Rotational Slide and Slumping

About

Slumping is similar to rotational slides. Slumps have rotational movements along a curved slide plane. The coherence of the mass is lost. the movement may take place as a flow near the toe of the slide. Theses processes result in an arcuate scar at the head where the material had evacuated, followed by a linear tongue of mobile material, and ending with a bulging toe dominated by flowage.

Causes

Heavy Rainfall :

• Water increases pore pressures ->reduces frictional forces between the soil particles & destabilises the slope
• Water adds to the weight of the soil and has a lubricating effect between layers of rock.

Geological Conditions :

• Rotational landslides are very likely if a permeable rock layer lies above an impermeable rock layer
• Water percolates through the porous rock and settles between the two rock layers
• Lubricates them & adds to their weight.

Earthquakes :

• Earthquakes cause vibration & trigger slope instability
• The vigorous shaking of an already unstable slope may cause it to fail

Human Activities :

• Over-steepening of slopes caused by construction of houses & roads results in removal of toes
• Effectively reduces the resisting mass, leading to slope instability & failure
• Building houses on the slope can add weight to it
• Human acitvities introduces a lot of ground water beneath the homes, lubricating and weakening the soil or rock layer beneath.
  
  Process
  
  
• Rotational slide will result in a series of rocks along a curved fracture with much of the materials moving as a coherent large slump block. A back tilted slope is a characteristic feature.
• Slumping also results in sliding of masses of rock not as a coherent whole. The coherence of the mass is lost and flow is observed at the toe. An arcuate scar at the head may be formed, with a bulging toe at the end of the slump.
• In both rotational slide and slumping, there are prominent crown, major scarp, minor scarps and a toe.
• Rotational slides will leave a series of concave slope forms
• Slumping results in a concave slope form and sometimes the undulating nature of the toe will depict a miniature series of concave and convex at the base of the slope.

Animation depicting the cross-section of a mudflow