Mass movement

Mass movement

Mass movement is the detachment of earth materials like soil; rock etc. due to different agents which on combination with water and under the influence of gravity moves down.

Mechanism of Mass movement

On the basis of factor of safety
Factor of safety = (Shearing resistance / Magnititude of shearing force) i.e. strength/stress
Case I
SR (Shear resistance) = MSF (Magnitude of shearing force), FOS(Factor of safety) = 1
This is the equilibrium condition

Case II
SR > MSF, FOS > 1
This is stable condition. This is because the shear resistance is greater than magnitude of shear force.
 

Case III
SR < MSF, FOS < 1
This is unstable condition. This is because the shear resistance is less than magnitude of shear force.
 

Factors affecting shear resistance and Magnitude of shear force

It can be seen from the above points that mass movement occurs if the shear resistance is low or the magnitude of shear force is high.

1. Weathering

Normally, the materials on the earth's surface have low resistance and hence, they weather very easily. This decreases the value of shear resistance.

2. Water

Water has the ability to weaken the binding between the particles. This ultimately decreases the strength. Moreover, when water (in pore) creates hydrostatic force, it tends to increase magnitude of shear force.

3. Earthquake

The lateral movement of the earth surface contributes to the magnitude of shear force.
To sum up, we can say that water brings changes to both shear resistance and magnitude of shear force. So, mass movement is more frequent in rainy season.

Types of mass movement

Mass movement has been classified into the following types.

1. Slope failure

In this type of mass movement, small dimension of mass detached on the steep slope moves down slope. This may be due to road slide (Cutting of steep slope). This type of slope failure causes more internal destruction and the sliding surface is not seen.

2. Landslide

In this type of mass movement, movement of large dimension of land along a plane is found. These are found to have low velocity along that plane due to friction and they have low internal effect and sliding surface is seen.

3. Debris flow

Debris refers to the mixture of rock, soil, air and water and flow refers to movement along a particular path which are viscous generally. Hence, Debris flow refers to the movement of the mixture of rock, soil, air and water along a channel with high speed. These are the most destructive type of mass movement. There is more internal destruction and covers huge area on deposition. They can cover kilometers in just few seconds. This occurs due to the accumulation of small channels of water due to flow of water content into huge one.