Question

Assuming that a river bed level does not change and the depth of water in river was 10m, 15 m and 8m during the months of February, July and December respectively of a particular year. The average bulk density of the soil is $20kN/m^3$. The density of water is $10kN/m^3$. The effective stress at a depth of 10 m below the river bed during these months would be

• A. $300kN/m^{2}inFebruary$ , $350kN/m^{2}inJulyand$ , $320kN/m^{2}inDecember$
• B. $100kN/m^{2}inFebruary$ , $100kN/m^{2}inJulyand$ , $100kN/m^{2}inDecember$
• C. $200kN/m^{2}inFebruary$, $250kN/m^{2}inJulyand$, $180kN/m^{2}inDecember$
• D. $300kN/m^{2}inFebruary$ , $350kN/m^{2}inJulyand$ , $280kN/m^{2}inDecember$

Answer 1

The correct option is B $100kN/m^{2}inFebruary$ , $100kN/m^{2}inJulyand$ , $100kN/m^{2}inDecember$

Given,

Average bulk density,

${\gamma }_t=20kN/m^3$

${\gamma }_w=10kN/m^3$

Since soil is saturated

Hence;

${\gamma }_t={\gamma }_{sat}=20kN/m^3$

${\gamma }_{submerged}={\gamma }_{sat}-{\gamma }_w$

= 20 - 10

= 10 kN/$m^3$

Effective stress at a depth 10m below the river bed

$\overline{\sigma }={\gamma }_{sub}\times H$

= 10 $\times$ 10

= 100 kN/$m^2$

Note : Effective stress at any depth does not depend upon the change of water level above the soil.