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160 Applied Petroleum Geomechanics

where E m is Young’s modulus of the rock mass; E r is Young’s modulus of
the rock matrix; k n is the normal stiffness of the structure plane; and s is the
spacing of structure planes.
The weak planes or fractures in a rock mass make the compressive
strength of the rock mass far less than that of the rock matrix. The presence
of bedding or other discontinuities also causes anisotropies in both me-
chanical properties and mechanical behaviors; for example, rock strength,
Young’s modulus, and deformation are considerably different in the parallel
and perpendicular directions of the bedding, as described in Chapters 2
and 3. These behaviors should be considered in the subsurface engineering
design and operations.

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