Design against stress-controlled instability 383high compressive radial stress induced beneath the bolt head (theoretically,
the magnitude is infinite beneath a point load), but this dissipates rapidly
as the distance along the bolt increases. At point A, the tangential stress is
tensile, ameliorating the eff eds of a compressive tangential stress induced
by the in situ stress field. At point B both the radial and tangential stresses
are high, and the sign of these stresses changes passing from the left-hand
side of the anchor point to the right-hand side. There are high deviatoric
stresses in the rock at this point which may be sufficient to induce failure
in the rock mass, a fact that is not often appreciated.
When rock bok are used to counteract any anticipated structurally
controlled instability, consideration should always be given to the stresses
induced by the bolts, whether they be mechanically anchored or fully
grouted bolts.
20.2.4 Ground response curve
In Section 16.4, we introduced the concepts of the ground response curve
and available support lines, illustrated in Figs 16.6-16.8. The philosophy
behind the ground response curve is that, under the action of the in situ
stress field, stresses may be induced around an opening that cause failure
of the rock material, either through the development of new discon-
tinuities, by yield of the intact rock, or damage to existing discontinuities.
Rock bolts anchored at
A&BDistribution of boundarv stressv = 0.25
b/a = 3Stress distribution along bolt
Figure 20.25 The influence of a tensioned rockbolt on the stress distribution around
a circular opening (from chapter by J. W. Bray in Brown, 1987).