Civil Engineering Reference
In-Depth Information
Comprehensive solution
wedge stability
III.1 Introduction
This appendix presents the equations and proce-
dure to calculate the factor of safety for a wedge
failure as discussed in Chapter 7. This compre-
hensive solution includes the wedge geometry
defined by five surfaces, including a sloped upper
surface and a tension crack, water pressures, dif-
ferent shear strengths on each slide plane, and
up to two external forces (Figure III.1). External
forces that may act on a wedge include tensioned
anchor support, foundation loads and earthquake
motion. The forces are vectors defined by their
magnitude, and their plunge and trend. If neces-
sary, several force vectors can be combined to
meet the two force limit. It is assumed that all
forces act through the center of gravity of the
wedge so no moments are generated, and there
is no rotational slip or toppling.
3
5
1
2
L
4
H
1
Line of
intersection
Figure III.1
Dimensions and surfaces defining size
and shape of wedge.
shorter analyses that can be used for a more lim-
ited set of input parameters. In Section 7.3, a
calculation procedure is presented for a wedge
formed by planes 1, 2, 3 and 4 shown in Fig-
ure III.1, but with no tension crack. The shear
strength is defined by different cohesions and fric-
tion angles on planes 1 and 2, and the water
pressure condition assumed is that the slope is
saturated. However, no external forces can be
incorporated in the analysis.
A second rapid calculation method is presen-
ted in the first part of appendix 2 in
Rock Slope
Engineering
, third edition. This analysis also
does not incorporate a tension crack or external
forces, but does include two sets of shear strength
parameters and water pressure.
III.2 Analysis methods
The equations presented in this appendix are
identical to those in appendix 2 of
Rock Slope
Engineering,
third edition (Hoek and Bray,
1981). These equations have been found to be
versatile and capable of calculating the stabi-
lity of a wide range of geometric and geotech-
nical conditions. The equations form the basis of
the wedge stability analysis programs SWEDGE
(Rocscience, 2001) and ROCKPACK III (Watts,
2001). However, two limitations to the analysis
are discussed in Section III.3.
As an alternative to the comprehensive ana-
lysis presented in this appendix, there are two
