Engineering Rock Mechanics

186 Testing techniques

and rates of shearing, allowing a wide range of parameters to be obtained
(c, 4, curvature of the failure locus, residual strength, variation with shear-
ing rate, and so on).
In order to measure the more complex discontinuity stiffness behaviour
described in Section 7.3.1 and to obtain more precise information on the
strength and failure behaviour, it is necessary to use a laboratory-based
shear or triaxial testing machine. Such tests are difficult to conduct, because:

(a) of the possibility of sampling disturbance causing premature failure of

the discontinuity;

(b) a rock specimen containing a discontinuity does not manifest uniform

behaviour in the triaxial cell, causing relative rotation of the specimen

halves or puncturing of the sleeve, as illustrated in Fig. 11.10;

(c) the discontinuity properties are likely to be anisotropic and so it is time

consuming and difficult to establish, for example, the 3 x 3 stiffness

matrix relating the normal and two shear stresses with the normal and

two shear displacements.

Notwithstanding these difficulties, it is important in many rock engineer-

ing projects to understand the mechanical behaviour of discontinuities in

detail, and to study the combined thermo-hydro-mechanical properties, In

the Hot Dry Rock geothermal energy project (where cold water is pumped

down one borehole, passes through a fractured zone within the hot rock,

and returns via another borehole), there is little experience to guide the

work so it must be driven by numerical analysis. Determination of the

discontinuity properties for these complex conditions is a vital ingredient

in supporting the analysis process. The TerraTek testing machine

illustrated in Fig. 11.11 has been used at Imperial College to study such

behaviour.

11.5 Tests on rock masses

The determination of rock mass properties can be approached in two ways:

(a) via the properties of the intact rock and the properties of the discon-

(b) via the properties of the rock mass as measured or estimated directly.

tinuities, which together make up the rock mass properties; or

'i'

t

Ul

1

Lil t

1

1 t

'3

t

Figure 11.10 Triaxial testing of discontinuities (after Brady and Brown, 1985).