28 Geological setting
Figure 2.25 (a) Joints caused by rapid brittle fracture. (b) Fold caused by slow ductile
deformation.Although geological activity is essentially long term, there exist both brittle
and ductile (i.e. rapid and gradual) behaviour which are manifested in quite
different geological structures as shown in Fig. 2.25.
In fact, one of our basic tools in mechanics is the theory of elasticity which
links stresses and strains by the instantaneous response of the rock. Because
there is no time component in elasticity, this theory is unlikely to fully
explain geological processes. The theory is, however, likely to be of
considerable assistance in engineering when we are interested in the initial
redistribution of the stress field upon excavation.
The influence of time is important additionally because of factors such
as the decrease in rock strength through time, and the effects of creep and
relaxation. Creep is increasing strain at constant stress; relaxation is