Page 462 - Rock Mechanics For Underground Mining

LONGWALL AND CAVING MINING METHODS

Figure 15.11 Distribution of ob-
served “good” roof conditions in
rectangular roadways with the an-
gle between the roadway axis and
the major principal in situ stress di-
rection, Southern Coal Field, New
South Wales, Australia (after Gale and
Blackwood, 1987).

be overlooked, especially in terms of the effects on roadway stability and support and
reinforcement requirements (see section 15.3.5). The influence of horizontal in situ
stresses and, in particular, their orientations with respect to those of the roadways and
panels was not well recognised until the last 20 years. This may have reflected the
approximately hydrostatic in situ stress conditions encountered in many coal mining
districts.
Gale and Blackwood (1987) carried out one of the pioneering studies of this issue
in the Southern Coalfield, New South Wales, Australia, where the major, intermediate
and minor principal in situ stresses are approximately horizontal, vertical and hori-
zontal, respectively. They used three dimensional boundary element stress analysis
to calculate the stress distributions induced around rectangular roadways driven with
their longitudinal axes at varying angles,
SR , to the major principal in situ stress.
Using known material properties, and values of the in situ principal stresses of 25, 16
and 15 MPa, they were able to calculate the distributions of factors of safety against
shear failure through the rock and shear failure along bedding planes. As shown in
Figure 15.11, Gale and Blackwood (1987) correlated the presence of “good” roof
conditions, defined by the absence of visible shear fracturing, with values of
SR .
They concluded that in stress fields having dominant horizontal stress components,
the driveage direction has a major effect on the type and geometry of potential rock
mass failure around the excavation.

15.3.3 Longwall caving mechanics
The discussion presented in the previous section shows that significant redistribution
of the in situ stresses occurs during and following the mining of a longwall panel.
As a result, the pre-existing compressive stresses are relieved above the mined-out
area and there is a concentration of compressive stresses and the development of
principal stress differences in the rock over and beyond the abutments of the panel.
These new stress distributions induce tensile fracturing, de-lamination and opening
of pre-existing fractures producing caving of the rock mass above the mined-out
area, and shear fracturing and slip on bedding planes and natural and mining-induced
discontinuities and fractures in the rock mass surrounding the panel. Figure 15.12
shows a widely accepted representation of these effects around a single longwall
panel. As in the case of longwall metalliferous mining discussed in section 15.2, the
shear fractures occurring ahead of the face will contribute to the caving mechanism

444