Question

A river 5m deep consists of a sand bed with saturated unit weight of $20kN/m^3$, ${\gamma }_w=9.81kN/m^3$. The effective vertical stress at 5 m from the top of sand bed is

• A. $41kN/m^2$
• B. $51kN/m^2$
• C. $55kN/m^2$
• D. $53kN/m^2$

Answer 1

The correct option is B $51kN/m^2$


Effective stress = Total stress - Pore water pressure

$\overline{\sigma }=\sigma -u$ ...(i)

Total stress at A,

$\sigma =5{\gamma }_w+5{\gamma }_{sat}$

= 5 $\times$ (9.81 + 20)

= $149.05kN/m^2$

Pore water pressure

u = (5 + 5) ${\gamma }_w$

= 10 $\times$ 9.81

= $98.1kN/m^2$

From (i)

$\sigma =149.05-98.1$

$=50.95kN/m^2$