Figure 9 - Relationships among different types of mass movements

Heave (heaving up of soils due to frost growth and other causes), flow and slide are the three types of mass movements (figure 9). Mass movements can be grouped under three major classes:

♤ Slow movements – the slow downhill movement of debris and soil on moderate slope is described as creep. Depending upon the type of material involved, several types of creep viz., soil creep, talus creep, rock creep, rock-glacier creep etc., can be identified. Leaning fence post, accumulation of earth on the upslope side of stone walls, etc. are example of creep. Also included in this group is solifluction which involves slow downslope flowing soil mass or fine grained rock debris saturated or lubricated with water. This process is quite common in moist temperate areas where surface melting of deeply frozen ground and long continued rain respectively, occur frequently. The permanently frozen ground prevents the downward percolation of water in summer, producing a highly saturated and mobile soil layer. Also, there is absence of deep-rooted vegetation to bind the soil. Solifluction can occur on slopes of 30 or less.

♤ Rapid movement – these depend on there being sufficient water to saturate comprehensively the soil mass. These movements are mostly prevalent in humid climatic regions and occur over gentle to steep slopes. Earthflow is movement of water-saturated clayey or silty earth materials down hillsides. When slopes are steeper, even the bedrock especially of soft sedimentary rocks like shale or deeply weathered igneous rock may slide downslope. Another type in this category is mudflow. In the absence of vegetation cover and with heavy rainfall, thick layers of weathered materials get saturated with water and either slowly or rapidly flows down along definite channels. It looks like a stream of mud within a valley. Mudflows occur frequently on the slopes of erupting or recently erupted volcanoes. A third type is the debris avalanche, which is more characteristic of humid regions. Avalanche can be much faster than the mudflow.

♤ Landslides – In these, as the velocity does not continually decrease downwards, there must be one or more shear surfaces on which movement takes place. Where the shear surface is approximately planar, the strict meaning of the term slide is appropriate. However, another common type of landslide takes place on arcuate shear planes, and these are called rotational slips. It results into slumping of debris with backward rotation. Most landslides usually occur fairly rapidly, often after excess groundwater following heavy rain has reduced soil strength. Over steep slopes, rock sliding is very fast and destructive.