Engineering Rock Mechanics

340 Underground excavation instability mechanisms

Figure 19.2 Measuring fracture surface roughness.

surface, e.g. the element at the top left of Fig. 19.1, the loading system
is soft: the load will not decrease significantly with displacements of
the rock mass, because of the 'dead weight' nature of the loading.
Conversely, at depth, for example an element of rock around the tunnel
being loaded by the surrounding rock, the loading system can be much
stiffer.
It is more difficult to obtain information about the rock mass properties
underground than at the surface. As we noted in Q2.8, the amount
of rock core obtained during a site investigation programme may be
only of the order of 0.0005% of the rock mass being considered. Also,
in addition to this small sampling volume for underground design
(compared to the information available from surface rock outcrops for
surface design), some of the rock properties are much more difficult to
measure, for example the persistence of fractures. These properties, such
as the fracture surface roughness shown being measured in Fig. 19.2, are
often limited to the core dimensions.
The rock mass will be disturbed by the rock excavation process, but
the elegant pre-splitting technique used to protect the remaining rock