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Rock Mechanics.pdf - Mining and Blasting

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Figure 8.8 Assumed distributions<br />

of axial compressive stress <strong>and</strong><br />

parabolic thrust line (after Diederichs<br />

<strong>and</strong> Kaiser, 1999a).<br />

EXCAVATION DESIGN IN STRATIFIED ROCK<br />

force, V , be balanced by the resisting moment MR of the distributed end loads,<br />

i.e.<br />

MA = 1<br />

8 ts2 = MR = 1<br />

2 fc ntz (8.5)<br />

or<br />

fc = 1 s<br />

4<br />

2<br />

(8.6)<br />

nz<br />

where fc is the maximum compressive stress acting in the beam, operating at the<br />

bottom edge at the abutment <strong>and</strong> at the top edge at the centre of the span.<br />

Analysis of the three modes of roof instability involves determination of fc, n <strong>and</strong> z.<br />

Following Sofianos (1996), Diederichs <strong>and</strong> Kaiser (1999) showed that the assumption<br />

of Evans of n = 0.5 was inadequate, that the iterative method proposed by Brady <strong>and</strong><br />

Brown (1985) was appropriate for small deflections, but that convergence <strong>and</strong> stability<br />

in the solution procedure could be improved.<br />

The solution procedure begins with an assumption of the initial moment arm prior<br />

to deflection, zo, which is given by<br />

zo = t 1 − 2<br />

3n (8.6)<br />

The length L of the parabolic reaction arch is given by<br />

L = s + 8 z<br />

3<br />

2 o<br />

(8.7)<br />

s<br />

To calculate the elastic shortening of the arch <strong>and</strong> the central deflection of the arch<br />

through equation 8.2, an assumption must be made about the distribution of axial<br />

compressive stress over the longitudinal vertical section of the beam. In their original<br />

relaxation analysis, Brady <strong>and</strong> Brown (1985) assumed the bilinear variation shown<br />

in Figure 8.8a. From various numerical studies, Diederichs <strong>and</strong> Kaiser proposed that<br />

a better approximation for the simple, two-member voussoir beam is the quadratic<br />

232

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