Question

The active earth pressure on the back of retaining wall is 80 kN/m. The height of wall is 5m. The backfill is cohesion less soil having effective angle of internal friction ${30}^0$. The degree of saturation of backfill (in percentage correct upto two decimal place) if backfill has a porosity of 30%. (Rounded upto single decimal place. Take specific gravity of solids 2.7)

• A. 22.4 %
• B. 18.4 %
• C. 37.9 %
• D. 8.05 %

Answer 1

The correct option is A 22.4 %

Given, H = 5m,

$Activepressure,P_a=80kN/m,\varphi ={30}^{\circ }$

$P_a=\frac{1}{2}{K}_a\gamma H^2$

$80=\frac{1}{2}\times \left(\frac{1-sin{30}^{\circ }}{1+sin{30}^{\circ }}\right)\times \gamma \times 5^2$

$\gamma =19.2kN/m^3$

$\left(\frac{G+Se}{1+e}\right){\gamma }_w=19.2$

$Now,voidratio,e=\frac{n}{1-n}$

$=\frac{0.3}{1-0.3}=\frac{3}{7}$

$\therefore \left(\frac{2.7+3S/7}{1+3/7}\right)\times 9.81=19.2$

$\frac{3S}{7}=0.096$

$S=0.224$

= 22.4%