Page 144 - Rock Mechanics For Underground Mining

ROCK STRENGTH AND DEFORMABILITY

Figure 4.43 Effect of shearing di-
rection on the shear strength of a wet
discontinuity in a slate (after Brown et
al., 1977).
displacements, u and v, respectively. When a normal compressive stress, n is applied,
the discontinuity will compress. This compressive stress–displacement behaviour is
highly non-linear (Figure 4.45a) and at high values of n , becomes asymptotic to a
maximum closure, V mc , related to the initial thickness or aperture of the discontinuity.
Suppose that a clean, rough discontinuity is sheared with no normal stress applied.
Dilatancy will occur as shown in the upper curve of Figure 4.45b. If the shear resis-
tance is assumed to be solely frictional, the shear stress will be zero throughout. For
successively higher values of constant normal stress, A, B, C and D, the initial normal
displacement will be a, b, c and d as shown in Figure 4.45a, and the dilatancy–shear
displacement and shear stress–shear displacement curves obtained during shearing
will be as shown in Figures 4.45b and c. As the normal stress is increased, the amount
of dilatancy will decrease because a greater proportion of the asperities will be dam-
aged during shearing.
Now suppose that a test is carried out on the same specimen with the normal stress
initially zero and no dilation permitted during shearing (i.e. v = 0 throughout). By
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