Studies have indicated that when the load on the footing reaches ultimate bearing capacity, the soil may fail in shear in any of the following three modes: 1. General Shear Failure. 2. Local Shear Failure. 3. Punching Shear Failure.
Mode # 1. General Shear Failure:
Experimental investigations have indicated that foundations on the stiff clay or dense sand, with relative density (ID) greater than 70%, fail suddenly with pronounced peak resistance when settlement reaches about 7% of the foundation width.
At this peak resistance, the settlement increases considerably at a very fast rate. The failure is accompanied by considerable bulging of the sheared mass of the soil. This type of failure is known as general shear failure. General shear failure occurs in dense sand with relative density ID > 70% and in stiff clays.
Important characteristics of general shear failure are as follows:
i. Well-defined failure surface.
ii. Distinct peak failure load.
iii. Sudden catastrophic failure accompanied by tilting of foundation.
iv. Bulging of ground surface adjacent to the foundation.
v. Low failure strain of about 7% of footing width.
vi. Large settlement after failure.
General shear failure is the only type of failure, which is amenable to theoretical solution. The bearing capacity equation developed by Terzaghi is applicable only for general shear failure.
Figure 18.6(b) shows the load intensity and settlement relationship for the footing shown in Fig. 18.6(a) subjected to load intensity q that fails in general shear. In dense sands and stiff clays, the load intensity increases rapidly with low settlement up to the point of failure. At this stage, the settlement increases rapidly with little or no increase in load intensity. The load intensity corresponding to the failure is the ultimate bearing capacity and this type of failure is known as general shear failure.
Mode # 2. Local Shear Failure:
In the case of loose sands or soft clays, settlement increases steadily with increase in the load intensity up to a certain point. The load intensity settlements relationship up to this point is nonlinear. Beyond this point, the settlement increases more rapidly with increase in the load intensity and the load intensity settlement relationship is more or less linear, as shown in Fig. 18.7(b). This type of failure is known as local shear failure.
In local shear failure, there is no clearly defined failure stress and the load intensity and the settlement keep on increasing but the settlement exceeds the permissible value. In local shear failure, the failure surfaces end somewhere inside the soil. However, some signs of soil bulging at the surface can be observed, as shown in Fig. 18.7(a). Local shear failure occurs in loose-to-medium sand with relative density of 35%-70% and in soft clays.
Important characteristics of local shear failure are as follows:
i. Well-defined wedge and slip surfaces only beneath the foundation.
ii. Slip surfaces not visible beyond the edges of the foundation.
iii. Slight bulging of the ground surface adjacent to the foundation.
iv. Significant settlement of the soil directly beneath the foundation.
v. The load-settlement curve does not indicate a distinct ultimate load.
For local shear failure, the bearing capacity is obtained from the following equation –
qu = c’Nc‘ + γDNq‘ + 0.5 γBNγ‘ …(18.37)
For local shear failure, the local shear parameters as given below are used to compute the ultimate bearing capacity –
c’ = (2/3)c …(18.38)
tanɸ’= (2/3) tanɸ’ …(18.39)
Nc‘, Nq ‘, and Nγ‘ are the bearing capacity factors corresponding to ɸ’.
Mode # 3. Punching Shear Failure:
Foundations on relatively loose sand with relative density less than 35% or clays of soft consistency penetrate into the soil without any bulging of the ground surface [Fig. 18.8(a)]. The base resistance gradually increases as the settlement progresses. The rate of settlement increases and reaches a maximum at a settlement of about 15%-20% of foundation width. Sudden jerks can be observed as soon as the settlement reaches about 6%-8% of the foundation width.
The failure surface [Fig. 18.8(a)], which is vertical or slightly inclined and follows the perimeter of the base, never reaches the ground surface. This type of failure is known as punching shear failure. Figure 18.8(b) shows the load- settlement relationship for a typical punching shear failure of a footing at ground surface and that at some depth below GL.
The failure surface occurs in loose sand with relative density less than 35%. The points given in Table 18.2 may be used as a guide to distinguish general and local shear.
Figure 18.9 shows the Vesic’s classification of shear failures based on the relative density of sand and relative depth of foundation