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Rock Mechanics.pdf - Mining and Blasting

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Figure 15.23 Vertical section<br />

through the longitudinal axis of an<br />

extraction drift, showing (a) ring<br />

burden, V , <strong>and</strong> slice gradient, , <strong>and</strong><br />

(b) truncated ellipsoid of motion.<br />

Figure 15.24 Flow pattern in the<br />

plane of the wall of a vertical slice.<br />

SUBLEVEL CAVING<br />

width increases <strong>and</strong> as the extraction velocity increases. If it is assumed that the flow<br />

may be described by equations 15.6 to 15.10, <strong>and</strong> that hN 2S where S is the slice<br />

height, then the semi-width of flow, r, can be found from equation 15.10 as<br />

r S[6(1 − ε 2 )] 1/2<br />

(15.11)<br />

where ε is the unknown eccentricity, dependent on the height of flow <strong>and</strong> the particle<br />

size distribution.<br />

It must be recognised that the classical concept of gravity flow in sublevel caving<br />

operations outlined above is based on a number of idealisations <strong>and</strong> cannot be expected<br />

to represent accurately all of the circumstances met in mining practice. Not only might<br />

the flow <strong>and</strong> limit “ellipsoids” be more like inverted tear-drops than true ellipsoids,<br />

457

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