Page 469 - Rock Mechanics For Underground Mining

LONGWALL COAL MINING

element are more suitable for some conditions than for others (Frith, 2000, Fuller,
1999). Mechanisms that may have to be taken into account include:
bed separation, roof sag and buckling, particularly under high horizontal stresses,

requiring roof beam formation and reinforcement and tying the roof beam back to
sufficiently strong and stable strata under weak roof conditions;
shear failure on bedding planes;

compressive (shear) failure of rock materials;

tensile failure of rock materials and opening of pre-existing joints under low or

relieved horizontal stress conditions;
falls of blocks of rock isolated by bedding planes, joints and stress-induced

fractures;
unravelling of closely jointed and fractured rock or coal; and

rib failures in coal under vertical stress concentrations or adverse geological

conditions.
Computational methods have been developed to analyse some of these mechanisms
(e.g. Tarrant and Gale, 1998) but, in practice, computational and analytical design
methods are usually supported by the observational method through monitored trials
and in-service performance monitoring (e.g. Frith, 2000, Hebblewhite and Lu, 2004).
Figure15.19showstheinstrumentationlayoutinstalledbyHebbelwhiteandLu(2004)
within2mofa main gate face at the Angus Place Colliery. Through the use of
this instrumentation system, they found that delamination and dilation occurred in
strata above the zone reinforced by the 2.1 m long bolts illustrated in Figure 15.18
causing significant roof deformations. Hebblewhite and Lu (2004) concluded that
longer primary roof bolts and, in some locations, cable bolts were required to prevent
deformation in the upper strata.

Figure 15.19 Instrumentation lay-
out used in heading 2, main gate 22,
Angus Place Colliery, Western Coal
Field, New South Wales, Australia
(SE = sonic extensometer; SG =
strain gauged rock bolts; WE = wire
extensometer; all dimensions in mm)
(after Hebblewhite and Lu, 2004).

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