Questions and answers: rock reinforcement and rock support^273(a) Assuming the friction angle of the
fractures, &, is 509 use the $i geomet-
rical construction in two dimensions to(c) The calculation refers only to the crown of the tunnel where the
rock is free to fall. The support pressure can be reduced away from
crown, because there is additional support from lower blocks, but bear
in mind the potential sliding of the-blocks as discussed in Q and
A 16.2 and 16.3. As a general guide,
radial bolting is appropriate for a
circular tunnel. For the conditions
assumed in this question, no rock-
bolts are needed in the floor, which
is not as strange a comment as it
may seem: sometimes rockbolts are
installed in the floor to counteract
the type of floor heave induced by a
combination of high stress and weak
rock, as illustrated to the left.A16.6 The principle of the #j geometrical construction is indicated
below.
--.Interface
I I
21
When a force is applied to an interface, slip can occur if the force, F,
is applied at a sufficiently low angle, 90-a, to the interface, i.e. slip will
occur for F sin a > tan # F cos a, or tan a > tan 4, which means a > 4.
In the excavation peripheral rock, one principal stress is perpendicular
to the excavation surface with a value of zero, and the other is parallel to
the excavation surface.
This enables the construction below to be used in which the regions
susceptible to interlayer slip can be readily identified - as those where
the angle between the tangent to the excavation surface and the normal
to the layers is greater than the angle of interlayer friction, dj.Goodman R. E. (1989) Introduction to Rock Mechanics. Wiley, New York, 2nd ed.,
562pp.