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Design Manual

GEOTECHNICAL DESIGN MANUAL

CHAPTER 10

SLOPE STABILITY ANALYSIS

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Table of Contents

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10.1 OVERVIEW .................................................................................................................. 10-4

10.2 REQUIRED INPUT DATA FOR SLOPE STABILITY ANALYSIS ........................... 10-5

10.2.1 Soil Profile ......................................................................................................... 10-5

10.2.2 Slope Geometry .................................................................................................. 10-6

10.2.3 Soil Shear Strength ............................................................................................ 10-6

10.2.4 Pore Water Pressure ........................................................................................... 10-8

10.3 METHODS OF STABILITY ANALYSIS .................................................................... 10-9

10.3.1 Limit Equilibrium Equations ............................................................................. 10-9

10.3.2 Preliminary Methods ........................................................................................ 10-10

10.3.3 Method of Slices .............................................................................................. 10-11

10.3.4 Methods Derived from Method of Slices ......................................................... 10-11

10.3.5 Comparison of Methods ................................................................................... 10-13

10.3.6 Back Analysis .................................................................................................. 10-14

10.3.7 Evaluation of Remedial Methods ..................................................................... 10-14

10.3.8 Computer Programs ......................................................................................... 10-14

10.3.9 Required Factor of Safety ................................................................................ 10-14

10.4 REFERENCES ............................................................................................................ 10-15

CHAPTER 10

Slope Stability Analysis

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10.1 OVERVIEW

Slope stability analysis should be used to determine whether a proposed slope meets the required

safety and performance criteria during design. This type of analysis is also utilized to determine

stability conditions of existing natural or constructed slopes and evaluate the influence of

proposed remediation methods if required. Slope stability analysis is used in a wide variety of

problems including, but not limited to:

Problems Slope Stability Analysis

Natural Slopes Evaluation of existing natural slopes.

Fill Slopes Determination of stable fill slopes.

Cut Slopes Determination of stable cut slopes.

Retaining Walls Assessment of overall stability (global and compound).

Deep/Shallow

Foundations Assessment of overall stability for structures located on slopes.

Landslides Assessment of landslides, remediation methods, & back-analysis.

Table 10-1 Slope Stability Analysis – Geotechnical Problems

An investigation should be made of all proposed fills supporting highway facilities and cuts that

support important adjacent lands or structures. It is also essential that locations of existing

landslides be identified before new highway alignment is fixed.

Once slope geometry has been modeled and subsurface conditions have been determined, the

stability of a slope may be assessed using a limit-equilibrium analysis, with appropriate drainage

conditions and shear strengths.

This chapter provides a basic outline of slope stability analysis. For detailed design guidance and

reference, geotechnical designers are directed to FHWA/NHI 05-123 Soil Slope and Embankment

Design.

Stability analysis techniques specific to rock slopes, other than highly fractured rock masses that

can in effect be treated as soil, are described in NYSDOT GDM Chapter 15. Detailed stability

assessment of landslides is described in NYSDOT GDM Chapter 16.

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10.2 REQUIRED INPUT DATA FOR SLOPE STABILITY ANALYSIS

It is expected that geotechnical designers performing slope stability analysis on NYSDOT

projects include the following basic data in the stability model:

Parameter Basic Input Data

Soil Profile Detailed Assessment of Soil Stratigraphy

Slope Geometry Derived from Topos, Photogrammetry and Survey

Soil Shear Strength Total and Effective Stress Analysis

Pore Water Pressure Determination of Water Regime

Table 10-2 Basic Input Data

The input data needed for slope stability analysis is also described in:

• NYSDOT GDM Chapter 2 Project Geotechnical Planning

• NYSDOT GDM Chapter 6 Engineering Properties of Soil and Rock

• NYSDOT GDM Chapter 13 Soil Cut Design

• NYSDOT GDM Chapter 15 Rock Cut Design

• NYSDOT GDM Chapter 16 Landslide Analysis and Mitigation

10.2.1 Soil Profile

Detailed assessment of soil stratigraphy is critical so that a representative soil profile can be

modeled. It is important to define any thin, weak layers present, (i.e. the presence of silt-clay

varves, etc.) as these fine details of the stratigraphy could control the stability of the slope in

question. Knowledge of the geologic nature of the units present at a site, including past

performance, is necessary for appropriate determination of ground conditions and the

development of a realistic geotechnical model.

The location and characteristics of the critical soil strata are usually obtained by taking

continuous split spoon or thin-walled tube samples. Borings should be progressed in sufficient

quantities to adequately define subsurface materials. In a potential slide area, borings should be

taken at the top and bottom of existing or proposed slopes if possible. The number of borings

required longitudinally depends on the continuity of the soil conditions and the extent of the

possible problems as discussed in NYSDOT GDM Chapter 4. Seismic investigation can be a

useful supplement if the rock surface is a major influencing factor.

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10.2.2 Slope Geometry

Representative slope geometry is required for slope stability analysis, and should include

accurate locations of streams, roads, structures, and utilities. Surfaces needed to reflect slope

geometry can be derived from photogrammetric mapping, field surveys, or published topographic

maps. For side hill conditions, cross sections should be carried above the existing road or to a

controlling feature such as rock outcrop and down to the bottom of the natural slope. If a

watercourse occurs at the bottom of the slope then the stream bed should be surveyed or

estimated. Cracks caused by earth movement should be mapped to provide a realistic failure

model.

10.2.3 Soil Shear Strength

The shear strength of the soil along the failure surface is a function of the effective stress at

failure. The effective stress can only be calculated indirectly if the pore water pressures at the

state of failure are known, both in the laboratory and in the field. Consolidated undrained triaxial

tests with pore pressure (CU/p) measurements are commonly performed by NYSDOT to

determine the drained friction angle of the critical soil. This test also provides a range of

undrained shear strengths at various confining stresses levels. See NYSDOT GDM Chapter 8 for

guidance on the development of shear strength parameters.

The geotechnical designer should determine if subject slope consisting of cohesive, impermeable

soils will experience loading and at what rate of load will be applied – short-term (rapid loading

condition, ie. construction loading), long-term (very slow loading condition), or a multi-stage

loading (placement of a portion of load, followed by waiting period). The rate of load application

will determine the selection of soil shear strength parameters used as input in the analysis as

follows:

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(Table 5-2 FHWA-NHI-05-123)

Table 10-3 Shear Strengths, Drainage Condition, Pore Pressure and

Unit Weights for Slope Stability Analysis (after Duncan, 1992)

For assessing the stability of large landslides that have moved over a period of years or for cuts in

highly overconsolidated clays, if the slope is relatively free to deform after the cut is made or is

otherwise unloaded, residual shear strength parameters should be obtained and used for the

stability analysis.

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10.2.4 Pore Water Pressure

A change in effective stress caused by pore water pressure is a major contributor of slope failures

because the pressures tend to alter the shear strength of the soil along the shear zone. An

understanding of pore water pressure and groundwater regime at the study site is critical for

establishing an accurate model for slope stability analysis.

Figure 10-1 Types of Aquifers (US Department of the Interior, 1981)

Detailed piezometric data at multiple locations and depths within and below the slope might be

required, depending on the geologic complexity of the stratigraphy and groundwater conditions.

The use of piezometers (vibrating-wire or open-well) should be considered if the existing or

future groundwater conditions cannot be readily determined by field observation and these

conditions would have a detrimental effect on the stability of the slope or cut based on the nature

of the subsurface materials or geometry of the site. Separate piezometers sealed into multiple soil

or rock deposits may be necessary to obtain a better estimate of the groundwater conditions.

Sudden drawdown is a rapid lowering in the level of water standing against a slope. This

condition can occur on the slopes adjacent to a reservoir, river, or canal either following a long

period of rainfall accumulation, planned lowering of water through control structures, or failure

of water impoundment structure. Sudden drawdown often occurs in clayey slopes in which the

excess pore water pressures do not dissipate as the water recedes, thereby keeping the overall

shear strength low.

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10.3 METHODS OF STABILITY ANALYSIS

Methods of stability analysis are detailed in Publication No. FHWA/NHI 05-123 Soil Slope and

Embankment Design.

10.3.1 Limit Equilibrium

A soil slope is commonly analyzed based on Limit (or force or moment) Equilibrium methods

that measure its stability by a factor of safety (FS) (ie. the ratio of resisting force to driving

force). A slope with a FS =1 is theoretically marginally stable, or just ready to move. The shear

strength (limit) required along a failure surface is calculated to just maintain stability and is then

compared with the available shear strength to provide the overall FS for the slope.

Figure 10-2 Various Definitions of Factor of Safety (FS) (FHWA, 1994)

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10.3.2 Preliminary Methods

The following figure shows limit equilibrium methods that can be readily solved by hand or

spreadsheet and do not depend on the distribution of the effective normal stress along the failure

surface. Published design charts and methods shown in following figure should be only used for

preliminary evaluation of slopes and are described in detail in FHWA/NHI 05-123.

Infinite Slope

Planar

Circular, Log Spiral

Block

Figure 10-3 Limit Equilibrium Methods

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10.3.3 Method of Slices

If mobilized strength for a soil is to be calculated, the distribution of the effective normal stresses

along the failure must be known. This condition is usually analyzed by sectioning the potential

failure mass into small vertical slices and treating each as a unique sliding block. This analysis is

known as the Method of Slices.

Figure 10-4 Method of Slices

10.3.4 Methods Derived from Method of Slices

A number of limit equilibrium methods are based on the Method of Slices. While originally

calculated by hand, these more rigorous methods were converted to computerized versions which

are commercially available to geotechnical designers doing work for NYSDOT.

The methods are generally divided into three categories, based on the number of equilibrium

equations to be satisfied:

1. Overall moment equilibrium methods,

2. Force equilibrium methods, and

3. Moment and force equilibrium methods.

Slope stability problems are usually statically indeterminate. Assumptions are generally involved

to render the problem determinate, with each method of analysis using a different assumption. As

a result, the computed factor of safety varies between the different methods. The following table

lists commonly available methods of analysis and the conditions of static equilibrium that are

satisfied in determining the factor of safety.

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Table 10-4 Static Equilibrium Conditions Satisfied by Limit Equilibrium Methods

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10.3.5 Comparison of Methods

The chart below shows a presentation of Fredlund and Krahn (1977) study comparing limit

equilibrium methods using a representative slope.

Figure 10-5 Comparison of Methods

(Fredlund and Krahn, 1977)

A detailed comparison of limit equilibrium methods is discussed in FHWA/NHI 05-123.

NYSDOT does not specify a slope stability method required for analysis of slopes. The final

selection of computerized method is left to the geotechnical designer, based on their experience,

accuracy of input data, and understanding of the limitations of the various methods.

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10.3.6 Back Analysis

After a slope failure, an analysis is usually performed in an effort to identify probable causes. By

using a known or assumed failure surface, some form of back analysis can be performed in order

to estimate the material shear strength, pore pressure or other conditions at the time of failure.

10.3.7 Evaluation of Remedial Methods

Once the failure surface is identified by back analysis, the soil strengths and pore pressures can

be adjusted until the calculated FS is equal to 1.0. The chosen soil strengths and pore pressures

can then be used to evaluate repair methods and determine relative improvement to the FS. Based

on incremental improvement of FS and other constraints (economical, environmental, real estate,

impacts to public, etc.) the geotechnical designer can then recommend appropriate remedial

actions.

Mitigation measures for slope failures are described in NYSDOT GDM Chapter 16 and Chapter

22.

10.3.8 Computer Programs

The Geotechnical Engineering Bureau currently uses SLOPE/W (2012) to perform slope stability

analysis. XSTABL is also used to calculate the factors of safety according to the Simplified

Bishop and Simplified Janbu methods of analysis.

10.3.9 Factor of Safety

The following table provides minimum NYSDOT required Factors of Safety if a geotechnical

designer has enough information to adequately define soil profile, slope geometry, soil shear

strength and pore water pressure in a slope stability model:

Geotechnical Element FS LRFD

Embankment Side Slopes 1.25 -

Embankment End slopes 1.30 -

Cut Slopes 1.50 -

Landslide Remediation 1.25 -

Bridge or Wall Structure on Slope* 1.50 .65

Minor Wall with Minimal Impact on Slope* 1.30 .75

*Load Factor of 1.

Table 10-5 Minimum Required Factors of Safety – Slope Stability Analysis

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10.4 REFERENCES

Abramson, L., Boyce, G., Lee, T., and Sharma, S., Advanced Technology for Soil Slope Stability,

Volume 1: Slope Stability Manual, Publication No. FHWA-SA-94-005, 1994.

Collin, James G, Leshchinsky, D., and Hung, C., Soil Slope and Embankment Design,

Publication No. FHWA/NHI-05-123.

Geotechnical Engineering Bureau, Design Procedure for Launched Soil Nail Shallow Slough

Treatment, Geotechnical Design Procedure GDP-14, New York State Department of

Transportation, Office of Technical Services,

https://www.dot.ny.gov/divisions/engineering/technical-services/technical-services-

repository/GDP-14b.pdf

McGuffey, V.G., Earth Cut Slope Design in New York State, Highway Research Record, Number

457, 1973.