10.3 Types of Slope Movement
There are different modes of deformation and failure that can exist within a steep slope. These failure mechanisms need to be understood in order to assist in a proper design of any monitoring scheme to be implemented for monitoring of a particular slope.
The prominent types of slope movement are as follows:
10.3.1 Initial response
When a slope is excavated or exposed, there is a period of initial response as a result of elastic rebound or relaxation of stress (Zavodni, 2000). This initial response is most common in open pit mines having rapid excavation rate. Martin (1993) reported that the amount of such initial response may vary from 150mm to 500 mm depending upon types of rock mass. The rates of movement during initial response period decreased with time and eventually indicate no movement.
10.3.2 Regressive and progressive movement
Following the period of initial response, slope failure can be indicated by development of tension cracks near the crest of the slope. The development of such cracks is evidence that the movement of the slope has exceeded the elastic limit of the rock mass. However, it is possible that mining can safely continue under these conditions with the implementation of a monitoring system. Eventually, an “operational slope failure” may develop which can be described as a condition, where the rate of displacement exceeds the rate at which the slide material can be safety mined (Call, 1982).
A clear distinction between regressive and progressive time– displacement curves (Figure 2) may be used as a practical means for differentiating plastic strain of the rock mass from operational failure of the slope. A regressive failure (curve A) is one that shows short-term decelerating displacement cycles if disturbing events external to the slope, such as blasting or water pressure, are removed. Conversely, a progressive failure (curve B) causes displacement at an increasing rate, unless stabilization measures are implemented (figure 2). Correct interpretation of the curves is valuable in understanding the slope failure mechanism and predicting the future performance of the slope.
Operations can be continued below slopes experiencing regressive movement, but it is necessary that the mining be conducted for short periods with frequent pullbacks, with care being taken to identify the transition to a progressive failure (Zavodni, 2000).
Figure 2: Types of slope movement: (a) typical repressive and progressive displacement curves; (b) structural geological conditions corresponding to types of slope movement (Broadbent and Zavodni, 1982).
10.3.3 Long-term creep
Long-term creep may occur where there is no defined failure surface, such as a toppling failure or where the change in slope geometry is very slow, for example, due to stress relief following glacial retreat or erosion at the toe by a river. Other causes of such long-term movement are historical earthquakes that causes displacement, and climatic changes that result in periods of high precipitation and increased pore water pressure within the slope. In most of these cases, there is no evidence of recent movement because the rock surfaces are weathered and there is undisturbed soil and vegetation filling the cracks. It is possible that very slow creep developed, but no long-term monitoring program was available to confirm it. In such cases, presence of tension cracks does not necessarily indicate risk of imminent collapse. However, the hazard may be significant if there is evidence of recent movement such as disturbance to the soil and movement of blocks of rock, or there is a proposed change to the forces acting on the slope, due to excavation at the toe.