4.8 EFFECT OF WATER TABLE

In the discussion presented in the preceding sections, it has been assumed that the water table is located below the failure surface in the soil supporting the foundation. However, if the water table is present close to the foundation, the terms q and ? in Eqs. (4.31), (4.35), (4.36), (4.37) to (4.39), Eq. (4.69) and Eq. (4.71) need to be modified. This can be explained with reference to Fig. 4.27, in which the water table is located at a depth d from the ground surface.

Figure 4.27: Effect of ground water table on the ultimate bearing capacity of shallow foundations.

For d=0, the term q= ? D _f should be changed to q= ?? D _f, and ? should be change to ?? (where ??=effective unit weight of soil). For 0 d ? D _f. q should be equal to ? d+ ??( D _f ? d), and ? should be changed to ??. Again, if D _f ? d ? B, q= ? D _f, and the term ? should he replaced by an average unit weight ?, or

For d D _f+ B, q is equal to ? D _f and ? is equal to ?. This implies that the ground water table has no effect on the ultimate capacity.