Wedges of rock are defined when two joints intersect each other in the vicinity of an excavation. Due to the variability of joint length, spacing and strength, probability methods are required to estimate the frequency of failure of wedges in a rock slope. The point estimate method allows rapid analysis of the effects of variability on strength parameters but the variation of geometry cannot be assessed Using this technique. The Monte Carlo method was used to evaluate geometrical variations of wedges. The frequency of wedge formation was determined by taking into consideration joint continuity and spacing. Finally the total volume of failure was estimated by taking into consideration the number of unstable wedges in different size intervals and accounting for small wedges which may be contained in larger wedges. The technique showed that the probability of failure of wedges is more sensitive to variations in joint geometry than joint strength. The frequency of wedge formation and hence total volume of failure was shown to be very sensitive to the continuity of joints.
Instability in rock slopes may occur if two joints intersect near the slope face and result in a wedge of rock which is able to slide into the excavation.
If jointing is continuous then very large wedges can be formed which could result in disastrous collapse of an entire slope face. The failure of relatively small wedges of rock could result in instability in individual benches.
This paper describes a method of identifying wedges in rock slopes from joint data so that the risk to the functionality of the slope and safety of Workers can be assessed. The method of analysis was encoded into a computer Program which allows rapid computation of potential wedges in rock slopes at any inclination or orientation.
Joint orientations vary considerably as do the strength parameters of joints. In any analysis it is important to take these variations into consideration. Probability methods were employed to account for variability. Probability methods can also be used to quantify the effect of uncertainty in input data, for it is seldom that one has accurate data on all the relevant parameters which affect wedge stability.
Joints are weak planes which are found in most rock masses. Joints can be a few centimeters long or can be several hundred meters long. Joints planes are assumed to consist of a discontinuity part and a rock bridge part, as shown in Figure 1. The discontinuity part can be assigned strength parameters according to the frictional properties of the two sides of the discontinuity.
The rock bridge is assumed to have the strength of the surrounding intact rock. Intact rock strength is at least an order of magnitude greater than the shear strength of a discontinuity. In this paper it is assumed that if a rock bridge exists, then shear will not take place through the rock bridge.
