10.2.3 Rock and Debris Slides

Rock slides and debris slides result when rocks or debris slide down a pre-existing surface, such as a bedding plane or joint surface. A rockslide is a type of landslide caused by rock failure in which part of the plane of failure passes through intact rock (figure 5).

A debris slide is characterized by unconsolidated rock and soil that has moved downslope along a relatively shallow failure plane.  Debris slides are most likely to occur on slopes greater than 65 percent where unconsolidated alluvium overlie a shallow soil/bedrock. The shallow slide surface is usually less than 5 m deep. The probability of sliding is low where bedrock is exposed, except, where weak bedding planes and extensive bedrock joints and fractures exists parallel to the slope.


Debris slides generally start with big rocks that start at the top of the slide and begin to break apart as they slide towards the bottom. Debris avalanches are very fast and the entire mass seems to liquefy as it slides down the slope. This is caused by a combination of saturated material and steep slopes (figure 6).




Figure 5: Movement of slope along rock layer.



Figure 6: Rock and Debris Slides occurs near roadway and damege or block the roadway









Figure 7: Different types sudden slope failure