Chapter 8, Problem 8.1P

(a)

To determine

Check whether the statement “In a soil, the effective stress can be greater or less than the pore water pressure” is true or false.

Answer to Problem 8.1P

The given statement is true.

Explanation of Solution

The clay has low hydraulic conductivity and the compressibility is high. The capillary pressure will occur in this condition. The pore water pressure in this condition is greater than the effective stress.

The hydraulic conductivity for the sand is high. The water will be squeezed out easily. The pore water pressure will be less than the effective stress.

Therefore, the given statement is true.

(b)

To determine

Check whether the statement “Upward seepage increases the effective stresses and downward seepage decreases the effective stresses” is true or false.

Answer to Problem 8.1P

The given statement is false.

Explanation of Solution

Write the equation for effective stress in upward seepage condition as follows:

${{\sigma }^{\prime }}_c=z{\gamma }^{\prime }-iz{\gamma }_w$

Refer to the upward seepage concept, when the seepage is upward, the effective stress decreases and the pore water pressure increases.

Write the equation for effective stress in downward seepage condition as follows:

${{\sigma }^{\prime }}_c=z{\gamma }^{\prime }+iz{\gamma }_w$

Refer to the downward seepage concept, when the seepage is downward, the effective stress increases and the pore water pressure decreases.

Therefore, the given statement is false.

(c)

To determine

Check whether the statement “Quick condition cannot occur when the seepage is downward” is true or false.

Answer to Problem 8.1P

The given statement is true.

Explanation of Solution

Write the equation for effective stress in upward seepage condition as follows:

${{\sigma }^{\prime }}_c=z{\gamma }^{\prime }-iz{\gamma }_w$

When the upward seepage pressure is equal to submerged weight of soil, the effective pressure is equal to zero. i.e., ${{\sigma }^{\prime }}_c=0$

$\begin{array}{l}0=z{\gamma }^{\prime }-i_{\text{cr}}z{\gamma }_w\\ z{\gamma }^{\prime }=i_{\text{cr}}z{\gamma }_w\\ i_{\text{cr}}=\frac{{\gamma }^{\prime }}{{\gamma }_w}\end{array}$

This condition is only possible in upward seepage.

Write the equation for effective stress in downward seepage condition as follows:

${{\sigma }^{\prime }}_c=z{\gamma }^{\prime }+iz{\gamma }_w$

In downward seepage condition it is not possible that, the downward seepage pressure is equal to submerged weight of soil.

Therefore, the given statement is true.

(d)

To determine

Check whether the statement “Seepage force does not depend on the permeability of the soil” is true or false.

Answer to Problem 8.1P

The given statement is true.

Explanation of Solution

Write the equation to find the seepage force $\left(P^{\prime }\right)$ as follows:

$P^{\prime }=z{\gamma }^{\prime }A$

Here, the thickness of the soil is z, the submerged unit weight is ${\gamma }^{\prime }$, and the cross sectional area of the soil is A.

Refer to the equation, the seepage force is independent of the permeability of the soil.

Therefore, the given statement is true.

(e)

To determine

Check whether the statement “Critical hydraulic gradient is greater for loose sands than dense sands” is true or false.

Answer to Problem 8.1P

The given statement is false.

Explanation of Solution

Critical hydraulic gradient occurs in the upward seepage condition.

When the soil pores are dense, the possibility of occurring the quick sand condition is high. This is due to the spacing between the particles are very less.

Therefore, the given statement is false.