soil can withstand without undergoing shear failure. qn = qf - qo Here, qo represents the overburden pressure at foundation level and is equal to үD for level ground without surcharge where ү is the unit weight of soil and D is the depth to foundation bottom from Ground Level. Safe Bearing Capacity (qs) : It is the safe extra load the foundation soil is subjected to in addition to initial overburden pressure. qs qn qo F Here. F represents the factor of safety. Allowable Bearing Pressure (qa) : It is the maximum pressure the foundation soil is subjected to considering both shear failure and settlement. Foundation is that part of the structure which is in direct contact with soil. Foundation transfers the forces and moments from the super structure to the soil below such that the stresses in soil are within permissible limits and it provides stability against sliding and overturning to the super structure. It is a transition between the super structure and foundation soil. The job of a geotechnical engineer is to ensure that both foundation and soil below are safe against failure and do not experience excessive settlement. Footing and foundation are synonymous. Modes of shear failure Depending on the stiffness of foundation soil and depth of foundation, the following are the modes of shear failure experienced by the
foundation soil. 1. General shear failure (Ref Fig. 7.1a) 2. Local shear failure (Ref Fig. 7.1b) 3. Punching shear failure (Ref Fig. 7.1c) Shear failure in foundation soil P – Δ curve in different foundation soils Fig. 7. 1 : Footing on ground that experiences a) General shear failure, b) Local shear failure and c) Punching shear failure General Shear Failure This type of failure is seen in dense and stiff soil. The following are some characteristics of general shear failure. 1. Continuous, well defined and distinct failure surface develops between the edge of footing and ground surface. 2. Dense or stiff soil that undergoes low compressibility experiences this failure. 3. Continuous bulging of shear mass adjacent to footing is visible. 4. Failure is accompanied by tilting of footing. 5. Failure is sudden and catastrophic with pronounced peak in P – Δ curve. 6. The length of disturbance beyond the edge of footing is large.
7. State of plastic equilibrium is reached initially at the footing edge and spreads gradually downwards and outwards. 8. General shear failure is accompanied by low strain (<5%) in a soil with considerable Φ (Φ>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. 1. A significant compression of soil below the footing and partial development of plastic equilibrium is observed. 2. Failure is not sudden and there is no tilting of footing. 3. Failure surface does not reach the ground surface and slight bulging of soil around the footing is observed. 4. Failure surface is not well defined. 5. Failure is characterized by considerable settlement. 6. Well defined peak is absent in P – Δ curve. 7. Local shear failure is accompanied by large strain (> 10 to 20%) in a soil with considerably low Φ (Φ<28o) and low N (N < 5) having low relative density (ID > 20%). Punching Shear Failure This type of failure is seen in loose and soft soil and at deeper elevations. The following are some characteristics of general shear failure. 1. This type of failure occurs in a soil of very high compressibility. 2. Failure pattern is not observed. 3. Bulging of soil around the footing is absent. 4. Failure is characterized by very large settlement.
5. Continuous settlement with no increase in P is observed in P – Δ curve. Fig. presents the conditions for different failure modes in sandy soil carrying circular footing based on the contributions from Vesic (1963 & 1973) Fig. Modes of failure at different Relative densities & depths of foundations Distinction between General Shear & Local or Punching Shear Failures The basic distinctions between general shear failure and punching shear failure are presented in Table Table : Distinction between General Shear & Local Shear Failures General Shear Failure Occurs in dense/stiff soil Local/Punching Shear Failure Occurs in loose/soft soil Φ>36o, N>30, ID>70%, Cu>100 Φ<28o, N<5, ID<20%, Cu<50 kPa kPa