A Preliminary Finite-Element Analysis 

of a Shallow Landslide in the Alki Area 

of Seattle, Washington

U.S. Department of the Interior

U.S. Geological Survey

By S. Debray and W.Z. Savage

Open-File Report 01-0357

2001

This report is preliminary and has not been reviewed for conformity with U.S. Geological

Survey editorial standards nor with the North American Stratigraphic Code. Any use of

trade names in this publication is for descriptive purposes only and does not imply

endorsement by the U.S. Government.
1

Introduction

The Seattle area has a long history of landslide problems (Tubbs, 1974, Thorsen, 1989,

Galster and Laprade, 1991, Baum, and others,

1998). Shallow landslides often initiate in colluvium derived from glacial deposits, are frequently triggered by winter rain storms (Tubbs,

1974), and may mobilize into debris flows that

can cause considerable damage (Baum and others, 1998).

In this report we present a preliminary finiteelement based stability analyses of a shallow

landslide that occurred during the period from

December 1996 to early January 1997 near Alki

Beach and Duwamish Head at the north end of

West Seattle (Figure 1). During this period a

series of snowstorms, freezing rain and warm

rain produced many debris flows and landslides

in the Seattle area (Baum and others, 1998). The

shallow landslide that we are analyzing was

18 m high and 75 m long. It mobilized into a

debris flow that traveled down a 35-meter-high

embankment below the slide damaging the bottom floors of two apartment buildings at the

foot of the embankment (Figure 2). In what follows, we briefly outline the geology and hydrology of the Alki landslide site, give a short

description of the computational algorithms

used in the finite element program, and present

the results of the finite element model of the

landslide. 

Geology and hydrology of the 

Alki landslide site

Galster and Laprade (1991) identify the following sequence of glacially derived surficial geologic formations in the Seattle area. They

are, from top to bottom, Recessional Outwash,

Vashon Till, Advance Outwash, Lawton Clay,

and the Pre-Vashon. Deposits in this sequence

are often overconsolidated, have a wide range of

hydraulic conductivities, are laterally heterogeneous, and form steep, landslide-prone, coastal

bluffs. 

According to Waldron and others (1962) only

the Esperance Sand and the Lawton Clay are

present at the Alki landslide site. However,

Figure 1. Alki landslide location. Shannon and Wilson (2000) identify, from boreFigure 2. View of the Alki landslide area. 

(Photograph taken August 2001 by S. Debray.)
2

hole logging, dense Pre-Vashon sands, silts, and

clays below the Lawton Clay at the Alki site.

The Esperance Sand is a unit in the Advance

Outwash that has been overridden by glacial ice.

Generally these outwash deposits are dense,

brown and gray, and permeable with fine to

coarse sand and silt. Lawton Clay is a glaciolacustrine deposit that consists of very stiff to

hard and relatively impermeable clays and silts.

Where pervious sand overlies the clay, this clay

acts as an aquiclude and ground water is

perched in the sand. The Pre-Vashon sediments

are interglacial fluvial and lake deposits.

The Alki landslide occurs in an 8- to 12-m thick

layer of colluvium, a very loose to medium density soil that commonly covers upper, flatter

slopes at the Alki site (Shannon and Wilson,

2000). Here the colluvium overlies Lawton clay

and is laterally continous with the Esperance

Sand. Its looseness results in a higher permeability than that of the underlying Lawton Clay, so

that ground water is perched at the contact or in

the lower part of the colluvial mantle (Shannon

and Wilson, 2000).

Resistant and relatively impervious Vashon Till

commonly caps Seattle-area hilltops (Tubbs,

1974, Galster and Laprade, 1991). This layer

impedes water infiltration. However, at the Alki

slide, till coverage is absent and, thus, surface

water can percolate into the underlying

Esperance Sand continuing downward until it

reaches the relatively impervious Lawton Clay.

Ground water then moves laterally in the

Esperance Sand reaching the colluvium and

emerging as springs on the hillside. If the rate of

infiltration exceeds the capacity of the hydraulic

gradient to transmit ground water to hillside

seeps, pore pressures rise and slope failure

ensues. 

Geotechnical properties of 

Alki slide materials

Adetailed geotechnical investigation of the

Alki glacial deposits is beyond the scope of

this work. Therefore Alki-site property values

are taken from a previously published summary

(Savage and others, 2000b) and from data

reported by Shannon and Wilson (2000). Peak

cohesion (Cp) for the Alki colluvium is taken by

Shannon and Wilson (2000) to be zero. We estimate it to be to be 5 KPa (see below). The peak

friction angle (Φp) for colluvium is 23° (Shannon

and Wilson, 2000). Strength values for the

Lawton Clay were those used by Savage and

others (2000a) in a finite-element limitingequilibrium analysis of stability of the Woodway

landslide. The estimated geotechnical properties

for the units at the Alki site that are used in the

finite-element modeling are given in Table 1.

Geologic Unit γdry γwet σ E MPa Cp KPa Φpo

KN/m3 KN/m3

Pre Vashon 18 20 0.33 250 40 32

Lawton Clay—Qc 17 20 0.49 250 20 24

Esperance Sand 17.6 19.2 0.30 200 20 32 

(Advance

Outwash)—Qva

Colluvium 15 19 0.33 160 5 24

Table 1. Estimated material properties used in finite-element modeling of the

Alki landslide. In this table, dry and wet unit weights are denoted by γdry , and

γwet, σ is Poisson’s ratio, E is Young’s Modulus, Cp is peak cohesion, and Φp is

the peak angle of internal friction.
3

Finite-element modeling of the 

Alki landslide

Stability of the Alki Landslide is modeled by

using Version 7 of the PLAXIS© finiteelement program. This program, designed for

geotechnical analysis, features automatic mesh

generation, pore pressure generation, a nonlinear elastic-plastic Mohr-Coulomb iterativesolution algorithm, and a phi-c-reduction procedure for calculation of safety factors. 

The factor of safety, FS, in PLAXIS is defined

by,

Where σn′ is effective normal

stress acting on a plane, C and

Φ are the input cohesion and

angle of internal friction, and

CR and ΦR are a reduced cohesion and reduced angle of

internal friction calculated in

the program as products of

the input values and a multiplier. The parameters CR and

ΦR are then iteratively

changed until they are just

large enough to maintain

equilibrium and then used in

the above equation to give the

factor of safety on an elementby-element basis. When the

factor of safety approaches 1,

failure is imminent in critically stressed elements. The factor of safety for a slope is the

mean of the factors of safety

for all elements approaching

failure in the finite-element

model. The safety factor procedure in PLAXIS gives

results similar to conventional

slip-circle analyses (Savage

and others, 2000a) where the

safety factor is the ratio of the

true strength to the minimum

strength required for equilibrium of a postulated slip surface.

The geometry of the site as used in the calculation is shown in Figure 3.

Since the Alki landslide failed after heavy precipitation in the area it is assumed that rising

ground water levels were responsible for slide

initiation. To simulate the effect of rising ground

water on the initiation of the Alki landslide a

perched water table near the top of the relatively

impervious Lawton Clay is raised in the finiteelement model from 0-4 meters head to give a

factor of safety of 1.07; a value close to the failure value of 1.

Precise values for the cohesion of the colluvium

are not available. Sensitivity results for a range

of cohesion between 5 KPa and 17 KPa for a

perched water table of 4.0 m yields a best model

Figure 3. Model of the geometry and geologic layers of the Alki site.

Double crosses indicate the model boundaries. Scale 1:3000.

Figure 4. Finite-element mesh used to model the Alki landslide. 

Scale 1:3000.
4

fit for a cohesion of 15.1 KPa (the factor of safety

for this value is 1.07).

Predicted displacement magnitudes, [ux

2 + uy2]

1

⁄

2,

(ux and uy are, respectively, horizontal and vertical displacements) when FS = 1.07 are shown in

Figure 5. Displacements in this case are calculated using the elastic-Coulomb plastic algorithm

in PLAXIS. 

Figures 6a and 6b show deformation of the mesh and concentration of displacements calculated by the PLAXIS© phi-c

reduction procedure when FS =

1.07. No magnitudes are given

for the displacements in Figures

6a and 6b as the phi-c reduction

procedure in PLAXIS© generates

additional, large, non-physical

displacements. However, the

shape of the deformed mesh and

the pattern of the displacements

give an indication of the shape

of the failure surface. The predicted failure-surface shape is in

reasonable agreement with that

produced during the winter of 1996-1997

(Shannon and Wilson, Inc., 2000).

Concluding discussion

The finite element modeling results give ageneral idea of the mechanism of landsliding at the Alki site in response to a rising

perched ground water table. The position and

shape of the failure surface are simple; failure

Cohesion of the colluvium Factor of safety

(KPa) (calculated with the 

phi/c reduction procedure)

5 0.302

9 0.485

14 0.85

15 0.891

15.1 1.07

15.2 1.12

16 1.23

17 1.45 

Table 2. Sensitivity of the factor of safety to cohesion (C) over the range 

5-17 KPa.

Figure 5. Predicted displacement magnitudes in millimeters. Only the area of interest outlined in

Figure 4 is shown. Displacements elsewhere are negligible.
5

occurred on a surface at the contact between the

colluvium and the clay and sand layers triggering a general movement of the colluvium which

then mobilized into destructive debris flows.

In conclusion, we suggest that the slide occurred

because of high pore pressures in the colluvium

resulting from the juxtaposition of the weak colluvium on the transition zone between impervious Lawton clay and overlying Esperance Sand.

Finally, although averaged and estimated geotechnical properties gave reasonable results for

the Alki slide, a more complete sensitivity analysis for evaluating the effect of geotechnical property variation should be performed. 

Figures 6a and 6b. Deformation of the mesh is shown in Figure 6a and displacement concentrations in regions at

incipient failure are shown in Figure 6b.

a b

References

Baum, R.L., Chleborad, A.F., and Schuster, R.L.,

1998, Landslides triggered by the winter

1996-97 storms in the Puget Lowland,

Washington: U.S. Geological Survey OpenFile Report 98-239, 16 p.

Galster, R.W., and Laprade,W.T., 1991, Geology

of Seattle, Washington, United States of

America: Bulletin of the Association of

Engineering Geologists, v. 28, no. 3, p. 235-

302.

Savage, W.Z., Baum, R.L.,Morrissey, M.M., and

Arndt B.P., 2000a, Finite element analysis of

the Woodway Landslide, Washington: U.S.

Geological Survey Bulletin 2180, 9 p.

Savage, W.Z., Morrissey, M.M., and Baum, R.L.,

2000b, Geotechnical properties for landslideprone Seattle-area glacial deposits: U.S.

Geological Survey Open-File Report 00-228,

5 p.

Shannon & Wilson, Inc., 2000, Geotechnical

Report, Alki Avenue/Duwamish Head, stabilization project, Seattle, Washington,

February 2000, 9 p.

Thorsen, G.W., 1989, Landslide provinces in

Washington, in Galster, Richard W., ed.,

Engineering Geology in Washington:

Washington Division of Geology and Earth

Resources Bulletin 78, v. I, p. 71-89. 

Tubbs, D.W., 1974, Landslides in Seattle:

Washington Division of Mines and Geology

Information Circular 52, 15 p., scale 1:31680.

Waldron, H.W., Liesch, B.A., Mullineaux, D.R.,

and Crandell, D.R., 1962, Preliminary geologic map of Seattle and vicinity, Washington:

U.S. Geologic Survey Miscellaneous Geologic

Investigations Map I-354, scale 1:31680.