Fractures, fracture structures

Planes of Weakness and Their Patterns

Rock mass behavior, which for engineering purposes can be described by its resistance (strength), deformability, and permeability, is affected by the combined behavior of continuous (or intact) rock and the planes of weakness (discontinuities) (see Vol. XIII: Residual Stress, Rocks). Hereafter, planes of weakness will be called discontinuities , because such planes represent a rupture of the continuity of intact or solid rock. Discontinuities are planar features (or surfaces) of smaller resistance and higher deformability and permeability than the intact rock. This section describes the practical effects of these discontinuities, their geometric properties and their observation in the field.

Figure 1 shows practical effects of discontinuities in rock masses. Horizontal stress relief joints restrict the size of granite blocks that can be cut from the quarry in Fig. 1a. Horizontal bedding plane discontinuities weakened this shale block (Fig. 1b)...