TM 5-818-1 / AFM 88-3, Chap. 7
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
8-4.
Slope stability charts.
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
8-2
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