Chapter 9, Problem 9.1P

(a)

To determine

State whether the statement “During consolidation, effective stress decreases” is true or false.

Answer to Problem 9.1P

The statement is False.

Explanation of Solution

Calculate the effective stress ${\sigma }^{\prime }$ using the relation.

${\sigma }^{\prime }\text{=}\sigma -u$

Here, $\sigma$ is the total stress and u is the pore water pressure.

Consolidation is the process to drain the water from the soil particle. The pore water pressure becomes zero during the consolidation. Hence, the total stress will increase due to zero pore water pressure.

Hence, the statement “During consolidation, effective stress decreases” is False.

(b)

To determine

State whether the statement “Compression index can be less or greater than the swelling index” is true or false.

Answer to Problem 9.1P

The statement is False.

Explanation of Solution

Consider the Boston blue clay soil.

Refer to Table 9.2 “Compression and Swell of natural soils” in the Text Book.

Take the value of compression index $\left(C_C\right)$ is $0.35$ for Boston blue clay.

Take the value of swell index $\left(C_S\right)$ is $0.07$ for Boston blue clay.

The value of compression index $\left(C_C=0.35\right)$ is greater than the value of swell index $\left(C_S=0.07\right)$.

Hence, the statement “Compression index can be less or greater than the swelling index” is False.

(c)

To determine

State whether the statement “The larger the coefficient of consolidation, the faster is the consolidation process” is true or false.

Answer to Problem 9.1P

The statement is True.

Explanation of Solution

“Coefficient of consolidation” $\left(c_v\right)$ is used to describe the rate at which the clay or soil undergoes consolidation, when subjected to an increase in pressure.

It is directly proportional to permeability, if the soil has high permeability then the “coefficient of consolidation” of the soil is high.

Hence, the statement “The larger the coefficient of consolidation, the faster is the consolidation process” is True.

(d)

To determine

State whether the statement “High plastic clays have higher $c_v$ than low plastic ones” is true or false.

Answer to Problem 9.1P

The statement is False.

Explanation of Solution

The “coefficient of consolidation” is less for high plastic clay due to the plastic limit, that is the expel of water from high plastic clay is not as much the low plastic clay.

Hence, the statement “High plastic clays have higher $c_v$ than low plastic ones” is False.

(e)

To determine

State whether the statement “The stiffer clay, the lower is the coefficient of consolidation” is true or false.

Answer to Problem 9.1P

The statement is False.

Explanation of Solution

The clay is stiff when the “coefficient of consolidation” is high and it is loose when the “coefficient of consolidation” is less.

Hence, the statement “The stiffer clay, the lower is the coefficient of consolidation” is False.