Modes of Shear Failure of Soil - General, Local, Punching Shear Failure

Modes of shear failure

There are three modes of shear failure, i.e. General, Local and Punching shear failures depending upon the compressibility of soil and depth of footing with respect to its breadth (i.e D/B Ratio). When the ultimate bearing capacity of the soil is reached, it may fail in one of the following three failure mode depending upon the type of soil and depth to width ratio of the footing (i.e. D/B)

• General Shear Failure
• Local Shear Failure
• Punching Shear Failure

1. General Shear Failure

• In this mode a slight downward movement of the footing develops fully plastic zones and a sudden failure takes place with a considerable bulging of the ground surface adjacent to the footing
• Characterized by well defined failure pattern, consisting of a wedge and slip surface and bulging (heaving) of soil surface adjacent to the footing
• This type of failure occurs in case of dense sand or stiff cohesive soil supporting the footing
• Failure load is well defined
• The load-settlement diagram is similar to stress-strain for dense sand or over-consolidated clay as shown
• The ultimate load is well defined on this curve as shown typically in figure given below
• Sudden collapse occurs, accompanied by tilting of the footing
  
  

  This type of failure occurs in case of dense sand or stiff cohesive soil supporting the footing Failure load is well defined The load-settlement diagram is similar to stress-strain for dense sand or over-consolidated clay as shown The ultimate load is well defined on this curve as shown typically in figure given below

2. Local shear failure

• Failure pattern consists of wedge and slip surface but is well defined only under the footing. Slight bulging of soil surface occurs. Tilting of footing is not expected. In this mode a large deformation takes place under the footing before the development of failure zones, i.e. large vertical settlement takes place before slight bulging of the ground surface Tilting of footing is not expected Ultimate load is not well defined It takes place in moderately compressible soils or loose sand i.e Occurs in soil of high compressibility Yielding takes place close to the lower edges of the footing Several yield developments may occur accompanied by settlement in a series of jerks The bearing pressure at which the first yield takes place is referred to as the first-failure pressure or first failure load

3. Punching shear failure

• Failure pattern is not well defined No bulging of ground surface and no tilting of footing occurs The yield surfaces are vertical planes immediately adjacent to the sides of the foundation The ground surface may be dragged down thus, no bulging of the surface takes place Failure take place immediately below footing and surrounding soil remains relatively unaffected Large settlements-ultimate load is not well defined Punching Shear Failure takes place in weak compressible soils with considerable vertical settlement i.e Occurs in soil of very high compressibility It also occurs in the soil of low compressibility, if the foundation is located at considerable depth. After the first yield the load-settlement curve will steepen slightly, but remain fairly flat Punching Shear Failure may also take place in soil of low compressibility, if the foundation is located at a considerable depth

• Modes of Shear Failure (Summary) General Local Punching Relative Settlement Less Large Large Bulging Significant Less No Tilting of Footing Expected Not expected Not expected Ultimate Load Well defined Not well defined Not well defined Failure Pattern Wedge + Slip Surface + Bulging Wedge + Slip Surface + Bulging(no or less) Not well defined Occurs in (Soil Type) Dense Less compressible Highly Compressible