Question 5.3: A trench is excavated in fine sand for a building foundation...

As per Fig. Ex. 5.3 the depth of the water table above the bottom of the trench = 10 ft. The sheeting is taken 6.5 ft below the bottom of the trench to increase the seepage path.

The equation for the critical gradient is i_{c}=\frac{G_{s}-1}{1+e}

If the trench is to be stable, the hydraulic gradient, i, prevailing at the bottom should be less than i_{c}. The hydraulic gradient i is

i=\frac{h}{L},

There will be no quick condition if,

From the given data

It is obvious that h / L>i_{c} There will be quick condition.

Remedy:

(i) Increase L to at least a 13 ft depth below the bottom of trench so that h/L = 0.77 which gives a margin of factor of safety.

or (ii) Keep the water table outside the trench at a low level by pumping out water. This reduces the head h.

or (iii) Do not pump water up to the bottom level of the trench. Arrange the work in such a way that the work may be carried out with some water in the trench.

Any suggestion given above should be considered by keeping in view the site conditions and other practical considerations.