Civil Engineering Reference
In-Depth Information
11
Stability of Rock Wedges and Excavation Surfaces
11.1
Introduction
In slopes and underground openings in rock, blocks or wedges are often formed by a
free surface and one or more discontinuities. Loads due to gravity, water pressure, earth-
quakes or other external forces may cause translations and rotations of rock wedges
endangering the local or even global stability of an engineering structure.
Whereas global stability is evaluated by finite element analyses as described in the previ-
ous chapter, the proof of local stability is normally carried out by means of limit equi-
librium analyses based on rigid-body mechanics -that is the rock wedges are assumed
to be rigid bodies.
Although rigid-body mechanics does not allow for conclusions to be drawn on stresses
and deformations arising in the rock mass due to the applied forces, the results of limit
equilibrium analyses in many cases lead to useful results about local and in some cases
even about global stability of engineering structures, and the required support mea-
sures. They may therefore provide valuable information when applied as a supplement
to finite element analyses.
The stability proof of rock wedges according to limit equilibrium analysis requires a
safety concept. In Sections 11.3 and 11.4, the stability of rock wedges is evaluated ac-
cording to the partial safety factor method which in Europe was generally introduced
for safety proofs in geotechnical engineering in 2005 (EC7-1 2005).
The stability of columns and layers of rock adjoining excavation surfaces of tunnels,
caverns and construction pits is examined in Section 11.6.
11.2
Potential Failure Modes of Rock Wedges
The principal modes of failure of rock blocks or wedges are transitional and rotational
movements referred to as “sliding” and “rotation” (Fig. 11.1). Also combinations of
these two failure modes can take place.
For rock slopes, wedge failure may be decisive not only for local but even for global
stability. In most cases, the determining factor with regard to stability of a rock slope
along with the loading due to seepage flow is the self-weight.
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