Types of shear failure of foundation soils
Depending on the stiffness of foundation soil and depth of foundation, the following are the modes of shear failure experienced by the foundation soil.
- General shear failure (Fig.1(a))
- Local shear failure (Fig.1(b))
- Punching shear failure (Fig.1(c))
Fig.1: Shear failure in foundation soil
Fig: Curve in different foundation soils
General Shear Failure
This type of failure is seen in dense and stiff soil. The following are some characteristics of general shear failure.
- Continuous, well defined and distinct failure surface develops between the edge of footing and ground surface.
- Dense or stiff soil that undergoes low compressibility experiences this failure.
- Continuous bulging of shear mass adjacent to footing is visible.
- Failure is accompanied by tilting of footing.
- Failure is sudden and catastrophic with pronounced peak in curve.
- The length of disturbance beyond the edge of footing is large.
- State of plastic equilibrium is reached initially at the footing edge and spreads gradually downwards and outwards.
- General shear failure is accompanied by low strain (<5%) in a soil with considerable (>36o) and large N (N > 30) having high relative density (ID> 70%).
Local Shear Failure
This type of failure is seen in relatively loose and soft soil. The following are some characteristics of general shear failure.
- A significant compression of soil below the footing and partial development of plastic equilibrium is observed.
- Failure is not sudden and there is no tilting of footing.
- Failure surface does not reach the ground surface and slight bulging of soil around the footing is observed.
- Failure surface is not well defined.
- Failure is characterized by considerable settlement.\
- Well defined peak is absent in curve.
- Local shear failure is accompanied by large strain (> 10 to 20%) in a soil with considerably low (<28o) and low N (N < 5) having low relative density (ID> 20%).
Punching Shear Failure of foundation soils
This type of failure is seen in loose and soft soil and at deeper elevations. The following are some characteristics of general shear failure.
- This type of failure occurs in a soil of very high compressibility.
- Failure pattern is not observed.
- Bulging of soil around the footing is absent.
- Failure is characterized by very large settlement.
- Continuous settlement with no increase in P is observed in curve.
Fig. 2 presents the conditions for different failure modes in sandy soil carrying circular footing based on the contributions from Vesic (1963 & 1973)