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Rock Mechanics.pdf - Mining and Blasting

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EXCAVATION DESIGN IN BLOCKY ROCK<br />

In analyzing the stability of the tunnel, the prospect of failure is defined by the<br />

formation of a key block of any size within the particular cross section of interest. A<br />

basic parameter in the analysis is the factor x, the key block size fraction of interest<br />

in the assessment of excavation stability. It is defined by<br />

x = minimum key block volume of interest/volume of maximum key block (9.3)<br />

The probability of failure is given by<br />

pf = volume of key block-forming region/volume of unit cell (9.4)<br />

The analysis also requires the definition of the geometric parameter, C, given by<br />

C = volume of key block-forming region/mean volume of unit cells<br />

= 1/6 length ∗ width ∗ altitude of the maximum key block/mean<br />

size of unit cells (9.5)<br />

The mean size of unit cells can be calculated from the mean spacings S1, S2, S3<br />

of the three joint sets.<br />

The unconditional probability of failure, defined as the probability that a key block<br />

larger than size x will intersect a r<strong>and</strong>omly selected tunnel cross section, is shown to<br />

be given by<br />

pf = C(1 − 3x 2/3 + 2x) (9.6)<br />

To assess key block sizes, the cumulative distribution function (cdf) FX(x) is obtained<br />

from<br />

FX(x) = 1 − C(1 − 3x 2/3 + 2x) (9.7)<br />

The probability density function (<strong>pdf</strong>) for key block sizes, fX(x), is given by<br />

fX(x) = 2C(x −1/3 − 1) (0 < x ≤ 1) (9.8)<br />

The <strong>pdf</strong> has a lumped mass at x = 0, given by p(x = 0) = FX(x = 0) = 1 − C.<br />

An alternative to considering the probability of key block occurrence in any r<strong>and</strong>omly<br />

selected cross section is to assess the number En of key blocks expected to be<br />

present along a given length of an excavation. This is evaluated as follows.<br />

From equation 9.8, the probability, pKB, that a particular tunnel cross section contains<br />

a keyblock within a differential size range dx is given by<br />

pKB = 2C(x −1/3 − 1) dx (9.9)<br />

Along a particular length of tunnel, Ltun, the fraction fKB of the tunnel cross sections<br />

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