Engineering Rock Mechanics

Mechanical properties 135

n

U

I Intact

6

U

By definition,

discontinuities

have zero

tensile strength

t

6

I I

777

7

Figure 7.21 A discontinuity loaded in compression, tension and shear.

stress-displacement curve that we would expect. The normal and shear
forces applied across a discontinuity can be scaled by the nominal area of
the discontinuity to give normal and shear stresses, respectively. However,
there is no 'length for scaling the displacement values to evaluate strain.
For this reason, we plot stress-displacement curves, with the result that
discontinuity stiffness has units of, say, MPdm, rather than the MPa units
of intact rock stiffness.
The three curves in Fig. 7.21 are of three different types. In compression,
the rock surfaces are gradually pushed together, with an obvious limit
when the two surfaces are closed. The stiffness associated with this com-
pression process gradually increases with applied stress or displacement,
again reaching a limit associated with the strength of the intact rock, as
indicated in the figure. In tension, because by definition discontinuities are
regarded as having no tensile strength, no tensile stress can be sustained
and hence the displacement increases as indicated. Finally, when a discon-
tinuity is subjected to shear stress or shear displacement, the curve is rather
like the complete stress-strain curve for compression of intact rock, except
of course that all failure is localized along the discontinuity. There is an
initial shear stiffness, a shear strength and a post-peak failure locus.
We can consider the stiffnesses for the cases of compression and shear.
As we have shown in Fig. 7.21, in neither case is there a linear relation