Engineering Rock Mechanics

Tests on discontinuities 185

Figure 11.8 The tilt test for measuring the angle of friction between discontinuity

surfaces (after Barton et al., 1985).

one irregular surface slides over another is complex. For good practical
reasons, in the past it has been assumed that there is only one friction angle,
i.e. there is no anisotropy in the roughness of the discontinuity. In fact,
because of the mode of formation of most discontinuities, there will be
anisotropy in the friction angle. The tilt test should be conducted in several
different directions with respect to any visible directional structure. Also,
if there is any stepping on the discontinuity surface, the friction angle will
vary with the direction of relative slip across the discontinuity. The direc-
tion of faulting causing natural slickensiding on discontinuity surfaces can
be detected by the roughness of the surface in different directions.
The most widely used apparatus for rapid determination of discontinuity
strength parameters is the field shear box, as illustrated in Fig. 11.9. A
sample of rock containing a discontinuity is cast in plaster and set in the
apparatus such that the discontinuity can be sheared between the two
halves of the box. This can be conducted at varying levels of normal stress

Ropc load cqualirer

.r(kPa)

400

::p-,

Gauge for meururement

shear displacement

Normal stress = 0.3MPa

loo

0 2 4 6 8 IO I2 14 I6

Wm)

Figure 11.9 The field shear box and typical results.