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
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