Engineering Rock Mechanics

1 38 Discontinuities

2.8 - Key

A A parallel to surface ridges

m B normal to surface ridges

2.4 -

2.0 -

h Shear direction B

A

/

/-

+’ = *-*” A- <hiar direction A corrected

to zero sample inclination

1

0 0.4 0.8 1.2 1.6 2.0 2.4

Normal load (kN)

shear direction A

Figure 7.22 Bi-linear Mohr-Coulomb failure criterion for rock discontinuities.

an asperity failure component and q& is the basic frictional component. We

have already discussed the size effect in general, and the shear strength of

discontinuities is no exception. Clearly, the roughness is not absolute as the

magnitude of the geometrical component will vary according to the sample

dimensions.

These ideas have been extended to include mechanical versus hydraulic

aperture and a coupling between the mechanical and hydraulic behaviour.

Factors such as the degree and type of infilling will also have an effect on

the strength of discontinuities. However, these effects have not yet been

quantified beyond being able to say that the strength of a discontinuity

approaches that of the filling when the filling is thick.

Here we do not consider the post-peak failure of discontinuities, as this

is included in the next chapter, particularly with respect to the shear

strength of discontinuities.

7.4 Discussion

Developments are possible in both the geometrical and mechanical charac-

terization of discontinuities as a result of research currently underway. We