Types of slope failure

The basic modes of failure of slopes are:

1. Rotational slip (slump), which has three sub-types: face, toe and base failure.
2. Translational slip (planar), along a shallow plane parallel to the slope.
3. Compound slip, with a combined curved and plane surface.
4. Wedge failure, sliding of a wedge along weak planes.
5. Infinite slope (surface) failure, a shallow slide parallel to a long slope.
6. Flow, fall and creep in which soil flows or falls.

1. Rotational slip (circular)

The failure surface is approximately a circular arc, and the soil mass rotates about a centre. It occurs in homogeneous cohesive soils (clay).

• Toe failure: the arc passes through the toe; common in steep slopes in soil with a high angle of friction.
• Face (slope) failure: the arc ends on the slope above the toe; occurs when a hard layer exists near the toe.
• Base failure: the arc goes below the toe into the base; occurs in flat slopes of soft clay with a deep soft layer.

        centre O
          *
        . . .  R
   _____.     .______  <- crest
        \  .'    <- slip circle
         \'
 toe  ----`-------------
  (toe failure: arc passes through the toe)

2. Translational slip (planar)

The soil slides along a nearly plane surface, often where a weak layer or a hard stratum lies at shallow depth parallel to the slope. The length is much greater than the depth. It is typical for cohesionless soil and infinite slopes.

  ______
        \\  soil mass
         \\\  -----> slides
          \\\\__ weak plane (parallel to slope)
   ----------------------------