Transition from Local to General Shear Failure in Sand.

As already explained, local shear failure normally occurs in loose and general shear failure occurs in dense sand. There is a transition from local to general shear failure as the state of sand changes from loose to dense condition. There is no bearing capacity equation to account for this transition from loose to dense state. Peck et al., (1974) have given curves for Nγ and Nq which automatically mincorporate allowance for the mixed state of local and general shear failures as shown in Fig. 12.8.

The curves for Nq and Nγ are developed on the following assumptions.

1. Purely local shear failure occurs when Ø < 28°.
2. Purely general shear failure occurs when Ø > 38°.
3. Smooth transition curves for values of Ø between 28° and 38° represent the mixed state of local and general shear failures.

Nq and Nγ for values of 0 > 38° are as given in Table 12. 1 . Values of  Nq and Nγ for 0 < 28°
may be obtained from Table 12. 1 by making use of the relationship

In the case of purely cohesive soil local shear failure may be assumed to occur in soft to medium stiff clay with an unconfined compressive strength qu <= 100 kPa.

Figure 12.8 also gives the relationship between SPT value Ncor and the angle of internal friction 0 by means of a curve. This curve is useful to obtain the value of Ø when the SPT value is known.

Figure 12.8 Terzaghi's bearing capacity factors which take care of mixed state of
local and general shear failures in sand (Peck et al., 1974)

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