Design against structurally-controlled instability 363Figure 20.2 The effect of excavation orientation in relation to discontinuity set
orientation.occur immediately above the roof. A simple elastic analysis of beam flexure
provides helpful indications of the stability of, and type of support that may
be required for, a stratified roof.
The equations resulting from this elastic analysis are shown in the lower
part of Fig. 20.4. The analysis is two-dimensional and thus is most realistic
for the cross-section of a long excavation. It must be remembered that these
equations apply to a CHILE material with the only discontinuities being
the planes of stratification. Furthermore, a high tensile strength for
the rock is required, not only in small zones of the beams, but on the
scale of the excavation itself. Despite these shortcomings, the equations
provide useful insight into the mechanical behaviour of stratified rock
structures.
Figure 20.3 The effect of excavation dimensions on the size of potentially unstable
blocks.