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Types of shear failure of foundation soilsDepending 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 FailureThis 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 FailureThis 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 soilsThis 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)Read More: What are Types of Foundation Failure Under Loads? What is Punching Shear? Punching Shear in Slabs and Foundations