A trench is excavated in fine sand for a building foundation, up to a depth of 13 ft. The excavation was carried out by providing the necessary side supports for pumping water. The water levels at the sides and the bottom of the trench are as given Fig. Ex. 5.3. Examine whether the bottom of the

Question

A trench is excavated in fine sand for a building foundation, up to a depth of 13 ft. The excavation was carried out by providing the necessary side supports for pumping water. The water levels at the sides and the bottom of the trench are as given Fig. Ex. 5.3. Examine whether the bottom of the trench is subjected to a quick condition if [latex]G_{s}=2.64 ext { and } e=0.7[/latex]. If so, what is the remedy?

Accepted Answer

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 [latex]i_{c}=\frac{G_{s}-1}{1+e}[/latex]

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

[latex]i=\frac{h}{L}[/latex],

There will be no quick condition if,

[latex]\frac{h}{L}<\frac{G_{s}-1}{1+e}[/latex]

From the given data

[latex]i_{c}=\frac{2.64-1}{1+0.7}=\frac{1.64}{1.7}=0.96[/latex]

[latex]\frac{h}{L}=\frac{10}{6.5}=1.54[/latex]

It is obvious that [latex]h / L>i_{c}[/latex] 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.

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

This Question has been Answered.

Already have an account?

Login

Related Answered Questions