The rise of the water table will influence the bearing capacity of the foundation resting on sand, due to the:

Concept:

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.

\({{\text{q}}_{\text{u}}}=\text{C}{{\text{N}}_{\text{c}}}+\text{q}{{\text{N}}_{\text{q}}}+\frac{1}{2}\text{B }\!\!\gamma\!\!\text{ }{{\text{N}}_{\text{ }\!\!\gamma\!\!\text{ }}}\)

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 (Df +B).

\({{\text{q}}_{\text{u}}}=\text{C}{{\text{N}}_{\text{c}}}+{{\text{ }\!\!\gamma\!\!\text{ }}_{\text{t}}}{{\text{D}}_{\text{f}}}{{\text{N}}_{\text{q}}}+\frac{1}{2}\text{B }\!\!\gamma\!\!\text{ }{{\text{N}}_{\text{ }\!\!\gamma\!\!\text{ }}}\)

In this case, γtDf 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.

\({{\text{q}}_{\text{u}}}=\text{C}{{\text{N}}_{\text{c}}}+\text{q}{{\text{N}}_{\text{q}}}+0.5{{\text{N}}_{\text{ }\!\!\gamma\!\!\text{ }}}\left[ \text{x }\!\!\gamma\!\!\text{ }+\left( \text{B}+\text{x} \right){{\text{ }\!\!\gamma\!\!\text{ }}_{\text{sub}}} \right]\)

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

\({{\text{q}}_{\text{u}}}=\text{C}{{\text{N}}_{\text{c}}}+\left[ {{\text{ }\!\!\gamma\!\!\text{ }}_{\text{t}}}\times \text{y}+{{\text{ }\!\!\gamma\!\!\text{ }}_{\text{sub}}}\left( {{\text{D}}_{\text{f}}}-\text{y} \right) \right]{{\text{N}}_{\text{q}}}+0.5\text{B}{{\text{ }\!\!\gamma\!\!\text{ }}_{\text{sub}}}{{\text{N}}_{\text{ }\!\!~\!\!\text{ }\!\!\gamma\!\!\text{ }}}\)

Explanation:

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

If the effect of cohesion is neglected, and if the water table rises from the great depth to the ground surface then the bearing capacity will be almost reduced to half (i.e 50%) as the Submerged unit weight of soil (γSub) is approximately half of surcharge unit weight of dry soil (γt).