Engineering Rock Mechanics

74 Intact rock: deformability, strength and failure

Figure 6.3 The trilithons at Stonehenge in the UK.

where fluids have circulated. When the engineer excavates rock and

creates new surfaces, the THMC processes are accelerated, because

the temperature changes, water flow is enhanced (possibly with pH

changes), and the rock stress is reduced to zero (see A3.5) on the

excavation surfaces resulting in a high stress gradient in the proximate

rock mass. Thus, the intact rock is in a state of change, the rate of which

depends on the rock type and the applied THMC gradients. The rock

has already undergone a variety of changes, and will do so again on

excavation or when adjacent to an excavation.

6.2 Questions and answers: intact rock

46.1 Predict how the volume of a specimen will change during a
uniaxial compression test, and sketch the variation of volumetric
strain versus axial strain. What physical processes are occurring in
the sample as the curve manifests significant gradient changes?

A6.1 We consider the volume change during the complete stress-strain
curve as illustrated on the next page (i.e. as it occurs during generation
of the curve in Fig. 6.2).
Initially, the compressive load closes any open, pre-existing cracks,
and so there is contraction of the specimen. Following this is a region
of elastic contraction during the linear portion of the pre-peak region
of the curve. From about 50% of the peak load onwards, new cracks
form at an increasing rate, causing a dilatation component to the volume
change and slowing down the contraction rate. Eventually, the dilatation
associated with new cracks is greater than the elastic contraction, and
the volume change versus displacement curve alters from a negative