# Design of RC Structures Questions and Answers – Pile Foundation – Set 2

This set of Design of RC Structures Multiple Choice Questions & Answers (MCQs) focuses on “Pile Foundation – Set 2”.

1. For the pile in cohesive soil, the point bearing is generally neglected for individual pile action. So the unit skin friction may be taken as equal to shear strength of the soil.
a) True
b) False

Explanation: For the pile in cohesive soil, the point bearing is generally neglected for individual pile action, since it is negligible as compared to friction resistance. The unit friction may be taken as equal to shear strength of the soil. This is given by:
rf = τf = c = (qu / 2).

2. What is the value for unit skin friction (rf) for non – cohesive soils?
a) (qu / 2)
b) K tanΦ (γZ + q)
c) (γB / 2)Nγq
d) 0.3 γ B Nγq

Explanation: The values adopted for unit skin friction for non – cohesive soil is:
rf = K tanΦ (γZ + q)
Here (K) is coefficient of lateral earth pressure,
(γ) is the density of soil and
(q) is the surcharge on the ground.
The depth of centre of gravity of the pile below ground surface is (Z).

3. While estimating the bearing capacity of a group of friction piles the value (nQf) is multiplied by which factor?
a) ηh
b) η
c) ηb
d) ηg

Explanation: A method of estimating the bearing capacity of a group of friction piles is to multiply the quantity (nQf) by a reduction factor called the efficiency of the pile group ηg. This depends on many factors such as characteristics of pile, spacing of piles, total number of piles (n) in a row, etc.

4. Which of the following is the Converse – Labarre formula for determining efficiency (ηg) of a pile group?
a) 1 – (θ / 90) {[(n – 1)m + (m – 1) n)] / mn}
b) 1 – (θ / 90) {[(n – 1)m + (m – 2)n)] / mn}
c) 1 – (θ / 90) {[(n – 3)m + (m – 3)n)] / mn}
d) 1 – (θ / 90) {[(n – 2)m + (m – 2)n)] / mn}

Explanation: The Converse – Labbare formula for determining the efficiency (ηg) of a pile group is given by:
ηg = 1 – (θ / 90) {[(n – 1)m + (m – 1)n)] / mn}
Here (θ) is tan – 1(d / s) degrees,
(d) is diameter of piles and
(s) is spacing of piles.

5. What is the value of maximum bending moment in a pile suspended at one point?
a) wL2 / 47
b) wL2 / 8
c) wL2 / 90
d) wL2 / 23

Explanation: For the pile suspended at one point it will be subjected to a maximum bending moment of (wL2 / 8) at the point of suspension. Here (w) is the weight of the pile per unit length and (L) is the length of the pile.

6. For the case piles suspended at two points what is the value of reaction at each point of suspension?
a) wL / 2
b) wL / 8
c) wL / 6
d) wL / 4

Explanation: When pile suspended at two points the value of reaction at each point of suspension will be (wL / 2). The maximum hogging and sagging moment at each cantilever end is:
(wx2 / 2) and
[wL / 2((L / 2) – x) – wL2 / 8)]
This gives the maximum bending moment as (wL2 / 47).

7. What is the equation for maximum bending moment when pile is suspended at three points?
a) wL2 / 47
b) wL2 / 8
c) wL2 / 90
d) wL2 / 23

Explanation: The equation for bending moment for pile suspended at three points is:
(wL2 / 90)
Here (w) is the weight of the pile per unit length and
(L) is the length of the pile.
In this the two points are located at a distance x = 0.15L from the ends so the bending moment is less. The third point is at the middle of the pile.

8. If the piles are erected from one end and the point x = 0.293L from the end, what is the value of maximum bending moment?
a) wL2 / 47
b) wL2 / 8
c) wL2 / 90
d) wL2 / 23

Explanation: When the one end of the pile is lifted the other end is resting on the ground. The distance of the point from end is x = 0.293L so that bending moment on the pile is least. The maximum bending moment will be equal to (wL2 / 23).

9. For what length of piles the suspension is made at two or three points?
a) Longer than 12 metres
b) Longer than 14 metres
c) Longer than 16 metres
d) Longer than 18 metres

Explanation: When the pile is of less than 12 metres (m) length, it is suspended from one point at its middle. Piles longer than 12 m are suspended at two or three points suitably spaced at its length so that handling moment is as small as possible.

10. The pile cap is designed for piles when the column is supported on more than one pile?
a) True
b) False

Explanation: When the column is supported on more than one pile, the piles should be connected through a rigid pile cap, to distribute the load to the individual piles. The pile cap consists of a rigid, deep, reinforced concrete slab which acts monolithically with the group of piles.

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