The Glennies Creek Dam spillway was designed as an unlined rock cutting in welded tuff 700 m long with a bedwidth of 37 m, a maximum cut height of 40 m and right-and left-hand side slopes of 3 in 1 and 2 in 1, respectively. Toppling failures in the 3 in 1 right-hand spillway batter occurred during construction. In order to maintain rockfill production, that part of the right-hand batter affected by the failures was battered back from 3 in 1 to a slope of 1 in 1; involving 33 000 m3 of additional excavation.
Geological investigations concluded that toppling became possible due to the removal of the column toe support due to plane sliding movements on joints dipping towards the cut at 24°. These sliding movements were caused by immediately adjacent heavy quarry blasting which reduced the available joint friction angle from the peak value of 38° to the residual value of 25°. An essential step in the analysis of slope stability is the identification of slopes susceptible to toppling. The application of standard methods of toppling analysis to slopes which are not susceptible to toppling even though it may be kinematically possible will result in unnecessarily conservative slope designs.