EFFECT OF WATER TABLE ON SAFE BEARING CAPACITY OF SOIL

The position of ground water has a significant effect on the bearing capacity of soil. Presence of water table at a depth less than the width of the foundation from the foundation bottom will reduce the bearing capacity of the soil.

The bearing capacity equation incorporating the ground water table correction factors is given below.

clip_image002

Where clip_image004 = Ultimate bearing capacity of soil in clip_image006

c = Cohesion of soil in clip_image006[1]

Nc, Nq, N? are Therzaghi’s bearing capacity constants.

clip_image008= depth of foundation in meters

B = Width of the foundation in meters

clip_image010 and clip_image012 are water table correction factors

The water table correction factors can be obtained from the equations given below.

1. When the water table is below the base of foundation at a distance ‘b’ the correction clip_image012[1] is given by the following equation

clip_image014;

when b =0, clip_image012[2] = 0.5

2. When water table further rises above base of foundation, correction factor clip_image010[1]comes in to action, which is given by the following equation.

clip_image016

when a =clip_image008[1], clip_image010[2] = 0.5

EFFECT OF WATER TABLE ON SAFE BEARING CAPACITY OF SOIL

Fig 1: Showing the influence of water table below foundation

The use of these equations is explained with the help of the Fig 1.

First let us begin with the correction factor clip_image012[3]

When water table is at a depth greater than or equals to the width of foundation, from the foundation bottom, the correction factor clip_image012[4] is 1. i.e. there is no effect on the safe bearing capacity.

Let us assume water table started rising then the effect of clip_image012[5] comes in to action. The correction factor will be less than 1. When the water table reaches the bottom of foundation, i.e, when b = 0, clip_image012[6] = 0.5.

Now let us assume water table further raises, above the depth of foundation. When the depth of water table is just touching the bottom of foundation, a = 0. This means clip_image010[3]= 1.0. On further rising, when the water table reaches the ground level, Rw1 becomes 0.5.

Hence, the assessment of ground water level is an important aspect in any site investigation.

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