Chapter 18, Problem 18.1P

State whether the following are true or false.

1. a. Load carrying capacities of timber piles are less than those of steel or concrete piles.
2. b. If the load carried by the pile cross section decreases linearly with depth, the frictional resistance per unit remains the same at all depths.
3. c. The point load is mobilized well before the shaft load.
4. d. Soil-pile friction angle δ′ can be greater than the friction angle of the soil ϕ′.
5. e. Bored piles are high displacement piles.

To determine

State whether the statement “Load carrying capacities of timber piles are less than those of steel or concrete piles” is true or false.

Answer to Problem 18.1P

The given statement is $\underset{\_}{\text{True}}$.

Explanation of Solution

The steel piles have a usual load carrying capacity of 300kN to 1200kN.

The concrete piles have a load carrying capacity of 300 kN to 3000 kN.

The timber pile has a load carrying capacity value of 300 kN to 500 kN.

Hence, the load carrying capacities of timber is less than concrete or steel.

Therefore, the given statement is $\underset{\_}{\text{True}}$.

To determine

State whether the load carried by the pile cross section decreases linearly with depth and the frictional resistance per unit remains the same at all depths is true or false.

Answer to Problem 18.1P

The given statement is $\underset{\_}{\text{True}}$.

Explanation of Solution

Determine the unit area frictional resistance $\left(f_z\right)$ using the relation.

$f_z=\frac{\Delta Q_z}{p\Delta z}$

Here, p is the pile cross section perimeter, $\Delta Q_z$ is the pile load at depth z, and $\Delta z$ is the difference in depth.

As per the formula, the change in pile load remains constant, when the pile load is increasing linearly with depth. Conversely, the perimeter and the change in depth have the inverse relationship with depth. Therefore, $f_z$ remains the same for all depths.

Therefore, the given statement is $\underset{\_}{\text{True}}$.

To determine

State whether the point load is mobilized well before the shaft load is true or false.

Answer to Problem 18.1P

The given statement is $\underset{\_}{\text{False}}$.

Explanation of Solution

If the shaft load is large, it depends on the pile diameter. The ultimate shaft resistance is not principally mobilized before the point load $Q_p$ reaches its capacity.

Therefore, the given statement is $\underset{\_}{\text{False}}$.

To determine

State whether the statement “soil-pile friction angle ${\delta }^{\prime }$ can be greater than the friction angle of the soil ${\varphi }^{\prime }$” is true or false.

Answer to Problem 18.1P

The given statement is $\underset{\_}{\text{False}}$.

Explanation of Solution

The magnitude of soil-pile friction angle ${\delta }^{\prime }$ can be usually found between $\frac{1}{3}{\varphi }^{\prime }\text{to}\frac{2}{3}{\varphi }^{\prime }$, hence ${\delta }^{\prime }$ is always less than ${\varphi }^{\prime }$.

Therefore, the given statement is $\underset{\_}{\text{False}}$.

To determine

State whether the given statement “bored piles are high displacement piles” is true or false.

Answer to Problem 18.1P

The given statement is $\underset{\_}{\text{False}}$.

Explanation of Solution

Bored piles cause the soil to be displaced radially as well as vertically as pile shaft is driven. For bored piles, the holes are initially bored in the ground and then pile is formed in this hole. Bored piles displace very less amount of soil.

Therefore, the given statement is $\underset{\_}{\text{False}}$.