c. Finite Element Method. This method is extensively used in more
complex problems of slope stability and where earthquake and vibrations are
part of total loading system. This procedure accounts for deformation and
is useful where significantly different material properties are encountered.
2. FAILURE CHARACTERISTICS. Table 1 shows some situations that may arise
in natural slopes. Table 2 shows situations applicable to man-made slopes.
Strength parameters, flow conditions, pore water pressure, failure modes,
etc. should be selected as described in Section 4.
SLOPE STABILITY CHARTS.
Rotational Failure in Cohesive Soils ([phi] = 0)
(1) For slopes in cohesive soils having approximately constant
strength with depth use Figure 2 (Reference 4, Stability Analysis of Slopes
with Dimensionless Parameters, by Janbu) to determine the factor of safety.
(2) For slope in cohesive soil with more than one soil layer,
determine centers of potentially critical circles from Figure 3 (Reference
4). Use the appropriate shear strength of sections of the arc in each
stratum. Use the following guide for positioning the circle.
(a) If the lower soil layer is weaker, a circle tangent to the
base of the weaker layer will be critical.
(b) If the lower soil layer is stronger, two circles, one
tangent to the base of the upper weaker layer and the other tangent to the
base of the lower stronger layer, should be investigated.
(3) With surcharge, tension cracks, or submergence of slope, apply
corrections of Figure 4 to determine safety factor.
(4) Embankments on Soft Clay. See Figure 5 (Reference 5, The Design
of Embankments on Soft Clays, by Jakobsen) for approximate analysis of
embankment with stabilizing berms on foundations of constant strength.
Determine the probable form of failure from relationship of berm and
embankment widths and foundation thickness in top left panel of Figure 5.
4. TRANSLATIONAL FAILURE ANALYSIS. In stratified soils, the failure
surface may be controlled by a relatively thin and weak layer. Analyze the
stability of the potentially translating mass as shown in Figure 6 by
comparing the destabilizing forces of the active pressure wedge with the
stabilizing force of the passive wedge at the toe plus the shear strength
along the base of the central soil mass. See Figure 7 for an example of
translational failure in rock.
Jointed rocks involve multiple planes of weakness. This type of problem
cannot be analyzed by two-dimensional cross-sections. See Reference 6, The
Practical and Realistic Solution of Rock Slope Stability, by Von Thun.
5. REQUIRED SAFETY FACTORS. The following values should be provided for
reasonable assurance of stability: