3.4 Toppling failure

Toppling failures occur when columns of rock, formed by steeply dipping discontinuities in the rock rotates about an essentially fixed point at or near the base of the slope followed by slippage between the layers (Figure 8 & 9). The center of gravity of the column or slab must fall outside the dimension of its base in toppling failure.  Jointed rock mass closely spaced and steeply dipping discontinuity sets that dip away from the slope surface are necessary prerequisites for toppling failure. The removal of overburden and the confining rock, as is the case in mining excavations, can result in a partial relief of the constraining stresses within the rock structure, resulting in a toppling failure. This type of slope failure may be further categorized depend on the mode such as flexural toppling, block toppling, and block flexural toppling.

 

Figure 8: Typical view of Toppling failure

Figure 9: Schematic view of Toppling failure

Block toppling

As illustrated in Figure 10a block toppling occurs when individual columns in a strong rock are formed by a set of discontinuities dipping steeply into the face. A second set of widely spaced orthogonal joints defines the column height. The short columns forming the toe of the slope are pushed forward by the loads from the longer overturning columns behind. This sliding of the toe allows further toppling to develop higher up the slope. The base of the failure generally consists of a stepped surface rising from one cross joint to the next. Typical geological conditions, in which this type of failure may occur, are bedded sandstone and columnar basalt in which orthogonal jointing is well developed.

 

Flexural toppling

The process of flexural toppling is illustrated in Figure 10b that shows continuous columns of rock separated by well developed, steeply dipping discontinuities, breaking in flexure as they bend forward. Typical geological conditions in which this type of failure may occur include thinly bedded shale and slate in which orthogonal jointing is not well developed. Generally, the basal plane of a flexural topple is not as well defined as a block topple. Sliding, excavation and erosion of the toe of the slope allows the toppling process to start and it retrogresses back into the rock mass with the formation of deep tension cracks that become narrower with depth. The lower portion of the slope is covered with disordered fallen blocks. Therefore it is sometimes difficult to recognize a toppling failure from the bottom of the slope.