where

A=

a peak ground acceleration, g's

g=

a acceleration of gravity

V=

peak ground velocity

The above equations are based on several simplifying assumptions: (a)

failure occurs along well defined slip surface, (b) the sliding mass behaves

as a rigid body; (c) soils are not sensitive and would not collapse at small

deformation; and (d) there is no reduction in soil strength due to ground

shaking.

Section 4.

EFFECTS OF SOIL PARAMETERS AND GROUNDWATER ON STABILITY

1. INTRODUCTION. The choice of soil parameters and the methods of analyses

are dictated by the types of materials encountered, the anticipated

groundwater conditions, the time frame of construction, and climatic

conditions. Soil strength parameters are selected either on the basis of

total stress, ignoring the effect of the pore water pressure, or on the

basis of effective stress where the analysis of the slope requires that the

pore water pressures be treated separately.

2. TOTAL VS. EFFECTIVE STRESS ANALYSIS. The choice between total stress

and effective stress parameters is governed by the drainage conditions which

occur within the sliding mass and along its boundaries. Drainage is

dependent upon soil permeability, boundary conditions, and time.

a. Total Stress Analysis. Where effective drainage cannot occur during

shear, use the undrained shear strength parameters such as vane shear,

unconfined compression, and unconsolidated undrained (UU or Q) triaxial

compression tests. Field vane shear and cone penetration tests may be used.

Assume [phi] = 0. Examples where a total stress analysis are

applicable include:

(1) Analysis of cut slopes of normally consolidated or slightly

preconsolidated clays. In this case little dissipation of pore water

pressure occurs prior to critical stability conditions.

(2) Analysis of embankments on a soft clay stratum. This is a

special case as differences in the stress-strain characteristics of the

embankment and the foundation may lead to progressive failure. The

undrained strength of both the foundation soil and the embankment soil

should be reduced in accordance with the strength reduction factors R+E, and

R+F, in Figure 10 (Reference 9, An Engineering Manual for Slope Stability

Studies, by Duncan and Buchignani).

(3) Rapid drawdown of water level providing insufficient time for

drainage. Use the undrained strength corresponding to the overburden

condition within the structure prior to drawdown.

7.1-331

Integrated Publishing, Inc. |