466 Questions 16. I- 16.10: rock reinforcement and rock supportIX t(a) The component of the bolt force acting parallel to the slope can be
considered as either a positive restraining component or as a negative
actuating component. Derive an expression for the factor of safety in
both these cases.
= 15", 1cI. = 42" and 4 = 36",
what bolt tension is required to give a factor of safety of unity for each of
the cases?
(c) Examine the behaviour of the two expressions for factor of safety
as the bolt tension varies from 25 kN less than, to 25 kN more than,
the force computed for each case above. Which of these two expressions
should we use for design purposes, when we require a factor of safety in
excess of l?
(d) Rock bolts function by developing a tensile force within them, and
are sometimes tensioned at the time of installation in order to generate
this force. In the case of untensioned bolts, how is this force generated?
Should we regard untensioned bolts as providing a positive restraining
component or a negative actuating component?
(e) Imagine that we require a factor of safety of three for the rock block
shown above. Should we use tensioned or untensioned bolts?
(b) For the situation when W = 1000 kN,
Q 16.4 A circular tunnel is being ex-
cavated in a blocky rock mass by dril-
ling and blasting. There is an Excav-
ation Disturbed Zone (EDZ) around
the excavated tunnel (defined on the
basis of a blast-disturbed zone where
there are loosened blocks which can
fall into the tunnel under the action
of gravity) which extends 0.75 m into
the rock from the excavation surface.
What support pressure is required at the crown to stabilize the loose
blocks of the EDZ given that the unit weight of the rock, y, is 25 kN/m3?416.5 (a) If the EDZ in Q16.4 is to be stabilized by the use of rockbolts
inserted into the roof as a supporting method, and the working capacity
of each bolt, T, is 150 kN, what area of the roof will each bolt support?
(b) Would you anticipate any ancillary problems with this bolting
pattern?
(c) In the circular tunnel, in which directions and at which locations
would you install the bolts?