The Effects of Ground Water, Slope 

Stability, and Seismic Hazard on 

the Stability of the South Fork Castle 

Creek Blockage in the Mount St, Helens 

Area, Washington

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 134ft
The Effects of Ground Water, Slope 

Stability, and Seismic Hazard on 

the Stability of the South Fork Castle 

Creek Blockage in the Mount St. Helens 

Area, Washington

By WILLIAM MEYER, M. A. SABOL, H. X. GLICKEN, and BARRY VOIGHT 

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1345

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1985
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Library of Congress Cataloging in Publication Data 

Main entry under title:

The effects of ground water, slope stability, and seismic hazard on the stability of the South Fork Castle Creek blockage in the 

Mount St. Helens Area, Washington.

(Geological Survey professional paper; 1345) 

Bibliography: p. 41 

Supt.ofDocs.no.:! 19.16:1345

1. Landslides Washington (State) Castle Creek, South Fork. 2. Slopes (Soil mechanics) Washington (State) Castle Creek, 

South Fork. 3. Saint Helens, Mount (Wash.) Eruption, 1980.1. Meyer, William, 1939- .II. Series.

QE599.U5E34 1985 551.3'53'0979784 84-600260

For sale by the Distribution Branch, U.S. Geological Survey 

604 South Pickett Street, Alexandria, VA 22304
CONTENTS

Page

Abstract ____________________________________ 1

Introduction __________________________________ 1

Acknowledgments __--____-___-_-___________---- 2

General description of the South Fork Castle Creek blockage . 2

Geologic setting _______________________________ 9

Preeruption geology ___________________________ 9

Posteruption geology --__---___-_____----__.-.- 9

Debris-avalanche deposit --____--____----__---. 12

Modern dacite, andesite, and basalt unit ---------- 12

Ancestral dacite unit _______________________ 12

Blast pyroclastic-flow deposits __________________ 12

Physical properties of blockage material _._________-.__ 13

Texture ___________________________________ 13

Unit weight and relative density __________________ 13

Strength __________________________________ 15

Ground-water conditions _____.___---.

Static slope-stability analysis ._____---. 

Modeled cross sections and assumptions _ 

Results of static slope-stability analysis _ _ 

Retrogressive failure analysis ________

Seismic hazard ____________________

Seismic history of the blockage _______

Dynamic slope-stability analysis ________

Modeled cross sections and assumptions . 

Calculation of ns .................

Results of dynamic slope-stability analysis 

Displacement of sliding masses _______

Conclusions -_____---___--_---_.--

References ______---___--_---_--.

Page

20

20

20

24

25

31

35

35

35

35

37

38

40

41

ILLUSTRATIONS

Page

FIGURES 1-6. Maps showing:

1. Location of the study area and the Mount St. Helens seismic zone _______________________ 3

2. Preeruption topography of the study area, prepared by the Weyerhauser Company in 1950 ______ 4

3. Posteruption topography of the South Fork Castle Creek blockage _______________________ 5

4. Thickness of the South Fork Castle Creek blockage _______________-_____----___----._ 6

5. Chronological appearance of slope-failure scarps on the South Fork Castle Creek blockage, as determined

from aerial photography _________________________________---___-------_-.--- 7

6. Geomorphic features mapped at South Fork Castle Creek blockage on September 22, 1982 _______ 8

7. Stereopair of aerial photographs taken of South Fork Castle Creek blockage on September 22, 1982, showing

slope-failure detachment scarp formed of multiple crescents _______________________________ 9

8. Map showing the geology of the study area ___________________-______-.----__-------_-_ 10

9. Photograph showing a typical exposure of blockage material ______________-_-___--_-_-._----. 11

10, 11. Graphs showing:

10. Results of direct shear-strength tests on the modern dacite, andesite, and basalt unit of the South Fork

Castle Creek blockage: A, peak (maximum) strength; B, ultimate strength ________________ 16

11. Results of direct shear-strength tests on the ancestral dacite unit of the avalanche debris from various

sites: A, peak (maximum) strength; B, ultimate strength ________________---___----..- 18

12. Map showing water-level contours for South Fork Castle Creek blockage, September 20, 1983 ___.----. 21

13. Hydrographs for two selected piezometers at Coldwater Creek blockage ____________.--___-----._ 22

14. Map showing location of cross sections on South Fork Castle Creek blockage that were analyzed for slope

stability .................................................................. 23

15. Diagram showing geology and September 1983 water levels at A, cross section AA'; B, cross section BB'; C,

cross sections CD, CE, and CF; and D, cross section GG' --.--_---_--------------------- 26

16. Graphs showing variation of factor-of-safety values (F8 ) versus seismic coefficients (ns) for selected water levels

at all cross sections ________________________________________--___----.-_--_--_ 28

17-21. Diagrams showing:

17. Critical failure surfaces predicted for A, cross section BB', and B, cross section GG', with water table

at land surface and zero cohesion __________________________--____------___.--- 30

18. Critical failure surface and potential failure surface encompassing the largest section of the blockage for

cross sections CE and CF with water table 25 ft above September 1983 levels and zero cohesion _ 31

19. Critical failure surfaces predicted for cross section AA' with water table at land surface and A, 200 lb/

ft2 cohesion and B, zero cohesion _____________________________________________ 32

20. Critical failure surface and potential failure surface encompassing the largest section of the blockage 

for cross sections CE and CF with water table at land surface and A, zero cohesion and B, 200 lb/ft2 

cohesion __________________________________________________---------.-.- 33

21. Predicted results of retrogressive slope-failure analysis on cross sections CD and CF with September

1983 water levels plus 25 ft and zero cohesion ____________________________________ 34
IV CONTENTS

FIGURES 22, 23. Diagrams showing:

22. Predicted results of retrogressive slope-failure analysis on cross sections CD and CF with water table 

at land surface and zero cohesion ____________-______--___-_---_____--____--_-_

23. Critical surfaces and potential failure surfaces encompassing the largest sections of the blockage for cross 

sections CD, CE, and CF with a seismic coefficient of 0.2 g, September 1983 water level, and zero 

cohesion _______________________________________________________________

24. Graph showing estimated displacement in the blockage resulting from an earthquake with an acceleration pulse 

of 0.5 g and duration of 0.1 and 0.2 seconds _________-______-_____---_____--____---__

25. Schematic of deformation expected with high ground-water level and maximum credible earthquake magnitude _

Page

36

39

40

41

TABLES

TABLE 1. Particle-size distribution in the South Fork Castle Creek blockage ___________--_____--___--.-__

2. Physical properties of the South Fork Castle Creek blockage and other sites on the debris-avalanche deposit _

3. Model-predicted factors of safety CFS *) for critical failure surfaces at South Fork Castle Creek blockage for 

selected positions of the water table and cohesion values ________________________________

4. Model-predicted factors of safety (F,) for selected seismic coefficients (ns) .......................

5. Estimated displacement of sliding masses for selected positions of the water table at South Fork Castle

Creek blockage _____________________________________________________________

Page

13

14

24

38

39

CONVERSION FACTORS AND ABBREVIATIONS

Multiply

inches (in.)

feet (ft) 

miles (mi) 

square feet (ft2) 

square miles (mi2) 

cubic feet (ft3) 

acre-feet (acre-ft)

gallons per minute (gal/min)

gallons per day (gal/d)

million gallons per day (Mgal/d)

feet per second (ft/s)

pounds (Ib)

pounds per cubic foot (lb/ft3)

Tons, short, per square foot (ton/ft2 )

millimeters (mm)

centimeters (cm)

By

25.4

2.540

0.0254

0.3048

1.609

0.09294

2.590

0.02832

1,233.0

0.001233

0.06309

3.785

3,785.0

0.3048

0.4536

16.02

9,765.0

0.03937

0.3937

To obtain

millimeters (mm)

centimeters (cm)

meters (m)

meters (m)

kilometers (km)

square meters (m2)

square kilometers (km2)

cubic meters (m3)

cubic meters (m3 )

cubic hectometers (hm3)

liters per second (L/s)

liters per day (L/d)

cubic meters per day (m3/d)

meters per second (m/s)

kilograms (kg)

kilograms per cubic meter (kg/m3)

kilograms per square meter (kg/m2)

inches (in.)

inches (in.)

acceleration of gravity (g)=32 feet per second squared, or 980.665 centimeters per second squared

National Geodetic Vertical Datum of 1929 (NGVD of 1929): A geodetic datum derived from a general adjustment of the first-order level nets of 

both the United States and Canada, formerly called mean sea level. NGVD of 1929 is referred to as a sea level in this report.

Tallany, Van Kuren, Gertis, and Thielman Datum of 1981 (TVGT of 1981): Datum derived from reference marks and surveyed at 1-mile intervals 

by TVGT and from auxiliary elevation control points surveyed by SPAN International, Inc. Specified accuracies were third order for reference 

marks and to 3 feet for auxiliary points.
THE EFFECTS OF GROUND WATER, SLOPE STABILITY, AND

SEISMIC HAZARD ON THE STABILITY OF THE

SOUTH FORK CASTLE CREEK BLOCKAGE IN THE

MOUNT ST. HELENS AREA, WASHINGTON

By WILLIAM MEYER, M. A. SABOL, 

H. X. GLICKEN, and BARRY VOIGHT

ABSTRACT

South Fork Castle Creek was blocked by the debris avalanche that 

occurred during the May 18, 1980, eruption of Mount St. Helens, 

Washington. A lake formed behind the blockage, eventually reaching 

a volume of approximately 19,000 acre-feet prior to construction of a 

spillway a volume sufficiently large to pose a flood hazard of unknown magnitude to downstream areas if the lake were to break out 

as a result of blockage failure. Breakout of lakes formed in similar 

fashion is fairly common; breakouts in several places around the 

world have posed hazards in recent times.

The South Fork Castle Creek blockage consists of poorly sorted, 

unconsolidated deposits ranging from clay- and silt-sized particles to 

large blocks tens of feet across, and has a matrix of predominantly 

sand and gravel particles. Slopes on the blockage range from about 

10 to 41 percent. Physical properties determined from field and laboratory tests suggest that the blockage debris is moderately compact 

and is probably typical of the depris avalanche.

In September 1983 a ground-water mound, 22 feet above lake level, 

caused flow from the mound to the lake and toward South Castle Creek. 

Depth to water below the crest to the blockage ranged from 20 to 60 feet. 

Ground-water levels are expected to fluctuate seasonally and may continue to increase annually until equilibrium is reached. September 1983 

water levels are thought to be at their seasonal low and are expected 

to fluctuate by approximately 15 feet annually.

Analyses of blockage stability included determining the effects of 

gravitational forces and of horizontal forces induced by earthquakes of 

credible magnitude from the Mount St. Helens seismic zone, which passes within several miles of the blockage. The blockage is stable at September 1983 water levels under static gravitational forces. If an earthquake with magnitude near 6.0 local Richter level occurred with September 1983 water levels, movement on the order of 5 feet on both upstream and downstream parts of the blockage over much of its length 

could occur. If the sliding blocks liquefied, retrogressive failure could 

lead to lake breakout, but this is not considered probable.

As ground-water levels increase in the blockage above September 

1983 levels, the overall possibility of slope failure on the blockage induced by gravitational and earthquake forces is increased. None of the 

analyses indicated that initial slope failure on the blockage would result 

in immediate lake breakout. Retrogressive slope failures could continue 

to lower the blockage, but lake breakout would occur only if liquefaction

of the sliding masses occurred during retrogressive failure, which is considered unlikely. The possibility of lake breakout as a result of slope failure induced on the blockage by gravitational or earthquake forces is 

therefore considered to be remote.

The most unstable conditions occur when the blockage is assumed to 

be saturated. A 50-foot rise in ground-water levels would saturate most 

of the blockage, in which case an earthquake near 6.0 in magnitude could 

cause movement of at least several tens of feet on both the upstream and 

downstream parts of the entire blockage. Factors of safety for slope failure induced by gravitational forces are less than 1.0 over several sections of the blockage when it is fully saturated.

INTRODUCTION

South Fork Castle Creek is a perennial stream that 

drains an area of 1.3 mi2 on the northwest flank of Mount 

St. Helens. The creek was dammed by a debris avalanche 

that occurred during the May 18,1980, eruption of Mount 

St. Helens. (The debris material is referred to in this report as the South Fork Castle Creek blockage or, simply, 

as the blockage.) A lake began forming directly behind the 

blockage and attained a volume of 19,000 acre-ft before a 

spillway constructed through the southeastern edge of 

the blockage in October 1981 stabilized its level. The volume of water in the lake (now known as South Fork Castle 

Creek Lake) is believed to be great enough to pose a flood 

hazard of unknown magnitude to downstream areas if the 

blockage should fail. The blockage could fail because of (1) 

slope failure induced by gravitational or earthquake 

forces, (2) liquefaction, (3) piping, and (or) (4) erosion.

This report contains the findings of a study aimed at estimating the possibility of slope failure in the blockage due 

to gravitational forces and earthquake-induced horizontal 

forces, and the effect on these forces of ground-water 

levels. The scope of the study included determing blockage thickness, mapping the geologic units in the blockage,
STABILITY, SOUTH FORK CASTLE CREEK BLOCKAGE, MOUNT ST. HELENS AREA, WASH.

determining the material properties of the units, determining ground-water levels in the blockage, and, finally, 

using a digital model to estimate the potential for slope 

failure in the blockage due to static gravitational or earthquake forces.

It is fairly common for lakes to form as the result of 

emplacement of volcaniclastic debris across stream valleys during explosive volcanic eruptions, as is their subsequent breakout following failure or overtopping of 

their blockages. Stratigraphic studies by K. M. Scott 

and R. J. Janda, (written commun., 1983) indicate that 

prehistoric lahars resulting from the breakout of large 

lakes occurred many times at Mount St. Helens. In 

modern times, lake breakouts resulting from the failure 

or overtopping of dams of volcaniclastic debris occurred 

at Bandai-san Volcano in Japan in 1888 (Knott and 

Smith, 1890), Katmai Volcano in Alaska in 1912 or 1913 

(Hildreth, 1983), and El Chichon Volcano in Mexico in 

1982 (Silva and others, 1982).

The techniques used to analyze the stability of the 

South Fork Castle Creek blockage should be useful in 

determining the stability of existing dams of volcaniclastic debris and of those that may be emplaced by future explosive volcanic eruptions. In addition, the techniques should be applicable in studies of the stability of 

dams composed of suddently emplaced, unconsolidated 

material from other large mass movements not associated with volcanic eruption. The dams formed by 

the Madison Canyon rockslide of 1959 in Montana (Hadley, 1978), the lower Gros Ventre slide of 1925 in 

Wyoming (Voight, 1978), the Mayunmarca rockslide of 

1974 in Peru (Kojan and Hutchison, 1978), and the 

Nevados Huascaran avalanche of 1971 in Peru (Plafker 

and Ericksen, 1978) are examples of such nonvolcanic 

large mass movements.

ACKNOWLEDGMENTS

We thank the many people, both within and outside 

the U.S. Geological Survey, who helped with the analysis of the stability of the South Fork Castle Creek 

blockage. Robert Schuster, T. Leslie Youd, and William Smith of the USGS provided technical guidance in 

the initial evaluation of potential hazard at the blockage 

and assistance throughout the project. Craig S. 

Weaver, geophysicist, USGS, and Stephen D. Malone, 

senior research assistant, University of Washington, 

aided in obtaining information on the seismic history of 

the study area and the potential for earthquakes to affect the stability of the blockage. The U.S. Army Corps 

of Engineers extended their cooperation; specifically, 

Duane Bankofier and Stephen Loren Stockton provided 

data and pertinent information and Timothy J. Seeman 

did laboratory work on material properties. Finally, 

Sunil Sharma of Purdue University provided guidance 

in the use of the STABL3 slope-stability model.

GENERAL DESCRIPTION OF THE 

SOUTH FORK CASTLE CREEK BLOCKAGE

Prior to the May 18, 1980, eruption, South Fork Castle Creek entered Castle Creek about 6 mi from the 

summit of Mount St. Helens. Material from the debris 

avalanche blocked South Fork Castle Creek at its junction with Castle Creek, creating South Fork Castle 

Creek Lake. Castle Creek flowed along the south side 

of the valley of the North Fork Toutle River and was 

a tributary of the North Fork Toutle River (fig. 1), 

joining it 2.2 mi northwest of the site of the blockage.

In its September 1983 condition, South Fork Castle 

Creek Lake has a surface area of 480 acres, an average 

depth of 90 ft, and a volume of 19,000 acre-ft. The lake 

level has been stabilized at an elevation of 2,577 ft by 

a spillway.

The material blocking the lake comprises poorly 

sorted unconsolidated deposits, from clay- and silt-sized 

particles to large rocks many tens of feet across and 

some wood debris. The blockage is about 2,000 ft long 

from ridge to ridge and is about 1,400 ft wide, on the 

average, from the lake to the downstream toe. The 

maximum height of the blockage from the crest to the 

downstream toe is 190 ft, and from the crest to the lake 

95 ft; vertical distance from the crest to the lake averages 60 ft. The altitude of the crest of the blockage 

ranges from 2,670 ft on the western side to 2,590 ft on 

the eastern side. Slopes average 28 percent from the 

crest of the blockage to the lake and range from 10 to 

41 percent from the crest toward the Toutle River. The 

steepest slopes are on the western half of the blockage.

The thickness of the blockage was estimated by comparing preemption and posteruption topography of the 

blockage area, using a 1:4800 topographic map prepared 

by the Weyerhouser Company in 1950 (fig. 2) and one 

prepared under contract with the USGS in 1981 (fig. 3), 

respectively. Comparison of stream and ridge locations 

on the two maps indicates a problem with horizontal 

control for at least one of the maps and suggests that 

the blockage thickness shown (fig. 4) for any point 

could be in error by an amount greater than the 10-foot 

contour interval perhaps as great as 100 ft in areas of 

steep relief.

Some instances of mass movement have occurred on 

the blockage, but to date they have all been relatively 

small compared with the volume of the blockage. 

Twenty-one slope-failure scarps were mapped on the 

blockage using aerial photographs taken between May, 

1980 and March, 1982 (figs. 5, 6). Additional features 

mapped on figure 6 include associated earthflow and debris-flow deposits; areas of standing water; fluvial surfaces (areas of running water or fluvial deposits); disturbed areas (roadways and other areas affected by 

construction of the spillway in the fall of 1981); and 

remnants of the original surface of the debris avalanche
GENERAL DESCRIPTION OF THE SOUTH FORK CASTLE CREEK BLOCKAGE

EXPLANATION

St. Helens seismic zone 

Debris- avalanche deposit

46°18'

46°15'

46°12'

Mount St. Helens

122°22'30" 122°15' 

FIGURE 1. Location of the study area and the Mount St. Helens seismic zone.

122°07'30"

that are relatively unaffected by human activities, 

water, or secondary mass movements. Using the classification of Varnes (1978), the slope failures include debris slumps; slides along a concave-upward rupture surface; and debris slides, where movement of material occurs along relatively planar detachment surfaces. The 

rims of the larger scarps are often formed by multiple 

smaller crescent-shaped scarps, as shown in figure 7, 

and therefore individual scarps mapped on figures 5 and 

6 do not always represent a single mass movement. 

There is no evidence, from either the sequential areial 

photographs or field observations, of continued retrogressive failure.

To better understand the processes that might contribute to weakening of the blockage, the approximate 

dates of appearance of slope-failure scarps were determined from aerial photographs taken at selected times 

between May 18, 1980, and March 6, 1982 (fig. 5). The 

first three failures occurred in locally steep areas of the 

blockage between May 18 and November 12, 1980. 

Fourteen more failures occurred between November 

12, 1980, and March 1, 1981. Four failures occurred between March 1, 1981, and March 6, 1982. None of the 

failures encompassed large areas of the blockage. The

failures that occurred between May 18, 1980, and 

March 1, 1981, are believed to have resulted mainly 

from a combination of locally oversteep relief and surface saturation from precipitation during the first 

period of winter rains following emplacement of the avalanche. The nearby Elk Lake earthquake that occurred on February 14, 1981, may have contributed to 

some of the failures, but this involvement cannot be 

documented. The final four scarps plotted are located 

along the bank of Castle Creek, suggesting that undercutting of the stream banks may have contributed to 

the instability. Were Castle Creek to migrate toward 

the blockage, erosion could change blockage geometry 

to the point that the blockage became unstable. This 

possibility is considered remote, but the morphology of 

the channel is nevertheless frequently monitored by aerial photography and by repeated cross-sectional surveys.

In general, mass movements on the blockage to date 

have occurred in areas where the topographic gradient 

was high; by smoothing the relief of the blockage such 

mass movements may have served to increase overall 

stability of the blockage. No surface manifestations of 

deep-seated instability are apparent.
STABILITY, SOUTH FORK CASTLE CREEK BLOCKAGE, MOUNT ST. HELENS AREA, WASH.

0 400 800 FEET

0 100 200 METERS 

Contour interval: 10 feet

FIGURE 2. Preemption topography of the study area; map prepared by the Weyerhauser Company in 1950. Scale, 1:4800.
GENERAL DESCRIPTION OF THE SOUTH FORK CASTLE CREEK BLOCKAGE

0 800 FEET

0 100 200 METERS 

Contour interval: 10 feet

FIGURE 3. Posteruption topography of the South Fork Castle Creek blockage.
STABILITY, SOUTH FORK CASTLE CREEK BLOCKAGE, MOUNT ST. HELENS AREA, WASH.

0 400 800 FEET

0 100 200 METERS 

Contour interval: 25 feet

FIGURE 4. Thickness of the South Fork Castle Creek blockage.
GENERAL DESCRIPTION OF THE SOUTH FORK CASTLE

- -X

/ TV

\rlr j^Ml - ^i **-/ ,~:~^~^ ," i

CREEK BLOCKAGE 7 

EXPLANATION

Contact 

 Inferred 

Designation

A 

B 

C 

D

E 

F 

G

0 400

0 100

Slope-failure between

5/1 8/80 and 6/1 9/80 

6/20/80 and 11/12/80 

11/13/80 and 2/02/81 

2/03/81 and 3/01/81 

3/02/81 and 7/27/81 

7/28/81 and 2/07/82 

2/08/82 and 3/06/82

800 FEET

200 METERS

Contour interval: 10 feet

FIGURE 5. Chronological appearance of slope-failure scarps on the South Fork Castle Creek blockage, as determined from aerial photography.
STABILITY, SOUTH FORK CASTLE CREEK BLOCKAGE, MOUNT ST. HELENS AREA, WASH.

EXPLANATION

W Standing water

F Fluvial surfaces (running water 

orfluvial deposits)

D Disturbed areas

DS Earthflow or debris-flow deposits

SF Slope-failure scarps

0 Original surface of blockage

 Contact

- Inferred contact

0 400 800 FEET

0 100 200 METERS 

Contour interval: 10 feet

FIGURE 6. Geomorphic features mapped at South Fork Castle Creek blockage on September 22,1982.
SETTING GEOLOGIC 

detach- slope-failure showing 22,1982, September on blockage Creek Castle Fork South of taken photographs aerial of 7. Stereopair FIGURE 

crescents. multiple of formed (arrow) scarp ment 

SETTING GEOLOGIC 

GEOLOGY PREERUPTION 

deep ft 130 channel a within contained was Creek Castle 

unconsoli- of fill northwestward-sloping broad a into cut 

2). (fig. Helens St. Mount from debris volcaniclastic dated 

of primarily composed is thickness, unknown of fill, This 

old. years 2,500 than more lahars and flows pyroclastic 

uncon- revealed canyon the along exposures However, 

augite- an and old years 180 than less deposits solidated 

(C. old years 2,200-2,500 flow lava andesite hypersthene 

at channel similar A 1983). commun., written Hopson, A. 

Fork North the contained fill the of edge northern the 

River. Toutle 

130-foot-deep a in flowed also Creek Castle Fork South 

unconsoli- southwestward-sloping into cut 2) (fig. channel 

more Helens St. Mount from deposits volcaniclastic dated 

commun., written Hopson, A. (C. old years 2,500 than 

unknown. also is deposits these of thickness The 1983). 

Castle of conjunction the of south ft 1,300 Approximately 

flat, swampy, a was Creek Castle Fork South and Creek 

2). (fig. Marsh Creek Castle called area alluvium-filled 

in- material volcaniclastic the of slope southwesterly The 

or flows pyroclastic thick time prehistoric in that dicated 

Cas- Fork South up flowed Helens St. Mount from lahars 

the at lake a forming it, blocked probably and Creek tle 

filled, lake ancient the When Marsh. Creek Castle of site 

blockage prehistoric the overtopped probably waters its 

some of that to similar manner a in suddenly, drained and 

deposit debris-avalanche 1980 the on lakes smaller the of 

the correct, is scenario this If 1981). others, and (Jennings 

Creek Castle Fork South 18,1980, pre-May the of channel 

breakout. lake ancient the of time the at cut was 

Fork South and Creek Castle containing valleys The 

the of ridges bedrock by bounded are Creek Castle 

The Formation. Ohanapecosh Oligocene Early or Eocene 

dacitic and andesitic of consists area this in Ohanapecosh 

as- and breccias, and flows lava both rocks, volcanic 

Ohanape- The rocks. sedimentary volcaniclastic sociated 

low- low-temperature, widespread undergone has cosh 

written Hopson, A. (C. alteration facies zeolite pressure 

1983). commun., 

GEOLOGY POSTERUPTION 

composed is blockage Creek Castle Fork South The 

of avalanche debris the of deposit the of entirely almost 

by described and mapped 9), 8, (figs. 1980 18, May 

deposit debris-avalanche The (1981). others and Voight 

(Lipman, deposits pyroclastic-flow blast by overlain is 

areas lower in but thick ft 1 than less generally 1981), 

from falling Ash thick. ft 15 than more blockage the of 

and 25, May 18, May of eruptions the during air the the 

blockage the at thick in. 1 than less is 1980, 7, August 

1981). others, and (Sarna-Wojcicki 
WASH. AREA, HELENS ST. MOUNT BLOCKAGE, CREEK CASTLE FORK SOUTH STABILITY, 10

EXPLANATION

running or standing with Areas W 

deposits fluvial and water 

construction during disturbed Areas D 

spillway of 

deposits pyroclastic-flow Blast bp 

unit dacite Ancestral da 

basalt and dacite/andesite, Modern dmu 

unit

Formation Ohanapecosh Eocene To 

(bedrock)

Contact 

contact Inferred 

location site Sample 

FEET 800 400 0

METERS 200 100 0 

feet 10 interval: Contour 

ForkCastte South 

Lake Creek 

area. study the of 8. Geology FIGURE 
GEOLOGIC SETTING 11

FIGURE 9. A typical exposure of blockage material.
12 STABILITY, SOUTH FORK CASTLE CREEK BLOCKAGE, MOUNT ST. HELENS AREA, WASH.

DEBRIS-AVALANCHE DEPOSIT

Voight and others (1981) indicated that the South Fork 

Castle Creek blockage is composed of lateral deposits 

(levee material) of the North Toutle unit of the debris-avalanche deposit. The units they mapped are identified 

primarily on the basis of unmodified morphologic characteristics. Because the blockage is composed of levee material, the slopes of the ridges and hummocks that make up 

the blockage are steeper than those of the bulk of the debris-avalanche deposit. The lithologic composition of the 

levee material is distinctly different from that of the bulk 

of the avalanche deposit.

The blockage consists of two major lithologic units that 

are informally referred to in this report as the ancestral 

dacite unit and the modern dacite, andesite, and basalt 

unit. In addition, a small area of blast pyroclastic-flow deposits that in places overlie ancestral dacite was mapped 

where the blast deposits are more than 5 ft thick (fig. 8). 

These units are described on the basis of field characteristics; detailed petrographic results are not included.

Modern Dacite, Andesite, and Basalt Unit

The modern dacite, andesite, and basalt unit (dmu on 

fig. 8) is an unsorted, mostly unstratified mixture of material ranging from silt-clay-sized particles to boulders up 

to 5 ft across. The rock types in the unit are predominantly dacite, andesite, and basalt from the modern (less 

than 2,500-year-old) cone of Mount St. Helens, as mapped 

by C. A. Hopson (written commun., 1983). (The distinction between the lavas of the modern cone and an underlying "old Mount St. Helens series" was noted by Verhoogen (1937). Hopson (written commun., 1983) referred 

to "modern" and "ancestral" lavas, and his terminology is 

adopted for this report.)

Minor amounts of ancestral dacite, dacitic pumice, and 

organic debris from the old mountain are also found in the 

modern dacite, andesite, and basalt unit, but the proportions are difficult to estimate because of the lack of good 

exposures. Examination of surficial float and selected outcrops cleared of slopewash suggests that dacite, andesite, 

and basalt from the modern cone of Mount St. Helens 

make up more than 75 percent of the unit.

The modern dacite must have come from the Goat 

Rocks or the Summit domes or from their volcaniclastic 

debris. These were the only dacite exposures on the 

Mount St. Helens edifice prior to May 18, 1980, and were 

the only two domes less than 2,500 years old that were incorporated in the debris avalanche (Voight and others, 

1981). The Goat Rocks and Summit domes were composed 

of augite-hornblende-hypersthene dacite that is grey 

when fresh and various shades of red or pink when hydrothermally altered. The dacite is nearly aphyric in hand 

specimen, but close examination reveals microphenocrysts of hornblende, pyroxene, and plagioclase, generally less than 0.1 in. long. Xenoliths of varying composition are common in these and other domes of Mount St. 

Helens (Hopson, oral commun., 1980).

The modern andesite and basalt rocks within the debris-avalanche deposit are derived from various lava 

flows on the pre-1980 edifice of Mount St. Helens (Verhoogen, 1937; Hopson, written commun., 1983), indistinguishable in hand specimen. The rocks are generally black 

or dark grey but are locally various shades of green or red 

when hydrothermally altered. The andesite is generally 

plagioclase phyric with varying amounts of hypersthene 

and augite. Olivine is rare and usually occurs as phenocrysts less than 0.05 in. across. The basalt is either 

aphyric or olivine-phyric and is much less abundant than 

the andesite. The andesite and basalt range from nonvesicular to scoriaceous.

Ancestral Dacite Unit

The ancestral dacite unit (da on fig. 8) is an unsorted, 

unstratified mixture of material ranging from silt- and 

clay-sized particles to clasts many feet in diameter. Much 

more homogeneous than the modern dacite, andesite, and 

basalt unit, the ancestral dacite unit consists almost entirely of the "ancestral dacite" lithology, often hydrothermally altered. However, lenses of andesitic breccia, generally measuring less than 1 ft2, are observed locally. 

These probably represent brecciated andesite dikes, 

created from dikes similar to the undeformed ones observed in the present crater of Mount St. Helens. Brecciated dikes may also be blended into the ancestral 

daciate; of 100 pebbles 1-4 in. in diameter that were 

examined at one exposure, 88 percent are ancestral dacite 

and 12 percent are other lithologies, primarily andesite.

The ancestral dacite originates from the complex of 

dacitic domes and associated breccias more than 2,500 

years old that make up the core of Mount St. Helens. 

These rocks are present in the light-colored exposures in 

the 1980 crater below 7,500 ft, and they form a few exposed domes around the flanks of the mountain. The ancestral dacite lithology is readily identified in the field. It 

is a hornblende-hypersthene dacite, grey when fresh and 

various shades of red, pink, and green when hydrothermally altered. It contains prominent phenocrysts of 

hornblende and plagioclase greater than 0.1 in. long, as 

well as less prominent hypersthene phenocrysts. 

Xenoliths of varying composition are present locally.

BLAST PYROCLASTIC-FLOW DEPOSITS

The blast pyroclastic-flow deposits (bp on fig. 8) are 

the hot, highly fragmented deposits of the directed 

blast of May 18, 1980. They were mapped only in the 

northwest corner of the study area where they are 

more than 5 ft thick and overlie ancestral dacite.
PHYSICAL PROPERTIES OF BLOCKAGE MATERIAL

TABLE 1. Particle-size distribution in the South Fork Castle Creek blockage

13

Particle-size classification

Sample 

number

82-826-1 - -

82-826-2 - -

82-827-8 - -

82-827-6 - -

82-827-7 - -

82-826-3 - -

82-826-4 - -

Geologic 

unit

dmu1

do.

do.

do.

do.

Ancestral dacite

Blast pyroclastic-flow deposit

Gravel 

(percent)

43.6

54.9

56.2

48.9

44.8

41.8

33.1

Sand 

(percent)

47.0

37.3

36.8

40.7

42.4

49.0

50.9

Silt-clay 

(percent)

9.4

7.8

7.0

10.4

12.8

9.2

16.0

Median 

diameter 

(millimeters)

0.7

1.3

3.5

1.7

1.2

1.1

0.5

Gradation

Uniformity 

coefficient2

42

42

91

74

66

37

25

Sorting 

coefficient*

5

6

8

8

8

6

6

'Modern dacite, andesite, and basalt unit.

2Uniformity coefficient (Hazen's): ratio of the diameter at the 60-percent-finer 

point and that at the 10-percent-finer point on a gradation curve.

'Sorting coefficient of Trask (1930).

Where exposed, the blast pyroclastic-flow deposits 

are generally a poorly sorted, unstratified mixture of 

angular material ranging from silt- and clay-sized particles to fragments several inches in diameter. They are 

identified by an undulating, wavy upper surface and 

olive-grey exposures that contain a blended mix of all 

pre-1980 Mount St. Helens rock types. They contain a 

characteristic prismatically jointed, semivesicular steelblue-grey hypersthene-hornblende dacite that is referred to as blast dacite and is thought to be juvenile material representing fragments of the pre-May 18, 1980, 

cryptodome (Hoblitt and others, 1981).

On the higher areas of the blockage, the blast pyroclastic-flow deposits generally are finer grained and do 

not contain clasts larger than half an inch in diameter. 

The unit here has characteristics similar to those of 

layers A2 and A3 of Waitt (1981) and of the pyroclastic 

surge and accretionary lapilli units of Hoblitt and 

others (1981).

PHYSICAL PROPERTIES OF 

BLOCKAGE MATERIAL

TEXTURE

Samples weighing from 4 to 11 Ib were collected in 

the blockage material at five sites in the modern dacite, 

andesite, and basalt unit (which composes most the 

blockage) and at one site each in the ancestral dacite 

unit and the blast pyroclastic-flow deposits to determine particle-size distribution. Only particles less than 

64 mm were considered in the analysis, although blocks 

meters in diameter are present locally. All the units are 

poorly sorted. Silt and clay content for the samples 

from the modern dacite, andesite, and basalt unit 

ranged from 7.0 to 12.8 percent, sand content from 36.8 

to 47.0 percent, and gravel content from 43.6 to 56.2 

percent. Corresponding values for the single sample

from the ancestral dacite unit were 9.2, 49.0, and 41.8 

percent, and for the blast pyroclastic-flow deposit 16.0, 

50.9, and 33.1 percent. Median diameters for the modern dacite, andesite, and basalt unit ranged from 0.7 to 

3.5 mm; the sample median diameter for the ancestral 

dacite unit was 1.1 mm and for blast pyroclastic-flow 

deposits, 0.5 mm. Uniformity coefficients (table 1) 

ranged from 25 to 91; those values are lower than the 

average of 104 reported for the avalanche as a whole by 

Voight and others (1981).

Particle-size data alone provide an incomplete picture 

of the texture of the modern dacite, andesite, and 

basalt unit. When vertical exposures of at least 10 ft2 

are cleared of loose material, many lithologically distinct blocks of breccia from the old mountain in various 

degrees of disaggregation can be seen. A typical exposure is shown in figure 9. Although the material properties differ slightly from block to block, the range is 

probably reasonably represented by the sampling.

UNIT WEIGHT AND RELATIVE DENSITY

Physical properties of particles measuring less than 

4.75 mm were determined for eight samples from the 

modern dacite, andesite, and basalt unit; one sample 

from the ancestral dacite unit; and one sample of blast 

pyroclastic-flow deposit from the blockage (table 2). 

Physical properties of samples from five sites on the debris avalanche outside the blockage are also shown for 

comparison.

True specific gravities of the modern dacite, andesite, and basalt unit samples just described ranged from 

2.57 to 2.70 (table 2). The samples of the ancestral dacite and blast pyroclastic-flow units had specific 

gravities of 2.68 and 2.69, respectively.

Dry unit weight (yd), determined by the sand-cone 

method, gave values for the avalanche in the range of
TABLE 2. Physical properties of the South Fork Castle Creek blockage and other sites on the debris-avalanche deposit 

[Particle size less than or equal to 4.75 mm; ydmin, laboratory-determined minimum unit weight, in pounds per cubic foot; ydmax, laboratory-determined maximum unit weight, in pounds per 

cohesion, in tons per square foot; 0', effective friction angle; 0*, apparent friction angle]

Sample 

number

South Fork Castle Creek Bl< 

82-826-1 

82-826-2 

82-827-6 

82-827-7 

82-827-8 - - - - 

CC14 ------

CC34

82-826-3 

82-826-4

Geologic unit

ockage: 

drnu8 

do. 

do. 

do. 

do. 

do. 

do. 

do. 

Ancestral dacite 

Blast pyroclastic flow

Specific 

gravity

2.70 

2.70 

2.57 

2.60 

2.66

2.68 

2.69

Dry weight 

(lbs/ft8)

99.3 

94.3 

89.9 

93.6 

108.6 

105.9 

119.6 

116.1 

90.5 

126.1

Field water content 

(pet)

5.6 

3.0 

8.2 

9.3 

5.2

3.4 

6.2

Void 

ratio2

0.70 

.79 

.79 

.73 

.53

.85 

.33

Porosity2 

(pet)

40 

43 

46 

44 

34

45 

24

ydmin 

(lbs/ft')

117.1 

115.0 

82.4 

97.2 

113.0

109.1 

136.7

ydmax 

(lbs/ft8)

136.3 

134.8 

93.2 

113.6 

132.4

129.0 

127.6

'A 

(pet)

0 

0 

55 

0 

0

0 

93

(to

0

Debris avalanche, various sites:

DX5-3 -----

DX3-20 ----

DX5-24

COE ------

( 'OF'

Modern dacite

Ancestral dacite

do.

do.

do.

2.69

2.71

2.75

2.73

2.73

115.4

129.8

116.1

130.0

118.9

 

 

 

68.0 .31

A O .43

24

30

95.5

108.2

85.2

108.1

107.5

117.0

137.3

112.6

132.6

128.5

95

26

100

91

0

2

'Results for South Fork Castle Creek blockage samples are suspect, as noted in text . 

Calculations using average specific gravity, 2.65. 

'Modern dacite, andesite, and basalt unit.

4L. Youd, R. Schuster, and B. Voight, U.S. Geological Survey, 1983. 

'Assumed water content.
PHYSICAL PROPERTIES OF BLOCKAGE MATERIAL 15

90 to 120 lb/ft3 (table 2). This wide range and the low 

average value of 102 lb/ft3 possibly reflects late and (or) 

postdepositional disturbances of portions of the blockage; materials in sheared or slumped levee walls and 

boundary zones have been observed locally to be less 

dense than less disturbed debris elsewhere on the avalanche (Voight and others, 1981). However, operator 

error is suspected in several of these measurements. 

Three measurements considered reliable give a range of 

106 to 120 lb/ft3 and an average of 114 lb/ft3 (Youd, 

Schuster, and Voight, written commun., 1983). One 

sample of the blast pyroclastic-flow deposit yielded a 

dry unit weight of 126 lb/ft3 . Water content, w, the weight of water divided by the 

weight of dry solids and expressed as a percentage, 

ranged from 3.0 to 9.3 percent and averaged 6.9 percent. Because samples were obtained near the surface 

following a dry period in August 1982, these values 

may be low in comparison with representative material 

at depth.

Including unreliable samples, the calculated average 

wet unit weight (the product of (l+w) and dry unit 

weight) for the South Fork Castle Creek blockage at 

the time of sampling was 109 lb/ft3. The average value 

of reliable measurements was 122 lb/ft3 .

By comparison, wet-unit-weight measurements for 

four large samples (6 ft in diameter and several feet 

thick) from the debris-avalanche deposit near Spirit 

Lake averaged 120 lb/ft3 for an average water content 

of about 8.8 percent (three samples; T. J. Seeman, 

written commun., 1983). On the basis of these measurements, the average dry unit weight is 108 lb/ft3 , a value 

somewhat less reliable than the wet weight because of 

assumptions made about water content. The average 

dry unit weight for 36 conventional sandcone measurements widely distributed over the avalanche is 119 lb/ 

ft3 (H. X. Glicken, written commun., 1983). The more 

reliable data from the blockage, yielding an average dry 

unit weight of 114 lb/ft3 , thus suggest that unit-weight 

values are similar to those at other sites on the debrisavalanche deposit.

Relative densities, Drt (Terzaghi and Peck, 1967) 

were calculated by comparing field dry unit weights, 

-yc?, with laboratory-determined maximum and minimum unit weights, ydmax and dmin, and are reported 

as percentage:

Dr =

-yd - cfanin \ max

-ydmax ydmin/ \ yd /

100.

Laboratory tests were performed on samples from 

the blockage using material 3 in. or less in diameter in 

0.5-cubic-foot cylindrical molds. Maximum unit weights

were determined using dry samples subjected to a 0.14- 

ton-per-square-foot surcharge on a vibratory table. 

Minimum unit weights were gained by gentle placement of debris into the mold, and the test was repeated 

until consistent results to within 1 percent were obtained-.

For the samples from the South Fork Castle Creek 

blockage, relative density of the blast pyroclastic-flow 

deposit sample was 93 percent; relative density of one 

sample from the debris-deposit avalanche was 55 percent. For the remaining samples from the debris-avalanche deposit, relative density was less than 0 (that 

is, dry unit weight was less than minimum unit weight) 

(table 2). The result, Dr <0, is probably caused by erroneous field determinations of dry unit weight. Laboratory determinations of dry densities are considered 

accurate (table 2); in these tests, average values of minimum and maximum dry unit weight for five samples 

from the modern dacite, andesite, and basalt unit were 

105.1 and 122.9 lb/ft3, respectively. Using these as extreme values, the average field dry unit weight of 113.9 

lb/ft3 for these samples (based on three measurements 

that were considered reliable) corresponds to a relative 

density of 53 percent. The average of 25 determinations 

of relative density distributed over the entire avalanche 

is 48 percent, a comparable value. Although fragmentary, the data suggest that the degree of compaction of 

the deposit at South Fork Castle Creek is probably typical of all avalanche debris.

STRENGTH

Laboratory-drained direct-shear tests were conducted 

at the North Pacific Division Material Laboratory, U.S. 

Army Corps of Engineers, Portland, Oreg., on representative splits of one sample from the blast pyroclastic-flow 

deposit and three samples from the modern dacite, andesite, and basalt unit (T. J. Seeman, written commun., 

1983). The fraction of the samples less than 4.75 mm was 

compacted to approximately field density, as estimated 

by sandcone measurements in a shear box 3-in. square 

and 0.75 in. thick. A normal pressure of 3 tons/ft2 was applied, and after a 24-hour soaking period, shearing was 

accomplished at a rate of 0.001 in. per minute to a displacement of 0.5 in. The procedure was repeated on fresh 

sample splits for normal pressures of 5 and 10 tons/ft2 .

The samples from the debris-avalanche deposit show 

slight development of peak strength and mainly contractive volumetric changes, characteristic of loose specimens 

of particulate material and in keeping with the low field 

estimates of dry unit weight. Linear Coulomb regressions 

for individual samples give effective stress-friction angles 

(cf>') of 35° to 38° and cohesion intercepts (<?') of from 0.10 

to 0.25 ton/ft2 (table 2). Forcing the Coulomb envelope 

through the origin gives apparent friction angles (cf>*) of
16 STABILITY, SOUTH FORK CASTLE CREEK BLOCKAGE, MOUNT ST. HELENS AREA, WASH.

30-1

O

O 

u_

LD 

CC

D 

8 20

DC 

ID 

CLc/) w

ID

cc.

ID

D

i

X

IOEXPLANATION

Least-squares linear regression 

forced through origin

and

95-percent confidence limits 

to mean value

Least-squares linear regression

and

95-percent confidence limits 

to mean value

Data point

0 

A

10

 T" 

20 30

NORMAL STRESS, IN TONS PER SQUARE FOOT

FIGURE 10. Results of direct shear-strength tests on the modern dacite, andesite, and basalt unit of the South Fork Castle Creek blockage:

A, peak (maximum) strength.

36° to 38.5° fitted through data points in the normal pressure range of 3 to 5 tons/ft2 (table 2).

Combined data for samples from the modern dacite, 

andesite, and basalt unit yield, by least squares analysis,

an average intercept of 0.16 ton/ft2 and peak <J>'of 36.4° 

(^=0.966; fig. IQA). Also for combined data, peak <}>* is 

37.1° (^=0.999) and ultimate <}>* is 34.9° (^=0.999; fig. 

1QB). The peak-strength estimates for these samples are
PHYSICAL PROPERTIES OF BLOCKAGE MATERIAL 17

30-1

o 

oLL

LU 

OC

D

2 20-

cc

LU 

Q.

z 

o

LU 

CC

LU

LU

<

i

ti

D

10-

EXPLANATION

Least-squares linear regression 

forced through origin 

and

95-percent confidence limits 

to mean value

Least-squares linear regression

and

95-percent confidence limits 

to mean value

Data point

10 20 

NORMAL STRESS, IN TONS PER SQUARE FOOT

30

FIGURE 10. Results of direct shear-strength tests on the modern dacite, andesite, and basalt unit of the South Fork Castle Creek blockage:

B, ultimate strength.

conservative, because the strength tests were carried out 

at a degree of compation believed to be small compared 

with the more reliable field estimates. A comparison sample, DXS-3, taken adjacent to the blockage from debris

composed mainly of modern dacite, gave comparable 

values (table 2).

The sample from the blast pyroclastic-flow deposit, 

with .Dr=93, was stronger than the sample from the mod-
18

30 H

O 

O

LL

LLJ 

OC

a 20-

CC 

LU 

0_

z

O

LLJ 

DC

QC 

<

x 

<

10-

STABILITY, SOUTH FORK CASTLE CREEK BLOCKAGE, MOUNT ST. HELENS AREA, WASH.

EXPLANATION

 Least-squares linear regression

forced through origin 

and

95-percent confidence limits 

"" to mean value

Least-squares linear regression

and

95-percent confidence limits 

to mean value

Data point

~r~ 

20

 r~ 

10 30

NORMAL STRESS, IN TONS PER SQUARE FOOT

FIGURE 11. Results of direct shear-strength tests on the ancestral dacite unit of the avalanche debris from various sites: A, peak (maximum)

strength.

ern unit debris-avalanche sample, with <}>'=41.50and 

c'=0.50 ton/ft2, and peak <f>*=44°. Combined results for 

five samples from the blast pyroclastic-flow deposit at 

various sites between Castle Creek and Spirit Lake (average A-=61) indicate peak <}>'=33.64° and c' = 1.42 tons/

ftV=0.721); peak <f>*=39° (r^O.932), and ultimate <}>* 

=33.6° (7^=0.966).

Strength determinations were not made for the ancestral dacite unit of the debris-avalanche deposit at South 

Fork Castle Creek, but the combined results for four sam-
PHYSICAL PROPERTIES OF BLOCKAGE MATERIAL 19

30-

O 

O

UJ

oc

D

Si 2°-

oc

UJ 

0.

UJ

cc

UJ 

1/5

5

D

EXPLANATION

Least-squares linear regression 

forced through origin

and

95-percent confidence limits 

to mean value

Least-squares linear regression

and

95-percent confidence limits 

to mean value

D O Data point

0

B

10 20

NORMAL STRESS, IN TONS PER SQUARE FOOT

30

FIGURE 11. Results of direct shear-strength tests on the ancestral dacite unit of the avalanche debris from various sites: B, ultimate strength.

pies from the ancestral dacite unit at other sites on the avalanche (table 2) are as follows: peak <J>'=31.6° and 

c' = 1.80 tons/ft2 (^=0.775); peak <J>*=39.4° (^=0.942), 

and ultimate <J>*=33.5° (^=0.975) (figs. IIA, B).

These results imply strength values somewhat lower 

than those given by Voight and others (1981) for samples 

distributed over the avalanche. They reported peak friction angles of about 38° to 44° for the same particle-size
20 STABILITY, SOUTH FORK CASTLE CREEK BLOCKAGE, MOUNT ST. HELENS AREA, WASH.

range for nine avalanche samples. The analysis of the latter samples assumed zero cohesion, and thus their friction 

angles are comparable to values of 0* determined during 

this study. Similarly, T. L. Youd and others (written commun., 1981) reported a peak angle of 42.5° for a single 

sample of debris-avalanche deposit. Both papers described reduction of friction angle (ultimate strength) as 

a function of displacement, and the validity of this concept 

is sustained by our results.

Several factors influence the cohesion values reported 

here. In some instances, sample heterogeneity has resulted in scattered data points on a Coulomb plot; linear 

regression results in a cohesion intercept that is more a 

reflection of data scatter than of finite strength at low normal pressure. In other instances, the data imply an intercept if a linear regression is assumed. Some workers 

might prefer to assume c'=0 and to restrict strength envelopes to linear, or nonlinear, regression lines passing 

through the origin (Voight and others, 1981). The cohesions are generally considered "apparent" values, mainly 

reflecting incomplete saturation of test specimens during 

shear. Although breakage of angular grains might have 

occurred in some tests (particularly in some samples from 

the ancestral dacite unit of the debris-avalanche deposit), 

the reported cohesions do not represent reproducible 

strength components that can be relied on in analyzing 

slope stability.

GROUND-WATER CONDITIONS

In July 1983, the U.S. Geological Survey installed 11 

piezometers in the South Fork Castle Creek blockage to 

determine ground-water levels. Water-level measurements taken on September 20, 1983 (after sufficient 

time had been allowed for the wells to recover from the 

effects of drilling and development), showed that a 

ground-water mound existed under the crest of the blockage (fig. 12). Water levels in the mound were as much as 

22 ft above lake level at the time. Depth to water below 

the crest of the blockage in September 1983 ranged from 

about 60 to 20 ft, the greatest depth to water occurring 

in the northwestern part of the blockage and the least in 

the southeastern part. Water levels in the mound above 

lake level indicate that water moves from the mound into 

the lake and toward Castle Creek.

Ground-water levels in the blockage can be expected to 

fluctuate seasonally as the factors that affect groundwater recharge and discharge vary, but the lack of waterlevel measurements before July 1983 precludes definite 

determination of the amplitude of the changes. However, 

the timing of fluctuations should be similar to that observed in the similar blockage that created Coldwater 

Lake, 2 mi north of South Fork Castle Creek Lake on the 

opposite side of the North Fork Toutle River valley (fig. 

1). Water levels in that blockage have been measured in

piezometers since July 1981 and have been observed to 

fluctuate seasonally, with annual highs in May and June 

and lows in September through October (fig. 13). The 

amplitude of these seasonal changes varies according to 

the location of the piezometer, but maximum changes of 

15 ft have been observed along the crest of the blockage.

The 2-year record of ground-water levels at the Coldwater Creek blockage also showed a net annual increase 

in water levels of 35 ft from August 1981 to August 1982 

and 15 ft from August 1982 to August 1983. This indicates 

that discharge from the ground-water system in the Coldwater blockage has yet to equilibrate to natural recharge; 

this is an important consideration in analyzing the stability of the South Fork Castle Creek blockage. Because 

slope stability varies indirectly with ground-water levels, 

seasonal changes and the potential for annual increases in 

water levels in the South Fork Castle Creek blockage are 

important. Water levels shown in figure 12 can be assumed to be near their seasonal low and were used to formulate baseline conditions for the stability analyses that 

follow. Stability analyses for higher ground-water levels 

were also made and are discussed in the next section.

The proximity of the Coldwater Creek blockage to the 

South Fork Castle Creek blockage and the similarity of 

their composition suggest that water-level changes similar to those at the Coldwater blockage may be occurring 

at the South Fork Castle Creek blockage. Owing to differences in thickness, geometry, and other features of the 

blockages, the magnitude of the changes may be different, however.

STATIC SLOPE-STABILITY ANALYSIS

Factors of safety (Fs) were determined for five cross 

sections on the blockage by using a digital computer program, STABL3, prepared by the Joint Highway Research Project, Engineering Experiment Station, Purdue 

University. The program contains a subroutine that 

searches for slip surfaces most likely to fail. The program 

allows use of either the Modified Bishop Method or the 

Janbu Method of Slices for determining factors of safety 

associated with the model-predicted failure surfaces. The 

Modified Bishop Method was used throughout this study. 

As an independent check on the results, an analysis using 

the Morgenstern and Price Method was performed on a 

selected number of failure surfaces generated by 

STABL3; the resultant factors of safety were virtually 

identical (W. K. Smith, oralcommun., 1982).

MODELED CROSS SECTIONS 

AND ASSUMPTIONS

Four of the five cross sections tested are on the downstream side of the blockage, in the area from the blockage 

crest toward Castle Creek (fig. 14). Of these four cross
STATIC SLOPE-STABILITY ANALYSIS 21

EXPLANATION 

2570' Water-table altitude

0 800 FEET

0 100 200 METERS 

Contour interval: 10 feet

FIGURE 12. Water-level contours for South Fork Castle Creek blockage, September 20,1983.
22 STABILITY, SOUTH FORK CASTLE CREEK BLOCKAGE, MOUNT ST. HELENS AREA, WASH. 

2480

2470

2460

2450

LLJ 

LLJ 

LL.

Z 2440

2430

2470

2460

O ND JFMAMJJASON DJFMAMJJASO

1981 1982 1983 

FIGURE 13. Hydrographs for two selected piezometers at Coldwater Creek blockage.
STATIC SLOPE-STABILITY ANALYSIS 23

EXPLANATION 

A A' Cross-section line

FIGURE 14. Location of cross sections on South Pork Castle Creek blockage that were analyzed for slope stability.
24 STABILITY, SOUTH FORK CASTLE CREEK BLOCKAGE, MOUNT ST. HELENS AREA, WASH.

sections, the three (AA', BB', and CD) on the western 

part of the blockage, where downstream surface relief is 

greatest, are representative of downstream areas most 

susceptible to slope failure. The fourth (GG') is representative of the eastern part of the blockage, where overall 

downstream relief is low. Surface geology, configuration 

of the preeruptive surface, and ground-water levels for 

September 8, 1983, were determined for these sections 

(figs. 15A-D). The modern dacite, andesite, and basalt 

unit is the predominant unit in all four cross sections.

Because slopes and subsurface conditions along the upstream side of the blockage, and the profile from the crest 

toward South Fork Castle Creek Lake, are generally uniform, one cross section, CF, was used to investigate the 

potential for slope failure in the upstream direction (fig. 

15OAt each cross section except GG', Fs was determined 

for six different conditions (four for GG'). In all cases, wet 

and saturated unit weights of 105 and 125 lb/ft3 , respectively, were assumed. These estimates were made before 

the field and laboratory test programs were completed, 

and the assumed unit-weight values are now considered 

low. A sensitivity analysis was conducted to examine the 

influence of unit weights on predicted factors of safety. 

Extreme values for unit weights produced a change in Fs 

no greater than 0.1.

A friction angle of 35° was used for all analyses. This 

value is probably conservative because it approximates 

the laboratory ultimate for material from the modern dacite, andesite, and basalt unit rather than the peak (or 

some intermediate) condition, and the laboratory values 

ignore strength contributions from larger clasts and from 

moderate compaction. Because the ultimate friction angle 

for the blast pyroclastic-flow deposit and ancestral dacite 

unit is about 33.5°, the use of 35° may not be entirely conservative. Because the modern dacite, andesite, and 

basalt unit constitutes most of the blockage, 35° is considered reasonable for analytical purposes.

Using the above assumption, cohesion was set to zero 

and stability was determined at each cross section for 

three positions of the water table: (1) the measured water 

level for September 1983, (2) the September 1983 water 

level plus 25 ft, and (3) the water level at land surface. 

Cohesion was next set to 200 lb/ft2 and stability was again 

determined for the three positions of the water table. At 

section GG', howe^7er, September 1983 water levels were 

about 20 ft below the crest, and therefore only water-level 

positions 1 and 3 were used the September 1983 water 

level and the water level at land surface. These positions 

had values of cohesion at 0 and 200 lb/ft2.

The program requires definition of a lower boundary 

that no generated failure surface will penetrate. The 

preemption surface underlying the blockage was chosen 

as this boundary. Cross sections AA' and BB' are underlain by the bedrock ridge on the western side of South

Fork Castle Creek valley (figs. 15A, B), so the assumption of nonpenetration along those sections is valid. Cross 

sections DF and GG' are underlain by prehistoric unconsolidated volcaniclastic deposits, however, and the assumption that this material represents a boundary that no 

generated failure surface could penetrate may not be 

valid. However, results of the computer analysis along 

these sections indicate that the surfaces most likely to fail 

are above the preemption surface; potential failure along 

these sections is therefore independent of the nature of 

the preemption surface.

Cross sections AA' and BB' terminate near the ridge 

bordering the western side of the lake. Slope failure 

along these sections would not result in a lake breakout 

unless this failure weakened the remaining blockage or 

unless it was accompanied by a failure back into the 

lake near these sections. The latter possibility does 

exist, and therefore stability in the areas of cross sections AA' and BB' is evaluated.

RESULTS OF STATIC 

SLOPE-STABILITY ANALYSIS

For each set of physical conditions used in a model 

simulation, the program generated 100 failure surfaces 

and calculated the factor of safety, Fs, for each surface. 

It also identified the surface with the lowest factor of 

safety, Fs*, hereafter referred to as the critical failure 

surface. Model-predicted Fs * values are summarized in 

table 3 for each set of physical conditions simulated at

TABLE 3 Model-predicted factors of safety (Fs* ) for critical failure 

surfaces at South Fork Castle Creek blockage for selected positions 

of the water table and cohesion values

[Stress friction angle, 0', equals 35°; factors of safety less than or equal to 1.2 are 

considered potentially unstable]

Position of the water table

September 1983 -----------

Cross 

section

A A'

BB'

CD

CE

CF

GG'

Fs* 

for zero 

cohesion

2.1 

2.2

2.1

1.7

1.6

4.1

Fs* for 

200 Ib/ft" 

cohesion

2.4 

2.4

2.3

1.9

2.0

4.3

September 1983 level plus 25 feet -----

AA'

BB'

CD

CE

CF

AA' /vn

BB'

CD

CE

CF

GG'

1.6

1.8

1.4

1.2

1.2

0

0

.8

1.4

1.3

0.8

0.6

2.4

1.9

2.0

1.9

1.4

1.5

1.1

1.6

1.5

1.0

1.0

2.8
STATIC SLOPE-STABILITY ANALYSIS 25

the cross sections. A value of Fs less than or equal to 

1.0 is considered to indicate potential instability. Although Fs values predicted by the model should be considered reasonable approximations of actual values, 

some uncertainties about material properties and analysis exist, and therefore factors of safety 1.2 or less are 

interpreted as identifying sections of potential instability.

Several general trends are apparent from the results 

of the static stability analysis. First, as ground-water 

levels rise, factors of safety for potential failure surfaces decrease, indicating that the blockage becomes 

less stable. This trend is illustrated in table 3. Second, 

rising ground-water levels also cause the range in 

values for the 10 lowest Fs surfaces to narrow (fig. 16), 

indicating that larger areas of the blockage become unstable. Third, for a given water level, analysis using a 

cohesion value of 200 lb/ft2 shows an average increase 

in Fs* of 0.3 over the Fs* obtained with zero cohesion.

Results of the static analyses indicate that massive 

slope failures on the blockage due to gravitational forces 

are unlikely when the ground-water table is at or near its 

September 1983 position. Fs* values for failure surfaces 

at all cross sections are above 1.2 for this condition. Potential failure is predicted on the blockage with ground-water 

levels 25 ft above September 1983 levels and zero cohesion. When the water table is assumed to be at land surface and zero cohesion is assumed, additional cross sections on the blockage are predicted to fail. These failures 

alone are not expected to initiate lake breakout because 

part of the blockage is left intact above lake level. Only 

cross sections BB' and GG' are stable for all three positions of the water table. The most conservative analyses 

for these sections, a water level at land surface and zero 

cohesion, are shown in figures 11A and B.

Prediction by the model of potential failure surfaces 

when the water table is assumed to be 25 ft higher than 

for September 1983 depends on the value used for cohesion. When a cohesion value of 200 lb/ft2 is assumed, no 

slope failures are predicted. However, when zero cohesion is assumed, slope failures are predicted along cross 

sections CE and CF (table 3, fig. 18). The critical failure 

surface on cross section CE, with an Fs* value of 1.2, extends 45 ft behind the crest of the blockage and 10 ft below 

lake level. On cross section CF, the critical failure surface 

has a value of ^ = 1.1. Two other potentially unstable 

shallow failure surfaces occur where Fs values equal 1.1 

and 1.2. Neither surface penetrates the blockage below 

lake level. Such failures occurring simultaneously along 

both cross sections would not result in lake breakout, because a substantial part of the blockage above lake level 

would be left intact.

With the water table at the land surface, potential failure surfaces predicted at cross sections AA', CE, and CF

when both zero and 200-pound-per-square-foot cohesion 

values are assumed. At cross section AA' the predicted 

failure surfaces are shallow and do not extend below lake 

level for1 either value of cohesion (figs. 19A, B). Therefore, 

these failures would have little influence on short-term 

blockage stability. At cross section CE, potential surfaces 

extend a maximum of 25 ft below lake level and 100 ft behind the blockage crest for zero cohesion (fig. 2QA) and 50 

ft behind the crest for 200 lb/ft2 cohesion (fig. 205). At 

cross section CF, potential failure surfaces are shallow 

and none extend below lake level when a 200-pound-persquare-foot cohesion value is assumed. However, assuming a cohesion value of zero, a potential failure surface of 

1.2 extends abaout 30 ft below lake level and 50 ft behind 

the blockage crest. The critical failure surface does not extend below lake level and has an Fs* value of 0.6. The potential failure surfaces along cross sections CE and CF 

that are predicted assuming both water level at land surface and zero cohesion, and that have Fs values <1.2, constitute a significant mass of the blockage, and some extend well below lake level. However, failure along both 

surfaces having Fs values of 1.2 (fig. 20-A) would still leave 

a substantial part of the blockage intact and above lake 

level. Thus, lake breakout is not predicted for initial slope 

failures on the blockage, regardless of the position of the 

water table.

RETROGRESSIVE FAILURE ANALYSIS

Retrogressive slope failure was analyzed for composite 

cross section DF on the part of the blockage that remained 

after initial slope failures, assuming both water levels 25 

ft higher than those in September 1983 and at land surface. The analysis included three assumptions considered 

to be conservative: (1) the material above the initial and 

all successive sliding surfaces is removed from the blockage, (2) retrogressive sliding occurs along the deeply penetrating surfaces having Fs values <1.2 and encompassing the greatest amount of blockage material, and (3) no 

slide mass will interfere with or influence the movement 

of successive slides. Zero cohesion was also assumed in all 

analyses.

Were the water table to rise 25 ft above its September 

1983 position, retrogressive analysis indicates that the 

blockage would remain above lake level, thus precluding 

lake breakout (fig. 21). With the water table at land surface, analysis for retrogressive failure indicates that the 

blockage might be lowered below lake level, thereby causing lake breakout (fig. 22).

The degree of mobility necessary for breakout from 

retrogressive failure is considered unlikely, because the 

blockage is composed of moderately compact and poorly 

sorted debris, contains large clasts, and is subjected to 

moderate confining pressure. Slope failure would be 

more likely to occur in the form of debris slides,
26 STABILITY, SOUTH FORK CASTLE CREEK BLOCKAGE, MOUNT ST. HELENS AREA, WASH.

EXPLANATION

da Ancestral dacite unit

dmu Modern dacite, andesite, 

and basalt unit

Water table, September 1983 

Pre eruption land surface

LU 

LU

u_

LU

O 

DO

LU

Q

2500 

2400

0

A

1,00 200 300 400

DISTANCE, IN FEET

EXPLANATION

dmu Modern dacite, andesite, 

and basalt unit

Water table, September 1983 

Preeruption land surface

600

B'

______ ^pyKtff^S^*^''^?^//'^^*^''

7^Z$7^

400 500 600 700 

DISTANCE, IN FEET 

FIGURE 15. Geology and September 1983 water levels at^4, cross section AA 'and B, cross section BB'

800 850
STATIC SLOPE-STABILITY ANALYSIS 27

EXPLANATION

dmu Modern dacite andesite, _..__ Water table, September 1983 

and basalt unit

2300

100 200 300 400 500 600 700 800

DISTANCE, IN FEET

1000 1100 1200 1250

UJ 

LLJ

G

2700 r

2600

2500

O

CO

UJ 

Q

D 2400

2300

EXPLANATION

dmu Modern dacite, andesite -. 

and basalt unit

Tbr Tertiary bedrock "/^

Water table, September 1983 

Preeruption land surface

dmu

dmu

Tbr

G'

dmu

Tbr

0 100 200 300 400 500 600 700 800

D DISTANCE, IN FEET

FIGURE 15. Geology and September 1983 water levels at C, cross sections CD, CE, and CFand D, cross section GG'.

900 1000
Cross section A-A' Cross section B-B'

EXPLANATION 

Sept. 1983 water levels 

Saturated blockage

0.1 0.2

n.

0.3

FIGURE 16B

0.4 0.1

Cross section C-D

I I I 

EXPLANATION

Sept. 1983 water levels 

Sept. 1983 water levels + 25' 

Saturated blockage

0.1 0.2

n.

0.3 0.4

0.3 0.4

FIGURE 16 A 

Cross section C-D'

0.1 0.2 0.3 0.4
STATIC SLOPE-STABILITY ANALYSIS 29

Cross section C-F Cross section G-G

1 r i

EXPLANATION 

Sept. 1983 water levels 

Sept. 1983 water levels+ 25' 

Saturated blockage

0.1 0.2 0.3 0.4

FIGURE 16C

FIGURE 16. Variation of factor-of-safety values (F8) versus seismic coefficients (ng) for selected water levels

at all cross sections.
30 STABILITY, SOUTH FORK CASTLE CREEK BLOCKAGE, MOUNT ST. HELENS AREA, WASH.

EXPLANATION

Preeruption land surface

B

2700 i1.4

 *" Critical failure surface and 

factor of safety

400 500 

DISTANCE, IN FEET

2.4

2700

2600

LU

> 2500

O

00

Q

D

2400

2300

EXPLANATION

Preeruption land surface 

Critical failure surface and 

factor of safety

800 850

0 100 200 300 400 500 600 700 800 900 1000

B DISTANCE, IN FEET 

FIGURE 17. Critical failure surfaces predicted for A, cross section BB'andB, cross section GG', with water table at land surface and zero cohesion.
~~1 2 

2700

2600

o

CO

LLJ 

Q

2500

2300

SEISMIC HAZARD

EXPLANATION

Preemption land surface 

Water table, September 1983,+ 25' 

Potential failure surface and factor of safety 

Critical failure surface and factor of safety

31

South Fork Castle 

Creek Lake

100 200 300 400 500 600 700

DISTANCE, IN FEET

800 900 1000 1100 1200 1250

FIGURE 18. Critical failure surface and potential failure surface encompassing the largest section of the blockage for cross sections CE and CF

with water table 25 ft above September 1983 levels and zero cohesion.

slumps, and slump flows, and flowage at the toe of the 

displaced mass would be slow. Driving forces decrease 

and resisting forces increase as a function of displacement on deep-seated, curved slide surfaces, so displacements would be limited and would seldom exceed a 

value of approximately 100 ft. Movements of this order 

of magnitude would not be sufficient to lower the blockage crest to lake level or below. Therefore, the possibility of actual lake breakout from retrogressive slope failure is considered remote.

SEISMIC HAZARD

Estimates of the seismic hazard at the South Fork 

Castle Creek blockage are based primarily on information about the St. Helens seismic zone (SHZ), which 

passes within several miles of the blockage (fig. 1). Defined by earthquakes of small to moderate magnitude 

(2.5-5.5 local Richter magnitude), the SHZ is interpreted as a 60-mile-long fault zone capable of generating a 

shallow (crustal) earthquake of moderate to large magnitude (Weaver and Smith, 1983). Earthquake focal 

mechanisms along the SHZ indicate nearly vertical 

right-lateral, strike-slip faulting with the preferred 

fault planes striking north-south. From these focal 

mechanisms, the direction of maximum compression 

would appear to be northeast, approximately parallel 

with the direction of convergence between the North

American and Juan de Fuca plates. The focal 

mechanism data suggest "locked" subduction, and this 

interpretation raises the possibility of a subduction 

earthquake of larger magnitude (Weaver and Smith, 

1983).

An earthquake of magnitude 5.5 (local Richter magnitude) occurred on February 14, 1981, near Elk Lake, 

6.5 mi north of the South Fork Castle Creek blockage 

(Grant and others, 1984). More than a thousand aftershocks up to magnitude 4.5 delineated a north-southtrending fault zone 4 mi long by 2 mi wide, ranging 

from 3 to 8 mi in depth. The probable low-stress drop 

of the main shock suggests that preexisting stress 

levels were not extensively lowered and that another 

moderate earthquake may still occur near Elk Lake 

(Grant and others, 1984).

The occurrence of an earthquake larger than 5.5 magnitude along the SHZ must also be considered a possibility. Using empirical regressions of magnitude with 

fault length (Mark and Bonilla, 1977), a magnitude 7 

earthquake would correspond approximately with a 

rupture of 50 percent of the length of the SHZ. However, Weaver and Smith (1983) recognized that there is 

no reliable procedure for estimating a maximum magnitude for the 1983 circumstances, and they considered 

a magnitude 7 earthquake unlikely for two reasons. 

First, Mount St. Helens is in the middle of the zone and 

the zone may be discontinuous across the mountain.
32 STABILITY, SOUTH FORK CASTLE CREEK BLOCKAGE, MOUNT ST. HELENS AREA, WASH. 

EXPLANATION

2700

LU 

LU

2600

O 

CO

LU 2500 

O

D

2400

A

2700 r 

LU 

LU

O

LU

O 

CO

LU 

O

D

2600

2500

2400

1.8

11

Preeruption land surface

Potential failure surface and 

factor of safety

Critical failure surface and factor 

of safety

200 300

DISTANCE, IN FEET 

EXPLANATION

Preeruption land surface

400 500 600

_ _ 1-2 _ _ Potential failure surface and

0.8

factor of safety

Critical failure surface and 

factor of safety

A 1

100 200 300 400 500 600

B DISTANCE, IN FEET

FIGURE 19. Critical failure surfaces predicted for cross section AA' with water table at knd surface and A, 200 Ib/ft2 cohesion and B, zero

cohesion.
SEISMIC HAZARD 33

2800

2700

CD

> 2600

1.2

EXPLANATION

Pre-eruption land surface

- ' Potential failure surface and factor of safety 

~* : *~ Critical failure surface and factor of safety

1000 1100 1200 1250

DISTANCE, IN FEET

LU 

LU

LU 

>

o

00

2800

2700

2600

2500

Q

D D

2400

2300

1.2

EXPLANATION 

Pre eruption land surface

- -'-- Potential failure surface of safety

-* ' *- Critical failure surface and factor 

of safety

200 300 400 500 600 700

DISTANCE, IN FEET

800 900 1000 1100 1200 1250

FIGURE 20. Critical failure surface and potential failure surface encompassing the largest section of the blockage for cross sections CE and CF

with water table at land surface and A, zero cohesion and B, 200 lb/ft2 cohesion.
EXPLANATION 

Preeruption land surface 

Water table, September 1983

100 200 300 400 500 600 700 800

DISTANCE IN FEET

900 1000 1100 1200 1250

2500

D

EXPLANATION

 Water table, September 1983

2500 

D

South Fork Castle 

Creek Lake

100 200 300 500 600 700 800

DISTANCE, IN FEET

900 1000 1100 1200 1250

South Fork Castle 

Creek Lake

2500

D

South Fork Castle 

Creek Lake

I I

100 200 300 500 600 700

DISTANCE, IN FEET

900 1000 1100 1200 1250
DYNAMIC SLOPE-STABILITY ANALYSES 35

Second, the earthquakes are in a zone several miles 

wide in which no surface faulting has been found; therefore, the crustal earthquakes could involve rupture of 

short fault segments, on the order of, say, 6 mi. 

Weaver and Smith did not specify an expected 

maximum magnitude but did caution that the Elk Lake 

earthquake does not represent the largest possible 

event on the SHZ. Beaulieu and Peterson (1981), in an 

evaluation of potential hazards at a nearby nuclear site, 

suggested that a maximum magnitude of 6.2 is more 

likely than one of 7.

The U.S. Army Corps of Engineers proposed a magnitude 6.8 design earthquake for their stability analysis 

of the Spirit Lake blockage (U.S. Army Corps of Engineers briefing, July 5, 1983, Portland, Oreg.). This 

design earthquake would lie on the SHZ and have a 

near-source horizontal acceleration of 0.55 g, a shaking 

velocity of 1.8 ft/s, and a duration of strong shaking of 

30s.

Earthquakes along the SHZ in the magnitude range 

of 6.2-6.8 are capable of producing peak horizontal accelerations of 0.55 g at the South Fork Castle Creek 

blockage (H. B. Seed, written commun., 1983), so this 

acceleration value seems reasonably conservative for 

seismic analysis of the blockage. For purposes of comparison, a magnitude 5 earthquake on the Spirit Lake 

segment of the SHZ could produce peak accelerations of 

0.3 g for two or three loading cycles at the blockage.

SEISMIC HISTORY OF THE BLOCKAGE

Since its emplacement on May 18, 1980, the South 

Fork Castle Creek blockage has been subjected to 

three periods of shaking. The first occurred during the 

10 minutes of emplacement of the debris avalanche 

when the levee constituting the blockage was deposited 

before the remainder of the avalanche had come to rest. 

Ground vibrations associated with avalanche emplacement were considerable (Stephen Malone, oral commun., 1983); it is conceivable that the blockage was 

subjected to accelerations on the order of 0.1 g for several minutes. The second occurred from 11:40 a. m. to 

5:30 p. m. (Pacific daylight time) on the same day, 

when numerous shocks associated with the Plinian 

eruption of Mount St. Helens produced strong shaking. 

The blockage was subjected to peak accelerations on 

the order of Q.05 to 0.1 g (Stephen Malone, oral commun., 1983). Finally, the Elk Lake earthquake of February 14, 1981, could have produced peak accelerations 

of 0.2 g at the blockage for several cycles.

FIGURE 21. Predicted results of retrogressive slope-failure analysis 

on cross sections CD and CF with September 1983 water levels plus 

25 ft and zero cohesion.

The effects of these events on the debris-avalanche 

deposit have not been studied, but it is presumed that 

the ground vibrations produced compaction of the deposit and increased its relative density. Ground-water 

levels in the blockage at the time of these shaking 

events were probably lower than those measured in 

September 1983.

DYNAMIC SLOPE-STABILITY ANALYSIS

STABL3 contains an optional program that allows 

determination of the approximate effect of an earthquake the factor of safety; this program was used to determine Fs values, for two levels of earthquakes, at all 

the cross sections ^previously tested for static stability. 

In using this program, the assumption is made that dynamic loading from an earthquake can be replaced by a 

static horizontal force that is obtained from the product 

of the weight of the potential sliding mass and a seismic 

coefficient, ns. Under this assumption, the section may 

be considered unstable if Fs approaches 1.0, but there 

is no theoretical basis to support this. Among other 

things, the value of ns varies directly with earthquake 

magnitude, and two values of ns were used in the analysis, 0.2 g and 0.4 g. As will be discussed subsequently, 

the larger value is believed to correspond closely to the 

seismic coefficient of the maximum credible earthquake 

for the SHZ. The maximum credible earthquake is believed to be near, but greater than, magnitude 6.0.

MODELED CROSS SECTIONS 

AND ASSUMPTIONS

Fs values for all cross sections were determined for 

seismic coefficients equal to 0.2 and 0.4 g and for water 

levels at their September 1983 positions. In addition, Fs 

values for cross section CD, CF, CE, and BB were determined for the range in seismic coefficients indicated 

above and for ground-water levels 25 ft above their 

September 1983 configuration, as well as for water 

levels at land surface. Wet and saturated unit weights 

of 105 and 125 lb/ft3 , respectively, were assumed in all 

analyses. For all analyses, zero cohesion and a drained 

friction angle of 35° were used with the additional assumptions previously discussed under "Static SlopeStability Analysis."

CALCULATION OF ns

Methods of assigning seismic coefficient values include selecting an empirical value, assuming rigid-body 

response, or using viscoelastic-response analysis. Because the last method provides a more rational approach, it was used to estimate a value for ns that could 

be assigned to the blockage for the maximum credible
ALTITUDE ABOVE TVGT, IN FEET ALTITUDE ABOVE TVGT, IN FEET

§ 8 I O§ 8 O:

ALTITUDE ABOVE TVG

§ 1 i 

ID §
DYNAMIC SLOPE-STABILITY ANALYSES 37

earthquake along the SHZ. The value was obtained 

from the relationship:

nt Ua (2/), (1)

where Wis weight of sliding mass, 772 is an incremental 

horizontal slice of the mass, and tia is the absolute acceleration at each instant, t, under consideration; y is the 

vertical coordinate direction (Seed and Martin, 1966). It 

is necessary to specify boundaries of the sliding mass in 

order to compute average seismic coefficients, leading 

to a different average ns value for each potential mass 

at any instant of time. The matter is simplified by representing each sliding mass as a triangular wedge, thus 

permitting the average seismic coefficent over a given 

depth to be computed directly. For given values of embankment shear-wave velocity, damping factor, and 

embankment height, values of ns(t) can be computed for 

any particular ground-motion record.

For the September 1983 case, the blockage is assumed to be homogeneous, with a shear-wave velocity 

of 1,000 ft/s and 20 percent critical damping. The 

north-south component of the 1940 El Centro earthquake, with duration of 0 s, is assumed for analysis, 

with the peak acceleration scaled to 0.55 g. Calculations 

presented by Seed and Martin (1966) then allow direct 

estimation of seismic paramenters. Seismic coefficients 

thus determined vary considerably as a function of time 

(Seed and Martin, 1966, with ordinate values multiplied 

by 1.70). For September 1983 purposes, the variation in 

seismic coefficients is approximated by an equivalent 

number of acceleration cycles of constant amplitude. 

The fundamental period of the blockage foundation system is about 0.5 s at cross section DF. An equivalent 

seismic force series might be approximated by 10 cycles 

of force with a predominant frequency of 1.8 cycles/s, 

yielding, to one significant figure, the following 

amplitudes of average seismic coefficient ns(t): 0.6 g (for 

potential slide masses in the upper quarter of blockage), 0.5 g (for the upper half), and 0.4 g (for the 

upper three-quarters or full blockage height).

Thus, average seismic coefficients vary considerably 

with position in the blockage material, as well as with 

time, and higher values occur with increasing altitude. 

This latter consideration is significant, because it influences the position of the most critical failure surface.

FIGURE 22. Predicted results of retrogressive slope-failure analysis 

on cross sections CD and CF with water table at land surface and zero 

cohesion.

RESULTS OF DYNAMIC 

SLOPE-STABILITY ANALYSIS

General results of all the dynamic analyses indicate 

that for a given position of the water table increasing 

earthquake magnitudes cause Fs and Fs* values to decrease and potential failure surfaces to occur deeper in 

the blockage than they do in the results of the static 

analysis. All cross sections on the blockage, including 

those that were stable for all positions of the water 

table under static analysis, become unstable during 

shaking induced by earthquakes near the maximum credible value, even for September 1983 water levels. In 

addition, for an earthquake of given magnitude, rising 

ground-watel* levels cause Fs and Fs* values to decrease, and in some cases they cause potential failure 

surfaces to penetrate deeper into the blockage. Decrease in values of Fs and Fs* as a result of increasing 

earthquake magnitude and ground-water levels is 

shown in figure 16. Each zone shown in the figure represents a given water-level condition and is bounded by 

the maximum and minimum Fs values for the 10 surfaces predicted by the model for each analysis. In addition to decreasing, the range in values for the 10 low Fs 

surfaces also narrows considerably.

Reduction in Fs and Fs* values caused by increasing 

earthquake magnitudes and by increasing ground-water 

levels is significant."Table 4 shows this reduction for both 

conditions, and, as can be seen, values of Fs* fall below 

unity along many sections for higher ns values and (or) 

higher* ground-water levels. Fs* values are below unity 

for all sections except GG' for na =0.49 g, regardless of the 

position of the water table. They are also below unity for 

water levels 25 ft higher than in September 1983 and 

7?s =0.2 g on all cross sections tested for this set of conditions. Narrowing of the range of the 10 low Fs surfaces 

shown in figure 16, combined with the reductions in Fs 

and Fs*, indicates that larger areas of the blockage become potentially unstable as earthquake magnitudes and 

ground-water levels increase.

Deformation over the entire blockage would be possible 

if an earthquake of a magnitude near 6.0 occurred when 

water levels were at their September 1983 position. Critical failure surfaces at all cross sections except AA' extend 

below lake level; the deepest extends approximately 120 

ft below lake level and 100 ft behind the crest of the blockage. The model does not predict that any of these initial 

failures would result in immediate lake breakout, because, even if slippage occurred simultaneously from the 

crest toward the lake and from the crest toward Castle 

Creek, a part of the blockage would remain intact and 

above lake level.

If a smaller earthquake (ns=0.2 g) occurred with water 

levels at their September 1983 position, considerable de-
38 STABILITY, SOUTH FORK CASTLE CREEK BLOCKAGE, MOUNT ST. HELENS AREA, WASH.

TABLE 4. Model-predicted factors of safety (Fs)for selected seismic coefficients (ns)

Cross 

section Water-table position MS = 0 ris = 0.2 MS = 0.4 Remarks on critical surfaces

AA' 

BB'

CD 

CE

Sept. 1983 level "2.10

Sept. 1983 level --------- *1.40

At land surface --------- 1.41

Sept. 1983 level --------- *2.10

Sept. 1983 level plus 25 ft - - - - *1.64

At land surface --------- *1.30

1.48

Sept. 1983level ---------

Sept. 1983 level plus 25 ft - - - -

CF -----

"1.70 

"1.20 

1.46 

At land surface --------- *.82

.86

Sept. 1983level --------- *1.60

Sept. 1983 level plus 25 ft - - - - *l.ll

1.24 

At land surface --------- * .63

1.18

GG' ----- Sept. 1983 level "4.10

*1.20

*1.10

* .76

*1.11 

.92

* .76

.78

*1.11

* .78 

.83

* .57 

.57

*1.05

* .76 

.78

* .47 

.70

*1.71

*0.90

* .70

* .52

* .69 

.63 

.53

* .53

* .76

* .57 

.58 

.44

* .43

*0.72 

.56

* .56

* .38 

.50

*1.05

Shallowest surface of 10 predicted.

Shallow Fs = 1.4 surface, figure 11 A.

Shallow surface passing below slope break.

Fs = 1.2 surface, fig. 18.

Fs = 0.8 surface, fig. 20.

Shallower than Fs = 0.8 surface, fig. 20.

Fs = 1.1 surface, fig. 18. 

Fs = 1.2 surface, fig. 18. 

Fs = 0.6 surface, fig. 20. 

Fs = 1.2 surface, fig. 20.

*Critical value of factor of safety.

formation of the blockage also would be possible. The 

model predicts potential failure surfaces at all sections except GG', where Fs* is 1.7. The greatest potential instability occurs along cross section DF, where potential failure surfaces reach their maximum distance below lake 

level (fig. 23). Once again, however, the model does not 

predict immediate breakout, because simultaneous movement on both the upstream and downstream parts of the 

blockage would still leave a section of the blockage intact 

and above lake level. Retrogressive analysis of this remaining section, using the same assumptions that were 

used for static conditions, suggests that after a series of 

three successive, highly mobile failures on both sides of 

the blockage, the crest would be lowered below lake level 

and the lake could be released. The same results occur 

when retrogressive analysis is conducted on the section 

remaining after an earthquake near magnitude 6.0.

Model analysis predicts that some part of the blockage 

would remain intact and above lake level for all positions 

of the water table tested and for all earthquake magnitudes up to the maximum credible. Although large masses of blockage material are predicted to move, the altitude of the blockage crest would remain the same under 

the worst conditions analyzed the water table at land 

surface level, a maximum credible earthquake, and simultaneous slippage from the crest toward the lake and from 

the crest toward Castle Creek along the deeptest potential failure surfaces. Retrogressive failure analysis on the

remaining sections, however, predicts lake breakout in all 

cases.

DISPLACEMENT OF SLIDING MASSES

The preceding analyses have indicated that retrogressive slope failure could result in lake breakout for static conditions if the blockage were saturated and for dynamic forces if water levels were at their September 1983 

positions or higher. As was concluded for static conditions, however, the degree of mobility required for the 

sliding masses during retrogressive failure is considered 

unlikely to occur.

The possible displacements associated with slope failure were estimated on the basis of concepts outlined by 

Newmark (1965). Calculations were based on the assumptions that the sliding mass moves as a coherent body and 

that resistance is initiated along the failure surface. An 

earthquake with a square acceleration pulse of 0.1-0.2- 

second duration and 0.5-g amplitude was assumed, the 

latter value corresponding to the average seismic coefficient produced by the maximum credible earthquake in 

the upper half of the blockage. Displacements were calculated for single pulses and for the sum of 10 pulses and are 

plotted agains the seismic coefficient of Fs=l (fig. 24). 

Displacements shown in figure 24 probably overestimate 

actual displacement for the assumed condition, because
DYNAMIC SLOPE-STABILITY ANALYSES 

EXPLANATION

39

2800,-

. 2700

UJ 2500 

§ D

< 2400

Preeruption land surface

:1_ Potential failure surface 

and factor of safety

Critical failure surface and factor of safety 

Water table, September 1983

South Fork Castle 

Creek Lake

600 700 800 900

DISTANCE, IN FEET

1000 1100 1300 1400 1500

FIGURE 23. Critical surfaces and potential failure surfaces encompassing the largest sections of the blockage for cross sections CD, CE, and CF

with a seismic coefficient of 0.2 g, September 1983 water level, and zero cohesion.

pulses in opposite directions are ignored and because the 

value of seismic coefficients required for Fs=l decreases 

with displacement.

Figure 16 shows values of Fs versus ns for selected positions of the water table and can be used in conjunction 

with figure 24 to determine the approximate displacement of a sliding mass along a given cross section for the 

assumed conditions. The value ns when Fs is equal to 1.0 

can be selected from figure 16 for various positions of the 

water table. Such a value can then be used in figure 24 to 

determine an approximate displacement value for slippage along a failure surface. Slide displacements for these 

conditions are summarized in table 5. Fs values, and 

therefore seismic coefficients shown in figure 16, were obtained from model analysis that assumed drained 

strength and are therefore not necessarily conservative. 

Displacements shown in table 5 are only on the order of 

5 ft or less for all cross sections assuming the September 

1983 position of the water table. As a result, lake breakout 

from retrogressive failure following earthquake-induced 

slope failure is considered highly improbable.

As the water table rises to 25 ft above September 

1983 levels, the analysis suggests displacements up to 

several tens of feet on cross sections CE and CF but 

less than 6 ft elsewhere. For fully saturated conditions, 

large displacements of sliding masses are possible at 

sections CE and CF, because the Fs of many trial surfaces drops well below unity even under static conditions. Such displacements cannot be estimated in the 

above analysis, but it is doubtful that movement would 

exceed 100 ft. Elsewhere, for fully saturated conditions, displacements of sliding masses of about 15 ft are

indicated for sections BB' and CD. The type of deformation expected along section DF, if the maximum credible earthquake occurred when the blockage was fully 

saturated, is shown in figure 25.

TABLE 5. Estimated displacement of sliding masses for selected positions 

of the water table at South Fork Castle Creek blockage

Cross 

section

AA'-

BB'-

CD -

CE -

CF -

Water-table position

- -Sept. 1983 level

- -Sept. 1983 level 

At land surface

- -Sept. 1983 level 

Sept. 1983 level plus 

25ft 

At land surface

- -Sept. 1983 level 

Sept. 1983 level plus 

25ft 

At land surface

- -Sept. 1983 level 

Sept. 1983 level plus 

25ft 

At land surface

ws for

0.31

.23 

.10

.22

.17 

.09

.24

.08 

<0

.20

.05 

<0

Displacement, in feet

1 pulse

0.1 to 0.2

0.1 to 0.4 

0.3 to 1

0.1 to 0.4

0.2 to 0.7 

0.4 to 2

0.1 to 0.3

0.4 to 2 

*(')

0.1 to 0.5 

0.7 to 3

*o

10 pulses

0.5 to 2

Ito4 

3 to 13

Ito4

2 to 6 

4 to 15

Ito3

4 to 17

*(')

Ito5

7 to 29 

*(')

GG' - - -Sept. 1983 .43 0.02 to 0.1 0.2 to 1

'Unstable under static forces.
40 STABILITY, SOUTH FORK CASTLE CREEK BLOCKAGE, MOUNT ST. HELENS AREA, WASH.

100

50 -

10 -

LLJ 

LLJ

LLJ 

O

CL 

(/) 

Q

0.5

0.1 -

I III

EXPLANATION

 Ten pulses

 One pulse

I_I

0.01 0.05 0.1 0.5

SEISMIC COEFFICIENT REQUIRED FOR ^ = 1

FIGURE 24. Estimated displacement in the blockage resulting from an 

earthquake with an acceleration pulse of 0.5 g and duration of 0.1 and 

0.2 seconds.

Seismic forces thus increase the possibility of deepseated movements, and as ground-water levels rise, the 

likelihood of displacement associated with a given 

earthquake increases. Displacements exceeding several 

tens of feet are possible along sections CE and CF, 

either from static forces or from seismic shaking when 

water levels are at land surface. At other locations, 

however, predicted displacements are only several tens 

of feet or less, even for maximum credible earthquake 

values. The magnitude of these displacements suggests 

that the possibility of lake breakout, indicated by retrogressive model analysis, is remote for all positions of 

the water table for earthquakes equal to or less than 

the maximum credible earthquake for the area.

CONCLUSIONS

South Fork Castle Creek, a tributary of the North Fork 

Toutle River, was blocked by the debris-avalanche deposit associated with the May 18,1980, eruption of Mount 

St. Helens. A lake that eventually reached a volume of 

19,000 acre-ft formed behind the blockage and now poses 

a flood hazard of unknown magnitude to downstream 

areas if the blockage fails. The blockage has steep slopes 

and consists of unsorted and unstratified debris-avalanche deposit material that ranges from silt- and claysized particles to blocks tens of feet in diameter. The St. 

Helens seismic zone, which is believed capable of producing an earthquake larger than magnitude 5.5 but less than 

7.0, passes within a few miles of the blockage. The blockage has been subjected to three historic periods of shaking, with accelerations of 0.1 or 0.2 g. These events are 

presumed to have caused compactions of the blockage, increasing its relative density, and should have increased 

the resistance of the blockage to liquefaction (Mori and 

others, 1978).

Unit-weight and relative-density tests suggest that the 

deposit at South Fork Castle Creek is moderately compacted; in this respect it is probably typical of the Mount 

St. Helens debris-avalanche deposit. Unfortunately, the 

available data are fragmentary. Further attention should 

therefore be given to this issue, because of its relevance 

to questions of liquefaction and slide mobility and to 

strength-pore-pressure-deformation behavior as estimated by laboratory tests on material compacted to field 

density.

Stability of the blockage depends heavily on groundwater conditions: as ground-water levels rise, stability of 

the blockage decreases. Ground-water levels were first 

measured in the blockage in September 1983. Depth to 

water below the crest of the blockage ranged from about 

20 to 60 ft at that time. Water levels are believed to have 

been near their seasonal low in September 1983, and they 

could also increase annually for several more years. The 

potential for failure of the blockage due to gravitational 

forces and to horizontal forces induced by an earthquake 

was evaluated for water levels at their September 1983 

position and higher, including saturation of the blockage. 

In general, results of these analyses indicate that if an 

earthquake with a seismic coefficient one-half that of the 

maximum credible occurred with water levels at their 

September 1983 position, substantial deformation on the 

blockage could occur. Slope failures could occur from the 

crest toward "the lake and from the crest toward Castle 

Creek. Potential failure surfaces are deep, extending as 

much as 120 ft below lake level. A nearby earthquake with 

magnitude near 6.0 could cause much more extensive deformation on the blockage. As ground-water levels rise, 

the factor of safety for critical failure surfaces decreases
REFERENCES 41

EXPLANATION

Preemption land surface 

Prefailure land surface

2800 r

2400

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500

DISTANCE, IN FEET

FIGURE 25. Schematic of deformation expected with high ground-water level and maximum credible earthquake magnitude.

and potential displacement associated with a given earthquake increases. As a result, potential deformation on the 

blockage also increases.

Slope failure from gravitational forces presents no 

significant problem unless the blockage becomes nearly 

saturated, although some deformation of the blockage is 

possible at lower ground-water levels. With the blockage fully saturated, slope failures extending as much as 

30 ft below lake level could occur. From the crest toward the lake and from the crest toward Castle Creek, 

there is a potential for deep-seated surfaces along 

which sliding could occur.

None of the analyses for static or dynamic slope failure indicate that immediate lake breakout would result 

from initial slope failure on the blockage, even if the 

failures occurred on both sides of the blockage simultaneously. Only when retrogressive failure is assumed 

using highly mobile sliding masses does the possibility 

of lake breakout result. For lake breakout to occur as 

a result of retrogressive failure, displacements along 

slippage faces would have to be on the order of hundreds to thousands of feet, whereas actual displacements even under the worst conditions analyzed would 

probably vbe on the order of only tens of feet.

None of the analyses indicated the potential for a 

breakout of South Fork Castle Creek Lake as a result 

of initial slope failures on the blockage, because some 

part of the blockage would remain intact and above lake 

level for all possible positions of the water table and for 

earthquakes up to the maximum credible magnitude for 

the area. Retrogressive failure in the remaining section

of the blockage could lead to lake breakout if an earthquake with a seismic coefficient in the blockage of 0.2 

g occurred when ground water was at existing (September 1983) levels or higher. Retrogressive failure 

could also lead to lake breakout following initial slope 

failures induced by gravitational forces with the water 

table at land surface. The assumptions used in the analysis for retrogressive failures are so conservative, however, that the possibility of actual lake breakout from 

slope failure is considered remote.

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*U.S. GOVERNMENT PRINTING OFFICE: 1986-^91-^09:20014