TM 5-818-1 / AFM 88-3. Chap. 7
SLOPE STABILITY ANALYSIS
depends on the strength of soil, its unit weight, the slope
This chapter is concerned with
height, the slope angle, and pore pressures. Failure
characteristics and critical aspects of the stability of
usually occurs by sliding on a deep surface tangent to
excavation slopes; methods of designing slopes,
the top of firm materials. For relatively high slopes that
including field observations and experience, slope
drain slowly, it may be necessary to analyze the stability
stability charts, and detailed analyses; factors of safety;
for three limiting conditions:
and methods of stabilizing slopes and slides. The
emphasis in this chapter is on simple, routine
condition. Analyze this condition using total stress
procedures. It does not deal with specialized problems,
methods, with shear strengths determined from Q tests
such as the stability of excavated slopes during
on undisturbed specimens.
Shear strengths from
unconfirmed compression tests may be used but
generally may show more scatter. This case is often the
Slope stability problems. Excavation slope
only one analyzed for stability of excavated slopes. The
instability may result from failure to control seepage
possibility of progressive failure or large creep
forces in and at the toe of the slope, too steep slopes for
deformations exists for safety factors less than about
the shear strength of the material being excavated, and
1.25 to 1.50.
insufficient shear strength of subgrade soils. Slope
(2) Long-term condition. If the excavation
instability may occur suddenly, as the slope is being
is open for several years, it may be necessary to
excavated, or after the slope has been standing for some
analyze this condition using effective stress methods,
time. Slope stability analyses are useful in sands, silts,
with strength parameters determined from S tests or R
and normally consolidated and overconsolidated clays,
tests on undisturbed specimens. Pore pressures are
but care must be taken to select the correct strength
governed by seepage conditions and can be determined
parameter. Failure surfaces are shallow in cohesionless
using flow nets or other types of seepage analysis. Both
materials and have an approximately circular or sliding
internal pore pressures and external water pressures
wedge shape in clays.
should be included in the analyses. This case generally
a. Cohesionless slopes resting on firm soil or
does not have to be analyzed.
rock. The stability of slopes consisting of cohesionless
soils depends on the angle of internal friction φ', the
(3) Sudden drawdown condition, or other
conditions where the slope is consolidated under one
slope angle, the unit weight of soil, and pore pressures.
loading condition and is then subjected to a rapid change
Generally, a slope of 1 vertical (V) on 1 1/2 horizontal (H)
in loading, with insufficient time for drainage. Analyze
is adequate; but if the slope is subjected to seepage or
this condition using total stress methods, with shear
sudden drawdown, a slope of 1V on 3H.is commonly
strengths measured in R and S tests. Shear strength
employed. Failure normally occurs by surface raveling or
shall be based on the minimum of the combined R and S
shallow sliding. Where consequences of failure may be
envelopes. This.case is not normally encountered in
important, required slopes can be determined using
excavation slope stability.
simple infinite slope analysis. Values of φ' for stability
c. Effect of soft foundation strata. The critical
analyses are determined from laboratory tests or
failure mechanism is usually sliding on a deep surface
estimated from correlations (para 3-6). Pore pressure
tangent to the top of an underlying firm layer. Short-term
due to seepage reduces slope stability, but static water
stability is usually more critical than long-term stability.
pressure, with the same water level inside and outside
The strength of soft clay foundation strata should be
the slopes, has no effect. Benches, paved ditches, and
expressed in terms of total stresses and determined
planting on slopes can be used to reduce runoff
using Q triaxial compression tests on undisturbed
velocities and to retard erosion. Saturated slopes in
specimens or other methods described in chapter 4.
cohesionless materials may be susceptible to
liquefaction and flow slides during earthquakes, while dry
Slopes In soils presenting special problems.
slopes are subject to settlement and raveling. Relative
densities of 75 percent or larger are required to ensure
a. Stiff-fissured clays and shales.
seismic stability, as discussed in Chapter 17.
shearing resistance of most stiff-fissured clays and
b. Cohesive slopes resting on firm soil or rock.
shales may be
The stability of slopes consisting of cohesive soils