Question

A 1.25 m layer of soil (n = 0.35, G = 2.65) is subjected to an upward seepage head of 1.85 m. What depth of coarse sand would be required above the existing soil to provide a factor of safety of 2 against piping? Assume that coarse sand has the same porosity and specific gravity as the soil and that there is negligible head loss in sand Take ${\gamma }_w=9.81kN/m^3$.

• A. 2.2

Answer 1

The correct option is A 2.2
1.85 m

Void ratio,

$e=\frac{n}{1-n}=\frac{0.35}{1-0.35}$

= 0.538

Submerged unit weight,

${\gamma }_{sub}=\left(\frac{G-1}{1+e}\right){\gamma }_w$

$=\left(\frac{2.65-1}{1+0.538}\right){\gamma }_w$

= 1.073 ${\gamma }_w$

We know that,

$FOS=\frac{Buoyantweight}{seepageforce}$

$2=\frac{(1.25+x){\gamma }_w\times 1.073}{1.85{\gamma }_w}$

x = 2.2 m .

Additional depth of coarse sand required = 2.2 m