Natural rock environments 15excavation of the surface rock, or whether the structure is underground.
Of course, a particular project may involve two or, indeed, all of these main
types, as in many hydroelectric schemes.
It is generally found that the fractures in the rock govern the stability of
near surface structures and the natural in situ stresses govern the stability
of deep structures. For example, the stability of a dam foundation will
depend critically on the deformability and permeability of the underlying
rocks, which in turn are dictated by the nature and geometrical con-
figuration of the fractures in the rock mass. This is also true for the stability
of the side slopes of surface excavations and the roof and sides of near
surface underground excavations. However, at medium depths in weak
rocks (for example the Channel Tunnel between England and France) and
at considerable depths in strong rocks (for example South African gold
mines) the natural stress, which is altered by the engineering, can be the
dominant problem.
Furthermore, these effects will be influenced by other factors; e.g.
whether the rock is wet or dry, cold or hot, stable or squeezing. Typical
circumstances where these factors are important are the degradation of
chalk and mudstones on either exposure to water movement or desicca-
tion, permafrost engineering, certain Japanese mines in which circulating
groundwater can be above boiling point, the difficulty of inducing roof
failure during longwall mining operations when the roof is too strong, and
loss of tunnel boring machines as they have attempted to cross squeeze
ground within major faults. It is the identification of these, and a whole host
of other geological factors, which is one of the keys to successful site
investigation and correct interpretation of the rock mass environment. Two
examples of the effects mentioned are shown in Figs 2.6 and 2.7.
Of course, different projects could be conducted in entirely different rock
environments and this would be taken into account utilizing the three-tier
Figure 2.5 Small-scale rock structure.