Page 231 - Rock Mechanics For Underground Mining

DELINEATION OF ZONES OF ROCK FAILURE

(c) The boundary stress at the centre of the crown would be approximately 0.72p,
estimated from equation 7.7. (The boundary element solution is 0.82p.)
(d) An estimate of the sidewall boundary stress, obtained by considering an in-
scribed ellipse and applying equation 7.6, yields A = 1.83p. For the sidewall
locations in the left wall shown in Figure 7.15, boundary element analysis gives
values of 1.87p, 1.75p and 2.08p. For the locations in the right wall, the A
values are 1.35p, 1.36p and 1.61p. The average of these six values is 1.67p.
Boundary element analysis also confirms conclusions (a) and (b). The extent of
the zone of tensile stress determined by the boundary element analysis is shown
in Figure 7.15.
The demonstration, in an elastic analysis, of a zone of tensile stress, such as in
the bench of the current excavation design, has significant engineering implications.
Since a rock mass must be assumed to have zero tensile strength, stress redistribution
must occur in the vicinity of the bench. This implies the development of a de-stressed
zone in the bench and some loss of control over the behaviour of rock in this region.
The important point is that a rock mass in compression may behave as a stable
continuum. In a de-stressed state, small imposed or gravitational loads can cause
large displacements of component rock units.
The conclusion from these studies is that a useful appreciation of the state of stress
at key sections of an excavation boundary can be established from simple, closed-
form solutions. Inscription of a simple excavation shape in the design cross section,
and determination of boundary curvature, are simple techniques allowing the key
features of the boundary stress distribution for an excavation to be defined. More
comprehensive definition of the boundary stress distribution would be required if
studies, such as those described, identified zones of mechanically unacceptable states
of stress around the excavation periphery.

7.5 Delineation of zones of rock failure

In assessing the performance of excavations and rock structures, it is useful to dis-
tinguish between failure of the structure, and failure or fracture of the rock mass.
Failure of a structure implies that it is unable to fulfil the designed duty requirement.
Failure of a rock structure in massive rock is synonymous with extensive rock frac-
ture, since the stable performance of the structure under these conditions cannot be
assured. In a mine structure, control of displacements in a fractured rock mass may
require the installation of designed support elements, or implementation of a mining
sequence which limits the adverse consequences of an extensive fracture domain.
On the other hand, limited fractured rock zones may pose no mining problem, and a
structure or opening may completely satisfy the design duty requirements. A simple
method of estimating the extent of fracture zones provides a basis for the prediction
of rock mass performance, modification of excavation designs, or assessing support
and reinforcement requirements.
In Chapter 4, it was observed that a compressive failure criterion for a rock mass
may be expressed in the form
(f) = F( 3 ) (7.8)
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