Page 244 - Rock Mechanics For Underground Mining

EXCAVATION DESIGN IN STRATIFIED ROCK
Figure 8.3 The effects of slip and
separation on excavation peripheral
rock.
the locations of bedding planes. The particular zones to be examined are the roof and
haunches of the excavation. It is then a relatively simple matter to map the domains
where the elastic stresses satisfy the criterion for slip. Of course, the mapped zones
do not indicate the complete extent of potential slip, but they can be used to obtain a
reasonable impression of the mining significance of the problem. The design process
seeks to limit the extent of the slip domain, while simultaneously restricting the extent
of any other adverse rock mass response.
Potential slip on bedding planes is a general problem in design in a stratified
rock mass. Its extent is clearly related to the pre-mining stress field and the planned
shape of the excavation. As a general rule, a problem configuration in which the
span/bed thickness ratio (s/t) is low will be subject to slip only in the haunch area.
This may be expressed in the rock mass as the opening of cracks subperpendicular
to bedding, perhaps coincident with any cross joints in the medium, as illustrated
in Figure 8.3a. For a configuration in which the s/t ratio is high (i.e. beds rela-
tively thin compared with excavation span), the zone of slip may include virtually
the complete span of the immediate roof. Since the sense of slip on bedding is such
as to cause inward displacement towards the span centreline of beds, the tendency
is for isolation of the lower bed, at its centre, from the one immediately above it.
Separation of a roof bed from its uppermost neighbour is highly significant because
it implies loss of support of the roof by the overlying beds, as can be appreciated
from Figure 8.3b. Prior to decoupling of the roof layer, its gravitational load is car-
ried in part by the more extensive volume of rock in which the layer is embed-
ded. After detachment of the roof, the bed itself must support its full gravitational
load.
Reference to Figure 8.3 gives some indication of the types of problem presented
by design of roof spans in strata-bound excavations. For thick roof strata, any slip
and cracking over the haunches would appear to introduce the possibility of failure
by shear displacement of the roof bed past the abutment. For thin roof strata, the
implied problem is one of stability of the roof bed under the deflection and lateral
thrust associated with detachment and gravity loading.
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