Engineering Rock Mechanics

2 76 Rock reinforcement and rock support

DEE These zones coincide with those determined by Daemen, also

shown in the diagram.

The #j method is quicker but only indicates the regions around the

excavation boundary where there is the potential for interlayer slip.

To establish the zones of distress within the rock mass, the rigorous

approach is required.

416.8 Two types of 'ground response curve', A and B, are illustrated

in the diagram below. The curves indicate the amount of support

pressure required to maintain an excavated rock surface at a given

value of displacement.

support

Rock pressure

displacement

Rock

displacement

With reference to these curves, explain:

(a) why the in situ stress condition before excavation cannot be

re-established by the support pressure applied after excavation;

(b) why it is inappropriate for an engineer to insist that the rock

should be supported in such a way that no displacement what-

soever occurs.

A16.8 (a) The principal stresses of the in situ stress state before excava-
tion will have certain magnitudes and directions. After excavation and
on the excavation periphery, these magnitudes and directions will have
changed, as indicated in A15.9 and in the sketch below.
Before After
excavation excavation No normal stress
or shear stress at
+------- excavation boundary

I >?y I I I

\ ,' Principal

'\ , stresses

'. -_----.a'

Recalling that stress is a tensor quantity with six independent com-
ponents, in the general case it is not possible to re-establish the in situ
stress by applying a normal stress, or pressure, to the excavation surface;
it would also be necessary to apply shear stresses, which is not practical.
(b) For either ground response curve, A or B, although it is not
possible to re-establish the in situ stress field by the application of