Page 229 - Rock Mechanics For Underground Mining

EXCAVATION SHAPE AND BOUNDARY STRESSES

Figure 7.13 Square opening with
rounded corners, in a medium subject
to hydrostatic stress.

boundarystresses.ThisisillustratedbytheproblemshowninFigure7.14a.Thearched
opening has a width of 4.0 m and a height of 4.5 m, which are not unusual dimensions
for a mine haulage. For a field stress ratio K of 0.3, an inscribed ellipse indicates
approximatesidewallstressesof2.5p,usingequation7.6.Iftheobservedperformance
of the opening involved crushing of the sidewalls, its redesign should aim to reduce
stresses in these areas. Inspection of equation 7.6 indicates this can be achieved by
reducing the excavation width/height ratio. For example, if the width/height ratio is
reduced to 0.5, the peak sidewall stress is calculated to be 1.7p. While the practicality
of mining an opening to this shape is not certain, the general principle is clear, that
the maximum boundary stress can be reduced if the opening dimension is increased
in the direction of the major principal stress. For this case, a practical solution could
be achieved as shown in Figure 7.14b, by mining an opening with a low width/height
ratio, and leaving a bed of mullock in the base of the excavation.

Figure 7.14 Effect of changing the
relative dimensions of a mine haulage
drive, to mitigate sidewall failure.

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