4.
STABILITY PROBLEMS IN SPECIAL MATERIALS
a. Controlling Factors. See Table 1, DM-7.2, Chapter 1, for primary
factors controlling slope stability in some special problem soils.
Strength Parameters.
b.
(1) Overconsolidated, Fissured Clays and Clayshales. See Table 2.
Cuts in these materials cause opening of fissures and fractures with
consequent softening and strength loss.
(a) Analysis of Cut Slopes. For long-term stability of cut
slopes use residual strength parameters c'+r, and [phi]'+r, from
drained tests. See Chapter 3. The most reliable strength information for
fissured clays is frequently obtained by back figuring the strength from
local failures.
(b) Old Slide Masses. Movements in old slide masses frequently
occur on relatively flat slopes because of gradual creep at depth.
Exploration may show the failure mass to be stiff or hard; but a narrow
failure plane of low strength with slickensides or fractures may be
undetected. In such locations avoid construction which involves regrading
or groundwater rise that may upset a delicate equilibrium.
(2) Saturated Granular Soils in Seismic Areas. Ground shaking may
result in liquefaction and strength reduction of certain saturated granular
soils. Empirical methods are available for estimating the liquefaction
potential. See DM-7.3, Chapter 1 for guidance. Methods of stabilization
for such soils are discussed in DM-7.3, Chapter 2.
(3) Loess and Other Collapsible soils. Collapse of the structure of
these soils can cause a reduction of cohesion and a rise in pore pressure.
Evaluate the saturation effects with unconsolidated undrained
tests, saturating samples under low chamber pressure prior to shear. See
Chapter 1 for evaluating collapse potential.
(4) Talus. For talus slopes composed of friable material, [phi]
may range from 20deg. to 25deg. If consisting of debris derived from
slate or shale, [phi] may range from 20deg. to 29deg., limestone about
32deg., gneiss 34deg., granite 35deg. to 40deg. These are crude estimates
of friction angles and should be supplemented by analysis of existing talus
slopes in the area.
Section 5.
SLOPE STABILIZATION
1. METHODS. See Table 4, for a summary of slope stabilization methods.
A
description of some of these follows:
a. Regrading Profile. Flattening and/or benching the slope, or adding
material at the toe, as with the construction of an earth berm, will
increase the stability. Analyze by procedures above to determine most
effective regrading.
7.1-335
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