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Some Slope-Movement Problems 

in Windsor County, Vermont, 1984 

By CHARLES A. BASKERVIllE and GREGORY C. OHlMACHER 

Prepared in cooperation with the 

Vermont Geological Survey 

A study of the variety of slope 

movements that developed in 

Windsor County, Vermont, involving 

a representative group of 

Pleistocene surficial materials 

U.S. GEOlOGICAl SURVEY BUllETIN 1828 
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

UNITED STATES GOVERNMENT PRINTING OFFICE: 1988 

For sale by the Books and Open-File Reports Section, U.S. Geological Survey 

Federal Center, Box 25425, Denver, CO 80225 

Library of Congress Cataloging in Publication Data 

Baskerville, Charles A. 

Some slope movement problems in Windsor County, Vermont, 1984. 

(U.S. Geological Survey bulletin ; 1828) 

Bibliography: p. 

Supt. of Docs. no.: I 19.3:1828 

1. Landslide-Vermont-Windsor County. I. Ohlmacher, Gregory C. 

11. Vermont Geological Survey. Ill. Title. IV. Series. 

QE75.B9 no. 1828 557.3 s 87-600417 

[QE599.U5] [551.3] 
CONTENTS 

Abstract 1 

Introduction 1 

Acknowledgments 1 

Physiography 3 

Geologic setting 3 

Weather conditions 3 

Slope failures in Vermont 4 

The problem 4 

Rising ground water as a triggering mechanism for slope movements 5 

Slope-movement classification 6 

Susceptibility of glacial lake-bottom deposits to sliding 6 

North Hartland Flood-Control Reservoir 10 

Earth flows 10 

Piping associated with earth flows 12 

Vermont Route II 13 

U.S. Route 5 14 

North Springfield Flood-Control Reservior 15 

Sharon slope failure 16 

Slope movements in ice-marginal deposits 20 

Slope movements in till deposits 21 

Brook Road 21 

Holbrook Hill Road 23 

Summary and conclusions 24 

References cited 25 

FIGURES 

1. Windsor County, Vt., locality map 2 

2. Bar graph showing mean monthly rainfall-snowfall for the period September I, 

I983, to June 30, I984, for all New England stations 3 

3. Classification of slope movements 7 

4. Photograph of the Glacial Lake Hitchcock rhythmite deposit at the North 

Springfield Flood-Control Reservoir 7 

5. Schematic cross section of a typical flood-reservoir slope configuration 9 

6. Map locating the North Hartland Flood-Control Reservoir slide area 10 

7. Photograph of slope movement on the northeastern wall of the North Hartland 

Flood-Control Reservoir 11 

8. Photograph of slickensides on a clay-smeared head-scarp surface of the slumpearth flow on the northeastern valley wall of the North Hartland Flood-Control 

Reservoir 11 

9. Block diagram showing rafts of intact material sliding on fluidized substrate 12 

10. Photograph of part of the southwestern valley wall of the North Hartland FloodControl Reservoir showing piping cavities 12 

III 
11. Map locating the Vermont Route 11 slide 13 

12. Schematic cross section of the terrace configuration at the Vermont Route 11 

landslide site 13 

13. Map locating the Route 5 and Brook Road slides 14 

14. Map locating the North Springfield Flood-Control Reservoir slide area 15 

15. Block diagram section sketches depicting the processes for the formation of 

multiple ridges or hummocks at the eastern abutment at the North Springfield 

Dam 17 

16. Map locating the Sharon slide area 18 

17. Views of the landslide site north of Sharon on Vermont Route 14 18 

18. Enlarged portion of aerial photograph of the Stoughton Pond area 20 

19. A block-section sketch of a portion of the eastern shore of Stoughton Pond that 

shows the reservoir-terrace kettle-rim relation 21 

20. Photograph of piping failures at Stoughton Pond 23 

21. Map locating the Holbrook Hill Road slide area 23 

TABLES 

1. Precipitation and ground-water data for Windsor County, Vt. 4 

2. Torvane and penetrometer tests taken in four glacial lake-bottom deposit s1tes in 

Windsor County, Sharon excluded 8 

3. Torvane and penetrometer tests of the varved deposits on the Sharon, Vt., 

slope 19 

4. Soil stratigraphy of three adjacent areas on the eastern shore of Stoughton 

Pond 22 

IV 
Some Slope-Movement Problems 

in Windsor County, Vermont, 1984 

By Charles A. Baskerville and Gregory C. Ohlmacher 

Abstract 

During 1984 in Windsor County, Vermont, an abnormal 

number of slope movements, including flows, slumps, and 

block slides in tills and glacial lake deposits, blocked main 

highways, closed secondary roads, and caused substantial 

damage to flood-control reservoirs. In one case, a home and 

its surrounding land were swept away, as the result of 

stream-bank failure due to heavy flooding. These slope 

movements were triggered by heavy rainfall and the melting 

of a thick snowpack. A statewide monthly average rainfall of 

3.39 inches (8.61 centimeters) from February through the 

end of April and an 8.96-inch (22.76-centimeter) average for 

May 1984 contributed to numerous floods and slope 

movements in the State. From February through May, the 

average monthly rainfall for the southeastern part of 

Vermont, which includes Windsor County, was 6.48 inches 

(16.47 centimeters). 

Mapping and sampling of the 19841andslides in glacial 

till and lake-bed deposits in Windsor County were carried 

out to determine the processes involved in these slope 

movements. Although specific landslide processes varied 

from site to site, the landslides can be subdivided into groups 

based on the surficial materials present. The most common 

slope movement was found to be the earth flow, which 

occurred in rhythmite deposits. In clayey till deposits, a 

mixture of blockslide, slump, and earth-flow slope movements was found. Failures involving bedrock also can be 

found in Windsor County, but none were directly attributable 

to the 1983-84 storms. 

INTRODUCTION 

Field studies were made of landslides that occurred 

during spring 1984 in Windsor County, Vt. Some of these 

slides were selected for presentation here as representative 

of the slope-movement problems found within the county. 

The area in which the slides occurred is bounded approximately by the White River valley on the north, the 

Connecticut River valley on the east, the Green Mountains 

on the west, and the Williams River valley on the south 

(see fig. I). 

The landslide sites chosen for study represent a 

variety of slope movements involving various types of 

surficial deposits found in Windsor County. The most 

extensive surficial deposits include Pleistocene till, glaciofluvial, and glaciolacustrine sediments (Stewart and Mac 

Clintock, 1969). Regardless of a slope's composition, the 

above-normal precipitation affected slope stability in all 

the soil types studied in the county. 

As used in this report, the term "slope movement" 

refers to various types of slides and flows in unconsolidated 

materials. Although rock falls and topples occurred in the 

county, they are not dealt with in this report. Slope 

movements studied in most of the valleys included slides, 

flows, and complex forms. These variations occurred in 

relatively small geographic areas and were primarily the 

result of differences in materials. Slope movements in the 

vicinity of theN orth Hartland and theN orth Springfield 

Flood-Control Reservoirs on the Ottauquechee and the 

Black Rivers.respectively, are of particular interest (fig. I) 

because several types of landslide mechanisms occur at 

these sites. 

New and reactivated landslides were studied. Some 

of the reactivated slides and flows examined were drilled 

by soil auger to determine any near-surface variation in 

the materials as well as to sample the materials present. 

Acknowledgments 

This landslide research project is a cooperative 

effort between the Vermont Geological Survey of the 

Agency of Natural Resources and the U.S. Geological 

Survey. Discussions with Charles A. Ratte, Vermont State 

Geologist; Frank J. Lanza, Chief Geologist; and Dan 

Cadiz of the State of Vermont Agency of Transportation 

have been very informative. We also wish to thank 

Michael Curran, upper Connecticut River Basin Manager, 

and Thomas Coen and James E. Dyer, Project Managers, 

North Springfield and North Hartland Dams, respectively, 

New England Division, U.S. Army Corps of Engineers, 

for their assistance, discussions of reservoir landslide 

problems, and general cooperation on this project. Our 

Introduction 
1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

10. 

11. 

12. 

13. 

14. 

15. 

16. 

17. 

18. 

19. 

20. 

I 

I 

) J 

,.. 

I 

\ 

) 

I 

44° ---4--~-----+-+-- 44° 1 

\ .... .-.......... - I 

\ lw1No~o"N I 

~ <' ,' I ~ / n· !_: 3 

"-.__11 I 

43° 

I --- L 

Green Mountains 

Brook Road 

Salt Ash Mountain 73° 

Skitchewaug Mountain 

Mount Ascutney 

Sharon 

Butterfield Hill 

Hartland Four Corners 

Brownsville 

Springfield 

Miller Hill 

Windsor 

Holbrook Hill Road 

Horseback Ridge 

Woodstock 

Cavendish 

Mill Brook 

U.S. Route 5 

Vermont Route 11 

Vermont Route 14 

I 

\ ---.J 

0 

0 

) 

I 

I 

25 50 MILES I 

25 KILOMETERS I ~---, I 

( 

I 

L_--, 

I 

L 

-----

0 

0 5 

------.........._ 

-------------....__ • 6 -....__ 

N 

\ 

14,18 

5 10 15 MILES 

10 15 KILOMETERS 

Figure 1. Windsor County, Vt., locality map. Numbers indicate places mentioned in the text. 

2 Some Slope-Movement Problems in Windsor County, Vermont, 1984 
gratitude goes out to our colleague, Carl Koteff, U.S. 

Geological Survey, who took the time to accompany us on 

visits to some of our sites. Discussions with Carl on the 

glacial history of the area were very enlightening. Special 

thanks also are due Dr. Ratte and William E. Davies, U.S. 

Geological Survey, for their critical reading and helpful 

suggestions on the original manuscript. We also appreciate 

the review and helpful suggestions given the final manuscript by David K. Keefer, U.S. Geological Survey. 

Physiography 

Windsor County is located in eastern Vermont to 

the east of the Green Mountains. The county is drained by 

the White, Ottauquechee, Black, and Williams Rivers and 

their tributaries and by Mill Brook. These streams flow 

eastward into the Connecticut River. The area is quite 

hilly, and the topography is controlled mostly by generally 

north-south-striking bedrock structures. Ridge-crest 

elevations range from as high as 3,286 ft (1,002 m) at Salt 

Ash Mountain (fig. 1, locality 3) near the northwestern 

corner of the county to slightly over 900 ft (274 m) at 

Skitchewaug Mountain (fig. 1, locality 4) along the 

Connecticut River near the southeastern corner of the 

county. Mount Ascutney (fig. 1, locality 5), a monadnock 

on the eastern side of Windsor County adjacent to the 

Connecticut River, is 3,144 ft (958 m) in elevation. Valley 

bottom elevations average from 1,100 to 1,300 ft (335-396 

m) in the western part of Windsor County and from 300 to 

500 ft (91.4-152 m) in the eastern part. 

The entire region has been glaciated, which gives 

much of the topographic surface a rolling aspect in many 

broad valley areas. The authors' observations show that 

the glacial deposits are thick in valleys and relatively thin 

on upper parts of ridges and mountain sides. 

Geologic Setting 

In the southern one-half of Windsor County, the 

bedrock includes the Precambrian Mount Holly Complex, 

which is composed of gneisses, quartzites, and marbles. 

The Silurian Waits River and the Silurian(?)-Devonian 

Gile Mountain Formations, which are stratigraphically 

above the Mount Holly Complex (Hepburn and others, 

1984), are found mostly in the northern one-half of the 

county. The Waits River Formation is composed of three 

major interbedded rock types-impure marbles, quartzites, 

and schists. The Gile Mountain Formation consists of 

micaceous and feldspathic quartzites and mica schists, a 

marble member, and fine-grained phyllites. Mount 

Ascutney is composed of Late Jurassic or Early Cretaceous 

granites, syenites, and volcanics (Faul and other, 1963). 

The surficial materials are mostly late Wisconsinan 

glacial drift (Stewart and Mac Clintock, 1969) and include 

! 

~ 

~ 

.: 

IL. 

~ z 

(/) 

..!. 

~ 

.: 

u.. 

z 

till, glaciofluvial, and glaciolacustrine deposits. The 

glaciolacustrine deposits are associated with Glacial Lake 

Hitchcock, which covered much of the Connecticut River 

valley and its tributaries during the retreat of the late 

Wisconsinan ice sheet. Many of the river valleys contain 

till overlain by rhythmically bedded silt and clay lakebottom deposits (rhythmites). In some places, the lakebottom deposits are overlain by late glacial deltas 

composed of fine sand, cobbles, and gravel and (or) by 

colluvium. Glaciofluvial deposits consisting of sand to 

coarse gravel are common in the upland valleys. 

Weather Conditions 

Slope stability is related closely to the amount of 

precipitation that falls in a given time interval (Zaruba and 

Mencl, 1982). The rate of rainfall coupled with soil or 

sediment properties of water absorption, infiltration, and 

retention are the critical hydrogeological factors associated 

with slope failure. The weather conditions prevailing in 

New England in general and in Windsor County in 

particular for the period of September 1, 1983, through 

June 30, 1984 (National Oceanic and Atmospheric 

Administration, 1983a-d, 1984a-f), are summarized in 

figure 2. 

The weather records for February 1984 show that 

the region, including Windsor County, had above-normal 

rainfall and that snow fell over most of the region from 3 

30 76.2 

25 63.5 

i 

~ 

20 50.8 ~ 

~ 

~ 

~ 38.1 0 15 z 

(/) 

..!. 

~ 10 25.5 u.. 

z :( :( 

a: a: 

5 12.7 

0 0 

1983 1984 

Figure 2. Mean monthly rainfall-snowfall for the period 

from September 1,1983, to June 30,1984, for all New England 

stations. Approximate snowpack equivalent to water is 3.95 

in. (10 em) of snow to 0.395 in. (1 em) of rain. From left to right 

at each month, the first (open) bar shows total unmelted 

snow and sleet for the month in New England. The second 

bar (hachures sloping down to the left) show the average of 

total liquid and melted frozen precipitation combined for 

New England. The third bar (hachures sloping down to the 

right) shows the normal precipitation in the form of rain for 

the month. Data from the National Oceanic and Atmospheric 

Administration (1983a-d, 1984a-f). 

Slope Failures in Vermont 3 
to 15 days of the month (National Oceanic and 

Atmospheric Administration, 1984b ). 

During winter and early spring 1984, the total 

snowfall recorded was 95.5 in. (242.6 em) at Woodstock, 

in the northern one-half of Windsor County, and was 85.7 

in. (217. 7 em) at Cavendish, in the southern one-half of the 

county; this snow cover is roughly equal to 9.6 and 8.6 in. 

(24.4 and 21.8 em), respectively, of rainfall. 

Abnormal precipitation raised ground-water well 

levels (table 1), which increased the moisture content to 

near saturation of the soil materials in Windsor County 

during fall 1983 through midspring 1984. Unusually 

intense spring storms in late May exacerbated the 

conditions by increasing the pore-water pressure in the 

sediments, thus triggering slides. 

Although table 1 shows some fluctuation in the 

ground-water levels, in general, they were above average 

for the period of interest. On the basis of these well 

records, it is assumed that the ground-water levels were 

higher than normal throughout the county. 

SLOPE FAILURES IN VERMONT 

The entire Green Mountain and Taconic Mountain 

ranges and their contained river valleys have been found 

to have slope failures in just about every geologic material 

and unit encountered. In rock terrane, large rock.falls 

have been observed in gneisses, schists, and quartzites; 

rock slumps, in quartzites; topples, in marbles; and debris 

slides, in schists. In unconsolidated sediments, earth flows 

and slumps, debris flows, and debris slides are common 

types of slope movements. In some instances where 

massive bare rock outcroppings dominate long ridges, 

rock falls and rock slides have triggered earth and (or) 

debris slides or flows, thus creating complex types of 

movement. Several hundred landslides have been located 

in the field-some were just noted on maps; others have 

been studied in detail, measured, sampled, and tested; and 

many more have been located and mapped by use of 

remotely sensed data. 

THE PROBLEM 

All the landslides discussed in this report occurred 

during and immediately following the heavy rains and 

rapid snowmelt that inundated the New England region 

during spring 1984. The culmination was exceptionally 

heavy rain that fell during the last few days of May. In 

Vermont, the damage that resulted from some slope 

failures involved flood-control reservoir slopes, road 

Table 1. Precipitation and ground-water data for Windsor County, Vt. 

[Precipitation data taken from U.S. Army Corps of Engineers records kept at the North Hartland and North Springfield Flood-Control Reservoir 

sites. Ground-water well readings supplied by David Butterfield of the Vermont Department of Water Resources. Chester, Hartland, and Rochester 

wells are in unconfined aquifers, and Queechee-West Hartford is confined partially in bedrock; recharge area is about I 00 to 200ft (30-60 m) from this 

well. All these wells are local quick-response types and cover no more than 1,000-acre drainage basins; all are within Windsor County (James Ashley, 

Vermont Department of Water Resources. personal commun. 1986)] 

Total precipitation 1 

North Hartland North Springfield 

Date 

1983: 

September ......................................... 1.97 

(5.00) 

October............................................... 2.66 

(6.76) 

November............................................ 6.35 

( 16.13) 

December.......................................... 5.89 

( 14.96) 

2.28 

(5.79) 

2.95 

(7.49) 

6.50 

( 16.51) 

6.65 

( 16.98) 

4 Some Slope-Movement Problems in Windsor County, Vermont, 1984 

Chester 

well 

3

6.09 

3( 1.86) 

4

-0.34 

4

(-0.10) 

5.83 

(I. 78) 

-0.48 

(-0.15) 

4.74 

( 1.44) 

+0.22 

(+0.07) 

5 :l.40 

5(1.04) 

+ 1.60 

( +0.49) 

Hartland 

well 

3

9.60 

\2.93) 

4

-0.21 

4

(0.06) 

9.63 

(2.94) 

-0.26 

(-0.08) 

9.29 

(2.83) 

-0.13 

(-0.04) 

7.47 

(2.28) 

+ 1.31 

(+0.40) 

Ground-water well readings 

Rochester 

well 

3

12.37 

3(3.77) 

4

-0.23 

4(-0.07) 

12.36 

(3.77) 

-0.95 

(-0.29) 

10.12 

(3.08) 

+0.81 

(+0.25) 

6.50 

(I. 98) 

+4.08 

( + 1.24) 

Quechee-West 

Hartford well 2 

35.32 

\ 1.62) 

6.38 

(I. 94) 

+0.18 

(+0.05) 
Table 1. Precipitation and ground-water data for Windsor County, Vt.-Continued 

Total precipitation 1 Ground-water well readings 

North Hartland North Springfield Chester Hartland Rochester Quechee-West 

Hartford well Date 2 

1984: 

January .. 0.80 0.59 

(2.03) ( 1.50) 

February .. 2.82 2.54 

(7.16) (6.45) 

March. --------------·- 2.9-5 3.70 

(7.49) (9.40) 

April. 3.60 4.42 

(9.14) ( 11.23) 

May .. --------------- 9.46 9.91 

(24.03) (25.17) 

June ------------------------------------ 1.41 1.64 

(3.58) (4.17) 

1Includes melted snow and ice in inches and centimeters(). 

2Flowmg well. + readings indicate level above ground surface. 

3Water level below ground surface in feet and meters(). 

4Departure from average for that month in feet± and meters()±. 

5New high record for December. 

well 

5.15 

( 1.0 I) 

-0.35 

(-0.11) 

3.30 

( 1.0 I) 

+1.40 

( +0.43) 

3.84 

( 1.17) 

-0.50 

(-0.15) 

2.98 

(0.91) 

+0.55 

(+0.17) 

7

1.57 

7(0.48) 

+2.79 

( +0.85) 

5.31 

( 1.62) 

+0.04 

(+0.01) 

well well 

8.59 10.41 +0.40 

(2.62) (3.17) (+0.01) 

-0.07 +0.37 

(+0.02) (0.11) 

8.17 8.08 +1.35 

(2.49) (2.46) ( +0.41) 

+0.36 +2.46 

(+0.11) (+0.75) 

7.96 9.44 +1.35 

(2.43) (2.88) (+0.41) 

+0.26 -0.48 

(+0.08) (-0.15) 

7.01 5.41 + 1.35 

(2.14) ( 1.65) (+0.41) 

+0.38 + 1.66 

(+0.12) (+0.51) 

7

5.96 74.61 +1.35 

7

( 1.82) 7

( 1.41) (+0.41) 

+ 1.83 +4.27 

(0.56) (+1.30) 

7

7.35 10.41 1.77 

7

(2.24) (3.17) 

+ 1.19 +0.79 

(+0.36) (+0.24) 

6+1.35-ft (0.41-m) readings are for flow from casing slot cut above ground level at that elevation. 

7New high record. 

NOTES: To save repetition, the values for the four water wells for every month follow the pattern for the values shown for 

September after footnotes 3 and 4. 

Average for the month can be determined by adding or subtracting the + or - amount indicated by the values before 

footnote 4 from the water level numbers before footnote 3 for any month. 

traffic disruptions, felled trees, and telephone lines. 

Fortunately, no lives were lost, and no one was injured. 

Because of the great number of slides that occurred, 

Windsor County was selected as a primary area of study. 

During a reconnaissance of the county in the first week of 

July 1984, it was observed that more than· one type of 

landslide had occurred in the various glacial deposits. The 

slope movements selected for detailed study were mainly 

earth flows and block slides. 

RISING GROUND WATER AS A TRIGGERING 

MECHANISM FOR SLOPE MOVEMENTS 

In every case of slope movement studied in the 

Windsor County lake bed and till deposits in 1984, abovenormal am-ounts of water played a major role in initiating 

and sustaining failure. Above-normal precipitation led to 

elevated ground-water levels, which. in turn, led to 

Rising Ground Water as a Triggering Mechanism for Slope Movements 5 
increased pore-water pressure. Related to the increased 

pore-water pressure was a decrease in effective normal 

stress and a corresponding decrease in shearing resistance 

in the soil (Terzaghi and Peck, 1948; Skempton and 

DeLory, 1957; Campbell, 1975). Failure occurs when the 

shearing resistance decreases below a critical level; this 

level is dependent on the type of material involved in the 

failure and the slope geometry. 

High water levels behind the North Springfield and 

the North Hartland Flood-Control Dams helped to raise 

ground-water levels and to increase pore-water pressures. 

Schuster ( 1980) stated that high water levels behind dams 

can lead to elevated pore-water pressures and can cause 

slope failures in the reservoirs. It was noted that all the 

slope movements that occurred in the flood-control 

reservoirs in Windsor County had head scarps at or below 

the high-water level for spring 1984. This fact may be 

related to rapid drawdown (Terzaghi, 1950, fig. 7). The 

pore spaces on the reservoir rim are filled to the head-scarp 

level. Following rapid reservoir drawdown, a time lag 

occurs between decreases in the reservoir level and in the 

water-table surface. In the toe area, this generates high 

seepage pressures, which produce slope instability and 

sliding (Terzaghi, 1950). 

Manmade slope alterations also played a major role 

in raising ground-water levels. Surface runoff tended to 

concentrate at the low points along the benches, which 

were old or abandoned unimproved roads and trails along 

some hillsides. With time, the soil slopes below these low 

points became saturated, and slope failures developed, 

sometimes beginning as creep. The failure along U.S. 

Route 5 at Windsor (fig. 1, locality 18) and those that 

occurred along Holbrook Hill Road (fig. L locality 13) 

near Hartland Four Corners (fig. 1, locality 8) are 

examples of slope movements that occurred below old 

roads and trails. 

SLOPE-MOVEMENT CLASSIFICATION 

This paper uses the slope-movement classification 

developed by Varnes ( 1978) (fig. 3), which has become 

widely accepted by landslide researchers. Slope movements 

are classified according to the type of material involved 

and the type of movement. 

All the materials involved in the slope movements 

discussed in this study are considered to be engineering 

soils. Such soils are unconsolidated, residual, or transported inorganic mineral matter and (or) rock fragments. 

The soils ranged from very fine grained silts and clays to 

coarser grained materials composed of sand, graveL 

cobbles, and boulders. In this report, the finer grained 

soils are referred to as earth, and the coarser mixtures are 

considered to be debris (Varnes, 1978). 

Within the study area, slope movements were diverse 

and identified as mostly rotational or translational slides, 

flows, and complex types. Some of the slope movements 

studied in flood-control reservoirs included piping in sand 

and gravel strata. This indicates a high-velocity waterflow 

as the water table, raised during flooding, attempted to 

reestablish itself at its normally lower level after reservoir 

drawdown. 

Slope movements that occurred during late spring 

and early summer 1984 in Windsor County were divided 

into groups based on the nature of the glacial deposits. 

The first, and by far the most common, group oflandslides 

occurred in deposits composed of varved silts and clays, or 

rhythmites. The second group of slope failures occurred in 

glacial till deposits. A few slope movements occurred 

along the sides of ice-marginal deposits; for example, 

kames. One slope movement of special interest, located in 

Sharon, Vt. (fig. 1, locality 6), is a complex deposit 

composed of stream-terrace sediments, above which overlie 

glaciolacustrine sediments, which, in turn, overlie esker 

deposits. 

SUSCEPTIBILITY OF GLACIAL LAKE-BOTTOM 

DEPOSITS TO SLIDING 

Rhythmites, which represent the greatest landslide 

problems found in the study area, generally are composed 

of alternating layers of silty-clay and silt to fine sand. 

Generally, the rhythmite deposits in the county range 

from nearly pure clays to silts and sand. Where layers of 

nearly pure clay are found, they are very thin, usually less 

than 1 in. (2.54 em). The average thickness of glacial 

lake-bottom deposits, as measured on lateral scarps and 

head scarps, at the sites investigated is 50 ft (15.24 m). 

Contained within a few rhythmite sequences are layers of 

medium to coarse sand and gravel. The medium to coarse 

sand layers are locally as thick as 4 ft ( 1.2 m). Also, thin 

calcite-cemented sand layers have been found at several 

locations (fig. 4). These indurated layers are 0.125 to 0.25 

in. (0.318-0.635 em) thick and spaced 5.12 to 5.9 in. ( 13-15 

em) apart. 

Some of the rhythmite deposits no longer show their 

original horizontal bedding (fig. 4). These materials were 

deposited on or around stagnant ice blocks. As the ice 

melted, the depositional units collapsed; an example 

occurs at the North Springfield Reservoir. Most of these 

rhythmites dip toward the failure face. 

Ground-water flow within the rhythmite sequences 

was mainly in the light-colored silt and sand layers. These 

silts and sands dominated as aquifers because the clayeysilt layers had lower hydraulic conductivities and acted as 

small confining beds. When fresh rhythmite surfaces were 

exposed, either by scraping off surface material or by 

digging small pits into the face, water would leak from the 

silt and sand layers. As water traveled through any of the 

silt and sand layers, it slowly saturated and weakened the 

silty-clay layers. 

6 Some Slope-Movement Problems in Windsor County, Vermont, 1984 
TYPE OF MATERIAL 

TYPE OF MOVEMENT ENGINEERING SOILS 

BEDROCK 

Predominantly coarse i Predominantly fine 

ROTATIONAL Rock slump Debris slump 

1 

Earth slump FEW I 

UNITS I 

SLIDES Rock block slide Debris block slide I Earth block slide TRANSLATIONAL ----- I 

MANY Rock slide Debris slide I Earth slide UNITS i 

LATERAL SPREADS Rock spread Debris spread I 

i Earth spread 

I 

Rock flow I 

FLOWS Debris Earth flow flow I 

(deep creep) (soil creep) 

COMPLEX Combination of two or more principal types of movement 

Figure 3. Classification of slope movements (from Varnes, 1978). The terms "Few units" and "Many units" beneath the 

"Type of movement" column refer to the degree of disruption in a slide; for example, a few blocks or many blocks. 

Figure 4. The Glacial Lake Hitchcock rhythmite deposit at 

the North Springfield Flood-Control Reservoir. The deposit 

is dipping to the west (left side of photograph) and has 

several calcite-cemented sand layers (hammer rests on one 

of these). 

In situ field strength measurements were taken in the 

above sediments using torvanes and pocket penetrometers 

(table 2). The moisture content, as determined in the field 

and based on visual procedures (American Society for 

Testing Materials, 1985), of the samples tested ranged 

from moist to wet-moist indicates damp but no visible 

water, wet indicates visible free water; no tests were run on 

completely dry materials. The field tests (table 2) indicated 

that some ofthe materials involved in sliding had relatively 

uniform engineering characteristics throughout their total 

thickness, as shown by penetrometer readings (unconfined 

compression). At the North Hartland and the North 

Springfield Dam sites, the silty-sand, clayey-silt, and siltyclay layers had penetrometer readings that averaged 3.16 

tons per square foot (3.16 kilograms per square centimeter). 

The low-cohesion fine sand at the U.S. Route 5 site at 

Windsor (fig. 1, locality 12) had an average penetrometer 

reading of 1.2 ton/ ft 2 ( 1.2 kg/ cm2

), and the clays at the 

Vermont Route 11 site at Springfield (fig. 1, locality 1 0) 

had an average of 0.3 ton/ ft 2 (0.3 kg/ cm2). 

Overall, the shear resistance (undrained shear 

strength) for all the materials was low; silty-clays and clays 

that had some silt content had greater cohesion and a 

higher shear strength than other slide materials; for 

example, cohesive clay and silt at North Springfield site 

No. 1 had an average vane shear of 0.545 ton/ft2 (0.545 

kg/ cm2

), and the brown clay and some silt at North 

Hartland site No. 1 averaged 0.423 ton/ft2). Vane shear 

readings (table 2) showed a zone of very low shear 

resistance 34.5 ft (1 0.5 m) below the top of the head scarp 

in the rhythmite section at North Hartland site No. 1; the 

crossbedded sand stratum in the rhythmite section was 

the thickest measured at the dam, and, as indicated in 

table 2, it was subject to piping. The average torvane 

reading for the zone referred to above was 0.038 ton/ ft 2 

(0.038 kg/ cm2). This material was sandy, containing very 

Susceptibility of Glacial Lake-Bottom Deposits to Sliding 7 
Table 2. Torvane and penetrometer tests taken in four glacial lake-bottom deposit sites in Windsor County, Sharon excluded 

[T I Sf F (ton per square foot) is numerically equivalent to kilograms per square centimeter using these instruments 1 

Location 

U.S. Route 5 

Windson, Vermont. 

North Hartland 

Flood-Control Dam. 

Site # 11---

Site #2 ---

Site #3 ---

Vermont Route ll, 

Springfield, Vt. 

North Springfield 

Flood-Control Dam. 

Site # 13---

Site #2---

Material 

Clay---

Dark-brown very fine 

sand. 

Dark clay layer---

Light silty fine sand---

Dark-brown clayey-silt 

(moist). 

Light-brown silty sand 

Yellowish-brown silty 

fine sand (moist). 

Brown clay some silt---

Light-brown silt (dry 

to moist). 

Gray-brown silty-clay 

Cross-bedded fine 

Torvane 

readings 

in T /5/F 

= (kgj cm2) 

0.1, 0.17 

0.06, 0.16, 0.19 

0.19, 0.20, 0.23 

0.06, 0.08, 0.08 

0.13, 0.17, 0.23 

0.21, 0.18, 0.14 

0.07, 0.04, 0.09, 

0.07 

0.43, 0.38, 0.46 

0.1, 0.06, 0.09, 

0.096 

0.20, 0.275, 0.30 

brown sand, some silt; 0.037, 0.038 

upper 3.9 in. (10 em) 

has interbeds of gray, 0.039 

silty-clay. 

Gray varved clay and 

silty-clay with fine 

sandstone concretions. 

Gray-brown silty-clay---

Gray silty-sand---

Gray silty-clay---

Wet clay---

Non wet clay---

Light-brown cohesive, 

moist, varved clay 

and silt. 

Gray clay layer---

---do---

Dark-brown clayeysilt. 

Both types of varves---

Gray silty-clay with 

clay layer. 

Light-yellowish-brown 

coarse silt and fine 

sand. 

Dark-brown varved 

clayey-silt. 

0.175, 0.28, 0.41 

0.21, 0.14, 0.175 

0.225, 0.23, 0.25 

0.42, 0.2, 0.23 

0.54, 0.55 

0.09, 0.1 

0.27, 0.28, 0.21 

0.48, 0.38, 0.32 

0.29, 0.29, 0.28 

Penetrometer 

readings 

in T/S/F 

= (kgj cm2

) 

l.3 

0.7, l.1, 1.3 

Remarks 

Readings taken on lateral scarp 

southern side of slide (moist to wet). 

l.l5, 1.55, 1.65 50ft above road. 

l.05,l.l5,l.l Do. 

l.2, 1.8, l.8 

3.95, 3.3, 3.25 

2.5, 2.35, 2.6 

2.4, 2.5, 2.15 

1.8, 1.45, 1.35, 

1.8 

2. 70, 2.80, 3.2 

l-2 ft below top of scarp. 

Slightly below above reading. 

4 ft below top of scarp. 

7 ft below top of scarp. 

9 ft below top of scarp. 

Piping with earthflow movement 

29.5 ft (9 m) below top of scarp. 

1.55, 1.45, 1.25 34.5 ft ( 10.5 m). This unit was subject 

to piping. 

1.9, 2.3, 2.6 42.7 ft (13m) (moist to wet-no visible 

water to visible free water). 2 

2.3, l.5, 1.7 Depth unknown. 

2.3, 2.5 Failure scarp on northern side of reservoir. 

l.8, l.9, 2.0 West of boat ramp. 

0.25, 0.3 Water seeps and springs occur in 

coarse sand layer above the 

0.3, 0.35 clay at 19.4 ft (5.9 m). 

14.8 ft (4.5 m) below top of scarp. 

2.5, 2.8, 3.3 Test made parallel to bedding. 

2.55, 3.1 Test made normal to bedding. 

0.8, 1.25, l.2 Central reclaimed area. 

3.0, 2.25, 2.5 Below above sample. 

3.6, 3.5, 3.55 Different location along scarp 

(12.5 ft below top of scarp). 

4.4, 4.2, 3.8 Stratum below above test ( 13 ft 

below top of scarp). 

3.0, 3.3, 3.15 Same section ( 14.1 ft below top 

of scarp). 

1 On south side of reservoir, I, 115.5 ft (340 m) west of intake tower at toe of dam and near boat ramp parking lot. 

2 American Society for Testing Materials, (1985, Designation 02488 in Annual Book of American Society for Testing Materials Standards, sect. 4, 

v. 04.08, Soil and Rock; Building Stones-see table 3, p. 415). 3

0n eastern side of reservoir 4,593.4 ft (I ,400 m) north-northeast of intake tower north-northeast and across reservoir east-southeast of boat ramp. 

8 Some Slope-Movement Problems in Windsor County, Vermont, 1984 
little clay, which gave the layer a very low cohesion. After 

the earth flow occurred, this unit was piped extensively by 

ground water escaping from the scarp along the length of 

the layer (fig. 5). The high water conductivity through 

these sandier layers and parallel to the surfaces of the clay 

layers helped add water to the ultimate failure surface. 

A 

\l 

B 

Draped colluvium 

Colluvium 

/---- Rhythmi~--

/ Noncohesive sand 

Rhythmite 

Water table 

Colluvium 

I 

? ----- RfiYtnmiteDraped colluvium j/ 

I Noncohesive sand 

Rhythmite 

Colluvium 

Seeps and pipes formed--------

Rhythmite 

\l 

Water table 

Water table 

Figure 5. A typical flood-reservoir slope configuration in which earth-flow and piping failures have developed. A, 

Preflooding configuration. 8, Flood stage. C, Postslide and piping configuration. The inverted delta symbol indicates the 

water surface. The sketches are not drawn to scale. 

Susceptibility of Glacial Lake-Bottom Deposits to Sliding 9 
Earth flows were the landslide type most commonly 

associated with the glacial lake deposits. Some of these 

slope movements had rotational or slump-type movement 

in the scarp area (slump-earth flow), and others had 

translational or block slide types of movement. The 

rhythmite deposits generally contained only minor 

amounts of gravel and larger detrital fragments. Slope 

movements occurring in these units were predominately in 

fine-grained materials, best classified as earth (Varnes, 

1978). In many examples, some gravelly eskers underlie 

the lake-bottom deposits. Although some of these esker 

materials became incorporated into the lower parts of the 

slide mass, they had little or no effect on the overall earthflow character of the slide material (for example, the 

Sharon site discussed in the section "Sharon Slope 

Failure," consisted of lake-bottom sediments overlying a 

gravelly deposit, but the earth-flow aspect was maintained 

during the event). 

North Hartland Flood-Control Reservoir 

Many earth flows, some having ptpmg failures, 

occurred along the rims and walls of tributary stream 

channels emptying into the North Hartland Reservoir 

(figs. 1, 5, 6). Earth flows with piping failures are treated 

separately in this section. 

Gary Wheeler, U.S. Army Corps of Engineers 

assistant project manager (personal commun., 1984), 

reported that, during the 1984 flood stage in the reservoir, 

seiches observed on the water surface were due to 

subaqueous landslides upstream. It was stated that the 

sound from upstream was audible at the dam. 

Earth Flows 

A slide located on the northeastern valley wall of the 

reservoir (fig. 7) near theN orth Hartland Dam emergency 

spillway was an example of the slump-earth flows encountered. This slope movement occurred in spring 1984 

along the face of a terrace that formed the northeastern 

valley wall. An unnamed stream flows southwestward into 

the reservoir from the west flank of Miller Hill (fig. 1, 

locality 11; A in fig. 6) along the base of the terrace. The 

toe of the failure temporarily blocked the stream, but, by 

eroding the toe , the stream has reoccupied its original 

channel. 

The slope failure had a differential elevation of 110 

ft (33.5 m) from the head scarp to the toe at the unnamed 

stream level. The toe of the failure was stopped by the 

stream channel just before reaching the lake. The failure 

length was about 580ft (176.8 m) from the head scarp to 

the toe along the axis of sliding and was 475ft (144. 78 m) 

wide normal to the axis. The upper boundary of this 

Base from U.S. Geological Survey 1 :24 000 

North Hartland. 1959 

0 .5 1 KILOMETER 

CONTOUR INTERVAL 20 FEET 

QUADRANGLE LOCATION 

Figure 6. The North Hartland FloodControl Reservoir slide area. The slide at A 

is on the northeastern side of the reservoir 

with the unnamed stream valley on the 

eastern edge. The slide at B is just southwest of the boat ramp. 

failure was marked by two undulating scarps. The two 

scarps (main and minor) had vertical offsets of2.5 ft (0.76 

m) each and horizontal tension openings of 4 to 9 in. 

(10.16-22.86 em). The slope angle of the scarps ranged 

from 50° to 62° ( 119-188 percent). Slickensides were 

observed on clay-smeared fresh scarps in July 1984 (fig. 8), 

but they were eroded gradually by subsequent rainstorms. 

The eastern side of the failure mass Uust out of view 

on the right side of fig. 7) had numerous en echelon 

transverse tension gashes and small vertical displacements. 

The western side of the mass was bounded by an irregular, 

curving scarp and one or more lateral shear planes. Except 

for some tension cracks, the central portion of the slope 

movement had a relatively smooth upper surface. 

The lateral tension cracks seen in the left foreground 

of figure 7 resulted from an increase in friction on the basal 

shear surface of the failure as the fluidized substrate 

pinched out, placing the basal shear surface in direct 

contact with the sole of the slide mass. This caused drag on 

the sole of the slide mass pulling it apart (extension) as 

shown in figure 9. Drag on the basal shear surface in the 

head scarp area generated tension crack openings (tensile 

10 Some Slope-Movement Problems in Windsor County, Vermont, 1984 
Figure 7. Slope movement on the northeastern wall of the North Hartland Flood-Control Reservoir west of the spillway. 

The head scarp is in the center. Tension cracks on the midsection of the flow can be seen on the lower left. The shrubs on the 

lower left were mud coated due to the high water during reservoir flooding. The smooth area in the foreground is the edge 

of a grass-covered hill on the southeastern side of the unnamed stream valley from which the photograph was taken. 

Figure 8. Slickensides on a clay-smeared head-scarp surface 

of the slump-earth flow on the northeastern valley wall of 

the North Hartland Flood-Control Reservoir. Tis the location 

of a horizontal tension crack. 

stress) normal to the direction of movement during sliding 

as shown at Tin figure 8. The entire upper surface of the 

slide mass has undergone some extension in the direction 

of movement. Hummocks and lobes of slide material 

formed the distal part ofthe toe ofthis earth flow. Tension 

cracks also were found throughout the toe area. 

A secondary earth flow developed within the toe 

area of the main movement. This failure occurred along 

the western side of the main earth flow where the valley of 

the unnamed stream widened. The toe of the secondary 

failure separated into small blocks of intact material riding 

on a layer offluidlike material. As the blocks came to rest, 

the fluid material oozed from below the blocks. 

In the upper part of the slide mass near the head 

scarp, the basal zone ofliquified material pinched out and 

was replaced by a planar failure surface (fig. 9). In the head 

scarp area of these earth flows, some small minor rotational 

slumps that were separate from the main failure mass were 

seen. 

Failure in both earth flows appeared to have 

occurred as rafts of intact material riding on layers of 

fluidized material. Some of the tension cracks referred to 

above were caused by the extension of the failure mass as 

discussed previously, but others could have been caused 

Susceptibility of Glacial Lake-Bottom Deposits to Sliding 11 
Figure 9. Rafts of intact material sliding on a 

fluidized substrate that pinches out headward. The 

failure surface above the pinchout point becomes a 

planar frictional surface. This friction creates a 

pulling apart of the slide material on tension cracks; 

as a result, some separated glide blocks developed. 

This sketch is not drawn to scale. 

by the bulging and buckling of the surface of the slide mass 

in the toe area as material piled up. 

Piping Associated With Earth Flows 

Of the many landslides that occurred on the 

southwestern valley wall ofthe North Hartland Reservoir, 

one in particular, just west of the boat ramp parking lot, 

had piping associated with the failure (fig. 1 0). The site, a 

terrace, consisted of a rhythmite sequence covered by a 

layer of colluvium. Near the top of the terrace, the 

rhythmite sequence was interrupted by a 4-ft ( 1.2-m)-thick 

medium sand layer. The noncohesive sand layer, which 

had a higher hydraulic conductivity than the surrounding 

rhythmites, acted as a confined aquifer as shown in figure 

5. 

As the water in the reservoir rose during spring 1984, 

it flowed into and through the colluvium and into the sand 

layer and the rhythmite sequences by way of their silt 

layers. Eventually, water within the confined sand reached 

a hydrostatic head roughly equal to the height of the 

reservoir pool above the confined sand layer. As the water 

level in the reservoir dropped, the colluvium above the 

rhythmite sequence maintained a perched water table, and 

the confined sand layer of medium grain size held water 

similar to an artesian aquifer. 

The slope failed as an earth flow, which was partially 

fed by water from the confined sand layer as well as from 

the saturated colluvium (fig. 5). The material involved in 

the failure included the colluvial cover along the slope face 

as well as some of the rhythmite sequence. It moved as a 

rapid fluid earth flow across a stream channel and was 

stopped by the parking lot embankment (fig. 10, foreground). The failure removed the colluvial cover, which 

exposed the confined sand layer and allowed the remaining 

water to escape from this layer. Because the sand had 

lower cohesion than the surrounding rhythmites, it was 

removed by the escaping water. Pipes as much as approximately 4ft (1.2 m) in diameter developed (A in fig. 

10). Most of this movement occurred in rapid succession; 

the initial earth flow was followed closely by the piping 

phase. 

From this example, it was apparent that groundwater flow in the slopes around the North Hartland 

Reservoir had a major effect on slope movement. When 

the reservoir filled, the ground-water table rose with it. As 

the reservoir level was drawn down, the then higher level 

ground water flowed toward the reservoir and exited the 

slope above normal pool level for some time (fig. 5). A 

high moisture content was still evident in the sand unit 

associated with the pipes. This ground-water flow generated high water pressure on the potential slide surface 

(see Costa and Baker, 1981). Slope movement initiation 

occurred through the increased pressure head created by 

this new ground-water configuration. 

Figure 10. Part of the southwestern valley wall of the North 

Hartland Flood-Control Reservoir showing piping cavities 

(A) in a sand layer confined between two rhythmite 

sequences. These cavities are up to 3.9 ft (1.2 m) in diameter. 

12 Some Slope-Movement Problems in Windsor County, Vermont, 1984 
Vermont Route II 

Another earth flow in glacial lake-bottom deposits 

developed along Vermont Route 11, southeast of the town 

of Springfield (fig. I, locality 10; fig. 11). The failure 

occurred on the face of an 80-ft (24.4-m)-high terrace on 

the southwestern wall of the Black River valley. Because 

the failure did not involve the present new alignment of 

Route 11, no corrective measures were taken, except for 

the removal of trees on the slide mass that threatened 

power and telephone lines. 

The terrace deposits consist of a medium to coarse 

sand overlying a rhythmite sequence. The subhorizontal 

terrace surface is 100ft (30.5 m) wide back to front. The 

slope angle of the terrace front is 42° (90 percent), and t 'le 

hill slope angle behind the terrace is 34° (67 percent). 

Numerous springs issue from the bedrock hill above 

the terrace. The terrace surface has a slight slope back 

toward the bedrock-defended hill. A small stream drains 

the swamp area along the proximal side of the terrace 

surface and runs in an easterly direction parallel to the 

bedrock hill. Water from these springs also soaks into the 

terrace sediments and reemerges as springs on the terrace 

front (fig. 12). In a trench excavated in the scarp face to 

Base from U.S. Geological Survey 1 :62 500 

Claremont. N.H.-VT. 1957 

.5 

I 

2 KILOMETERS 

CONTOUR INTERVAL 20 FEET 

QUADRANGLE LOCATIO N 

Figure 11. The Vermont Route 11 slide. Slide is in the 

upper left of the map at the arrow. 

Figure 12. The terrace configuration at the Vermont 

Route 11 landslide site near Springfield showing strata and 

water-flow direction. This sketch is not drawn to scale. 

obtain a clean exposure of the rhythmite sequence, water 

flowed from the silty sand layers. 

Many features of this failure were similar to those 

described in the section "North Hartland Flood-Control 

Reservoir." The head scarp measured 12 ft (3.66 m) 

vertically and 140ft (42.67 m) across normal to the slide 

axis. The scarp had a steep face, and the failure moved 

along a single slide plane in the head-scarp area; some 

rotation also was noted. The surface of the failure mass 

had separated into discrete blocks. 

The toe crossed what had been the original alignment 

and remnant pavement of Route 11. The material in the 

toe, including blocks containing trees, flowed out over the 

old pavement without disturbing it. A fluid mass of graybrown silt came out from under the failed mass and spread 

over the old road beyond the toe. It was apparent to the 

authors that the main slide mass moved on a fluidized 

zone. This fluid layer was fed by seeps issuing from the 

terrace face at the water-table level. Water still was flowing 

from the failure mass onto the old road at a volume 

equivalent to a very small creek when the failure was 

examined in mid-July 1984. 

After the movement, trees on the slide mass were cut 

down, ostensibly to prevent their interference with communications lines crossing the site normal to the slide axis 

should sliding commence again. Examination of tree rings 

from the stump of one of these cut trees showed that this 

area had prior creep movement. During a few years on 

either side of 1950, the tree rings indicated that asymmetric 

growth developed, which suggested a period of stress for 

the tree when the slope may have slipped a bit or at least 

developed accelerated creep. 

Susceptibility of Glacial Lake-Bottom Deposits to Sliding 13 
The slopes on Route 11 that had not failed showed 

signs of creep, and the pipestem tree trunk structure was 

common. Phipps (1974) suggested that the leaning or 

curving of tree trunks was the result of geotropic response 

of the trees. Sharpe ( 1938) indicated that bent tree trunks 

may be one of the indicators of slow flowage or creep on 

slopes. Tree spacing was such that phototropic response 

could not be a satisfactory answer for the number of trees 

with these shapes on the unfailed adjacent slopes as well as 

on the one that failed. 

Penetrometer tests showed that the rhythmites had 

extremely low compressive strength. The silt layers yielded 

readings of 0.3 ton / ft 2 (0.3 kg / cm2

), compared to the 

average of 2.5 ton / ft 2 (2.5 kg/ cm2

) for silts tested in other 

slope-movement areas. Low compressive strength readings 

at this location may have been the result of the soils being 

wetter than those encountered at other sites. 

U.S. Route 5 

The site of this landslide was located in the 

Connecticut River valley on the western side of U.S. 

Route 5 on the eastern slope of Horseback Ridge just 

Base from U.S. Geological Survey 1:62 500 Claremont. N.H.-VT. 1957 

.5 

south of the town of Windsor (fig.13). The slope failed just 

after the heavy rains at the end of May 1984. The head 

scarp had cut halfway across an old overgrown road that 

runs parallel to Route 5 about 110ft (33.5 m) upslope. The 

slope angle below the old road and along the slide axis 

measured 30° (58 percent), and the slope angle above the 

old road was 36° (73 percent). 

Several old landslide scars were seen to the south of 

this failure. All the head-scarp crowns were at the same 

elevation. This elevation defined a terrace level that 

extended to the east of the old road in a southerly 

direction. 

From the top down, the stratigraphy of the Route 5 

slide is as follows. The topsoil was a spongy dark-brown to 

black silty clay and had about 40 percent fine mica; this 

layer was about 1ft (0.30 m) thick. Beneath the topsoil was 

31.5 in. (0.80 m) of rhythmically bedded sticky gray clay, 

and very fine dark-brown sand containing about 30 

percent silt; the top 6 in. (0.15 m) of this horizon was 

saturated. The sand layers were damp but showed no 

visible water; the clay showed visible water at the time of 

observation. The next soil unit below contained a fine to 

medium brown sand that had about 30 percent silt and 

fine pea gravel. Beneath this layer, which is about 10ft (3 

2 KILOMETERS 

CONTOUR INTE RVAL 20 FEET 

QUADRANGLE LOCATION 

Figure 13. The Route 5 and Brook Road slides (fig. 1, localities 2,18 slides). The slope failures are designated 

by "sf." 

14 Some Slope-Movement Problems in Windsor County, Vermont, 1984 
m) below the top of the scarp, was a saturated dark-brown 

gravelly fine sand and silt. The toe material was a brown to 

gray silt and clay quite churned up from the sliding. 

When we examined the site on July 24, the saturated 

failed toe area still was exuding water. Springs were noted 

to be issuing from numerous locations at the base of the 

head scarp and in the scarp areas of the older landslides to 

the south. These springs marked a level along this entire 

slope just below the terrace top that defined the contact 

between an upper aquifer composed of the dark-brown 

gravelly fine sand and a lower layer of gray clay, which was 

observed in the failed head scarp. Heavy flow from these 

springs after intense precipitation apparently triggered the 

slides. 

Torvane tests taken in the sticky gray clay were 0.10 

to 0.17 ton jft 2 (0.10-0.17 kg j cm2), and the penetrometer 

gave a reading of 1.3 ton / ft 2 ( 1.3 kg / cm2

), indicating very 

low shear resistance and compressive strength. The darkbrown fine sand, which was weaker, had torvane readings 

of0.06 to 0.19 ton j ft 2 (0.06-0.19 kg/ cm2) and penetrometer 

readings of 0.07 to 1.3 ton / ft 2 (0. 7-1.3 kg/ cm2). 

The Route 5 slide was an earth flow. The flow mass 

had a width that varied from 150 to 200ft ( 45.7-61 m) and 

was not channelized. No tension gashes were noted as in 

other slides of this type; for example, North Hartland. 

Some slickenslides were visible on lateral scarps and the 

head scarp. Good evidence for this event having been a 

flow was the nondescript slushlike toe material that still 

was issuing water weeks after the slide. Trees on the 

surface of the slide mass ranged from saplings to 12 to 36 

in. (30.5-91.4 em) in diameter at breast height; most of 

these had a pipestem trunk structure, indicating long-term 

creep on the slope. 

North Springfield Flood-Control Reservoir 

Since the first filling of the North Springfield 

Reservoir in 1960, slope movements of serious proportions 

have been occurring due to floods that year and all 

subsequent such events (Michael Curran, U.S. Army 

Corps of Engineers, oral commun., 1984). Slope movements also occurred before construction of the reservoir, 

as evidenced by pre-1960 landslide sites. Trees on these 

earlier landslide sites showed creep evidence based on 

pipestem structure and that their convex sides faced 

downslope near the bases of their trunks. Eroded slide 

scarps were also present. Of the many old landslides that 

predate the construction of the flood-control system, some 

were reactivated in 1984. 

Three older landslide areas of the eastern slope 

within the flood pool of the reservoir had been stabilized 

by the U.S. Army Corps of Engineers. This stabliization 

was acccomplished by grading the slope back to an angle 

of 26° (approximately a 2 on 1 slope) and the planting of 

selected types of vegetation. The southern part of the 

eastern slope was stabilized in two stages beginning in fall 

1975 and completed in summer 1976; the northern part of 

this slope was stabilized inl980 (Thomas Coen, U.S. Army 

Corps of Engineers Project manager, oral commun., 

1985). However, these slopes were reactivated, and 

numerous new slides were activated elsewhere in the basin 

(fig. 14) in spring 1984. 

The failures at theN orth Springfield Reservoir can 

be best classified as earth flows that had some local 

variations. Two special features found at North Springfield 

were the configuration of the slide toes and the nature of 

Base from U.S. Geological Survey 1 :24 000 

Chester. 1972 

.5 1 KILOMETER 

CONTOUR INTERVAL 20 FEET 

QUADRANGLE LOCATION 

Figure 14. The North Springfield FloodControl Reservoir slide area. The location of 

the slide on the eastern side of the reservoir 

at which changes of attitude of the deposit 

were noticed over the slide length is 

designated by A. The site of the Corps of 

Engineers slope stabilization project is shown 

by B. 

Susceptibility of Glacial Lake-Bottom Deposits to Sliding 15 
the deposit. The deposit at one failure studied on the 

southeastern side ofthe reservoir (A in fig. 14) had a clayeysand, gravel, and silt layer at the top. These materials were 

probably stream deposits laid down after the level of 

Glacial Lake Hitchcok dropped. This would make the 

material that covered the slope face a reworked soil and 

(or) slope wash. A lake-bottom rhythmite sequence 

underlies these deposits for about 60ft (18m). This latter 

sequence overlays a coarse gravel, cobble, and sand unit. 

These lower deposits may be those of an esker. The 

attitude (or strike and dip) of this lower unit, and the 

overlying rhythmites, changed along the slope length from 

north to south as follows: N. 55° E. dipping 30° NW., N. 

60° E. dipping9° NW., and N. 20° E. dipping75° E. These 

attitudes indicate collapse structures possibly caused by 

stagnant ice that subsequently melted beneath these 

deposits. An angular unconformity exists between the 

dipping rhythmite layers and the top clayey-sand, gravel, 

and silt strata. 

The coarse gravelly bottom deposits provided a zone 

of high permeability and high shear resistance beneath the 

slide mass. As was the case with the piping failures at the 

North Hartland Reservoir, water entered the coarse 

materials when the reservoir level was high. When the lake 

level was lowered, these coarse deposits provided water to 

the overlying material that was involved in the failure. The 

coarse bottom deposits did not become part of the slide 

mass except for slight mixing along the contact; the sliding 

mass passed over these materials and left them almost 

intact. 

Toes on failures at theN orth Springfield Reservoir 

were somewhat different from what was observed at the 

North Hartland Reservoir. At North Springfield, multiple 

ridges or hummocks of slide material were aligned parallel 

to the toe, whereas many of the toe areas at North 

Hartland had relatively smooth surfaces with en echelon 

tension gashes normal to the slide axis. At the site 

stabilized by the Corps of Engineers on the eastern valley 

wall (B in fig. 14) closest to the dam, the hummocky toe 

consisted of a rigid layer that buckled upon sliding; small 

crevasses formed by the buckling became filled with finegrained fluidized material from beneath the flow (fig. 

I SA). This buckled layer had an appearance similar to a 

rug that has been pushed against a wall and folded. 

The buckling in some of the toes appeared to go a 

step further, which probably was due to a greater velocity 

or a greater fluidity in the sliding material. As the distal 

part of the toe stopped moving, slide material in the main 

track behind the toe still would be travelling on a fluidized 

substrate. The still-moving mass then probably formed an 

overthrust sheet and ramped up onto the first sheet. This 

process apparently repeated itself several times (fig. 

IS B). At the leading edge of a thrust sheet, a linear trough 

occurred that was filled with material from the fluidized 

fine-grained substrate. 

Sharon Slope Failure 

The Sharon landslide is located in the White River 

valley on the eastern side of Vermont Route 14 about 2 mi 

(3.2 km) north of the village of Sharon (figs. I, 16). This 

valley has been modified into aU-shape by glacial erosion. 

While the glacier still occupied the valley, kames and 

eskers were deposited along the valley walls and bottom. 

After the glacier receded, the valley was occupied by 

Glacial Lake Hitchcock. Lake beds were deposited that 

had some deltaic sediments emplaced above them. The 

present valley configuration was established when the 

glacial lake drained and the White River eroded the lakebed sediments and other glacial deposits. The White River 

presently flows along bedrock in this section of the valley. 

The failure occurred in a terrace deposit along the 

eastern valley wall. The stratigraphy of the terrace from 

the top down is as follows: 0 to 12 in. (0-0.305 m), lightbrown sandy loam; 12 to 24 in. (0.305-0.610 m), tan to 

grayish-tan silt, a slight trace of clay and very fine sand; 

and 24 to 30 in. (0.610-0.762 m), crossbedded fine to 

coarse light-brown sand, some fine to medium gravel. This 

unit varies in thickness parallel to the valley trend and 

shows definite foreset and bottomset beds. These deposits 

unconformably overlie about 56ft (17.1 m) of rhythmically 

bedded gray silts and clays. Below the rhythmites is an 

esker deposit composed of stratified cobbly sand and 

gravel about 25 ft (7.62 m) thick. 

The rhythmically bedded units at Sharon are Glacial 

Lake Hitchcock bottom sediments. The rhythmite units 

have a slight dip to the east and are displaced by reverse 

faults (fig. 17 A) that dip steeply to the west. 

The sand units overlying the rhythmites are either 

deltaic deposits from a lake that formed later than Glacial 

Lake Hitchcock or stream sediments deposited on the 

rhythmite surface immediately after the lake drained. In 

either case, they fill in the low spot along the upper part of 

the terrace. The sand unit presents a cross section that has 

a concave bottom surface from west to east across the top 

of the terrace, similar to a section across a stream-channel 

filling. 

Thin layers of late or post-Wisconsinan colluvium 

(Carl Koteff, U.S. Geological Survery, oral commun., 

1984) are draped over the deltaic deposits and rhythmites. 

A discontinuous clay layer within the colluvium is stretched 

and, in some instances, pulled apart in boudinlike fashion. 

These structures are the result of tension in cohesive 

materials due to downslope creep (Weller, 1960). 

Before the 1984 failure, two borrow pits had been 

excavated into portions of the terrace. The first of these 

had removed the esker sediments from the lower part of 

the terrace facing on Vermont Route 14 (fig. 17 B). The 

other borrow pit was dug into the top of the terrace where 

the deltaic sediments were the excavated materials. Both 

pits were inactive when the failure took place. 

16 Some Slope-Movement Problems in Windsor County, Vermont, 1984 
'II 

Shoreline 

\ M 

lit 

A 

B 

\I, 

~I 

~, 

\11 

\I/ 

'I 

Fluidized material 

beneath ramp 

Topsoil, sand, silt, 

and gravel 

Rhythmite sequence 

Coarse gravel (esker?) 

2 

Fluid soil from fluidized zone 

(mudflow) 

Topsoil, 

sand, silt, 

and gravel 

Topsoil, sand, silt, and gravel 

Rhythmite sequence 

Fluidized zone on 

failure surface 

Fluid soil from 

fluidized zone 

(mudflow) 

Figure 15. The processes for the formation of multiple ridges or hummocks at the eastern abutment of the North 

Springfield Dam. A, The hummocks developed in the intact layer are shown with the fine-grained fluidized material 

from beneath penetrating the cracks in the upper layer. B, Mode of overthrusting and ramping in the toe area of slope 

movements is shown. The numbers represent the order of the development of the failures. These sketches are not 

drawn to scale. 

Susceptibility of Glacial Lake-Bottom Deposits to Sliding 17 
Base from U.S. Geological Survey 1 :24 000 

Sharon, 1981 

.5 1 KILOMETER 

CONTOUR INTERVAL 20 FEET 

IJ ( VT.( 

L/ 

QUADRANGLE LOCATION 

Figure 16. The Sharon slide area. 

The width of the failure was approximately 100ft 

(30.5 m) across at the head scarp and about 250ft (76.2 m) 

across at the toe. At the top, near the head scarp. blocks 

rotated in the sand units and the rhythmites (note tilted 

trees at right-centers of fig. 17 C). Also. in the slide mass 

are large chunks [±3 ft (±0.9 m)] of the rhythmite that 

failed and remained on the slope or near the point of 

failure as disoriented or jumbled blocks; a few of these 

have been rotated. 

Before the excavation of the upper borrow pit, 

surface-water runoff flowed off the terrace and did not 

saturate the deltaic sediments. This was due, more than 

likely, to the colluvium, which has a low enough 

permeability to have acted as a protective cap. After 

excavation, the upper pit became a catchment basin, but 

no channel drained out. Water that was impounded in the 

pit saturated the sand unit above the rhythmic sequence, 

which acted as an impermeable bottom. As water in this 

pit deepened, it seeped through the rim and overflowed the 

rhythmite sequence, inundating the slope. Previously, 

evaporation and underflow drainage along the long axis 

of the pit, which slopes slightly southward, may have kept 

an equilibrium between inflow and outflow. However, 

intense rainfall in late spring 1984 may have saturated 

A 

B 

Figure 17. The landslide site north of Sharon on Vermont 

Route 14. A, This fault is one of several reverse faults 

exposed at this site. 8, How the site looked as a borrow pit 

facing Route 14 before the failure. C, The breach channel 

that developed during the failure is seen to the left of 

center. (Photograph 8 courtesy of Charles A. Ratte.) 

18 Some Slope-Movement Problems in Windsor County, Vermont, 1984 
c 

Figure 17. Continued. 

completely the sand layers and raised the impounded 

water level to the rim at the face of the terrace. 

Representative samples were taken down the face of 

the rhythmite unit. Torvane and penetrometer tests for 

shear resistance and compressive strength, respectively, 

were performed at each sampling station; these stations 

were 6.54 to 9. 79ft (2-3m) apart vertically. The tests show 

a low shear resistance for rhythmites in general and the 

lowest resistance in the upper silty-clays (table 3). The 

clayey-silts have the greatest shear resistance of all the 

rhythmite units. Shearing resistance in the clays shows an 

increase as moisture decreases and the clayey-silts show an 

increase in strength as clay content decreases. The 

compressive strength of the rhythmite was relatively 

high- 2 to over 5 ton / ft 2 (2-5 kg/ cm2). 

Two other field tests were used to determine whether 

the fine-grained materials were predominantly silt or clay. 

The first of these tests was based on the portion of the 

fine-grained soil identification test used for plasticity 

determinations (American Society for Testing Materials, 

1985, 02488). A small sample of the material was rolled 

between the palms of the hands into a 0.125-in. (0.32-cm) 

thread as described in the test procedures (American 

Society for Testing Materials, 1985) and in Mathewson 

( 1981 ). If the material could not be rolled, then it was 

considered to be nonplastic and silt dominated. If the 

material could be rolled easily to its plastic limit [near the 

0.125-in. (0.32-cm) diameter], then the material was 

considered to be plastic and clay dominated. We also used 

the DILATANCY TEST (American Society for Testing 

Materials, 1985; see also Costa and Baker, 1981 ). Relative 

particle size was determined by shaking a small cake of 

remolded rhythmite material in the hand to see whether 

water would come to the surface. If the water subsequently 

disappeared back in the cake, then the specimen was 

mostly silt; if the sample was mostly clay, then moisture 

would remain on the surface of the cake. The results of 

these tests are shown in table 3 in the "Dominant Particle 

Size" column. 

Table 3. T orvane and penetrometer tests of the varved deposits on the Sharon, Vt., slope 

[T / S / F (ton per squa re foot) is numerically equivalent to kilograms per square centimeter using these instruments] 

Depth Field Field Dominant 

below top torvane penetrometer particle 

of varve readings, readings, size 

unit, in meters in T / S/ F in T/ 5/ F (clay/ silt) 

0.40 0.41 5.0+ Clay. 

0.30 5.0+ Clayey-silt. 

I. 70 0. 15, 0.30 3.4, 4.3 Clay. 

0.26, 0.45 Clayey-silt. 

5.0+ Sand. 

3.70 0.09, 0. 1 4.0 Clay. 

5.0+ Silty-clay. 

5. 70 0.47, 0.49, 0.55 3.1' 3.3, 3.35 Clayey-silt. 

8.70 0.35. 0.43 2.25, 2.50, 2. 75, 2.8 Clay. 

4.0, 4.5 Silty-clay. 

12.70 0.35, 0.37, 0.39 1.8, 2.05, 2. 15 Clay and silty-clay. 

16.40 0.46, 0.48, 0.56 4.3, 4.65, 5.0+ Clayey-silt. -

0. 76. 0.07, 0.68 4.3, 4.65 Fine sand. 

17.00 5.0+ Very fine sand, 

some silt 46-61 

em thick. 

Susceptibility of Glacial Lake-Bottom Deposits to Sliding 19 
The failure took place in two phases. The first phase 

was a slump-earth flow along the back face of the lower 

borrow pit. The failure extended from the top of the 

terrace to the floor of the borrow pit, which is only slightly 

above the level of Vermont Route 14. The second phase 

started when the upper pit rim was breached by the firstphase failure. A deep channel was cut down the face of the 

lower borrow pit by a slurry of sandy material as the upper 

pit drained. The material from this failure crossed Vermont 

Route 14, damaging a heavy trunk telephone cable 

paralleling the eastern side of the road as it passed. This 

second-phase failure can be classified as a wet sandy earth 

flow. 

SLOPE MOVEMENTS IN ICE-MARGINAL 

DEPOSITS 

A number of slope failures occurred in ice-marginal 

deposits at Stoughton Pond (fig. 1; fig. 18) on theN orth 

Branch Black River (fig. 1). Stoughton Pond is part of the 

Army Corps of Engineers North Springfield FloodControl Project. Slope angles measured on the eastern 

slope of Stoughton Pond averaged 37° (75 percent). 

The surface of the glacial terrace on the eastern 

shore of Stoughton Pond near the dam contains an 

elongated closed depression (kettle) (fig. 19) that strikes N. 

30° E. The floor of the kettle is about 15ft ( 4.57 m) above 

the normal pond level. The crest of the western rim of this 

depression, which is also the eastern rim of the reservoir, is 

approximately 20ft (6.09 m) above the depression floor. 

The rim width averages about 15ft (4.57 m). The rim has 

been breached several hundred feet (tens of meters) north 

of the eastern abutment of the dam (fig. 19). The breach 

extends 5 to 6ft (1.52-1.83 m) below the top of the rim. 

The terrace materials are crossbedded, fine to 

coarse, glaciofluvial deposits. Most of these materials 

contain little silt or clay. The stratigraphic variation from 

north to south along the eastern shore is indicated in the 

three descriptions shown in table 4. 

A few elongate lenses [3.0 ft (0.91 m) thick] of fine 

red-brown sand containing gray clayey-silt layers several 

inches (centimeters) thick can be seen in section on some 

slide surfaces. The lenses range from approximately 16.0 ft 

( 4.88 m) across to as much as the entire width of a failure, 

which may be 100ft (30m) across. 

Water flow from the reservoir into the kettle through 

the breach is indicated by broad aprons of cobbles and 

gravel on the kettle floor leading from the breach. Saplings 

growing in the kettle were killed by drowning; in spring 

1984, new leaves had sprouted but subsequently wilted. 

Mud on shrubs and tree trunks in the depression indicated 

that water stood at least 20 ft (6.1 m) deep. Also 

indications of backflow into Stoughton Pond from the 

kettle as the pond level decreased were found. 

Distributary channels, leading from the breach 

down the slope on the reservoir side of the rim, contained 

gravels from which the fines had been winnowed. When 

the pond level dropped below the floor of the breach, a 

pool of water about 14ft ( 4.28 m) deep remained in the 

kettle. 

The Stoughton Pond slope to the north ofthe kettle 

rim showed evidence of long-term creep. Here were found 

small slope movements of approximately 3 to 26ft (0.9-7.9 

m) across; head scarps were 10 to 25 ft (3-7 .6 m) upslope 

from pool level. 

0 1 MILE 

0 1 KILOMETER 

Figure 18. The Stoughton Pond area showing the location of the north and south limits of slide sites on the 

eastern shore. (Photograph courtesy of AeroGraphics 

Corp., Bohemia, N.Y.) 

20 Some Slope-Movement Problems in Windsor County, Vermont, 1984 
Terrace 

Rim 

Breach 

Stoughton Pond 

Figure 19. A portion of the eastern shore of Stoughton 

Pond that shows the reservoir-terrace kettle-rim relation. 

This sketch is not drawn to scale. 

A piping of the glacial deposits also was observed 

just south of the kettle slides (fig. 20). The pipes varied 

from a little over 1 to over 3ft (0.3-over 1 m) in diameter. 

In some of the incipient slide areas, evidence of highvelocity underground flow was seen. Several piping 

failures beneath the slope surface were evidenced by 

collapse and (or) sagging of the ground surface over some 

of these pipes. The soil cover over piping tubes was 

composed of mats of topsoil and vegetation, underlain by 

a layer of loam, all of which was held together by a 

network of plant root systems. Beneath these sagging mats 

were large cavities (fig. 20A). Sand and gravel on the 

cavity floors had a flow-produced appearance similar to 

that seen on some large open slope movements. 

Sand and gravel deltas were built up along the 

eastern shore of the pond near normal pool level, below 

where these pipes exit the slope (fig. 20B). Below the water 

level of the pond just off the mouth of these pipe-flow 

deltas, subaqueous pits were seen. These obviously were 

excavated by bottom scour of a high-velocity slurry 

composed of water, sand, and gravel concentrated on the 

lake bottom at the locations of the pipe exits. 

Slope movements at Stoughton Pond were mainly 

debris flows, approximately 3 to 25 ft (0.9-7.6 m) wide, 

and were composed of sands, gravels, boulders, and 

vegetation. The failures were initiated by the drawdown of 

the high water in the reservoir created by the May 1984 

flooding. Wave erosion along the shoreline at normal pool 

level had oversteepened the slopes (noted at the time of 

our study), which aided instability. The red-brown sand 

lenses have a much lower permeability than the surrounding sands and gravels. Localized high pore-water 

pressure zones formed in the materials immediately above 

the lenses and initiated slope movement along lens 

contacts. 

Narrow distributary channels crossing the slide toe 

area and floored with gravel and cobbles suggested a 

winnowing out of fines, topsoil, silt, and fine sand by 

relatively rapid ground-water outflow following reservoir 

drawdown. The drawdown of the flood-filled reservoir 

was rapid compared to the drop in the new high groundwater level in the slopes surrounding the pond. The soils 

were probably already near saturation due to spring 

snowmelt and rains before reservoir flooding. 

As the reservoir was drawn down, the water table 

remained high due to a lag in the discharge rate from the 

soil. This situation caused excess pore-water pressure that 

aided in a decrease in shear resistance of the slope 

materials (see table 2). The high pore-water pressures also 

were fed by the water that had ponded in the kettle. The 

pressure head provided by the water in the kettle and the 

increased pore pressure in the bank soils allowed subsequent drawdown to weaken the materials on the slope. 

Failures then were initiated along the southeastern rim of 

Stoughton Pond. 

Other ice-marginal deposits occurred throughout 

eastern Vermont. These deposits should be considered to 

have similar failure characteristics as those at Stoughton 

Pond. 

SLOPE MOVEMENTS IN TILL DEPOSITS 

Till deposits were involved in slope failures at two 

locations within the county. The first location was along 

Brook Road between Brownsville (fig. 1, localities 2, 9) 

and Windsor, and the other, along Holbrook Hill Road 

(fig. 21) between Hartland Four Corners and Brownsville. 

The latter site is in the vicinity of the Hartland-West 

Windsor town line. 

Brook Road 

Along Brook Road (fig. 13), five slope movements 

were examined that appear to have been active at various 

times during the last few years and were reactivated by the 

1984 flooding. The materials at these failures were basal or 

lodgement tills that varied from dark-gray-brown and 

gray clayey till to dark-brown fine to medium sand and silt 

and some fine to coarse gravel, cobbles, and boulders that 

have a trace of some clay; variations were extensive 

throughout these deposits. An ablation till lacking silt and 

Slope Movement in Till Deposits 21 
Table 4. Soil stratigraphy of three adjacent areas on the eastern shore of Stoughton Pond 

Depth Depth 

from from 

terrace (1) Southern side of boat terrace 

surface, ramp approximately surface, (2)Approximately 200 

in feet, 3,937.2 feet {1,200 m) in feet ft (61 m) north of 

() = m north of site (2) () = m eastern abutment 

North 

0-0.167 Brown sandy topsoil 0-0.167 Gray-brown topsoil 

(0-0.051) (0-0.051) 

0.167-22.95 Dark-brown fine to 0.167-.67 Brown silt with a trace 

(0.051-6.996) coarse sand, some fine (0.051-0.203) of grit and gravel. 

to coarse gravel, trace 

to some cobbles and 

boulders, trace of silt. 

0.67-1.25 Layer of gravel with a 

(0.203-0.381) trace of cobbles. 

1.25-22.95 Brown fine to coarse 

(0.381-6.966) sand, some fine to 

coarse gravel, trace of 

silt and boulders. 

22 Some Slope-Movement Problems in Windsor County, Vermont, 1984 

Depth 

from 

terrace 

surface, 

in feet 

() = m 

0-0.167 

(0-0.051) 

0.167-1.58 

(0.051- 0.483) 

1.58-3.67 

(0.483-1.118) 

3.67-3.99 

(1.118-1.219) 

3.99-4.17 

( 1.219-1.270) 

4.17-5.83 

(1.270-1.778) 

5.83-6.67 

(I. 778-2.032) 

6.67-8.42 

(2.032-2.565) 

8.42-9.09 

(2.565-2. 769) 

9.09-9.58 

(2.769-2.921) 

9.58-9.67 

(2.921-2.946) 

9.67-10.08 

(2.946-3.073) 

10.08-10.34 

(3.073-3.150) 

10.34-10.83 

(3.150-3.302) 

(3)Approxi mately 

150ft (46 m) 

north of eastern abutment 

South 

Gray-brown sandy 

topsoil. 

Yellowish-brown 

fine sand, some 

silt, trace of 

gravel and clay. 

Medium to coarse 

sand, some gravel, 

trace of silt. 

Brown gravel, some 

coarse sand, trace 

of cobbles. 

Yellowish-brown, 

cross bedded, 

fine to medium 

sand, some silt, 

trace of gravel. 

Brown cobbly gravel, 

some boulders and 

coarse sand-damp. 

Coarse to medium brown 

sand, some gravel 

trace of silt. 

Cobbly gravel, some 

boulders and coarse 

brown sand (cobbles 

and boulders are 

well rounded). 

Medium and coarse 

light-gray clean 

sand, trace of silt 

becoming more silty 

toward the base. 

Light-brown to tan 

silt and fine sand, 

trace of clay. 

Brown clay. 

Tan to white fine 

sand and silt. 

Yellow-brown 

fine to coarse 

sand, some siltdamp; white clean 

fine sand near 

the bottom-dry. 

Tan silt, some 

fine sand, trace 

of clay, lightgray gravelly 

medium sand-dry; 

light-gray medium 

to coarse sand-dry. 
Figure 20. Piping failures in the eastern slope of 

Stoughton Pond; an impoundment. A, Collapse pits 

over pipes. B, Outflow delta deposits from mouth of 

pipe at pond leveL 

clay overlaid the lodgement till. The natural slope angles 

along Brook Road averaged 32° to 40° (62-84 percent). 

About 0.5 mi (0.8 km) east of Brownsville along 

Brook Road was a small slump. The failure was 100 ft 

(30.5 m) wide and extended vertically upslope from the 

road for 55ft (16.8 m). The scarp was about 9ft (2.7 m) 

high and had an angle of 76° (about 164 percent). The 

material in the scarp was dry to moist ablation till. Seeps 

and springs were common in the toe area of the failure and 

appeared to be emerging along the contact between the 

upper ablation and a lower basal till unit. 

A large landslide occurred about 0.4 mi (0.6 km) 

west of the eastern end of Brook Road. The failure was a 

series of shallow block slides. The slope angle was 43° (93 

percent), and the failure measured 200ft (60.9 m) wide by 

about 50 ft ( 15.2 m) upslope. The material was a graybrown sand and silt and had some gravel- to boulder-sized 

fragments and traces of clay. At the level of the road, the 

material had piled up and rotated slightly. Highway crews 

had been removing the toe material, which had flowed out 

onto the road. 

The other slope movements along Brook Road were 

in the same till sequence but were not as thick and rested 

on steeply dipping bedrock surfaces. These tills apparently 

absorbed large amounts of water as a result of the aboveaverage precipitation (table 1) during the period beginning 

in fall 1983 and ending in spring 1984. Additional heavy 

rains in late spring (May 27 through 31 in particular) 

precipitated sliding by saturating the soil, thus reducing 

frictional forces. 

Holbrook Hill Road 

The materials involved in the Holbrook Hill Road 

slides (fig. 21) were very compact lodgement tills overlain 

by relatively loose and permeable ablation till with a 

variable thickness of approximately 60 to over 100 ft 

(18-over 30.5 m). Numerous landslides have occurred 

previously in this valley, as indicated by hummocky 

profiles on the slopes. We noted that several slides in this 

hummocky terrain were reactivated in spring 1984; one 

Base from U.S. Geological Survey 1 :24 000 

Hartland. 1959 

.5 1 KILOMETER 

CONTOUR INTERVAL 20 FEET 

QUADRANGLE LOCATION 

Figure 21. The Holbrook Hill Road slide 

area. The letters "sf" point out the slope 

failures. 

Slope Movement in Till Deposits 23 
slide took out a telephone pole. The head scarp of this 

latter slide is at the edge of an old abandoned road that is 

part of a larger interconnected network of abandoned 

roads upslope of the failure. The road appears to have 

channeled water from higher elevations into the permeable 

ablation till. The ablation till extended for at least 100ft 

(30.5 m) upslope from the slide and was composed of a 

dark-brown fine to coarse sand that has 5 (trace) to 30 

(some) percent silt and fine to coarse gravel and cobbles. 

These soils, which had a more open texture, were damp at 

the time of sampling. The lodgement till underlying the 

ablation till was composed of sticky gray clay that had a 

trace to some fine to medium sand and gravel and some 

cobbles and small boulders. 

Within a distance of 0.2 mi (0.322 km), six active 

slides were observed; the largest of these was 60ft (18.3 m) 

wide and 105ft (32m) upslope. Many trees up to 1ft (0.305 

m) in diameter at breast height were taken down by the 

failure. After the failure, water seeped out of the head 

scarp along the contact between the ablation and lodgement 

tills, and the slide plane was a wet slick mud. 

Although a small stream is located at the toe of the 

slope, it apparently had not cut the toe and, thus, was not 

the cause of failure. This appeared to be the case for other 

slope movements in similar basal till deposits in the towns 

of Hartland and West Windsor. The streams in this area 

appeared to erode till with difficulty, whereas slower 

moving ground water soaked these soils a few inches 

(millimeters) at a time. The loose open-textured ablation 

till had a high hydraulic conductivity. Ground-water flow 

through the ablation till eventually soaked into the 

lodgement till for a short distance beneath the contact, 

producing a thin layer of soupy, sticky mud. This layer 

formed the zone of failure and allowed the overlying 

materials to move downslope. The actual slope movement 

was a combination of earth flow and block slide. 

SUMMARY AND CONCLUSIONS 

Slope movements studied in Windsor County, Vt., 

indicated that susceptibility to slope instability is high in 

practically every soil type in the area, especially when large 

volumes of water are added rapidly. The greater-thannormal precipitation in Windsor County for November 

1983 through April 1984 (table 1) saturated slope soils. 

The absence of significant drying during the period and 

abnormal rainfall between May 29 and 31, 1984, 

exacerbated the problem (table 1; National Oceanic and 

Atmospheric Administration, 1984e). The occurrence of 

multiple failures during and immediately following the 

intense rainstorms of May 1984 (J. E. Dyer and Thomas 

Coen, Project Managers, North Hartland and North 

Springfield Flood-Control Dams, respectively, U.S. Army 

Corps of Engineers, personal commun., 1984; C. A. Ratte, 

Vermont State Geologist, personal commun., 1984) 

indicates that the landslides were triggered by a buildup of 

pore pressure resulting from very high water tables. 

Lake-bottom sediments, or rhythmites, were found 

to present the greatest landslide problem in the study area. 

Earth flows were the landslide type most commonly 

associated with these deposits. The earth flows occurred as 

rafts of intact material riding on layers of fluidized 

material. Some failures had either rotational or slump 

movement in the scarp area; others had either translational 

or block slide movement. The surface layer of slide 

material at theN orth Springfield Flood-Control Reservoir 

had buckled in the toe area. In some instances, the velocity 

of the slide mass and fluidity of the material developed 

overthrust sheets that would ramp up onto the sheets 

ahead that had slowed or stopped in their advance 

downslope. At the leading edge of these thrust sheets, a 

trough paralled to this edge contained the fluidized finegrained substrate. 

Ground-water flow in the reservoir slopes had a 

major effect on movement in these slopes. Water flow in 

the rhythmite deposits in these slopes was found to be 

confined to the silt and sand layers; instances of piping 

were associated with some of the earth flows. At both 

reservoirs, filling due to heavy rain and snowmelt runoff 

raised the water table. As the reservoirs were drawn down, 

the higher ground-water level flowed toward them, which 

exited the slope above pool level for some time. Where 

thick, noncohesive sand layers were exposed by slope 

failures along the rims of the reservoirs, sand was removed 

as the water flowed out and formed pipes. 

At several locations, pipestem structure on trees was 

observed. Pipestem structure can be a phototropic response 

by the trees or caused by hillslope creep. The number of 

trees involved at the sites investigated indicates the latter 

to be the case. This is supported by tree-ring data from the 

Vermont Route 11 landslide. 

The slope failure at Sharon appears to have occurred 

in two phases. The first, a slump earth flow, took place 

along the back face of the lower borrow pit, which 

separated it from the coarse deposits in the lower pit and 

allowed the upper pit to fill with water. The second-phase 

failure began when the upper pit rim was breached by the 

first-phase failure, which released water contained in that 

pit. A deep channel, which cut down the face of the lower 

pit, resulted from the second failure as a slurry of sandy 

material ran down when the upper pit drained. 

Slope movements in the ice-marginal deposits were 

mainly debris flows initiated by the drawdown of the high 

waters in reservoirs after the May 1984 flooding. These 

failures were accompanied by piping in places. At the 

Stoughton Pond site, a high-pressure head was maintained 

by water ponded in the terrace kettle on the eastern rim of 

the Stoughton Pond Reservoir; this probably aided the 

24 Some Slope-Movement Problems in Windsor County, Vermont, 1984 
piping. Drawdown of the reservoirs was rapid as compared 

to the rate the water-table level declined in the surrounding 

slopes, thus creating excess pore-water pressure that 

triggered slope failures. 

Slope movements in till deposits at the Brook Road 

site resulted from reduced frictional forces brought on by 

saturation after the prolonged and intense precipitation 

infiltrating these sediments that were on bedrock slopes. 

At the Holbrook Hill Road site, the till deposits were a 

combination of an ablation till overlying a clayey lodgement till. Water traveling through the open network, highconductivity ablation till saturated the surface of the 

clayey lodgement till below, thus creating a layer of soupy, 

sticky mud that became the slide surface. 

Some of the significance in these studies is in the 

similarity of failed material types over a great distance 

relative to grain size and engineering characteristics. The 

shear strengths in the various types of deposits tested 

ranged from a low of 0.032 to over 0.5 ton/ ft2 (0.032-0.5 

kg/ cm2), the average shear resistance was 0.208 tonjft2 

(0.208 kg/ cm2). The finer sediments also had the greatest 

cohesiveness, whereas the sandier layers had the least. 

Because the sandier layers have a higher hydraulic conductivity than the finer sediments, they behave as aquifers. 

After slope failure occurred, it was common to find water 

seeping from the sandier layers. If the sandy layer was 

thick enough and became exposed during slope failure, 

then it was subject to piping by escaping ground water. 

Compressive strengths are related to moisture 

content and grain size of the materials; whether the field 

tests were taken normal or parallel to the bedding seemed 

to have no affect on the results. Overall, the silty-sands, 

silty-clays, and clayey-silts had the highest compressive 

strength, which averaged 3.16 ton/ ft2 (3.16 kg/ cm2). 

REFERENCES CITED 

American Society for Testing Materials, 1985, Annual book of 

ASTM standards, Sect. 4 Construction, soil and rockBuilding stones: Philadelphia, Pa., American Society for 

Testing Materials, 970 p. 

Campbell, R. H., 1975, Soil slips, debris flows, and rainstorms in 

the Santa Monica mountains and vicinity, southern 

California: U.S. Geological Survey Professional Paper 

851,5lp. 

Costa, J. E., and Baker, V. R., 1981, Surficial geology-Building 

with the earth: New York, John Wiley & Sons, Inc., 498 p. 

Faul, Henry, Stern, T. W., Thomas, H. H., and Elmore, P. L. D., 

1963, Ages of intrusion and metamorphism in the northern 

Appalachians: American Journal of Science, v. 261, p. 

1-19. 

Hepburn, J. C., Trask, N.J., Rosenfeld, J. L., Thompson, J. B. 

Jr., 1984, Bedrock geology of the Brattleboro quadrangle, 

Vermont-New Hampshire: Vermont Geological Survey, 

Agency of Environmental Conservation, Bulletin No. 32, 

162 p. 

Mathewson, C. C., 1981, Engineering geology: Columbus, Ohio, 

Charles E. Merrill Publishing Co., 450 p. 

National Oceanic and Atmospheric Administration, 1983a, 

Climatological data New England, September: National 

Oceanic and Atmospheric Administration, v. 95, no. 9. 

-- 1983b, Climatological data New England, October: 

National Oceanic and Atmospheric Adminstration, v. 95, 

no. 10. 

-- 1983c, Climatological data New England, November: 

National Oceanic and Atmospheric Administration, v. 95, 

no. 11. 

-- 1983d, Climatological data New England, December: 

National Oceanic and Atmospheric Administration, v. 95, 

no. 12. 

-- 1984a, Climatological data New England, January: National Oceanic and Atmospheric Administration, v. 96, no. 1. 

-- 1984b, Climatological data New England, February: 

National Oceanic and Atmospheric Administration, v. 96, 

no. 2. 

1984c, Climatological data New England, March: 

National Oceanic and Atmospheric Administration, v. 96, 

no. 3. 

1984d, Climatological data New England, April: 

National Oceanic and Atmospheric Administration, v. 96, 

no. 4. 

-- 1984e, Climatological data New England, May: National 

Oceanic and Atmospheric Administration, v. 96, no. 5. 

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Oceanic and Atmospheric Administration, v. 96, no. 6. 

Phipps, R. L., 1974, The soil creep-curved tree fallacy: Journal of 

Research, U.S. Geological Survey, v. 2, no. 3, p. 371-377. 

Schuster, R. L., 1980, Landslides related to reservoirs: Geological 

Society of America, Abstracts with Program, v. 12, p. 303. 

Sharpe, C. F. S., 1938, Landslides and related phenomena: New 

York, Columbia University Press, 137 p. 

Skempton, A. W.,and DeLory, F. A., 1957, Stability of natural 

slopes in London Clay, in International Conference, Soil 

Mechanics Foundation Engineering, 4th, London, 1957, 

Proceedings, v. 2, p. 378-381. 

Stewart, D. P., and Mac Clintock, Paul, 1969, The surficial 

geology and Pleistocene history of Vermont: Vermont 

Geological Survey, Department of Water Resources, 

Bulletin No. 31, 251 p. 

Terzaghi, Karl, 1950, Mechanism of landslides, in Application of 

geology to engineering practice (Berkey volume): Geological Society of America, p. 83-123. 

Terzaghi, Karl, and Peck, R. B., 1948, Soil mechanics in 

engineering practice: New York, John Wiley & Sons, 566 

p. 

Varnes, D. J., 1978, Slope movement types and processes, in 

Schuster, R. L., and Krizek, R. J., editors, Landslides 

analysis and control: Transportation Research Board, 

Special Report 176, p. 11-33. 

Weller, J. M., 1960, Stratigraphic principles and practice: New 

York, Harper & Brothers, Publisher, 725p. 

Zaruba, Quido, and Mencl, Vojtech, 1982, Landslides and their 

control: New York, Elsevier, and Prague, Academia, 324 p. 

References Cited 25 



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