Engineering Rock Mechanics

358 Underground excavation instabiliv mechanisms

Mohr - Coulomb criterion for the intact rock

1 + sin 0

Criterion for fractured rock

1 + sin 0

u 1 -u - 3 (i)=ugd 1 -sin ef

u1=‘3(w)+( ;“_“s::: )=bu3+c0

tttt

Stresses within Stresses within Radus of fractured zone:

fractured zone: elastic zone:

ur =pi (<)*-I

ug = dpi (+r-’ ue=p (^1 +e ;“)-P1$

where

Figure 19.19 Development of fracture zones around circular openings.

the orientation of the discontinuity with respect to the field stresses, the
proximity of the discontinuity to the excavation, and the strength para-
meters of the discontinuity itself.
The analysis, although idealized, is useful for evaluating the likely
influence of the parameters on construction.

Special case of stratified rock. There can be cases of composite instability in
which the stress induces slip on pre-existing planes of weakness, with the

d

For each d, calculate 01, r, ur, u8, T,@

transform ur. u8 and T~~ to u,,, and T

tang T1unbm .... ...........

I!

I-----I d

Zone of slip

Plot dun, vs. d, compare to tan@ to determine

zone of slip. Slip causes redistribution of

elastic stresses, which may lead to further

slip. This method is an approximation.

Figure 19.20 Slip on pre-existing discontinuities.