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.

3.

SLOPE STABILITY CHARTS.

Rotational Failure in Cohesive Soils ([phi] = 0)

a.

(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 analysis in soil and Figure 8 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:

7.1-318

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