Question

A flow net below a dam consists of 24 equipotential drops and 7 flow channels. The difference between the upstream and downstream water level is 6m. the length of the flow line adjacent to the toe of the dam at exit is 1 m. The specific gravity and void ratio of the soil below the dam are 2.70 and 0.70, respectively. The factor of safety against piping is

• A. 1.67
• B. 2.5
• C. 3.4
• D. 4

Answer 1

The correct option is D 4

$N_d=24$

$N_t=7$

$\Delta H=6m$

G = 2.70

e = 0.70

Factor of safety against piping

$FOS=\frac{i_c}{i_e}$

$i_c=\frac{G-1}{1+e}=\frac{2.7-1}{1+0.7}=1$

$i_e=\frac{\Delta h}{L}=\left(\frac{H}{N_d\times l}\right)=\frac{6m}{24\times 1}=\frac{1}{4}$

$FOS=\frac{1}{1/4}=4$