Stability of Rock Wedges and Excavation Surfaces - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 288

The most frequently occurring failure modes in slopes are sliding on one discontinuity
(Fig. 11.1, left) and sliding parallel to the intersection of two discontinuities. The latter
will be comprehensively treated in Section 11.3.2.
A rotation over a discontinuity or a toppling about one edge of the support (Fig. 11.1,
right) can occur in steeply inclined rock slopes and in cases where a horizontal load,
such as a load due to earthquake, is acting on the slope. Also, the rotation about a vari-
able axis must be considered. This mode of failure frequently remains unrecognized and
will be dealt with in Section 11.4.
Figure 11.1 Potential failure modes of rock slopes
In tunnels, shafts and caverns, rock wedges may be formed by the roof, the sidewalls
or the temporary face and two or more discontinuities. Dislocation of rock wedges is
usually observed in the roof area (Fig. 11.2, upper) while wedges in the sidewalls may
slide on one or two discontinuities (Fig. 11.2, lower). Particularly in the latter case the
rock wedge may be partly loaded by a vertical stress in addition to its self-weight. The
magnitude of this stress and the resulting additional load can be calculated by finite
element analyses.
For a gravity dam the decisive loads with regard to stability are normally the weight
of the dam {G}, the water load on the dam {W} and the loading due to seepage flow
consisting of hydrostatic uplift {F U } and seepage forces {F S }. In regions of high seismic
activity earthquake loads are also of importance. A frequently occurring case is that
discontinuities dip horizontally or shallowly and the sliding of rock wedges is possible
by either topographical conditions or a second discontinuity set (Fig. 11.3, left). When
three discontinuity sets exist at the foundation or at the abutments of an arch dam, slid-
ing parallel to the intersection of any two of these may occur (Fig. 11.3, right).
With regard to stability analyses of rock wedges we can distinguish between two- and
three-dimensional problems. In two-dimensional problems only failure modes are con-
sidered that occur parallel to a discontinuity, that is the geometry of the wedge does not