Engineering Rock Mechanics

2 74 Rock reinforcement and rock support

Stress is parallel to the ,

Region where

interlayer slip

can occur

cannot occur

F

In the case for q5j = 50°, the left-hand diagram below shows that
interlayer slip can occur over most of the roof and all the floor, but not
at the walls. If the value of q5j is reduced to 20", however, interlayer slip
can occur at any location around the tunnel roof, walls and floor. This
is one example of the extra problems that can arise with, for example,
weak rocks and changing water conditions.

a) @ = 50' /

I

b) @ = 20'

9

'#I ,'I I

,*' :!

I ; ,#'

9'

II I I ,'

We encourage you to become adept at using this construction by

mentally constructing sketches such as those shown here: it is a powerful

technique to use when observing problems in underground excavations.

Note that the solutions for several fracture sets can be superimposed.

Ql6.7 In a paper discussing the potential slip on fractures in the

rock around a tunnel or shaft, Daemen (1983)'j uses the solution for

the stresses around a circular opening in a CHILE rock to establish

the normal and shear stresses on the fractures. Applying the Mohr-

Coulomb criterion to these stresses enables the potential zones of

distress around the excavation to be established.

The zones of distress shown in Daemen's diagram below are for

an isotropic stress field and an angle of friction of <b = 20°.

6Daemen J. J. K. (1983) Slip zones for discontinuities parallel to circular tunnels of

shafts. Int. J. Rock Mech. Min. Sci. Geomech. Abs. 20,135-148.