ 32

22. Longitudinal and transverse sections of the Loveland Basin landslide _________.._ 33

23. Map showing plaza area and buttress placed on toe of Loveland Basin landslide _____________ 36

24. Sections for analysis of the Loveland Basin landslide _________ ___ 38
GEOLOGICAL, GEOPHYSICAL, AND ENGINEERING INVESTIGATIONS

OF THE LOVELAND BASIN LANDSLIDE, 

CLEAR CREEK COUNTY, COLORADO, 1963-65

By CHARLES S. ROBINSON, FITZHUGH T. LEE, and others

INTRODUCTION

By CHARLES S. ROBINSON and FITZHUGH T. LEE

The Loveland Basin landslide that occurred in the 

Spring of 1963 was caused by an excavation made 

for the approach road to the east portal of the 

Straight Creek Tunnel. The site of the tunnel is 

about 55 miles west of Denver (fig. 1) on the north 

side of Loveland Basin and northwest of the Loveland Basin ski area.

A contract for the construction of 2 miles of Interstate 70 that will serve as the east approach road 

for the Straight Creek Tunnel was awarded in August 1962. In June 1963, much of the construction 

under this contract had been completed, and the cut 

for the east portal of the Straight Creek Tunnel 

was nearly finished. At that time, F. W. Fitzpatrick, project engineer for the Colorado Department 

of Highways, while examining the slope above the 

top of the cut, noted landslide scarps. He notified G. 

N. Miles, district engineer, who in turn notified the 

division personnel of the U.S. Bureau of Public 

Roads. The U.S. Geological Survey was alerted by 

these agencies because its personnel were then engaged in a research project for the Straight Creek 

Tunnel pilot bore, which had a direct bearing on 

conditions at the east portal.

Action had to be taken immediately to stabilize 

the landslide because the Colorado Department of 

Highways are planning to let a contract early in the 

fall of 1963 for the construction of the pilot bore 

for the Straight Creek Tunnel. The landslide was 

above and north of the east portal. Before remedial 

procedures could be planned, the volume and mass 

of the landslide had to be determined, and if possible, some evaluation of its rate of movement had to 

be made. Joint conferences of the Colorado Department of Highways, the U.S. Bureau of Public 

Roads, and the U.S. Geological Survey resulted in 

decisions to map the landslide geologically and to 

try to determine the thickness of the slide by geophysical methods. The authors believed that drilling 

the slide would be impractical because of the relative inaccessibility and the nature of the geology of 

the slide area. R. Woodward Moore, assisted by 

other members of the U.S. Bureau of Public Roads 

and by members of the Colorado Department of 

Highways, made the resistivity measurements in

39° 

40'

Base from L.^. ^.^w&.^a. ^, u rve 

Loveland Pass, 1:24,000,1958 

0 2 MILES

J

CONTOUR INTERVAL 400 FEET

DATUM IS MEAN SEA LEVEL 

106°

FIGURE 1. Index maps of the Loveland Basin landslide 

area, Clear Creek County, Colo. Stipple indicates area 

of Loveland Pass fault zone.
LOVELAND BASIN LANDSLIDE, COLORADO

the slide area. R. D. Carroll and J. H. Scott, assisted by other members of the U.S. Geological Survey, made the seismic measurements on the slide. C. 

S. Robinson and F. T. Lee mapped the geology of 

the slide area and surveyed the resistivity and 

seismic lines by planetable methods. Control for the 

planetable mapping was established by a transit 

survey around the slide by the Colorado Department of Highways. At that time the department 

also had a topographic map of the landslide area 

prepared from aerial photographs taken in 1961.

Approximately 3 weeks in July 1963 were required to obtain the geologic and geophysical data. 

During this time, the Colorado Department of 

Highways had two holes drilled in the plaza area at 

the bottom of the cut. (See fig. 2.) The department 

inserted plastic casing in these holes and probed the 

holes daily for any movement. It also established a 

first-order grid on 50-foot centers in the plaza area 

and kept records of the movement of these points.

The geological and geophysical data were compiled and interpreted, and a report giving the volume and mass of the slide was prepared and submitted by Robinson, Carroll, and Lee (1964) to the 

U.S. Bureau of Public Roads and the Colorado Department of Highways in September 1963. These 

data were then used by R. A. Bohman, U.S. Bureau 

of Public Roads, in analyzing the stability of the 

landslide and in calculating the load necessary to 

stabilize the slide. Late in September 1963, compacted fill material was placed at the base of the 

slide to serve as a buttress. The load was emplaced

by October 1963. Low-angle drainage holes were 

drilled into the slip zone at various locations near 

the western and southern perimeter of the slide, 

and these were lined with perforated casing. Additional drainage was accomplished by the construction of French drains and the installation of corrugated-metal pipe culverts. The contract for the 

construction of the Straight Creek Tunnel pilot bore 

was awarded late in October. For about 2 years, periodic observations of the slide area were made, 

mainly by John D. Post of the Colorado Department 

of Highways.

The authors have little firsthand information regarding the behavior of the landslide since 1965. 

The Colorado Department of Highways (oral commun., 1969) believes that the lower part of the 

landslide, northwest of the buttress, has deepened 

and that the northwest and northeast bordering 

scarps have lengthened during 1968 and 1969. Steel 

supports in the east portal of the Straight Creek 

Tunnel have tilted eastward near the crown line, 

possibly owing to landslide movement. The landslide 

remains a great concern to those involved in the 

safety and economic completion of the Straight 

Creek Tunnel.

The purpose of this report is to present the methods used to investigate the Loveland Basin landslide 

during the period of June 1963 through June 1965 

and the results of those investigations. The work is 

presented in seven topical chapters; each chapter is 

written by those persons principally responsible for 

the work.
Geological Investigations

By CHARLES S. ROBINSON and FITZHUGH T. LEE

GEOLOGICAL, GEOPHYSICAL, AND ENGINEERING INVESTIGATIONS 

OF THE LOVELAND BASIN LANDSLIDE, CLEAR CREEK COUNTY, COLORADO,

1963-65

GEOLOGICAL SURVEY PROFESSIONAL PAPER 673-A

462-882 O - 72 - 2
CONTENTS

Page

Abstract ________________________________________________ 3

Introduction ___________________________________________ 3

Physiography _____________________________________________ 6

Regional geology ________________________________________ _ 6

Bedrock _________________________________________________ 6

Metasedimentary rocks ___________________________________ 6

Granite ______________________________________________ 7

Surficial deposits ___________________________________________ 8

Structure ________________________________________________ 8
GEOLOGICAL, GEOPHYSICAL, AND ENGINEERING INVESTIGATIONS OF THE LOVELAND 

BASIN LANDSLIDE, CLEAR CREEK COUNTY, COLORADO, 1963-65

GEOLOGICAL INVESTIGATIONS

By CHARLES S. ROBINSON and FITZHUGH T. LEE

ABSTRACT

The Loveland Basin landslide occurred in a high mountain 

valley near the Continental Divide about 55 miles west of 

Denver during construction of the east portal area for the 

proposed Straight Creek Tunnel. The landslide developed in 

Precambrian bedrock, consisting of Silver Plume Granite 

with inclusions of metasedimentary rocks that are chiefly 

biotite gneiss and schist. Thin deposits of surficial material, 

consisting of moraine and soil mixed with talus, overlie the 

bedrock.

The entire area is within the Loveland Pass fault zone. 

Individual faults within this zone are characterized by 

masses of breccia and gouge, ranging in width from less 

than 1 foot to 1,000 feet; these masses are separated by 

masses of less intensely sheared rock. The preexisting structure partly determined the shape of the landslide.

The landslide is outlined by distinct scarps. The upper 

two-thirds of the landslide is downdropped in relation to the 

surrounding rock. The lower one-third of the landslide has 

been thrust upward and outward over the surrounding 

topography. Within the landslide mass, movement is indicated by tension fractures, which are most numerous near 

the margins of the mass.

INTRODUCTION

Probably not all landslides can be prevented, even 

after the most careful examination and interpretation of the geology of an area in advance of construction. When a landslide occurs, it must be denned rapidly and stabilization methods must be 

designed. Construction activity was at a critical 

stage in Loveland Basin when the landslide occurred. The Colorado Department of Highways' 

construction schedule called for completion of the 

tunnel plaza (portal) area by midsummer 1963 so 

that construction of the Straight Creek Tunnel pilot 

bore could be started by late summer or before the 

start of winter. The landslide had to be denned and 

stabilized to ensure the safety of men and equipment at the plaza, to forestall any effects of the 

landslide on construction of the pilot bore, and to

prevent rejuvenation of movement of the landslide 

by construction of the pilot bore.

The geologic studies of the Loveland Basin landslide, including a written report, were completed in 

approximately 2y% months. Persons concerned with 

the studies came to realize that more definitive results could have been obtained in a shorter period 

of time had the personnel and the necessary equipment been available on a full-time basis.

Knowledge of the geology, particularly the landforming processes, is necessary for the prevention 

or analysis of a landslide. Geologic studies revealed 

the framework of the Loveland Basin landslide. The 

bedrock of the area consists of Precambrian igneous 

and metasedimentary rock that locally has been extensively faulted and altered. During the Pleistocene the stream valleys at the altitude of Loveland 

Basin were glaciated and their sides were steepened. When the glaciers retreated, they deposited 

morainal material in the bottoms of the valleys and 

plastered it along the oversteepened valley sides. 

Since the retreat of the glaciers, the morainal material locally has slid part way or completely downslope, and the oversteepened bedrock walls have 

been retreating by weathering processes. As a result, 

many slopes in the high mountain valleys are not 

stable under present conditions, and changes of these 

slopes caused by construction may trigger serious 

landslides. When the slope of the bedrock in Loveland Basin was oversteepened, the rock failed along 

preexisting faults and moved with its load of surficial material toward the cut. The Loveland Basin 

landslide is but one of several landslides in the high 

mountains that are the result of failure of highly 

sheared and altered bedrock loaded with surficial 

material. Another such slide, but a much larger one, 

occurred about 1 mile west of the west portal of the 

Straight Creek Tunnel pilot bore along the west approach road.
LOVELAND BASIN LANDSLIDE, COLORADO

~~- ""--, C' 7 / _ ~'.frV- _ '^-^ '>-2S: ~^' '.' -**£* '' *-~~s-^t^-

-----

CONTOUR INTERVAL 5 FEET 

DATUM IS MEAN SEA LEVEL

Prelandslide topography by Continental 

Engineers Inc., Denver, Colo., based on 

aerial photographs taken September 1961 

and modified to show landslide, August 

1963; base further modified and generalized by C. S. Robinson and F. T. Lee, 1964

Geology by C. S. Robinson and F. T. Lee, 

assisted by M. D. Lee, August 6-10, 

21-22, 1963. Mapped by planetable 

methods based on transit traverse by 

Colorado Department of Highways, July 

1963, and adjusted to topographic base

FIGURE 2. Geologic and topographic map of Loveland Basin landslide, Clear Creek County, Colo.
GEOLOGICAL INVESTIGATIONS

The extent of the Loveland Basin landslide was 

difficult to define. The slide area, particularly the 

upper part, was inaccessible to drilling equipment 

without the construction of a long access road. The 

authors believed that the surface of the slide should 

be disturbed as little as possible and that drilling 

should only be considered if alternate methods of delineation proved inconclusive. The construction of

EXPLANATION

Area of soil and talus and scattered bedrock outcrops

Area predominantly of surficial deposits

Area covered predominantly by morainal material

Area predominantly of granite

Area predominantly of metasedimentary rocks 

In cut

Contact, approximately located

Landslide fault scarp

Showing dip (66), plunge ofstriations (45), and height (8), 

in feet. Dashed where approximately heated

Vertical landslide fault scarp

Landslide thrust fault

Showing height ofoverthrust roll in feet. Dashed 

where approximately located

Zone of minor fractures

-uOverturned roll in soil 

Showing trace of axial plane and direction of dip of limbs

0 DDH 1

Diamond-drill hole

-A'

Line of section shown on figures 14 and 22

roads across the slide would have required the removal of vegetation and forest debris, which would 

have increased the absorption of surface water and 

would have contributed to the instability of the 

landslide mass.

A more compelling reason for placing a low 

priority on a drilling program was the nature of 

the material involved in the landslide. Owing to the 

heterogeneous mixture of altered and sheared metasedimentary and granite bedrock and blocks of unaltered and unsheared metasedimentary rocks and 

granite, the material could not be expected to be 

cored and a slip plane identified. The alteration and 

gouge along preexisting faults and shears in the 

landslide mass and in the undisturbed bedrock 

below the landslide mass probably would have resembled the clay gouge commonly found along slip 

surfaces. The large blocks of relatively unaltered 

and unsheared rock in the landslide mass would 

have resembled that in the undisturbed bedrock. 

The authors concluded that a slip surface, or surfaces, could not be identified by drilling.

From these considerations, and because of the 

need for immediate information, participating agencies decided that the landslide could best be defined 

by careful surface geologic, geophysical, and engineering studies.

The principle considered in the use of geology 

and geophysics in defining a landslide is that landslides are natural phenomena controlled by certain 

natural properties. These properties can be identified, measured, and interpreted. By mapping the 

natural features such as scarps, crevices, and rolls 

 that are associated with a landslide, the landslide 

can be defined in two dimensions. From a knowledge of the composition of a landslide mass, which 

can be determined from the regional geology, geophysical measurements can be interpreted to define 

the third dimension. A landslide mass has, in general, a greater porosity and permeability and 

therefore lower density than the surrounding material. This fact means that, depending upon hydrologic conditions, the landslide mass can be expected 

to exhibit an increased or decreased electrical conductivity and a decreased seismic velocity as compared to the surrounding stable area. The degree of 

accuracy of the geologic and geophysical interpretations can be checked by engineering calculations. 

For example, for a given landslide mass to be able 

to move at a known slope angle, it must have a 

cohesion and internal friction that can be estimated 

within certain mathematical limits. The driving 

force must be greater than the restraining force
LOVELAND BASIN LANDSLIDE, COLORADO

and is limited by the observable dimensions of the 

landslide. Therefore, the interpretations of geophysical results are tempered by data or by estimates of 

geometric and mechanical properties of the landslide mass.

The geology of the Straight Creek Tunnel site, including the area where the Loveland Basin landslide 

developed, was mapped at 1:12,000 in 1962 by the 

authors as part of a research project on the pilot 

bore for the Straight Creek Tunnel. (See Robinson 

and Lee, 1962.) After the development of the landslide, the geology of the landslide area was mapped 

(fig. 2) by planetable methods at 1:1,200.

PHYSIOGRAPHY

The landslide occurred near the head of Clear 

Creek in the high mountains of the Front Range of 

Colorado. Clear Creek valley was occupied by glaciers during the Pleistocene and the valley walls 

were oversteepened by glacial erosion. Morainal and 

talus deposits are common on the valley walls, and 

talus is at the present time still accumulating along 

these slopes. The level of the principal glacial erosion in the valley extended to an altitude of about 

11,500 feet. Above this level the slopes are more 

gentle and are covered by frost-heaved rock and the 

products of solifluction (fig. 3). The annual precipitation occurs principally as snow from January to 

April and yields about 25 inches of moisture. Thundershowers, however, are common in the afternoons 

during the summer months. Timber, principally 

spruce, grows to an altitude of about 11,700 feet, 

but above this altitude vegetation is limited to 

mountain grasses and willows. The top of the landslide was approximately at timberline.

REGIONAL GEOLOGY

The bedrock of the landslide area consists principally of Silver Plume Granite that contains inclusions of metasedimentary rocks. The granite and 

the metasedimentary rocks are of Precambrian age. 

A few augite diorite dikes of probable Tertiary age 

occur about 1 mile west of the landslide.

The landslide is within the northeast-trending 

Loveland Pass fault zone. The fault zone, which is 

about 2 miles wide, consists of numerous faults and 

shear zones that are separated by relatively unsheared rock. Individual faults or shear zones range 

in width from less than 1 foot to 1,000 feet. The 

Loveland Pass fault zone is considered to be an area 

of Precambrian shearing and faulting that was resheared during the Laramide orogeny in a manner 

similar to the reshearing of the faults and shear 

zones described by Tweto and Sims (1963).

BEDROCK

Before the excavation of the east portal area and 

the development of the landslide, outcrops of bedrock in the area of the landslide were few. Small 

knobs of granite and metasedimentary rock cropped 

out in the stream valley just west of what is now 

the central part of the landslide and in the low cliffs 

in what is now the central third of the northeast 

edge of the landslide. Construction of the cut near 

the toe of the landslide exposed granite in the upper 

part of the cut face (fig. 4) and to the east and 

south of the cut where an excavation had been made 

for the approach road. The center of the cut exposed altered metasedimentary rock. Geologic mapping in the vicinity of the tunnel site (Robinson 

and Lee, 1962) revealed that the bedrock in the tunnel area consisted of about 75 percent granite and 

about 25 percent metasedimentary rocks; the percentage of the two rock types probably is similar in 

the landslide area. The metasedimentary rocks 

occur as inclusions in the granite which range in 

maximum dimension from less than 1 foot to about 

1,000 feet.

METASEDIMENTARY ROCKS

The metasedimentary rocks in the landslide area 

consist of several types of fine-grained biotite gneiss 

which are typically interbanded with granitic material. The most common varieties of the gneiss are 

biotite-quartz-microcline gneiss and biotite-quartzplagioclase gneiss. The rocks have a distinct foliation as a result of the concentration and orientation 

of the minerals in layers. The layers range in thickness from about one-tenth of an inch to several 

inches. Layers of granitic material principally 

quartz and microcline of irregular thickness occur 

in the metasedimentary rocks. These layers commonly constitute 25 percent of the rock.

The metasedimentary rocks are commonly altered 

 more so than the granitic material. Most of the 

minerals, with the exception of quartz, in these 

rocks are altered to clay over a distance of less than 

,1 inch to several inches on each side of joints and 

faults. Where large bodies of metasedimentary rock 

are within a shear zone, the rock is commonly altered to masses of chloritic clay in which the foliation, though contorted, can still be recognized. The 

interlayered granitic material is generally less altered and forms layers of sheared, but relatively 

competent, rock within the clay mass.
GEOLOGICAL INVESTIGATIONS

FIGURE 3. Unfinished cut near east portal of the Straight Creek Tunnel prior to development of landslide. Depth of cut is

about 150 feet.

GRANITE

Petrographic analyses of samples of the granite 

collected during the investigation of the Straight 

Creek Tunnel site show that the granite typical for 

the area consists of about equal percentages of microcline, plagioclase feldspar (sodic oligoclase), and 

quartz and 1-15 percent biotite. Muscovite is a 

common accessory mineral. Other accessory minerals are apatite, zircon, magnetite, and pyrite. The 

size of mineral grains ranges from less than 1 mm 

to 10 mm; the average grain size is about 3 mm.

The microcline grains in the granite are commonly subparallel; this orientation gives the rock an 

indistinct foliation. Most of the biotite is not oriented ; however, in biotite-rich varieties of the granite, the orientation is parallel to the microcline 

grains. The foliation of the granite is believed to be 

the result of flow at the time of the rock's emplacement. Microshearing of the grains locally imparts 

an indistinct cataclastic foliation. In the granite 

outcrops along a stream west of the landslide, feldspar grains have been cut and offset by many microfractures, and biotite grains are alined parallel 

with the microfractures. These features give the 

rock a cataclastic foliation that is generally not parallel to the flow foliation.

The mineral grains of the granite are commonly 

altered adjacent to faults and joints, but their alteration is not as conspicuous in the landslide area as 

in other areas near the Continental Divide to the
LOVELAND BASIN LANDSLIDE, COLORADO

FIGURE 4. Cut near east portal of Straight Creek Tunnel after start of landslide.

west. The ferromagnesian minerals and the feldspars are commonly partially altered to clay minerals out from faults and joints to distances ranging 

from less than an inch to several inches. In this area 

there is no evidence of sulfide mineral deposition; 

therefore the authors believe that the alteration is 

primarily the result of the action of ground water 

rather than of hydrothermal solution.

SURFICIAL DEPOSITS

The landslide surface is mantled by a thin layer 

of morainal material, soil, and talus (fig. 2). Morainal materials occur along the west side, across 

the southeast end, and to the east of the landslide. 

The geophysical investigations indicated that the 

morainal material along the west side of the landslide was deposited, in part, in a preglacial valley. 

The morainal material ranges in thickness from less 

than 1 foot to about 15 feet and consists of an unsorted mixture of clay, sand, gravel, and boulders. 

In general, it is well compacted.

The higher parts of the landslide to the east are 

mantled by less than 1 foot to about 3 feet of soil 

mixed with talus. The soil is sandy and silty and, in 

the forested areas, is masked by an inch or so of decayed organic material and evergreen needles. 

Mixed with this soil in varying amounts are talus 

blocks ranging in largest dimension from less than 

1 inch to about 3 feet. Talus forms a small cone 

below the steep slope on the northeast margin of 

the landslide between altitudes of 11,500 and 11,575 

feet.

STRUCTURE

The extent of the Loveland Basin landslide is 

principally controlled by preexisting faults that 

form a wide shear zone. The shear zone in the area 

of the landslide was previously mapped by Robinson
GEOLOGICAL INVESTIGATIONS

FIGURE 5. Landslide fault scarp at the northwest border of the landslide. Note striations.

and Lee (1962) as about 600 feet in width and 

about 4,000 feet in length. It trends about N. 15° E. 

and is terminated on the west by the fault that 

forms the western boundary of the landslide. This 

fault can be traced for more than half a mile and 

extends northwest of the northern end of the landslide as it existed in 1965 (fig. 2).

The northeastern boundary of the landslide was 

determined by two preexisting en echelon faults. 

The northern fault was not recognized before the 

development of the landslide. The scarp of the 

southern fault was exposed in the vicinity of the 

curve in the fault near the northeast boundary of 

the landslide (fig. 2). Before 1963, this scarp was 

about 6 feet high and the fault surface showed silicified slickensides with striations bearing S. 25° E.

and plunging 25°, indicating that the south side 

was downdropped relative to the north side.

The landslide was outlined by landslide fault 

scarps and thrusts as of 1965. The scarps occur 

where the landslide mass has been displaced downward. The thrusts occur where the landslide mass 

has moved up and over the preexisting land surface.

The landslide fault scarps are on the western and 

northeastern sides of the landslide. At the time of 

geologic mapping, the heights of the scarps were 

measured and are shown in figure 2 by the number 

adjacent to the scarps. The heights ranged from 

less than 1 foot to 11 feet. The dip of the scarps 

and the bearing and plunge of striations on the 

scarp surface were also measured and are shown in 

figure 2. For several years after the geologic map462-882 O - 72 - 3
10 LOVELAND BASIN LANDSLIDE, COLOEADO

ping was completed in 1963, the heights of the 

scarps increased, and weathering removed the striations. The striations on the scarp surface near the 

northwest corner of the landslide (fig. 5) were well 

formed.

The landslide thrust faults occur principally 

along the southeast margin of the slide and in the 

interior of the slide near the southwest margin. The 

heights of the thrusts range from less than 1 foot to 

about 6 feet (fig. 2). Rolls in the soil with amplitudes of about 1 foot exist in the landslide mass adjacent to the thrusts. During the initial period of 

landslide movement the thrusts were connected to 

the scarps by open fractures that had little or no 

vertical displacement. These thrusts were the most 

spectacular discernible feature of the landslide during the early days of the investigation. At their 

margins flowers and small trees could be seen in the 

process of being overturned and covered by the 

mass of moving soil and rock of the landslide. A 

photograph of the thrust sheet near the southeast

FIGURE 6. Landslide thrust fault at southeast edge of landslide. Note destruction of plant life as landslide advanced.

corner of the slide (fig. 6) shows a small tree being 

pushed over by the landslide mass.

During the early period of landslide movement 

(1963), the ground surface in the central part of 

the landslide showed little evidence that the mass 

was moving. However, near the west margin and 

upslope (northwest) from the main thrust, there 

developed some relatively small scarps and thrusts 

and some zones of small fractures that locally cut 

across the landslide mass. Along the southwest margin of the landslide a zone was formed that is 

bounded by a generally parallel line of scarps, open 

fractures, and a thrust. In this zone the landslide 

mass was considerably disturbed, and probably the 

greatest amount of movement has taken place there. 

Upslope from the southeastern marginal thrust is 

another zone of scarps and open fractures that are 

probably the result of tension caused by the development of the thrust. Associated with the other 

marginal scarps and thrusts, and locally in the middle of the landslide mass, are zones of minor fractures. The fractures in these zones range in length 

from a few inches to several feet and in width from 

less than 1 inch to about 3 inches. Few of these 

minor fractures show any relative vertical displacement.

The magnitude, orientation, and location of the 

scarps, thrusts, and fractures indicate that the landslide did not move as a single homogeneous mass, 

but rather as several units, each somewhat dependent upon the others. The principal movement was 

parallel to the long axis of the landslide, but the 

southwest side of the slide moved faster, or more, 

than the east side. The lower part of the east side 

probably moved more than the main mass, as indicated by the fractures to the northwest of the 

thrust. After completion of the geologic mapping, 

survey points were established by J. D. Post, of the 

Colorado Department of Highways, and records 

were maintained as to the relative amount of movement of different parts of the landslide. This work 

is discussed in chapter D.
Electrical Resistivity Investigations

By R. WOODWARD MOORE

GEOLOGICAL, GEOPHYSICAL, AND ENGINEERING INVESTIGATIONS 

OF THE LOVELAND BASIN LANDSLIDE, CLEAR CREEK COUNTY, COLORADO, 

1963-65

GEOLOGICAL SURVEY PROFESSIONAL PAPER 673-B
CONTENTS

Page

Abstract ________________________________________________ 11

Introduction ______________________________________________ 11

Theory and interpretive methods _________________________________ 11

Test results ______________________________________________ 13

Summary _____________________________________________ 14

II
GEOLOGICAL, GEOPHYSICAL, AND ENGINEERING INVESTIGATIONS OF THE LOVELAND 

BASIN LANDSLIDE, CLEAR CREEK COUNTY, COLORADO, 1963-65

ELECTRICAL RESISTIVITY INVESTIGATIONS

By R. WOODWARD MOORE 2

ABSTRACT

A rapid subsurface survey, involving 26 earth-resistivity 

depth determinations, was made primarily on two longitudinal traverses along the Loveland Basin landslide mass. These 

measurements, involving two field parties and a 3-day period 

in July 1963, yielded preliminary information on the probable 

location of the active slip surface, or zone of slip surfaces. 

From this information a tentative estimate was obtained of 

the volume of unstable material; approximately 500,000 

cubic yards of material was indicated either to be moving 

or to be potentially unstable. The data from this investigation, together with information from 12 subsequent resistivity depth determinations on a third longitudinal traverse, 

were used in the subsequent comprehensive geological, geophysical, and engineering landslide analysis.

INTRODUCTION

Landslides have plagued construction work wherever man has disturbed nature's balance by making 

excavations or by destroying stabilizing conditions. 

Determining the zone of slippage in an active landslide by means of borings and test pits can be time 

consuming and costly. Moving heavy equipment to 

drill sites for testing constitutes a difficult, if not 

insurmountable, problem in some areas, particularly 

when time is short. Use of light weight portable 

geophysical equipment, therefore, is generally advantageous in making a rapid reconnaissance of 

subsurface conditions. Use of the earth-resistivity 

method for subsurface exploration seems to have 

merit for such preliminary surveys of landslide conditions.

In demonstrating the usefulness of the resistivity 

technique the Bureau of Public Roads has made 

tests of 44 landslide areas in 19 States (Moore, 

1954, 1957, 1958, 1961a, b; Trantina, 1963). For 

most areas only a single section taken through the 

approximate center of the moving mass was studied. However, at one location in Ohio in 1951, and

2 Formerly highway research engineer, U.S. Bureau of Public Roads.

at two locations in Tennessee in 1961, more complete surveys of relatively large landslide areas 

were made. The successful results obtained in these 

investigations by the resistivity method prompted 

the use of that method at the Loveland Basin landslide.

Two resistivity parties were made up of Colorado 

Department of Highways and Bureau of Public 

Roads personnel. During 3 days, beginning on July 

17, 1963, the two groups made resistivity-depth 

tests at 26 locations along two sections (A and B in 

fig. 7) spaced about 200 feet apart and passing 

through the slide area. Subsequently, an additional 

12 measurements conducted by Bureau of Public 

Roads personnel were made along a third section (C 

in fig. 7).

THEORY AND INTERPRETIVE METHODS

The earth-resistivity measurement is made by 

passing electrical current through the ground and 

measuring the resistance of earth materials to current flow. This current flow is largely electrolytic 

and is dependent on the moisture and dissolved salts 

within soils and rocks for direction of its path 

(Tagg, 1964, p. 5).

A Wenner configuration, consisting of four 

equally spaced electrodes with the center of the system set at a fixed point (fig. 8), was used in this investigation. The outer two electrodes, d and c2 , pass 

current into the ground, and the difference in potential due to this current is measured between the 

inner two electrodes PI and p2 (Howell, 1959, p. 

371).

As the spacing of the electrodes increases, 

greater amounts of underlying material are influenced. The average measured (apparent) resistivity 

will trend toward the resistivity of the underlying 

layer. The resulting average resistivities are plotted

11
12 LOVELAND BASIN LANDSLIDE, COLOKADO

EXPLANATION

A__\ ____,__A 

i i

Seismic line and shotpoints

o B 0

Resistivity line and stations

CONTOUR INTERVAL 5 FEET 

DATUM IS MEAN SEA LEVEL

Prelandslide topography by Continental 

i Engineers Inc., Denver, Colo., based on 

aerial photographs taken September 1961 

and modified to show landslide, August 

1963; base further modified and generalized by C. S. Robinson and F T. Lee, 1964

FIGURE 7. The Loveland Basin landslide showing location of geophysical traverses.
ELECTRICAL RESISTIVITY INVESTIGATIONS 13

BATTERY AMMETER

o

FIGURE 8. Principle of earth-resistivity measurement using 

the Wenner configuration ci, cz, current electrodes; pi, pz, 

potential electrodes; a, electrode separation.

against the electrode separations, resulting in a 

curve. Figure 9 shows a typical resistivity curve 

which was obtained by expansion of the electrode 

system to include the depths shown.

From the earliest days of surveying by resistivity 

technique, the interpretation of the curves has been 

a matter of great concern and, indeed, of controversy. Two schools of thought exist regarding the 

interpretation of resistivity curves. The first of 

these schools is primarily empirical and is based 

partly on experience. The second is primarily based 

on theory and employs several methods which make 

use of theoretically derived depth curves.

Whatever the interpretive method employed, pertinent geological and additional pertinent geophysical data are necessary to explain changes in resistivity with depth. When calibration tests can be

30,000

25,000

£ 20,000 ~i

t p

^ £ 15,000

o: o

LU u_ 10,000

> o

£ 2000 -

- > E 1500 -

^ 1000 -

30 40 50 

DEPTH,IN FEET

FIGURE 9. Resistivity-depth curve (dashed line) for talus 

overlying weathered bedrock and cumulative resistivity 

curve (solid line) used to empirically determine depths to 

subsurface layers. Depth, in feet, of effective current 

penetration is assumed to equal the surface electrode 

spacing.

made over known subsurface conditions, resistivity 

changes can be conclusive. A major drawback with 

the theoretical analyses is that they assume ideal 

conditions; that is, uniform isotropic layers with interfaces parallel to the ground surface. The advantage in empirical interpretations is that they endeavor to treat the real, and always anisotropic, 

situation. The author attempted to use theoretical 

depth curves to analyze the data obtained from previous near-surface studies performed by the Bureau 

of Public Roads. The results were generally discouraging owing to the frequency with which the field 

conditions failed to be adequately described by the 

results obtained by use of the theoretical curves.

The author favors the empirical interpretation of 

depth curves on the basis of over 30 years' experience which yielded largely successful results; this 

method, however, also has shortcomings. Tagg 

(1964, p. 21) showed that for an ideally isotropic 

material the current penetration varies with depth 

but is always' greater than the electrode spacing. 

However, for anisotropic heterogeneous material 

there is closer correspondence between depth of current penetration and distance of electrode spacing. 

As Barnes (1952, p. 31) pointed out, current flow 

follows an irregular path and is governed by the 

anisotropies of natural materials. Such irregular 

paths could shorten the effective depth penetration 

of current flow in anisotropic material. Thus the 

electrode spacing could nearly equal, if not equal, 

the depth of current penetration. Studies conducted 

to correlate electrode spacing with depth of current 

penetration indicated that, for depths above 100 

feet, depth of current penetration is nearly equivalent to the distance between adjoining electrodes. 

Malott (1965) found that the results of 34,000 shallow resistivity soundings and of more than 4,000 

correlation borings indicated that the electrode 

spacing is equal to the depth of investigation (current penetration).

In an attempt to show more clearly the geologic 

boundaries on depth curves the author (Moore, 

1945) introduced the cumulative resistivity method. 

In the present report both the individual value 

curves and the cumulative curves are shown. The 

cumulative method has been most successful in 

areas containing a sand and gravel deposit having a 

shallow water |;able and overlying clay.

TEST RESULTS

Resistivity measurement is a logical technique for 

use in studies of landslides in which excess water is
14 LOVELAND BASIN LANDSLIDE, COLORADO

likely to be associated with the zone of sliding. 

When porous talus material and sheared bedrock 

slide out over less pervious material such as undisturbed sheared bedrock, one would expect evidence 

in the plotted resistivity curve of the effect of the 

trapped excess water at the base of the disturbed 

material. The change in slope shown in the cumulative resistivity curve of figure 9 at a depth of about 

35 feet is an example of the type of effect to be expected when the less pervious deeper material is 

less resistant to current flow than the overlying 

slide material (Moore, 1957). Although no confirming borings had been made in this slide area as of 

1965, the 35-foot depth shown in figure 9 correlated 

well with the depths at which slope changes were 

detected in eight other depth tests made along a section through the center of the moving mass. In the 

tests to be discussed, such slope changes were taken 

as evidence of the probable slip zone.

Figure 10 shows three resistivity depth curves 

considered to be rather typical for the 38 depth 

tests that were made over the slide area. Slope 

changes are present in each curve at depths believed 

to coincide with the elevation at the top of the slip 

zone. Slope changes similar to those appearing in 

the three curves of figure 10 at depths of about 27, 

25, and 16 feet were observed in most of the resistivity-depth curves plotted for the 38 tests.

The cross section shown in figure 11 is typical of 

the three sections plotted from the data obtained in 

the several tests, showing the relation of the existing ground surface to the inferred position of the 

slip surface. The three lines of tests from which the 

three sections were prepared were spaced at intervals of 200-300 feet across the slide area from east 

to west.

Figure 11 shows all the significant subsurface resistivity changes along line A. The longer the 

dashed lines, the more laterally persistent were the 

resistivity layers. The most significant and the 

most persistent resistivity layers are shown by the 

heavier dashed lines. These heavier lines also indicate the location of the probable slip surface. The 

author believes that the subsurface conditions at the 

depths indicated by these lines are the weak zones 

that are the most continuous and therefore the most 

susceptible to movement. Near the toe of the slide 

two such zones or surfaces were defined from the 

resistivity measurements. Therefore, the author believes that probably two separate slip surfaces were 

present, one above the other, in this part of the 

slide.

These data were made available to the Colorado 

Department of Highways and the U.S. Geological 

Survey and were used by Robinson, Carroll, and 

Lee (1964) to prepare the maps of the landslide and 

to calculate the volume and mass of the landslide.

SUMMARY

The resistivity method has considerable merit for 

use in landslide studies. Its use in this study, as 

well as in many other tests made over other landslide areas, has served worthwhile purposes in providing, rapidly and economically, an approximation 

of the total quantities of material undergoing movement and in suggesting depths to which corrective 

measures must be made in attempts to control fur18,000

16,000

14,000

£ 12,000

i 

1 10,000 t z.

V 8000

x

o

u_ 6000

o

4000

2000

0

20 30 40 50 60 

DEPTH,IN FEET

TEST 3 

RESISTIVITY LINE B

I 1I I T

TEST 3 

RESISTIVITY LINE C

50 60 70 0 10 

DEPTH. IN FEET

20 30 40 50 60 70

FIGURE 10. Resistivity-depth curves (dashed lines) and cumulative resistivity curves (solid lines) typical of data 

obtained on Loveland Basin landslide area. Circled zone is believed to indicate effect of water just above slip surface. Depth, in feet, of effective current penetration is assumed to equal the surface electrode spacing.
ELECTRICAL RESISTIVITY INVESTIGATIONS 15

11,700

11,600'-

11,500'

11,400' 

11,300

11,200' 

11,100' 

11,000'

\ \ \

Resistivity changes at depth<^\ \ \ \

\ \.\\VA8

FIGURE 11. Section through east part of sliding mass (resistivity line A) Loveland Basin landslide. A1-A13, 

resistivity-depth test locations. Line of section shown in figure 7. (Vertical exaggeration, X 2.)

ther sliding. When used in conjunction with other 

geophysical techniques, such as the seismic refraction method, and with available geologic information, the resistivity method helps to provide more 

complete subsurface data and to select places where 

boring results should prove most profitable.

462-882 O - 72 - 4
Seismic Refraction Studies

By RODERICK D. CARROLL, JAMES H. SCOTT, and FITZHUGH T. LEE

GEOLOGICAL, GEOPHYSICAL, AND ENGINEERING INVESTIGATIONS 

OF THE LOVELAND BASIN LANDSLIDE, CLEAR CREEK COUNTY, COLORADO,

1963-65

GEOLOGICAL SURVEY PROFESSIONAL PAPER 673-C
CONTENTS

Page

Abstract _________________________________________________ 17

Introduction ______________________________________________ 17

Method _______________________________________________ 17

Results and conclusions ______________________________________ 18

II
GEOLOGICAL, GEOPHYSICAL, AND ENGINEERING INVESTIGATIONS OF THE LOVELAND 

BASIN LANDSLIDE, CLEAR CREEK COUNTY, COLORADO, 1963-65

SEISMIC REFRACTION STUDIES

By RODERICK D. CARROLL, JAMES H. SCOTT, and FITZHUGH T. LEE

ABSTRACT

Seismic refraction measurements were made in July 1963 

along 13 lines in the area of the Loveland Basin landslide. 

Velocity layering detected on the landslide consists of a nearsurface layer exhibiting compressional velocities from 1,000 

to 2,700 fps (feet per second) and an overlying 3,000 to 

6,500-fps layer. Thickness of the low-velocity layer ranged 

from 8 to 66 feet, tending to be greater in the central and 

western parts of the slide. The low-velocity layer probably 

represents the material above the top of the slip zone, the 

volume of which was 440,000 cubic yards. Severe energy 

attenuation and complex arrival times on several lines 

rendered the seismic interpretation uncertain at several 

locations when used without other criteria to aid in the depth 

determination.

INTRODUCTION

The U.S. Geological Survey undertook a seismicrefraction investigation of the Loveland Basin landslide in July 1963 to delineate the depth of unstable 

material. The seismic refraction method of investigation provided a third independent method of obtaining an estimate of the geometry and volume of 

the material involved in the landslide.

Published data on the delineation of landslides by 

seismic methods are meager. Trantina (1963) reported on two landslides in California that had 

depths to the slip plane of approximately 12 and 23 

feet; these slides were successfully delineated by the 

use of a sledge hammer as an energy source and by 

means of seismic-refraction techniques. In these reported landslides the slip surface represented a 

well-defined geologic and seismic boundary between 

two different materials; this boundary enabled excellent resolution of arrival times. However, geologic conditions in the Loveland Basin landslide 

area differ in that the slip surface or slip surfaces 

are associated with very incompetent zones within 

an inhomogeneous and moderately incompetent

mass of material. Consequently, the delineation of 

the slip zone by the detection of seismic contrasts in 

the media can be expected to be strongly dependent 

on localized geologic conditions.

METHOD

Because of the high altitude, steep slopes, and inaccessibility to motor transport, portability was a 

major consideration in the selection of seismic 

equipment; consequently, a portable seismic instrument (Portaseis ER-75) was used. Seismic signals 

were recorded on Polaroid film. The entire recording apparatus was contained within a 24-pound 

package. Standard seismic cables and detectors 

were used and 12 geophones were spaced at 25-foot 

intervals. Measurements were made along 13 

seismic lines in the landslide area (fig. 7). Shotpoints were located at distances of 25 and 50 or 25 

and 100 feet from the ends of the lines. On several 

lines a shotpoint was located at the midpoint of the 

line to obtain more information on variations in the 

overburden velocity.

Holes for the emplacement of the seismic charges 

were driven with a punch bar; and charges, averaging five sticks of dynamite (40-percent gel) per 

hole, were placed at depths of 1.5-3 feet. Energy 

input to the ground ranged from good to poor, depending on the nature of the near-sur/ace material. 

A typical record obtained on line N is shown in 

figure 12. The aerated and disturbed condition of 

the overburden material above the slip zone yielded 

poor first arrivals on many lines. Several lines 

crossed fracture zones visibly open at the surface to 

depths of a few inches to several feet. The poor energy-transmission characteristics of the overburden 

would have made sledge hammer refraction methods 

unsuccessful.

17
18 LOVELAND BASIN LANDSLIDE, COLORADO

Shot break

Time

FIGURE. 12. A part of seismic record obtained on seismic 

line 2V. Charge was five sticks of dynamite at 3-foot depth. 

Interrupted segments of traces represent time intervals of 

10 milliseconds.

Interpretation of the results obtained on several 

lines was hampered both by severe attenuation of 

high-frequency energy and by weak energy in the 

first arrivals. Interpretation was further complicated by the occasional presence of an irregular refracting surface as well as abrupt velocity variations within the near-surface layer. These two 

features were sometimes inseparable and caused 

considerable scatter of points on several traveltime 

plots. Two examples, typical of the extremes of difference in the traveltime plots, are shown in figure 

13.

Traveltime curves for line K shown in figure 13 

are such that a unique interpretation may be made 

of the thickness of the low-velocity layer in this 

area. The traveltime curves for line D, on the other 

hand, exhibit the effects of complex subsurface conditions. On several other lines, especially those parallel with the long axis of the landslide, traveltime 

curves indicated complex seismic ray paths. One 

possible explanation for these complex paths is the 

presence of relatively competent zones of high velocity within the low-velocity layer. These competent zones may possibly act as isolated buttresses 

within the sliding mass, inhibiting the development 

and movement of the slide.

100

100 200 

DISTANCE, IN FEET

300

FIGURE 13. Time-distance graphs obtained on seismic lines 

K and D. Location of lines shown in figure 7.

RESULTS AND CONCLUSIONS

The results of the seismic refraction survey indicate that, in general, two velocity layers exist 

within the landslide an upper layer of soil and disturbed incompetent bedrock in which the velocity 

ranges from 1,000 to 2,700 fps (feet per second) 

overlying a layer which has a velocity ranging 

from 2,000 to 6,500 fps and which probably represents less disturbed but incompetent bedrock. An 

exception to this generalization occurred on line F 

in the plaza area. Velocities in the overlying material along this line ranged from 5,500 to 6,700 fps 

to a depth of about 30 feet. The material composing
SEISMIC REFRACTION STUDIES 19

the underlying high-velocity layer was characterized 

by velocities in the range of 14,000 to 15,000 fps; 

these suggest the presence of firm, granitic bedrock. 

Subsequent core drill data furnished by the Colorado Department of Highways substantiated this 

interpretation.

Thickness of the low-velocity layer of the slide 

ranges from 8 to 66 feet (fig. 14). This layer is thin 

in the eastern and southern parts of the slide and 

thick in the western and central parts. Interpretation of the seismic data in the vicinity of the old 

preglacial stream channel on the western margin of 

the slide was hampered by velocity variations in the 

subsurface bedrock and in the surficial material. 

The data suggest, however, that the low-velocity 

layer was thickest near the western edge of the 

slide.

The thickness of the low-velocity layer (fig. 14) is 

considered to represent the probable minimum 

amount of material involved in landslide movement.

The volume of this material was calculated to be 

440,000 cubic yards. Data from other investigative 

methods used in the study of the landslide suggest 

that a greater volume of material may be involved 

than is indicated by the seismic data. At many locations the seismic data were in general agreement 

with estimates made by other means.

The results of this investigation indicate that the 

seismic refraction method is helpful in delineating 

landslides of a complex nature similar to the one 

discussed in this report. At some locations, however, 

the reliability of the interpretation of seismic data 

is doubtful without independent criteria for checking the thickness of the sliding mass. By using several independent criteria for estimating the geometry of the sliding mass in this instance, seismic 

and electrical resistivity soundings coupled with detailed geologic mapping a greater measure of confidence can be achieved in making calculations.

11,200'-

11,100'-

11,000

LONGITUDINAL SECTION

FIGURE 14. Longitudinal and transverse sections of the Loveland Basin landslide showing thickness of low-velocity layer.

Location of sections shown in figure 2.
Engineering Investigations

By JOHN D. POST and CHARLES S. ROBINSON

GEOLOGICAL, GEOPHYSICAL, AND ENGINEERING INVESTIGATIONS 

OF THE LOVELAND BASIN LANDSLIDE, CLEAR CREEK COUNTY, COLORADO,

1963-65

GEOLOGICAL SURVEY PROFESSIONAL PAPER 673-D
CONTENTS

Page

Abstract ______________________________________________ 21

Introduction ___________________________________________ 21

Surveys _________________________________________________ 21

Core holes in plaza _______________________________________ 24

Drainage _____________________________________________ 24

Conclusions ____________________________________________ 25

II
GEOLOGICAL, GEOPHYSICAL, AND ENGINEERING INVESTIGATIONS OF THE LOVELAND 

BASIN LANDSLIDE, CLEAR CREEK COUNTY, COLORADO, 1963-65

ENGINEERING INVESTIGATIONS

By JOHN D. POST* and CHARLES S. ROBINSON

ABSTRACT

After recognition of the Loveland Basin landslide on June 

28, 1963, the Colorado Department of Highways established 

control points around and on the landslide mass in order to 

observe the amount and rate of movement. Measurements 

between pairs of these points established that different parts 

of the landslide were moving at different rates and that the 

chief movement was during the summer months. To define 

the toe of the landslide and to measure its movement, a survey grid was established in the plaza area to the south of 

the landslide. The records of measurements of the grid 

showed that the toe of the landslide was restricted to the 

northwest corner of the plaza area.

Two core holes were drilled and logged south of the landslide to determine if a slip plane existed below the plaza 

area. Plastic pipe was packed into these holes and the alinement of the pipe was checked periodically.

To drain surface and ground water from the landslide, 

five near-horizontal holes were drilled in the west margin of 

the slide. The water from two springs was collected and 

piped off the slide, and an 18-inch pipe was placed in an 

area of unusually heavy snow accumulation to conduct the 

melt water from the landslide.

INTRODUCTION

The Colorado Department of Highways, upon recognition of the landslide and after conferences 

with the U.S. Bureau of Public Roads and the U.S. 

Geological Survey, established surveyed reference 

points on and around the Loveland Basin landslide 

to determine the amount and direction of movement 

of the landslide. Subsequently, two holes (fig. 2) 

were cored in order to detect any landslide movement near the site of the portal of the Straight 

Creek Tunnel pilot bore. In addition, efforts were 

made to drain the accessible parts of the landslide 

mass. Many persons of the Colorado Department of 

Highways participated in this work. In general, 

however, the department's work was under the 

direction of the senior author, who made periodic

4 Colorado Department of Highways.

records of the movement of the landslide through 

1966.

SURVEYS

Early in July 1963, a transit and chain survey 

was run around the periphery of the slide. After 

this survey was run, 15 pairs of transit and tape 

measurement points were established at or within 

the slide margin, and 32 other points, to be observed periodically by transit, were established 

across the slide mass. The locations of some of the 

pairs of transit and tape measurement points across 

the breaks in the slide are shown in figure 15. 

Figure 16 shows records of the relative change in 

distance between the pairs of points between July 

1963 and January 1965.

The measurement points were established after 

the major movement of the landslide had occurred; 

consequently, the total vertical change in distance 

between a pair of measurement points can only be 

approximated by adding the vertical change as 

measured since July 1, 1963, to the height of the 

scarps as shown in figure 2. A systematic schedule 

of observations of the measurement points could not 

be made throughout the year because of the deep 

winter snows. As a result of the movement and 

breakup of the landslide mass, some points were destroyed and had to be reestablished (new points, fig. 

16). The records show, however, that both the horizontal and the vertical movements of the slide mass 

were greatest between July and November of 1963. 

Movement continued, principally during the summer months, in 1964 and 1965. A buttress load was 

placed on the toe of the slide in September and October 1963. Much of the slide mass was frozen during the winter and spring months (1963-65), and 

little movement took place. Noticeable movement occurred on the east and west sides of the slide in 

June and July 1964, and then decreased as the

21
22 LOVELAND BASIN LANDSLIDE, COLORADO

EXPLANATION

Landslide fault scarp Location of survey grid point in plaza

Tape and transit measurements

Landslide thrust fault °

Sawteeth on upper plate Location of horizontal movement point

_ _ _ _ on bench above plaza area

 -~-=- =~^=^ Tape measurements

Zone of minor fractures

.DDH 2

Diamond-drill hole

7 < .Location of tape and transit measurement line and pairs of points

Ap p roxi m ate 

3-inch drill hojepj (ft

CONTOUR INTERVAL 5 FEET 

DATUM IS MEAN SEA LEVEL

FUNNEL ^iLOl

REFERENCE POINT

Prelandslide topography by Continental 

Engineers Inc., Denver, Colo., based on 

aerial photographs taken September 1961 

and modified to show landslide, August 

1963; base further modified and generalized by C. S. Robinson and F. T. Lee, 1964

FIGURE 15. The Loveland Basin landslide; the location of the measurement points at the margins of the landslide and the 

approximate locations of the surface and subsurface drainage pipes are shown. Prepared by J. E. Gay, Colorado Department of Highways.
ENGINEERING INVESTIGATIONS 23

4.0

u. 3.5

o

3.0

2 '5

2.0 

- 1.5

1.0

0.5 -

More than one horizontal 

measurement plotted

-1.2

-1.4

JULY AUG SEPT OCT NOV DEC JAN FEB MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC JAN FEB MAR APR MAY JUNE JULY AUG 

1963 1964 1965

FIGURE 16. The relative vertical and horizontal changes in distance between pairs of points outside and within the landslide mass. Locations of surveyed lines are shown in figure 15. Dashed lines indicate inferred change.
24 LOVELAND BASIN LANDSLIDE, COLORADO

water table lowered. The central part of the slide 

did not move as much as the margins which were 

deformed more during the original movement.

On July 11, 1963, six points were established on 

the first cut bench above the plaza area. The horizontal distances from these points to a reference 

point south of the landslide area were established 

with a 500-foot chain at a tension of 60 pounds. The 

approximate locations of these points are shown in 

figure 15. Measurements between these points and 

the reference point were made between July 11 and 

September 3, 1963. The total horizontal movements 

of these points during this period are shown in the 

following table.

Horizontal movement of points on bench above plaza area, 

July 11 to September 3,1963

[500-ft chain under 60-lb tension]

Point

2

3

4

5

6

7

Total 

horizontal 

movement 

(feet)

0.02

2.97

2.65

.05

.03

.02

These measurements show that the horizontal movement of the cut bench at the front of the landslide 

during this period was chiefly restricted to the 

northwest side (points 3 and 4) and that the maximum movement averaged about 0.06 foot per day.

At the start of the landslide investigation the location of the toe of the landslide in the plaza area 

was considerably in doubt. This uncertainty was 

partly because the contractor was continuing to 

lower the grade in the plaza area. A survey grid 

was established in the northwest corner of the plaza 

to determine the limit of the movement of the landslide so that the portal of the Straight Creek pilot 

bore could be relocated in stable ground. Figure 

17A shows the location and original dimensions of 

the survey grid, which is shown in figure 15. The 

points of the grid were x's on steel caps threaded 

on 1%-inch steel pipes grouted into holes drilled 

into the bedrock of the plaza floor. The grid system 

was completed on August 12, 1963, and daily horizontal and vertical measurements (referenced to a 

point south of plaza, fig. 15) were made from August 12 to August 23, 1963. Figure 17 shows the results of the initial and final horizontal and vertical 

measurements of the grid system. It should be noted

that point Ml, in the northwest corner of the plaza, 

moved upward 0.23 foot over a period of 11 days. 

Point O3 in the southwestern part of the plaza 

moved 0.24 foot upward in the same time.

CORE HOLES IN PLAZA

From August 5 to 10, 1963, two core holes were 

drilled and logged in the plaza area (figs. 2, 15). 

These holes were drilled to determine if a slip surface of the landslide existed below the plaza level or 

if a slip surface developed below the plaza with 

time.

The holes were drilled with an NX-size diamond 

bit. Hole 1 was drilled in competent granite to a 

total depth of 90.0 feet. It was cased, after reaming, 

for the first 10 feet with uncemented 4-inch casing. 

Hole 2 was drilled to a depth of 82.6 feet, also in 

competent granite, and was left uncased. (See chap. 

C, seismic line F.) No slip plane was recognized in 

either drill hole. Plastic pipe, 1 inch in diameter, 

was placed in each hole and the space between the 

pipe and the wall of the hole was packed with cinders.

The assumption was that, if any movement took 

place in the rock below the plaza area, the plastic 

tubing in the drill holes would be bent and eventually sheared. In order to test for bending of the 

plastic tubing, a 30-inch-long 3/8-inch-diameter rod 

was lowered on a nylon line to the bottom of the 

holes. These tests were made daily until mid-August, and then at various times until the start of 

construction of the rockfill buttress in the first week 

of September. During this time interval there was 

no indication that the plastic tubing had been bent.

DRAINAGE

In order to lower the water level in the landslide 

and increase stability, as much ground water as 

possible needed to be drained from the landslide 

mass and as much surface water as possible needed 

to be collected and diverted from the landslide. In 

mid-September 1963, five nearly horizontal 3-inch 

holes were drilled into the west side of the landslide 

from a point about 25 feet northeast of the flume 

west of the landslide (fig. 15). The flume had been 

installed during the construction of the cut to divert 

a small stream around the area of the cut. Following is a tabulation of the lengths, bearings, and inclinations of the drill holes and of the initial flows 

from each.
ENGINEERING INVESTIGATIONS 25

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FIGURE 17. The location and initial (A, Aug. 12, 1963) and final (B, Aug. 23, 1963) dimensions of survey grid established in the plaza area southeast of the landslide mass. Vertical 

measurements are shown to the left of the reference points (X). Horizontal measurements 

are shown between reference points. Units are feet. +, lengthened dimension; , shortened 

dimension.

Lengths, bearings, inclinations, and initial flows from drill 

holes in western part of the Loveland Basin landslide

Hole

1

2

3

4

5

Length 

(feet)

97

168

50

- , 60

110

Bearing

N.25°W.

N. 10° E.

N. 40° E.

N.30°E.

N. 16° E.

Inclination

10° SE.

5° SE.(?) co q-w/-

19° QW

91 ° SW

Initial flow 

(gallons per 

minute)

1

0

On £,

2

1 Plugged.

The holes were drilled into bedrock and pipes were 

inserted into the holes for part of their length. In 

addition, the water from two small springs near the 

west margin of the landslide was impounded and 

conducted through pipes to the large flume west of 

the landslide. In July 1964, water was running from 

two of the drill holes, and one of the springs that 

had been tapped was flowing at an estimated rate 

of 8 gallons per minute.

In mid-October 1963, an 18-inch pipe was laid on 

the ground near the west margin of the slide (fig. 

15) to divert surface runoff from snow which collects during the winter months in a depression on 

and above the present head of the landslide. The 

snow accumulation amounts to as much as 21 feet, 

and the snow commonly persists into July of each 

year. The runoff from this area of snow accumulation was anticipated to be collected by the pipe and 

then to be diverted off the landslide to the large 

flume to the west of the landslide. Actually, very little water was diverted because the numerous fractures around the inlet caused the runoff to go underground.

CONCLUSIONS

The engineering investigations of the landslide 

were carried out under difficult conditions. The establishment of control points and the periodic observations of these points were hampered by topography and climate. The measurements made, 

however, proved to be valuable in defining the limits 

of the toe of the slide and in evaluating the effect of 

the buttress load on the toe of the slide. The at-
26 LOVELAND BASIN LANDSLIDE, COLORADO

tempts to drain the surface and ground water from 

the slide were not as successful as had been hoped. 

Because of the topography, much of the slide area 

is not easily accessible to drilling equipment for 

drilling near-horizontal drain holes. Most of the 

precipitation on the landslide is snow, which accumulates to depths of as much as 250 inches. When

the snow melts, very little of this moisture runs off 

the landslide; most of it melts from below and seeps 

into the landslide mass or evaporates. The data 

from results of engineering investigations and observations to September 1963 were available to R. 

A. Bohman and were used in his considerations of 

methods to stabilize the landslide.
Compilation of Results of Geological, 

Geophysical, and Engineering Investigations

By CHARLES S. ROBINSON and FITZHUGH T. LEE

GEOLOGICAL, GEOPHYSICAL, AND ENGINEERING INVESTIGATIONS 

OF THE LOVELAND BASIN LANDSLIDE, CLEAR CREEK COUNTY, COLORADO,

1963-65

GEOLOGICAL SURVEY PROFESSIONAL PAPER 673-E
CONTENTS

Page

Abstract ______________________________________________ 27

Introduction ______________________________________________ 27

Compilation of geological and geophysical data _____________________ 27

Calculations of volume and weight of the landslide ____________________ 28

Conclusions ____________________________________________ 28

II
GEOLOGICAL, GEOPHYSICAL, AND ENGINEERING INVESTIGATIONS OF THE LOVELAND 

BASIN LANDSLIDE, CLEAR CREEK COUNTY, COLORADO, 1963-65

COMPILATION OF RESULTS OF GEOLOGICAL, GEOPHYSICAL, AND

ENGINEERING INVESTIGATIONS

By CHARLES S. ROBINSON and FITZHUGH T. LEE

ABSTRACT

The compilation of data from the geological and geophysical investigations of the Loveland Basin landslide indicates 

that the movement of the landslide mass took place through 

a relatively wide zone in comparison to the total thickness 

of the landslide mass. Isopach and structure contour maps 

prepared from the geological and geophysical data indicate 

that the volume of the landslide above the slip zone is 470,000 

cubic yards weighing 990,000 tons and that the volume of 

the landslide to the base of the slip zone is 770,000 cubic 

yards weighing 1,600,000 tons. It is believed that, in any 

sudden failure of the landslide, the volume and weight of 

material that would be involved would be in amounts between these figures.

INTRODUCTION

Geologic mapping and engineering measurements 

defined the shape, behavior, and composition of the 

Loveland Basin landslide at the surface. Geophysical investigations gave data on the possible composition of the landslide with depth and a probable 

thickness at several points. The determination of 

the remedial measures that should be taken to stabilize the landslide required a calculation of the mass 

and volume of the landslide.

The authors of this chapter made the final interpretation of the geological, geophysical, and engineering data and the calculations of the mass and 

volume of the landslide. A summary of the investigations and the results of the interpretation of the 

data were made available by Robinson, Carroll, and 

Lee (1964) about 4 weeks after the start of the investigations and were the basis for the engineering 

analysis of the landslide and the recommendations 

for the stabilization of the landslide that are presented in chapter F.

COMPILATION OF GEOLOGICAL AND 

GEOPHYSICAL DATA

During the geophysical investigations, numbered 

stakes were placed for the resistivity work at the 

center of each spread and for the seismic work at 

each geophone location and shothole. At the time of 

the geologic mapping, these lines were located and 

plotted on the map (fig. 7). R. Woodward Moore 

furnished the authors with plots of the apparent resistivity and two interpretive sections along the resistivity traverses (lines A, B; fig. 7). Subsequently, 

the authors obtained from R. A. Bohman the plots 

of the apparent resistivities for the third traverse 

(line C, fig. 7). The authors furnished topographic 

profiles along each of the seismic lines (fig. 7), 

which were used to interpret the seismic data. On 

copies of the geologic map that showed the locations 

of all points where geophysical measurements were 

made, the authors plotted the depth information 

supplied by R. Woodward Moore and R. D. Carroll.

A controlling factor in the interpretation of the 

possible depth of the slip surface was the geometry 

of the landslide. From the geologic mapping and the 

engineering measurements, the outlines of the landslide mass and the relative rates of movement of 

different parts of the landslide were known. The dip 

of the scarps and the plunge of the striations on the 

scarp surfaces limited the depth of a possible slip 

surface and defined the direction of movement of 

the various parts of the landslide.

The geometry of different types of landslides has 

been well defined and illustrated by Varnes (1958). 

According to Varnes' classification, the Loveland 

Basin landslide is partly type IIA, a slump, and 

partly type IIB, a debris slide. Slump features are 

best displayed in the upper parts, whereas debris 

sliding and earthflows predominate toward the base.

27
28 LOVELAND BASIN LANDSLIDE, COLORADO

Because of these features and because the Loveland 

Basin landslide consisted of several units moving at 

different rates, all of which were dependent upon 

each other, this landslide could best be described as 

a composite slide.

In order to determine the probable depth of the 

slip surface, the geophysical data, geological data, 

and geometry of the slide were carefully considered 

for each point of measurement. To aid in this consideration, longitudinal and transverse vertical sections were constructed that intersected most of the 

points of measurement. On these sections were plotted the geophysical and geologic data. At several 

points, the geophysical data had indicated more 

than one possible depth to the slip surface. From 

the plots of these data in section and because the 

landslide consisted of a central, virtually unbroken 

mass bordered particularly on the north and west 

sides by a zone of intensely sheared material, a 

conclusion was reached that the movement of the 

landslide was taking place throughout a wide zone, 

or on a series of closely spaced slip surfaces, rather 

than on a single slip surface. Resistivity data at 

several locations indicated subsurface changes at 

greater depths than the depths calculated from the 

seismic velocity data. The different possible depths 

for the slip zone as given by the geophysical data, it 

was presumed, indicated the possible top and bottom of the slip zone. From the comparison and evaluation of the resistivity and seismic data with the 

geologic information and the limiting geometry of 

the slide, a depth for the top and bottom of the slip 

zone was picked for each point of measurement.

CALCULATIONS OF VOLUME AND WEIGHT 

OF THE LANDSLIDE

The depths to the top and bottom of the slip zone 

were plotted on separate copies of the topographic 

and geologic map. With this control, construction of 

isopach maps was then possible for the thickness of 

the landslide above the slip zone and for the thickness of the landslide to the bottom of the slip zone. 

The two isopach maps so constructed are figures 18 

and 19.

The volume and weight of the landslide above the 

slip zone and to the bottom of the slip zone were 

then calculated from the isopach maps. The volume 

was calculated by superimposing a 10x10- 

divisions-per-inch grid over the isopach maps and 

then averaging the thickness for each square inch 

or part thereof and multiplying the thickness by the 

area in square inches. The weight was calculated by 

multiplying the volume, in cubic yards, by a value

of 2.1 tons per cubic yard. The weight per cubic 

yard was determined, assuming complete water saturation, from density and porosity measurements 

and density-of-fracture calculations previously made 

during the study of the geology of the Straight 

Creek Tunnel pilot bore (Robinson and Lee, 1962). 

With the isopach maps plotted on the topographic 

base, structure contour maps of the top and the 

base of the slip zone could be constructed. These 

maps are figures 20 and 21. The greater detail of 

the contours on these maps as compared to the isopach maps is the result of the greater detail of the 

contours on the topographic base. The purpose of 

the structure contour maps was to give the approximate elevation of the top and bottom of the slip 

zone at any point within the landslide mass. These 

maps permit the rapid calculation of the necessary 

length and attitude of any holes drilled to intersect 

the top or bottom of the slip zone for drainage or 

any other purpose.

CONCLUSIONS

The volume of the landslide to the top of the slip 

zone was calculated to be about 470,000 cubic yards 

weighing about 990,000 tons. The total volume of 

the landslide to the bottom of the slip zone was calculated to be about 770,000 cubic yards weighing 

about 1,600,000 tons. The authors believed that, 

should the landslide fail suddenly, the volume and 

weight of material involved would be between these 

two sets of figures.

Figures 18 through 21 are modified from the 

original isopach and structure contour maps made 

available by Robinson, Carroll, and Lee (1964). The 

original maps were compiled on a topographic base 

with a 1-foot contour interval. The contour interval 

has been reduced to 5 feet on the maps included 

with this report. A 5-foot contour interval is more 

realistic in achieving accuracy in the calculated 

probable thickness of the landslide and the elevation 

of the top and bottom of the slip zone at any one 

point. These data on the maps are considered to be 

accurate to within 10 percent.

Longitudinal and transverse sections (fig. 22) 

were prepared from the structure contour maps to 

show the configuration of the slip zone and its relation to the entire landslide and the toe load. The 

central and the western parts of the slide, in general, are deepest. The changes in thickness of the 

slip zone and the sliding mass and the irregularities 

along the base of the slip zone are due mainly to 

variations in surface topography and bedrock competency.
COMPILATION OF RESULTS OF INVESTIGATIONS 29

EXPLANATION

Isopachs 

Interval 10 feet

Point where thickness determined by 

electrical resistivity method

Point where thickness determined by 

seismic refraction method

CONTOUR INTERVAL 5 FEET 

DATUM IS MEAN SEA LEVEL

Prelandshde topography by Continental 

Engineers Inc., Denver, Colo., based on 

aerial photographs taken September 1961 

and modified to show landslide, August 

1963; base further modified and generalized by C. S. Robinson and F. T Lee, 1964

Compiled by C. S. Robinson,

R. D. Carroll, and F. T. Lee, 1963

FIGURE 18. Isopach map of the Loveland Basin landslide to the top of the slip zone.
30 LOVELAND BASIN LANDSLIDE, COLORADO

EXPLANATION 

0

Isopachs

Interval 10 feet

Point where thickness determined by 

electrical resistivity method

Point where thickness determined by 

seismic refraction method

100 150 200 FEET

Compiled by C. S. Robinson,

R. D. Carrol I, and F. T. Lee, 1963

CONTOUR INTERVAL 5 FEET 

DATUM IS MEAN SEA LEVEL

Prelandslide topography by Continental 

Engineers Inc., Denver, Colo., based on 

aerial photographs taken September 1961 

and modified to show landslide, August 

1963; base further modified and generalized by C. S. Robinson and F. T. Lee, 1964

FIGURE 19. Isopach map of the Loveland Basin landslide to the base of the slip zone.
COMPILATION OF RESULTS OF INVESTIGATIONS 31

EXPLANATION 

11050

Structure contours 

Interval 10 feet

Point where thickness determined by 

electrical resistivity method

Point where thickness determined by 

seismic refraction method

0 50 100 150 200 FEET 

I I I l i

CONTOUR INTERVAL 5 FEET 

DATUM IS MEAN SEA LEVEL

Prelandslide topography by Continental 

Engineers Inc., Denver, Colo., based on 

aerial photographs taken September 1961 

and modified to show landslide, August 

1963, base further modified and generalized by C. S. Robinson and F. T. Lee, 1964

Compiled by C. S. Robinson,

R. D. Carroll, and F. T Lee, 1963

FIGURE 20. Structure contours drawn on the top of the slip zone of the Loveland Basin landslide.
32 LOVELAND BASIN LANDSLIDE, COLORADO

EXPLANATION 

 / 1050 

Structure contours 

Interval 10 feet

Contour enclosing area of 

lower structural relief

Point where thickness determined by 

electrical resistivity method

Point where thickness determined by 

seismic refraction method

0 50 100 150 200 FEET

I____|____|____|____|

CONTOUR INTERVAL 5 FEET 

DATUM IS MEAN SEA LEVEL

Prelandslide topography by Continental

Engineers Inc., Denver, Colo., based on

aerial photographs taken September 1961

and modified to show landslide, August

| 1963; base further modified and general-

! ized by C. S. Robinson and F. T. Lee, 1964

Compiled by C. S. Robinson,

R. D. Carroll, and F. T. Lee, 1963 I

FIGURE 21. Structure contours drawn on the base of the slip zone of the Loveland Basin landslide.
COMPILATION OF RESULTS OF INVESTIGATIONS

B

33 

B'

11,200' 

11,100' 

11,000

LONGITUDINAL SECTION

FIGURE 22. Longitudinal and transverse sections of the Loveland Basin landslide. Lines of sections shown in figure 2.
Stability Analysis

By ROBERT A. BOHMAN

GEOLOGICAL, GEOPHYSICAL, AND ENGINEERING INVESTIGATIONS 

OF THE LOVELAND BASIN LANDSLIDE, CLEAR CREEK COUNTY, COLORADO,

1963-65

GEOLOGICAL SURVEY PROFESSIONAL PAPER 673-F
CONTENTS

Page

Abstract ______________________________________________ 35

Introduction ______________________________________________ 35

Analysis ______________________________________________ 35

Conclusions ______________________________________________ 39

II
GEOLOGICAL, GEOPHYSICAL, AND ENGINEERING INVESTIGATIONS OF THE LOVELAND 

BASIN LANDSLIDE, CLEAR CREEK COUNTY, COLORADO, 1963-65

STABILITY ANALYSIS

By ROBERT A. BOHMAN 5

ABSTRACT

A preliminary analysis of the Loveland Basin landslide 

indicated that a fill or buttress containing approximately 

100,00 cubic yards of material properly placed in the plaza 

area at the toe of the landslide would provide a factor of 

safety against further movement of approximately 1.4. During September and October 1963, a buttress of 61,775 cubic 

yards of material was placed at the toe of the landslide. An 

analysis of this buttress indicates that the factor of safety 

against failure was about 1.1 at the base of the slip zone 

and about 1.2 along the top of the slip zone. The factor of 

safety against failure of the landslide mass directly above 

the level of the buttress in the cut slope was calculated as 

about 1.0. The effect of water was not considered because of 

the lack of hydrologic data.

The probability of failure of the landslide above the present buttress might be reduced by raising the level of the 

buttress. If the landslide should fail above the level of the 

present buttress, however, the landslide material would probably be retained on top of the present buttress and could aid 

in further stabilization.

INTRODUCTION

Prior to the preparation of the report by Robinson, Carroll, and Lee (1964), a stability analysis of 

the Loveland Basin landslide was made only from 

preliminary electrical resistivity data, measured dimensions, and assumed physical properties of the 

sliding material. This analysis indicated that a fill 

containing approximately 100,000 cubic yards of 

material properly placed in the plaza area at the toe 

of the landslide would provide a factor of safety 

against further movement along the full length of 

the existing slip zone of approximately 1.4. The fill 

was expected to act primarily as a buttress to receive pressure of the sliding material. The stability 

analysis of the buttress, after its construction, was 

based on the delineation of the slide and slip zone 

by Robinson, Carroll, and Lee (1964).

As a result of the analysis of all available data 

and after numerous meetings, field inspections, and

5 Assistant regional materials engineer, region 9, U.S. Bureau of Public 

Roads.

office projections, the decision was made to shift the 

position of the Straight Creek Tunnel pilot bore to 

the south, to lower the pilot bore and approach 

grade, and to shift the portal to the east of the location originally planned. The basic reasons for these 

changes were to place the portal of the pilot bore as 

far from the active landslide as possible and to develop all material possible for constructing the recommended buttress. The original plaza area, centerline locations, and other pertinent features are 

shown in figure 23. Also shown are the changes 

from the original plan.

The buttress was constructed in September and 

October 1963 and contains a volume of approximately 61,755 cubic yards of compacted material 

consisting primarily of shot and ripped rock obtained from the section immediately east of the 

pilot bore portal where the grade was lowered approximately 16 feet. This amount of material was 

all that was readily available at the time and was 

far less than that recommended for the buttress. A 

second analysis to be based on the actual buttress 

was considered essential; this analysis was made in 

1964 and is presented here.

ANALYSIS

Data on the constructed buttress, together with 

the information presented by Robinson, Carroll, and 

Lee (1964), form the bases for the stability analysis 

presented in this report. The method of analysis is 

virtually that given by Baker and Yoder (1958).

Assuming the main driving force is generally 

parallel to the axis of the zone of slippage in a vertical plane, the available factor of safety against 

failure at the lower limit of the slip zone may be 

calculated as follows (fig. 24) : 

Approximate unit weight of landslide material=

150 Ib per cu ft (pound per cubic foot). 

F.S. (factor of safety) =0.98. (The factor of safety

35
36 LOVELAND BASIN LANDSLIDE, COLORADO

I '!, 61,M'du.yd. \ ji;L Al 

,'j. | Average freight 46 ft |\ OJj° Toe

-

Prelandslide topography by Continenta 

Engineers Inc., Denver, Colo., based on 

aerial photographs taken September 1961 

and modified to show landslide, August 

1963; base further modified and generaljze_d_by_C.^Robinson and F. T. Lee, 1964

50 

__L

100

I

150 200 FEET

_I

CONTOUR INTERVAL 5 FEET 

DATUM IS MEAN SEA LEVEL

FIGURE 23. The plaza area and buttress placed on toe of 

Loveland Basin landslide. A-A' and B-B', sections used to 

calculate factor of safety against failure at the lower limit 

of the slip zone; shown in figure 24.

of a slope just prior to failure is 1.0. A slightly 

more conservative value of 0.98, before placement 

of the buttress, is assumed here.)

e (average slope angle) =26° (Sin 0 = 0.438, tan 0 = 

0.488, cos B- 0.899).

Volume of landslide to lower limit of slip zone: 

Section A-A'=61,800 cu ft, 1-ft-wide slice. 

Section £-£'=54,000 cu ft, 1-ft-wide slice.

W (weight of landslide): 

Section A-A':

150x61,800 W=

2,000

Section B-B':

W= 150X54,000 

2,000

=4,640 tons.

=4,050 tons.

T (driving force) = W sin 26°: 

Section A-A':

T=4,640X0.438 = 2,030 tons. 

Section B~B':

T-4,050 X 0.438 = 1,770 tons. 

S (shear resistance) =!FxF.S. (assumed): 

Section A-A':

5=2,030X0.98 = 1,990 tons. 

Section B-B':

5=1,770X0.98=1,730 tons.

Because no hydrologic data were available, this 

analysis omitted any specific or tangible allowances 

for the effects water undoubtedly has on the stability of the mass. The assumption was that the hydrologic conditions existing at the time of failure 

(F.S. = 1.0 or 0.98) would be repeated about annually and that this repetition would represent the 

worst hydrologic condition likely to be present for 

the ensuing few years. During this time further 

physical, geologic, and hydrologic data could be 

gathered, and the slide corrective measures could be 

reevaluated on the basis of new information.

Because of the lack of hydrologic data a possible 

drainage solution could not be evaluated. In addition, where no drainage solution is involved, no values of friction and cohesion for the sliding material 

in redesigning the failed slope or buttress need be 

determined.

It is, however, necessary to estimate the shear 

strength of the material used in the buttress. We 

assumed that the material would be granular material similar to shot rock, talus, or sand and gravel. 

We further assumed this material would have no 

cohesive strength but would have an angle of internal friction of about 34° or about a IrlVa slope. 

This value is considered to be conservative but realistic. Therefore, for the buttress (fig. 24) :

(j> (angle of internal friction) =34°. Tan ^, = 0.67.

Approximate unit weight of buttress material= 

150 Ib per cu ft.

WB = estimated weight of a 1-ft-wide slice of 

buttress material on sections A~Af and B-B'.

WB = N cos <j>, <£ = 0°, cos 0° = 1, W=force normal to surface.

TB (tangential component of WB ) = 0 (no 

driving forces) :

Section A-A':

W 150X5,480 K =-

2,000 =411 tons.
STABILITY ANALYSIS 37

Section B-Bf :

TTy 150 X 5,820

WB= =436 tons. 2,000

Factor of safety with buttress at lower limit of slip 

zone is:

S+Ws tan<j> F-S- = = -

Section A-A':

2,030

Section B-B':

^ o 1,730 + 436x0.67 F.S. = = 1.14. 

1,770

The factor of safety against failure along the 

upper limit of the slip zone may be calculated as 

follows:

Volume of landslide to upper limit of slip zone: 

Section A-A = 36,300 cu ft, 1-ft-wide slice. 

Section B-B' = 36,400 cu ft, 1-ft-wide slice.

Section A-A':

150X36,300

W= 2,000 =2'72° t0nS' 

7=2,720x0.4380 = 1,190 tons. 

5=1,190X0.98 = 1,170 tons. 

__ 1,170 + 411x0.67

1,190

Section B-B':

150x36,400

W= 2,000 =2'73° ^ 

7=2,730X0.4380=1,200 tons. 

5=1,200X0.98 = 1,180 tons. 

1,180 + 436x0.67

F S 1^00 _______________ 190 1>23-

If we assume that the force of the landslide at 

the lower limit of the slip zone acts on the buttress 

and, in the plaza area in general, is a horizontal 

one, the factor of safety is raised slightly:

Horizontal component of driving force (7^=7 cos 

26°):

Section A-A':

7fc=2,030X0.899 = 1,820 tons.

& = 7\xF.S. = l,820x0.98=l,780 tons.

_ 1,780+411X 0.668 _ 

'* 1,820 'l '

Section B-B':

7^=1,780X0.899 = 1,600 tons. 

5^=1,600X0.98 = 1,570 tons.

1,570 + 436x0.67 F.S.»=- = 1.16. 

1,600

From the appearance of conditions shown on sections A-A' and B-B', figure 24, shear failure 

through the landslide mass might possibly occur in 

a zone between the approximate altitudes of 11,080- 

11,150 ft. If we assume that the weak zone is at an 

altitude of 11,080 ft and that failure would most 

likely occur along a horizontal surface at the top of 

the buttress, then all material below this point, 

including the buttress, can be omitted from further 

analysis and the force components are as follows:

Wi = Estimated weight of a 1-ft-wide slice of slide 

material on section A-A' and B-B' if we assume failure will most likely occur at an altitude of 11,080 ft. 

Section A-A'

Wi= - - (61,800-2,700)X150 =4,430 tons. 

2,000

Section B-B':

(54,000-2,000)X150

Wi = n nnn = 3,900 tons. 

2,000 '

The negative areas in the above equations are 

shown as crosshatched areas in figure 24.

Section A-A':

T!= (tangential component of Wi) = Wi sin 26°. 

7\=4,430X0.438 = 1,940 tons. 

Horizontal component of Ti = 1,940x0.899 = 

1,740 tons.

Section B-B':

7\ = 3,900X0.438 = 1,710 tons. 

Horizontal component of 7\ = 1,710x0.899 = 

1,540 tons.

Wx = Estimated weight of 1-ft-wide slice of the resisting block of slide material (stippled 

areas in fig. 24) if we assume failure is most 

likely to occur at an altitude of 11,080 ft.

Section A-A':

950X150 Wg= =71 tons. 

2,000

Section B-B':

600X150

2,000

=45 tons.
38 LOVELAND BASIN LANDSLIDE, COLORADO
STABILITY ANALYSIS 39

TU (resisting forces) = horizontal component of 

TiXF.S. + Wx tan </>:

Section A-A'':

7*=1,740X0.98 + 71x0.67=1,750 tons. 

Section B-B':

7^=1,540X0.98 + 45X0.67=1,535 tons.

The factor of safety, then, is: 

Section A~Af :

1,750

F.S.= =1.006. 

1,740

Section B-B':

1,535

F.S.= =0.997. 

1,538

These figures indicate that the factors of safety are 

only about 1 and that conditions are such that additional movement can be expected.

Areas used in all calculations were measured by 

planimeter. Calculations were made to slide-rule accuracy.

One method of minimizing the possibility of failure above the buttress would be the construction of 

another buttress on top of the existing one. The desirability of doing this was recognized and discussed during earlier stages of investigation and 

analysis. Lack of material and cost were the primary reasons given for not building an additional, 

second-story buttress during the period JulyOctober 1963. Another reason was that the author 

believed that shear failure along a plane at an approximate altitude of 11,080 feet would not result 

in an instantaneous movement of the entire landslide mass that might override the buttress and spill 

into the new plaza or portal area. Rather, he believed that such shear failure would result in a slow 

movement of landslide material out onto the surface 

of the buttress, thereby forming a natural secondstory buttress that would effectively resist further 

movement before it reached the edge of the buttress 

where it could spill over into the plaza area. Any 

movement of material onto the buttress would result in a decreased driving force and increased resistance to further movement.

No data are available for use in determining the 

effect of water on this slide. The analysis presented

makes no specific allowance for the effect of water, 

although water unquestionably has considerable influence on the overall condition of stability and 

should receive attention. The author believes that 

the most critical condition of stability will coincide 

with, or shortly follow, periods of rapid thawing or 

heavy rainfall.

Control points were placed and observed regularly by personnel of the Colorado Department of 

Highways to detect and determine magnitude and 

direction of movement (chap. D). A review of these 

data in July 1964 revealed that movement was still 

occurring in certain segments of the landslide, particularly near the head and eastern flank. The direction of movement near the head was generally parallel to the longitudinal axis of the slide. The 

movement along the east flank, however, was more 

in an easterly direction approximately parallel to 

the highway and tunnel alinement; this direction 

would indicate that natural buildup of resistance 

above the buttress and additional resistance provided by the buttress itself had shifted the point of 

weakness toward the east flank, which in turn resulted in a change of direction of the landslide 

movement away from the critical portal area.

In effect, the east flank was acting as a "relief 

valve," thereby reducing the stresses or driving 

forces assumed in this report to have been acting 

directly on the buttress and the part of the landslide immediately above the buttress.

CONCLUSIONS

Specific recommendations regarding additional 

landslide control measures or final location and design of the proposed vehicular tunnels are beyond 

the scope of this report. However, the author believes that any further studies supporting ultimate 

design of the vehicular tunnels and portal areas 

should not overlook the probable desirability of extending the height of the buttress and providing additional drainage for the landslide mass.

Further, the analysis contained herein is somewhat simplified and is based on many assumptions 

which may or may not be generally accepted. The 

aim here has been to reasonably evaluate the landslide conditions as they existed in 1963 and 1964 at 

the east portal of the Straight Creek Tunnel pilot 

bore.
Summary and Recommendations

By CHARLES S. ROBINSON and FITZHUGH T. LEE

GEOLOGICAL, GEOPHYSICAL, AND ENGINEERING INVESTIGATIONS 

OF THE LOVELAND BASIN LANDSLIDE, CLEAR CREEK COUNTY, COLORADO,

1963-65

GEOLOGICAL SURVEY PROFESSIONAL PAPER 673-G
CONTENTS

Page

Summary ________________________________________________ 41 

Recommendations ___________________________________________ 42

II
GEOLOGICAL, GEOPHYSICAL, AND ENGINEERING INVESTIGATIONS OF THE LOVELAND 

BASIN LANDSLIDE, CLEAR CREEK COUNTY, COLORADO, 1963-65

SUMMARY AND RECOMMENDATIONS

By CHARLES S. ROBINSON and FITZHUGH T. LEE

SUMMARY

The Loveland Basin landslide occurred in an area 

of extensively faulted and altered Precambrian igneous and metasedimentary rock. During Pleistocene time, morainal material was deposited in valley 

bottoms and on glacially oversteepened valley walls. 

After the glaciers retreated, the morainal material 

locally moved downslope, and the oversteepened and 

incompetent valley walls began receding. Thus, before the Loveland Basin landslide occurred, the 

slope was in a weakened and potentially unstable 

condition. Slope failure occurred when surficial material and bedrock were excavated at the base of the 

slope, effectively decreasing the resisting forces and 

triggering movement along preexisting faults and 

joints toward the plaza cut.

The heterogeneity of the geologic framework was 

expressed in the nonuniform geometry of the landslide mass. Both slump and debris slide features developed indicating the composite nature of the slide. 

Engineering surveys established that the mass consisted of several parts that were moving at different 

rates and that each part was dependent upon its 

neighbor. The geometry of the landslide influenced 

estimates of depth of the slip surface. Geologic 

mapping established the outline of the slide, dip of 

scarps, and the plunge of striations on the scarp 

surfaces. This information helped to limit estimates 

of the depth to a slip surface and, with the engineering measurements defined the direction of 

movement of the various parts of the slide.

The probable depth to the slip surface was determined by considering at each point of measurement geophysical, geological, and engineering 

data. At many points the seismic data and, to a 

greater extent, the resistivity data indicated more 

than one possible depth to a slip surface. This information combined with the extremely nonuniform

character of the bedrock was interpreted as indicating that landslide movement was occurring nonuniformly throughout a zone rather than occurring on 

a single slip surface. Seismic depth interpretations 

were generally more shallow than resistivity depth 

interpretations. Although many seismic and resistivity depth determinations at similar locations were 

in good agreement, many resistivity plots indicated 

additional, deeper changes. These deeper changes 

helped establish the presumed lower limit of the slip 

zone. A depth for the top and bottom of the slip 

zone was obtained for each point of measurement 

by comparing resistivity and seismic data with the 

geologic information and the limiting geometry of 

the landslide.

By means of the most significant shallow resistivity depth observations, a volume of 500,000 cubic 

yards for the slide was calculated. The generally 

shallow seismic depth determinations yielded a calculated volume of 440,000 cubic yards. The relatively close agreement of results from these two 

methods, when compared with geologic and engineering measurements, resulted in a calculated minimum volume of 470,000 cubic yards. This represented the volume of moving material above the top 

of the slip zone. The deeper resistivity depth 

changes combined with the observed bounding 

scarps, particularly as displayed in the plaza cut, 

determined the base of the slip zone. From this information the total volume of the sliding mass was 

calculated as 770,000 cubic yards.

A compacted buttress load of 61,775 cubic yards 

was placed on the toe of the landslide. A stability 

analysis of this buttress was made on the basis of 

the above slide mass volumes. The load was calculated to provide a factor of safety against failure of 

1.1 at the base of the slip zone and a factor of 

safety of 1.2 at the top of the slip zone. The factor

41
42 LOVELAND BASIN LANDSLIDE, COLORADO

of safety against failure of that part of the landslide mass directly above the level of the buttress in 

the plaza cut slope was calculated as about 1.0. If 

the landslide failed above the buttress level, slide 

material would probably be retained on top of the 

buttress and could aid in the further stabilization of 

the landslide.

RECOMMENDATIONS

The frequency of landslides related to construction could be greatly reduced if adequate geologic 

and engineering investigations were made in advance of design. This procedure would not prevent 

all landslides related to construction because the 

prediction of landslides is far from an exact technique. When a landslide does occur, experienced 

personnel and necessary equipment must be available full time until a solution is reached.

The first essential in the study of a landslide is 

an accurate topographic base map. The base map 

used for the study of the Loveland Basin landslide 

was prepared from earlier photographs taken for 

another purpose and lacked pertinent definition of 

the topography. The authors recommend that upon 

recognition of a landslide, or potential landslide, 

ground control be immediately established, the area 

be photographed, and a topographic and photogeologic map be prepared. Several bench marks placed 

at the time of establishing topographic control on 

the landslide mass and on the adjacent stable 

ground would be useful for future reference. An overlay which could be prepared during photogrammetric compilation of the topography and photogeology and which could show the distribution of 

various types of vegetation for interpretation of soil 

and ground-water conditions and as an aid in planning geologic and geophysical work would be helpful.

The geological and geophysical work should be 

conducted simultaneously and the interpretation of 

the results should be done jointly by the geologist 

and geophysicist. Ideally, the geologist and geophysicist should have the opportunity of returning to 

the field after preliminary compilation of data to 

check anomalous results. For the investigations of 

most types of landslides, both electrical resistivity 

and seismic studies are recommended. The combination of the two types of data allows a more accurate 

definition of the landslide than do the results of 

only one method.

Any landslide study should include repeated and 

systematic measurements of the locations of accurately located survey points established on the landslide mass and on stable ground adjacent to the 

mass. These measurements would not only show the 

direction and amount of movement before remedial 

work but would indicate, if measurements were continued after completion of the remedial work, the 

degree of success of the remedies.

The conclusions from this study are that the combination of geological, geophysical, and engineering 

methods of investigations of the Loveland Basin 

landslide during its early history had several advantages over physical methods of investigation, such 

as drilling and trenching. In summary, these advantages are: (1) less investigation time, (2) lower 

cost, (3) less disruption of the landslide mass, and 

(4) a more complete definition of the sliding mass. 

Ideally, a combination of geological, geophysical, engineering, and physical investigations should give 

the best results, but if time is important and the 

area has only limited accessibility, good results can 

be obtained by geological, geophysical, and engineering methods.
REFERENCES CITED

Baker, R. F., and Yoder, E. J., 1958, Stability analyses and 

design of control methods, Chap. 9 of Eckel, E. B., ed., 

Landslides and engineering practice: Natl. Research 

Council, Highway Research Board Spec. Rept. 29, p. 

189-216.

Barnes, H. E., 1952, Soil investigation employing a new 

method of layer-value determination for earth resistivity 

interpretation: Highway Research Board Bull. 65, p. 

26-36.

Howell, B. F., Jr., 1959, Introduction to geophysics: New 

York, McGraw-Hill Book Co., 399 p.

Malott, D. F., 1965, Application of geophysics to highway 

engineering in Michigan, in Geophysical methods and 

statistical soil surveys in highway engineering: Highway 

Research Rec., no. 81, p. 20-41.

Moore, R. W., 1945, An empirical method of interpretation 

of earth resistivity measurements: Am. Inst. Mining and 

Metall. Engineers Trans., v. 164, p. 197.

 1954, Subsurface exploration by earth-resistivity 

method a boon to the highway engineer: Western Assoc. 

State Highway Officials Proc., p. 229-241.

 1957, Application of electrical resistivity measurements to subsurface investigations: Southeastern Assoc. 

State Highway Officials Proc., p. 192-210; Public Roads, 

v. 29, no. 7, p. 163-169.

 1958, The application of geophysical methods of subsurface exploration to highway engineering problems: 

Tennessee Highway Conf., 40th, Proc., p. 105-132.

 1961a, Geophysics efficient in exploring the subsurface: Am. Soc. Civil Engineers Proc., Jour. Soil 

Mechanics and Found. Div., v. 87, paper 2838, no. SM 3, 

pt. 1, p. 69-100.

 1961b, Observations on subsurface explorations 

using direct procedures and geophysical techniques, in 

Geology as applied to highway engineering, 12th annual 

symposium, 1961: Tennessee Univ. Eng. Expt. Sta. Bull. 

24, p. 63-87.

Robinson C. S., Carroll, R. A., and Lee, F. T., 1964, Preliminary report on the geologic and geophysical investigations of the Loveland Basin landslide, Clear Creek 

County, Colorado: U.S. Geol. Survey open-file rept., 5 p.

Robinson, C. S., and Lee, F. T., 1962, Geology of the Straight 

Creek Tunnel site, Clear Creek and Summit Counties, 

Colorado, and its predicted effect on tunnel construction: 

U.S. Geol. Survey open-file rept., 41 p.

Tagg, G. F., 1964, Earth resistances: London, George 

Newnes, Ltd., 258 p.

Trantina, J. A., 1963, Investigation of landslides by seismic 

and electrical resistivity methods, in Symposium on field 

testing of soils: Am. Soc. Testing Materials Spec. Tech. 

Pub. 322, p. 120-134.

Tweto, Ogden, and Sims, P. K., 1963, Precambrian ancestry 

of the Colorado mineral belt: Geol. Soc. America Bull., 

v. 74, no. 8, p. 991-1014.

Varnes, D. J., 1958, Landslide types and processes, Chap. 

3 of Eckel, E. B., ed., Landslides and engineering practice: Natl. Research Council, Highway Research Board 

Spec. Rept. 29, p. 20-47.

43

U. S. GOVERNMENT PRINTING OFFICE : 1972 O - 462-882