There are three principle modes of shear failures:
- General shear failure
- Punching shear failure
- Local shear failure
1. General shear failure: It results in a clearly defined plastic yeild slip surface beneth the footing and spreads out one or both sides, eventually towards the ground surface. Failure is sudden and will often be accompained by severe tilting. Generally associated with heaving. This types of failure occurs in dense sand and stiff clay.
Fig.1 General shear failure
2. Local shear failure: It results in considerable vertical displacement prior to the development of noticeable
shear planes. These shear planes do not generally extend to the soil surface, but some adjacent bulging may
be observed, but little tilting of the structure results. This shear failure occurs for loose sand and soft clay.
Fig. 2 Local shear failure
3. Punching shear failure: This type of failure occurs in very loose sands and soft clays and there is little or no development of
planes of shear failure in the underlying soil. Slip surfaces are generally restricted to vertical planes adjacent
to the footing, and the soil may be dragged down at the surface in this region.
Fig.3 Punching shear failure
Fig. 4 Load settlement curves for different shear
From the curves the different types of shear failures can be predicted :
- For general shear failure there is a pronounced peak after which load decreases with increase in settlement. The load at the peak gives the ultimate stress or load.
- For local shear failure there is no pronounced peak like general shear failure and hence the ultimate load is calculated for a particular settlement.
- For punching shear failure the load goes on increasing with increasing settlement and hence there is no peak resistance.
Fig.5 Variation of the nature of bearing capacity failure in sand with
Relative density (Dr) and relative depth (D/B) (Vesic 1963).
