Underground works and in particular tunnels and caverns require characterising the rock mass, say matrix and discontinuities, in which the excavation takes place. Characterising the matrix is generally quite easy but the characterisation of the discontinuities and in particular faults is much more difficult. In underground works, a practical but real difficulty is added because the space is often confined and the structures cannot often be observed and analysed accurately and completely. This lecture aims at illustrating how structural observation can increase the quality of the observations, provide help for understanding the origin, the formation mode, the chronology, etc. of faults and fractures and finally lead to a better characterization of these defects. A second objective of this lecture is the presentation of practical rules using structural observations and applied for implementing rock mass classifications in tunnels and caverns.
The characterization of faults and fractures is a key point in rock mechanics, discontinuities being one of the most important aspects when describing a rock mass or when modelling it. Characterization is defined as the description of characters or qualities or peculiarities. In geosciences and more specifically in rock mechanics, the characterization of fractures and faults covers two distinct aspects:
The visual observation and description of discontinuities: this is the qualitative part of the analysis
The measurements and the quantification of certain parameters, related to the discontinuities; this is the quantitative part of the analysis.
Structural geology allows describing discontinuities in terms of origin, movement, chronology, setting (including cross-cutting) and in this sense provides an accurate description of the concerned discontinuities. The following sections present various aspects of the description and the analysis that can be carried out using structural geology as a tool for rock mechanics. The two aspects, description and quantification, are limited to what is ordinarily done during tunnelling or underground works. It is obvious that much more applications and much deeper analyses can be carried out, for example in the frame of a general or research structural study. Some aspects of possibilities given by the structural analysis are not presented here because difficult or even impossible to implement in underground works, due to the specific constraints of the underground environment.