Engineering Rock Mechanics

The nature of rock masses 1 19

Figure 8.1 (left and above) Examples of fractured rock masses. (a) Folded rock mass in
Crete illustrating that the mechanical behaviour will be a function of both the intact
rock and the fracture geometry. (b) Fractured limestone rock mass, relatively unaffected
during excavation by tunnel boring machine. (c) Jointed granitic rock mass with joints
dilated by bulk blasting. (d) Heavily jointed chalk rock mass, with joint dilation resulting
from weathering.

ozz

Rock fracture

Figure 8.2 Normal and shear stresses applied to a fracture.

kxx, kyy, and kzz. The reciprocals of these are the fracture compliances,

sxx, syy, and szz. There will also be interactions between the axes; for

example a shear stress in the x-direction can cause a displacement in the

z-direction, requiring a kxz or sxz term. This leads to nine stiffnesses or

compliances and the relations

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Usually, these nine stiffnesses (or compliances) are reduced to two: a