Page 156 - Rock Mechanics For Underground Mining

ROCK STRENGTH AND DEFORMABILITY

many types of in situ compression test including uniaxial compression, plate bearing,
flatjack, pressure chamber, borehole jacking and dilatometer tests.
The results of such tests must be interpreted with care particularly when tests are
conducted under deviatoric stress conditions on samples containing discontinuities
that are favourably oriented for slip. Under these conditions, initial loading may
produce slip as well as reflecting the elastic properties of the rock material and the
elastic deformabilities of the joints. Using a simple analytical model, Brady et al.
(1985) have demonstrated that, in this case:
(a) the loading–unloading cycle must be accompanied by hysteresis; and
Figure 4.52 Determination of the (b) it is only in the initial stage of unloading (Figure 4.52) that inelastic response is
Young’s modulus of a rock mass from suppressed and the true elastic response of the rock mass is observed.
the response on initial unloading in a
cyclic loading test (after Brady et al., Bieniawski (1978) compiled values of in situ modulus of deformation determined
1985). using a range of test methods at 15 different locations throughout the world. He found
that for values of rock mass rating, RMR, greater than about 55, the mean deformation
modulus, E M , measured in GPa, could be approximated by the empirical equation

E M = 2(RMR) − 100 (4.43)
Serafim and Pereira (1983) found that an improved fit to their own and to Bieni-
awski’s data, particularly in the range of E M between 1 and 10 GPa, is given by the
relation

RMR−10
E M = 10 40 (4.44)
Figure 4.53 shows equations 4.43 and 4.44 fitted to Bieniawski’s (1978) and Serafim
and Periera’s (1983) data, respectively. It also shows further data provided by Barton
(2002) fitted to the equation

E M = 10 Q 1/3 (4.45)
c
where Q c = Q c /100.
Following Hoek and Brown (1997), Hoek et al. (2002) proposed the more complex
empirical relation
((GSI−10)/40)
E M = (1 − D/2) ( c /100) · 10 (4.46)
which is derived from equation 4.44 but gives an improved fit to the data at lower
values of RMR (≈ GSI for RMR > 25), and includes the factor D to allow for the
effects of blast damage and stress relaxation.
It must be recognised that equations 4.43 to 4.46 relate rock mass classification
indices to measured static deformability values that show considerable scatter. Ac-
cordingly, it cannot be expected that they will always provide accurate estimates of
E M . It must also be remembered that, as indicated earlier in this section, rock mass
moduli may be highly anisotropic. They also vary non-linearly with the level of ap-
plied stress and so can be expected to vary with depth. Because of the high costs of
carrying out in situ deformability tests, geophysical methods are often used to esti-
mate in situ moduli. These methods generally involve studies of the transmission of

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