Engineering Rock Mechanics

Questions and answers: rock reinforcement and rock support^271

and so untensioned bolts only provide restraint. As such, we consider
untensioned bolts as providing a positive restraining component. Ten-
sioned bolts, on the other hand, provide a component of force before any
movement takes place, and so we regard these as providing a negative
actuating component.
(e) On the basis of the definition of factor of safety we gave earlier,
a factor of safety of 3 means that we are providing three times more
strength to the interface than is required. This implies that we are
preventing any movement occurring, and hence we must use tensioned
bolts. The bolt force required to develop a factor of safety of 3, is then

Fasin@ -cos@tan@

sin B tan @ + Fa cos

T, = W = 476 kN,

416.4 A circular tunnel is being

excavated in a blocky rock mass

by drilling and blasting. There is

an Excavation Disturbed Zone

(EDZ) around the excavated tun-

nel (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.

Ti

What support pressure4 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?

A16.4 The support pressure, p, is calculated as W/A, where W is
the weight of the loose blocks and A is the area of the tunnel being
considered. Taking the EDZ volume, V, above a 1 m2 area of tunnel roof,
the weight of the EDZ is y V, so W = 25 x (0.75 x 1 x 1) = 18.75 kN and
A = 1 x 1 = 1 m2, giving p = 18.75 kN/m2 = 18.75 kPa.
To provide some comparison with this value, a car tyre pressure is
typically about 170 kPa, which is about nine times larger. Note that this
calculation is only for the loosened blocks in the roof; blocks in other
parts of the EDZ would be partially supported by the blocks beneath
them, and so would require a lower support pressure.

476.5 (a) If the EDZ in 416.4 is to be stabilized by the use of

rockbolts inserted into the roof as a supporting method, and the

Note that the words 'pressure' and 'stress' have different meanings. Pressure occurs

in a fluid, is a scalar quantity, and acts equally in all directions. Stress occurs in a

solid (because a solid can sustain a shear stress), is a tensor quantity and has different

components in different directions. Here we use the word 'pressure' - in line with

conventional usage for tunnel support - to mean the force/unit area acting upwards at

the crown of the tunnel.