# Pavement Design Questions and Answers – Stresses in Flexible Pavement – 1

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This set of Pavement Design Multiple Choice Questions & Answers (MCQs) focuses on “Stresses in Flexible Pavement – 1”.

1. How does the vertical stress affect the pavement?
a) Rutting
b) Fatigue
c) Shear
d) Deflection

Explanation: When the pavement layer is subjected to vertical stress, it gets compressed. The compression leads to the crushing of the materials. Depressions are formed on the pavement surface, which is called the rutting of pavement.

2. The responses to moving traffic loads in the flexible pavement are influenced by what factors?
a) Traffic survey data
b) Type of flexible pavement
c) Thickness of pavement layer
d) Climatic conditions

Explanation: The responses to moving traffic loads in the flexible pavement are influenced by two factors. The factors are the type of flexible pavement and the ratio of moduli of the different layers of the pavement.

3. Stresses are the response of ______ of the pavement.
a) Deflection
b) Weathering
d) Bending

Explanation: The pavement is subjected to loading from the traffic and the response of the same is denoted as the stress. The subgrade material and environmental conditions can also lead to stress.
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4. According to Boussinesq’s theory, the soil mass is not considered to be ______
a) Elastic
b) Infinite
c) Homogeneous
d) Isotropic

Explanation: Some assumptions were laid down by Boussinesq for the method of finding vertical stress of soil. The assumptions for the soil mass under consideration are that it is elastic, isotropic, homogeneous and semi-infinite.

5. Fatigue cracking leads to the development of radial stresses in the pavement.
a) True
b) False

Explanation: When the bottom layers of the pavement are subjected to tension due to seepage and removal of materials, fatigue cracking occurs in the pavement. This fatigue cracking, later on, leads to the development of radial stresses in the pavement.

6. Which layer has the highest elasticity?
a) Surface
c) Base
d) Sub-base

Explanation: According to the two-layer theory by Burmister, the topmost layer of the pavement has the highest elasticity. The two layers considered are flexible layers and the subgrade. Surface is the topmost flexible layer and therefore has the highest elsticity.

7. Which of the below options represents the equation to compute the vertical stress under a uniformly distributed circular load?
a) $$σ=p \left[1+\frac{z^3}{(a^2+z^2 )^{3⁄2}}\right]$$
b) $$σ=p \left[1-\frac{z^3}{(a^2+z^2 )^{1⁄2}}\right]$$
c) $$σ=p \left[1-\frac{z^3}{(a^2+z^2 )^{3⁄2}}\right]$$
d) $$σ= \left[1-\frac{z^3}{(a^2+z^2 )^{3⁄2}}\right]$$

Explanation: $$σ=p \left[1-\frac{z^3}{(a^2+z^2 )^{3⁄2}}\right]$$ is the equation that is used to compute the vertical stresses under a uniformly distributed circular load. In the equation, the stress at depth ‘z’ is found out by considering surface pressure ‘p’ and the radius of loaded area ‘a’.

8. Compute the vertical stress under the centre of the load at a depth of 40 cm from the surface. A circular load of radius 16 cm with a uniform contact pressure of 7 kg/cm2 is applied on the pavement surface.
a) 1.3 kg/cm2
b) 1.3 kg/mm2
c) 1.4 kg/mm2
d) 1.4 kg/cm2

Explanation: The vertical stress is obtained by the equation $$σ=p \left[1-\frac{z^3}{(a^2+z^2 )^{3⁄2}}\right]$$.
Substituting the values of p = 7 kg/cm2, a = 16 cm and z = 40 cm in the above equation.
$$σ=7 \left[1-\frac{40^3}{(16^2+40^2 )^{3⁄2}}\right]$$=1.4 kg/cm2

9. What would be the vertical stress on top of the pavement if the radius of the circular load is 15 cm and the uniform contact pressure exerted is 6 kg/cm2?
a) 2.67 kg/cm2
b) 1 kg/cm2
c) 6 kg/cm2
d) 0 kg/cm2

Explanation: $$σ=p \left[1-\frac{z^3}{(a^2+z^2 )^{3⁄2}}\right]$$ is used to find vertical stress. The vertical stress on top of the pavement would mean that the depth at which stress is to be computed is zero. The value for a = 15 cm and p = 6 kg/cm2, so the vertical stress would be $$σ=p=6 kg/cm^2$$.

10. In a one-layer system, the load is considered to be a point load.
a) True
b) False

Explanation: The load coming on the pavement in the one-layer system is considered to be distributed over an elliptical area. The contact area can be approximated as a circular area instead of the elliptical area for the ease of doing the calculations.

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