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Rock Mechanics.pdf - Mining and Blasting

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Figure 9.21 Graph to determine factor<br />

A from rock strength <strong>and</strong> rock<br />

stress (after Potvin et al., 1989).<br />

STOPE WALL DESIGN – THE MATHEWS STABILITY CHART METHOD<br />

stable under given conditions of rock stress, rock structure <strong>and</strong> orientation of rock<br />

surfaces, <strong>and</strong> the shape factor or hydraulic radius, S, which accounts for the geometry<br />

of the stope excavation surface. The stability number is a derivative of Barton’s Q<br />

factor, while the shape factor is identical to the so-called ‘hydraulic radius’ used in<br />

Laubscher’s cavability assessment. The shape factor S is determined from<br />

S = (Stope wall area)/(Stope wall perimeter)<br />

When N <strong>and</strong> S are plotted on a graph, it is possible to map the complete domain<br />

into zones which represent recorded observations of stability <strong>and</strong> instability of stope<br />

spans.<br />

The stability number, N, is defined by the product<br />

N = Q ′ xAxB xC (9.37)<br />

In this expression, Q ′ is calculated from the results of structural mapping or geotechnical<br />

core logging of the rock mass using the method proposed for the tunnelling<br />

quality Q-classification system (Barton et al., 1974), taking the joint water reduction<br />

parameter <strong>and</strong> the stress reduction factor as unity. The rock stress factor, A, is determined<br />

from the ratio of the intact rock strength (the uniaxial compressive strength,<br />

c) to the induced compressive stress i , calculated at the centre of the stope wall;<br />

i.e.<br />

A = c<br />

i<br />

(9.38)<br />

The induced stress i can be found by numerical stress analysis or estimated from<br />

published stress distributions. A graphical relation between the strength to stress<br />

ratio <strong>and</strong> factor A has been developed empirically, as shown in Figure 9.21. The joint<br />

orientation adjustment factor, B, is a measure of the relative difference in dip between<br />

the stope surface <strong>and</strong> the critical joint set affecting wall stability, <strong>and</strong> is estimated using<br />

Figure 9.22. The gravity adjustment factor, C, reflects the effect that the orientation of<br />

the stope surface has on its stability, under the influence of gravity, <strong>and</strong> is determined<br />

from Figure 9.23.<br />

267

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