2.2 Geological discontinuities

The stability of rock slopes is significantly influenced by the structural discontinuity in the rock in which the slope is excavated. A discontinuity is a plane or surface that marks a change in physical or chemical characteristics in a soil or rock mass. A discontinuity can be in the form of a bedding plane, schistosity, foliation, joint, cleavage, fracture, fissure, crack, or fault plane. This discontinuity controls the type of failure which may occur in a rock slope. The properties of discontinuities such as orientation, persistence, roughness and infilling are play important role in the stability of jointed rock slope. Discontinuities may occur multiple times with broadly the same mechanical characteristics in a discontinuity set, or may be a single discontinuity.  It makes a soil or rock mass anisotropic.

The orientation of a major geological discontinuity relative to an engineering structure also controls the possibility of unstable conditions and mode of failure. The mutual orientation of discontinuities determines the shape of the individual blocks. Orientation of a discontinuity can be defined by its dip (maximum inclination to the horizontal) and dip direction (direction of the horizontal trace of the line of dip, measured clockwise from north). The strike is at right angles to the dip direction, and the relationship between the strike and the dip direction is illustrated in Figure 1.  Figure 2a explain the possibility if plane failure at lower value of dip  angle with respect of slope angle however, as the dip angle of discontinuity  increase  and become sub parallel to the slope angle the slope become relatively stable (figure 2b). However further increase in dip angle in discontinuity make is liable to undergo toppling failure (figure 2c).



Figure 1: Terminology defining discontinuity orientation (a) isometric view of plane (dip and dip direction, (b) plan view of plane (c) isometric view of line (plunge and trend).




                (a)                                                      (b)                                         (c)

Figure (2a-c) illustrates the effect of discontinuity orientation on the types of slope failure

A Jointed rock exhibits a higher permeability and, reduced shear strength along the planes of discontinuity apart from increased deformability and negligible tensile strength in directions normal to those planes. The degree of fracturing of a rock mass is controlled by the number of joint in a given direction. A rock mass containing more joints is also considered as more fractured. The spacing of adjacent joints largely controls the size of individual blocks controlling the mode of failure. A close spacing of joints gives low cohesion of rock mass and responsible for circular or even flow failure. It also influences the rock mass permeability.

Persistence of discontinuities defines, together with spacing, the size of blocks that can slide from the face (figure 3). Furthermore, a small area of intact rock between low persistence discontinuities can have a positive influence on stability because the strength of the rock will often be much higher than the shear stress acting in the slope.

Roughness of joint surface is a measure of the inherent unevenness and waviness of the surface of discontinuity relative to its mean plane. The friction angle of a rough surface comprises two components the friction of the rock material (φ), and interlocking produced by the irregularities of the surface (i).

Figure 3: Effects of persistence on slope stability