Stress -con trolled ins tabiliv mechanisms 347Figure 19.9 Identification of kinematically stable blocks at overhanging and non-
overhanging inclined surfaces.
a body force and is unidirectional. Consequently, the blocks either move
vertically downwards or along some preferred sliding direction. In this
section, however, instability mechanisms are discussed which are stress
controlled. Thus, the activating quantity is not a single force, but a tensor
with six independent components and hence the manifestations of stress-
controlled instability are more variable and complex than those of
structurally-controlled instability.
For example, considering the stresses around a circular opening, the
same stress concentrations can theoretically be experienced at opposite
ends of any diameter of the opening, which could be the floor and the
roof-with the result that both of these locations experience the same
manifestation of instability. So it is not surprising that not only may the roof
have to be supported, but the floor may also have to be supported. This
illustrates one of the fundamental differences between structurally- and
stress-controlled instability mechanisms.
Although in the case of structurally-controlled instability there is sim-
plicity in the mechanisms, it is necessary to consider the complexity of the
three-dimensional geometry of the rock mass. On the other hand, in the
case of stress-controlled instability, the fundamental ‘complexity’ of the