3.5 Rockfalls

In rockfalls, a rock mass of any size is detached from a steep slope or cliff along a surface on which little or no shear displacement takes place, and descends mostly through the air either by free fall, leaping, bouncing, or rolling (Figure 11). It is generally initiated by some climatic or biological event that causes a change in the forces acting on a rock. These events may include pore pressure increase due to rainfall infiltration, erosion of surrounding material during heavy rain storms, freeze-thaw processes in cold climates, chemical degradation or weathering of the rock, root growth or leverage by roots moving in high winds etc. In an active construction environment, the potential for mechanical initiation of a rockfall may probably be one or two orders of magnitude higher than the climatic and biological initiating events described above. 

Movements are very rapid to extremely rapid. Rock fall may involve a single rock or a mass of rocks, and the falling rocks can dislodge other rocks as they collide with the cliff.  Rockfalls are a major hazard in rock cuts for highways and railways in mountainous terrain (figure 12). Once movement of a rock perched on the top of a slope has been initiated, the most important factor controlling its fall trajectory is the geometry of the slope. In particular, dip slope face, such as those created by the sheet joints in granites are important, because they impart a horizontal component to the path taken by a rock after it bounces on the slope or rolls off the slope.

 

 

Figure 11: Typical view of Rock fall

           

Figure 12: Damage due to rock fall