Page 232 - Rock Mechanics For Underground Mining

EXCAVATION DESIGN IN MASSIVE ELASTIC ROCK

Figure 7.16 Prediction of the extent
of boundary failure around a circu-
lar excavation, using the rock mass
failure criterion and the elastic stress
distribution.
This indicates that for any value of the minimum principal stress at a point, a major
principal stress value can be determined which, if reached, leads to local failure of
the rock. As discussed in Chapter 4 and Section 7.1, when assessing the possibility of
rock failure at an excavation boundary, the applicable compressive strength parameter
of the rock mass is the crack initiation stress, ci . For design purposes, the tensile
strength of the rock mass is taken to be universally zero.
Prediction of the extent of boundary failure may be illustrated by reference to a
circular excavation in a biaxial stress field, as shown in Figure 7.16. Boundary stresses
are given by the expression

= p[1 + K + 2(1 − K)cos 2
]
For a rock mass with a crack initiation stress, ci , of 16 MPa (perhaps corresponding
to a uniaxial strength of the rock material of about 50 MPa), the data of Figure 7.16
indicate that compressive failure or spalling of the boundary rock occurs over intervals
defined by
7.5[1.3 + 1.4 cos 2
] ≥ 16

i.e. for
given by
◦
◦
−26 ≤
≤ 26 ◦ or 154 ≤
≤ 206 ◦
Similarly, boundary tensile failure occurs over intervals satisfying the condition

7.5[1.3 + 1.4 cos 2
] ≤ 0
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