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Rock Mechanics.pdf - Mining and Blasting

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MINING-INDUCED SURFACE SUBSIDENCE<br />

where w(, ) is a weighting factor introduced to take account of variations in thickness<br />

of extraction or the effect of convergence control measures, <strong>and</strong><br />

r = [(x − ) 2 + (y − ) 2 ] 1/2<br />

By writing dA = d d, the subsidence due to an area of extraction, A, can be<br />

found by integrating equation 16.13 to give<br />

<br />

s(x, y) = w(, ) f {[(x − ) 2 + (y − ) 2 ] 1/2 } d d<br />

A<br />

Brauner (1973) discusses a range of influence functions used mainly in Germany<br />

<strong>and</strong> eastern Europe. They are generally trigonometric or exponential functions of the<br />

form<br />

p(r) = k1Sf(B, r, k2)<br />

where S is the subsidence at the panel centre, B = h tan is the critical radius of<br />

extraction, <strong>and</strong> k1, k2 are constants. One of the most widely used functions is<br />

p(r) = nSmax<br />

B2 <br />

r<br />

<br />

2<br />

exp − n<br />

B<br />

where n is a parameter which characterises the strata properties.<br />

It is apparent that by integration over a large area A, a profile function can be derived<br />

from an influence function. Thus the two types of function are not mathematically<br />

distinct. Profile functions appear to have the advantage of greater simplicity, but<br />

influence functions are more adaptable <strong>and</strong> can be more useful for irregularly shaped<br />

mining panels.<br />

Peng(1992) provides a detailed account of the use of influence functions of the<br />

exponential type for subsidence analysis in the USA. Lin et al. (1992) used asymmetrical<br />

influence functions involving both trigonometric <strong>and</strong> exponential terms with<br />

variable functional parameters to derive solutions giving good fits to NCB data for<br />

seams having inclinations of up to 30 ◦ . Sheorey et al. (2000) modified the conventional<br />

influence function method to take account of asymmetrical subsidence, multi-seam<br />

mining involving extraction below existing goafs, the effect of hydraulic stowing,<br />

<strong>and</strong> to allow more accurately for the effects of the edges of the extraction zones.<br />

They applied their approach to a number of cases in India, including cases involving<br />

inclined seams, complex extraction shapes <strong>and</strong> multi-seam mining.<br />

16.5.3 Trough subsidence analysed as elastic deformation<br />

If in mining deep tabular deposits, fracture or plastic deformation of the rock mass<br />

is restricted to a relatively small zone surrounding the excavation, it may be assumed<br />

that most of the superincumbent strata deforms elastically, at least to a reasonable<br />

approximation. As a further idealisation, the problem of excavating a thin seam may<br />

be represented as one of a crack in an elastic medium. The problem is then one of<br />

determining the stresses <strong>and</strong>, through the stress–strain equations of elasticity, the<br />

strains <strong>and</strong> hence the displacements, induced by the creation of the crack or slit in a<br />

previously stressed semi-infinite elastic body.<br />

510

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