Engineering Rock Mechanics

Underground

79 excavation instability

mechanisms

19.1 Underground instability

In Chapters 17 and 18, we presented information relating to the potential
for rock instability at or close to the ground surface. When an excavation
in rock is made entirely underground, there are somewhat different rock
mechanics circumstances primarily related to the nature of the rock mass
and stiffness of the loading system - as indicated in Fig. 19.1.
At depth, the weathering and rock fracturing will be less intense, so
the rock mass will be more coherent, stiffer and stronger. However, the
rock stress and water pressure increase with increasing depth, so there
will be a greater potential for stress-induced damage and water pressure
effects such as water pressure-induced block movements. At the ground

Soft ,

loading

system

k slope

water presaure and fraeturi

Figure 19.1 Rock engineering at the surface and at depth.