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Rock Mechanics.pdf - Mining and Blasting

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SYMMETRIC TRIANGULAR ROOF PRISM<br />

(or the length of tunnel expected to contain blocks of this size) is<br />

fKB = 2CLtun(x −1/3 − 1) dx (9.10)<br />

The length (measured along the tunnel axis) of a key block of size x can be shown to<br />

be related to the length of the maximum key block by the expression<br />

length = length of maximum key block ∗ x 1/3<br />

(9.11)<br />

From equations 9.10 <strong>and</strong> 9.11, the expected number of key blocks of size x is given<br />

by<br />

En = 2CLtun(x −1/3 − 1)/(length of maximum key block ∗ x 1/3 ) dx (9.12)<br />

It is convenient to introduce a second geometric problem parameter defined by<br />

<strong>and</strong> En is then given by<br />

C2 = 6CLtun/length of maximum key block (9.13)<br />

For a finite size interval (x1, x2), En is given by<br />

x2<br />

En =<br />

x1<br />

= C2<br />

En = C2 1<br />

3 (x −2/3 − x −1/3 ) dx (9.14)<br />

1<br />

C2<br />

3 (x −2/3 − x −1/3 ) dx (9.15)<br />

<br />

x 1/3<br />

<br />

1 2/3 1/3 1 2/3<br />

2 − x 2 2 − x1 + x 2 1 (0 ≤ x1 ≤ x2 ≤ 1) (9.16)<br />

The expressions in equations 9.6, 9.7 <strong>and</strong> 9.16 may be applied in various ways in tunnel<br />

design. In discussing how to apply them, Kuszmaul (1999) reiterates the importance<br />

of bearing in mind the assumptions made in their derivation: that there are three<br />

discontinuity sets, the sets are well defined <strong>and</strong> widely spaced (on the scale of the<br />

excavation), the discontinuities are persistent <strong>and</strong> the rock mass characteristics remain<br />

uniform along the planned excavation length. If these conditions are not met, such<br />

as closely spaced discontinuities <strong>and</strong> limited persistence, the unit cell calculations<br />

overestimate the number of key blocks in the excavation. If there are more than three<br />

joint sets, the joint sets can be considered separately in different sets of three.<br />

The specific value of the unit cell method is that it provides a method for making<br />

design decisions based on likely key block sizes, rather than assuming the worst case<br />

of taking the dimensions of the maximum key block. Alternatively, a design approach<br />

could be based on seeking to minimize the probability of failure of an excavation<br />

support system.<br />

9.3 Symmetric triangular roof prism<br />

Having identified the feasible block collapse modes associated with joint orientations<br />

<strong>and</strong> excavation surface geometry, it is necessary to determine the potential for block<br />

displacement under the conditions that will exist in the post-excavation state of the<br />

255

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