Test: Effect of water table - Civil Engineering (CE) MCQ

When the water table is above the base of the footing, the submerged weight ‘γ’ should be used for the soil below the water table for computing the effective pressure or the surcharge.

Effect of water table on bearing capacity:

Case 1: when the water table is at a depth equal to or greater than ‘B’ from the base of the foundation.

i.e water table does not affect the bearing capacity of soil below the wedge zone.
Case 2: When the water table is at a depth equal to or greater than ’Df’ but less than (D_f +B).

In this case, γ_tD_f does not affect due to the water table but the soil lying below the foundation level will get affected hence concept of effective unit weight is used.

Case 3: When the water table lies between ground level and foundation level.

The two equations Nc = (Nq – 1) cot φ and Nq = tan2 (45 + φ/2) ex tan φ have been adopted by i) Terzaghi and Peck ii) Hansen iii) Vesic iv) Meyerhof and Bureau of Indian standards.

The Nγ term does not contribute significantly to qf so that almost any reasonable value can be used. Bureau of Indian standards (BIS) recommends using the value of Nγ, given by Vesic.

In the third method (Bowles, 1988), no water reduction factor is used, but effective unit weight (γe) is used for the soil in the wedge zone.

The value of Nc and Nq are same for Meyerhof, Hansen and Vesic method, while subscripts for Nγ identify the author, i.e. N γ(H) = valued by Hansen etc.

The essential difference in Vesic’s and Hansen’s procedure are i) use of slightly different values of Ny and ii) a variation on some of Hansen’s inclination, base and ground factors.

The Nγ factor has the widest suggested range of any value of N factors. A literature value reveals that for φ = 40°, 38 ≤ Nγ ≤ 192.

M Meyerhof’s extended the analysis of plastic equilibrium of a surface footing to Shallow and Deep foundation according to both Terzaghi and Meyerhof analysis.

The factor N γ(M) gives values by Meyerhof, as it has the subscript (M) in it.