surcharge loading, tension cracks. The effect of partial

far less than suggested by the results of shear tests on

submergence of a slope is given by a factor w in figure

undisturbed samples. This result is due, in part, to prior

8-2; seepage is given by a factor w' in figure 8-2;

shearing displacements that are much larger than the

displacement corresponding to peak strength. Slope

surcharge loading is given by a factor q in figure 8-2;

failures may occur progressively, and over a long period

and tension cracks is given by a factor t in figure 8-3.

of time the shearing resistance may be reduced to the

Compute safety factor from the following:

residual value-the minimum value that is reached only at

extremely large shear displacements. Temporary slopes

F = w w' q t N0 C

(8-1)

in these materials may be stable at angles that are

γH + q - γwHw'

steeper than would be consistent with the mobilization of

where

only residual shear strength.

The use of local

γ

=

total unit weight of soil

experience and empirical correlations are the most

q

=

surcharge loading

reliable design procedures for these soils.

N0 =

stability number from figure 8-1

b. Loess. Vertical networks of interconnected

If any of these conditions are absent, their corresponding

channels formed by decayed plant roots result in a high

i factor equals 1.0; if seepage out of the slope does not

vertical permeability in loess.

Water percolating

occur, H. equals IH.

downward destroys the weakly cemented bonds between

b. Stratified soil layers, φ = O, rotational

particles,

causing rapid erosion and slope failure.

Slopes in loess are frequently more stable when cut

failure.

vertically to prevent infiltration. Benches at intervals can

(1) Where the slope and foundation

be used to reduce the effective slope angle. Horizontal

consist of a number of strata, each having a constant

surfaces on benches and at the top and bottom of the

shear strength, the charts given in figures 8-1 through 8-

slope must be sloped slightly and paved or planted to

3 can be used by computing an equivalent average

prevent infiltration. Ponding at the toe of a slope must be

shear strength for the failure surface. However, a

prevented. Local experience and practice are the best

knowledge of the location of the failure surface is

guides for spacing benches and for protecting slopes

required. The coordinates of the center of the circular

against infiltration and erosion.

failure surface can be obtained from the lower diagrams

c. Residual soils. Depending on rock type and

of figure 8-1. The failure surface can be constructed,

climate, residual soils may present special problems with

and an average shear strength for the entire failure

respect to slope stability and erosion. Such soils may

surface can be computed by using the length of arc in

contain pronounced structural features characteristic of

each stratum or the number of degrees intersected by

the parent rock or the weathering process, and their

each soil stratum as a weighing factor.

characteristics may vary significantly over short

(2) It may be necessary to calculate the

distances. It may be difficult to determine design shear

safety factor for failure surfaces at more than one depth,

strength

parameters

from

laboratory

tests.

as illustrated in figure 8-4.

c. Charts for slopes in uniform soils with φ > 0.

Representative shear strength parameters should be

determined by back-analyzing slope failures and by using

(1) A stability chart for slopes in soils with

empirical design procedures based on local experience.

φ > 0 is shown in figure 8-5. Correction factors for

d. Highly sensitive clays. Some marine clays

surcharge loading at the top of the slope, submergence,

exhibit dramatic loss of strength when disturbed and can

and seepage are given in figure 8-2; and for tension

actually flow like syrup when completely remolded.

cracks, in figure 8-3.

Because of disturbance during sampling, it may be

(2) The location of the critical circle can be

difficult to obtain representative strengths for such soils

obtained, if desired, from the plot on the right side of

from laboratory tests. Local experience is the best guide

figure 8-5. Because simple slopes in uniform soils with φ

to the reliability of laboratory shear strength values for

> 0 generally have critical circles passing through the toe

such clays.

of the slope, the stability numbers given in figure 8-5

e. Hydraulic fills. See Chapter 15.

were developed by analyzing toe circles. Where subsoil

conditions are not uniform and there is a weak layer

a. Uniform soil, constant shear strength, φ =

beneath the toe of the slope, a circle passing beneath

0, rotational failure.

the toe may be more critical than a toe circle.

(1) Groundwater at or below toe of slope.

d. Infinite slopes. Conditions that can be

Determine shear strength from unconfined compression,

analyzed accurately using charts for infinite slope

or better, from Q triaxial compression tests. Use the

analyses shown in figure 8-6 are-

upper diagram of figure 8-1 to compute the safety factor.

If the center and depth of the critical circle are desired,

obtain them from the lower diagrams of figure 8-1.

(2) Partial slope submergence, seepage

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