Toppling failure is one of the modes of failures specifically related to rock slopes. Since toppling involves overturning of blocks or rotation of a system parallel steeply tipping columns about Pivotal points, this mode of failure is quite common in rock masses with regular bedding planes or joints. There are several classes of toppling which may occur in rock slopes and a detailed discussion on this topic can be found elsewhere Goodman and Bray,1976).
Among the various kinds of toppling described by Goodman and Bray(1976), the flexural toppling, the block toppling and the block-flexure toppling belong to one class. Another important class of toppling is known as secondary toppling in which the failures are initiated by some undercutting agent. The primary failure mechanism involves sliding or physical break-down. The various mechanisms under secondary toppling are:
Slide toe toppling due to the loading of the potentially toppling rocks by another instability.
Slide base toppling due to the beds being dragged along by overlying material.
Slide head toppling where movements cause blocks to topple higher up a slope.
Toppling and slumping by weathering of underlying rock.
Tension crack toppling 'due to the formation of new cracks above steep slopes.
The problem investigated in this paper falls into the main category of ‘secondary toppling’ and into the sub-classes of (4) and (5). Among the various stability analyses used to investigate toppling failures in rock slopes, the limit equilibrium method has been adopted by Attewell and Farmer (1976) as well as Goodman and Bray(1976). In the limit equilibrium method, the mode of failure is assumed prior to the calculation of factor of safety whereas in many cases, the mode of failure is the unknown factor. Furthermore, the limit equilibrium method considers only the equilibrium of forces and does not take into account the properties of the joint as well as the continuum. Also, it cannot realistically model the progressive failure of the slope. The discrete element method has been adopted by G. Hocking (1978) for the analysis of toppling-sliding mechanisms. This method is better than the equilibrium method because the failure surface is not assumed a priori and the movements of blocks are determined to evaluate the stability of the slope due to toppling mechanism. However, this method, at present, lacks in predicting the cracking of the rock mass.
The finite element method has been used for investigating the toppling failure by Kalkani and Piteau(1976) and Brown et al (1980). Kalkani and Piteau adopted a two dimensional elastic analysis which included the effects of raising the water table to investigate the toppling failure at Hell's Gate Bluffs in Canada. The zones of tensile stresses for low and high groundwater were determined.
Brown et al(1980) used the finite element technique to study the Nevis Bluff rock slope failure in New Zealand. Their investigation is, in fact, a back analysis of the slope which failed in 1975. The results of the analysis indicated that the failure was due to initial flexural toppling which propagated cracking and then sliding.
