Page 243 - Rock Mechanics For Underground Mining

ROCK MASS RESPONSE TO MINING

Inaddressingthesethreedifferentdesignfactors,itisnecessarytoconsiderdifferent
deformation modes of the excavation near-field rock. For example, the first factor
reflects concern with surface spalling and internal fracture in the rock medium, in the
post-excavation stress field. The second factor involves the problem of detachment
of the immediate roof from the host medium, and its loading and deflection into the
mined void under gravity loading. The third problem, floor heave, is an issue where
the floor rocks are relatively weak and the material yields under the stresses operating
beneath the excavation side walls. It is a problem more frequently encountered in a
room-and-pillar mining layout, rather than in designing a single excavation.
Since an excavation design must satisfy different rock mass performance criteria
for the various modes of rock response, it is clear that a number of different analytical
methods are to be employed in the design process. It also implies that it may be neces-
sary to iterate in the design process, to satisfy the various performance requirements
simultaneously.

8.2 Rock mass response to mining

Adverse performance of the rock mass in the post-excavation stress field may be
caused by either failure of the anisotropic medium or slip on the pervasive weakness
planes. The initial phase of the design process involves determining the elastic stress
distribution in the medium around the selected excavation configuration. Following
the procedure proposed for an excavation in massive elastic rock, one can then define
any zones of tensile stress, or compressive stress exceeding the strength of the rock
mass. The excavation shape may be modified to eliminate or restrict these zones,
or alternatively, the extent of domains requiring support and reinforcement may be
defined. Concurrently, it is necessary to determine the extent of the zone around the
excavation in which slip can occur on bedding planes.
The criterion for slip on bedding planes is obtained from the shear strength of the
surfaces. For the reference axes illustrated in Figure 8.2, interbed slip is possible if

| zx |≥ zz tan + c (8.1)

Hence, evaluation of the extent of slip requires that the stress components zz and zx
be determined, from the results of the elastic stress analysis, at points coinciding with

Figure 8.2 Slip-prone zones around
an excavation in stratified rock.

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