# Geotechnical Engineering Questions and Answers – IS Code of Practice for Design of Raft Foundations

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This set of Geotechnical Engineering Questions and Answers for Campus interviews focuses on “I.S. Code of Practice for Design of Raft Foundations”.

1. According to IS codes, the maximum differential settlements should not exceed ___________ in the foundation on clayey soil.
a) 25 mm
b) 100 mm
c) 25 mm
d) 40 mm

Explanation: According to IS: 2950-1965 code of practice, the maximum settlement should not exceed 40 mm in the foundation on clayey soils and 25 mm in foundations on clayey soils.

2. The possible method adopted for designing of raft foundation is ___________
a) Conventional method
b) Elastic method
c) Soil line method
d) All of the mentioned

Explanation: The behavior of a raft foundation being complicated, a number of simplifying assumption has to be made in design. There are two approaches for design-conventional method and the elastic method or the soil line method.

3. The conventional method for designing raft foundation is based on which of the following assumptions?
a) Foundation is infinitely rigid and Soil pressure is assumed to be planar
b) Overburden pressure is assumed as zero
c) None of the mentioned
d) All of the mentioned

Explanation: The conventional method is based on the following two basic assumptions:
i) The foundation is infinitely rigid and therefore, the actual deflection of the raft does not influence the pressure distribution below the shaft.
ii) The soil pressure is assumed to be planar such that the centroid of the soil pressure coincides with the line of action of the resultant force of all the loads acting on the foundation.

4. The actual reinforcement provided should be ___________ of that worked out theoretically by conventional method.
a) Twice
b) Thrice
c) Lesser
d) None of the mentioned

Explanation: The actual reinforcement provided should be twice of that worked out theoretically in the case of design by conventional method.

5. The method that can be used for designing raft, based on elastic method?
a) Simplified elastic foundation and Truly elastic foundation
b) Conventional elastic foundation
c) None of the mentioned
d) All of the mentioned

Explanation: A number of methods have been proposed based primarily on two approaches of a simplified and truly elastic foundation, i) simplified elastic foundation ii) truly elastic foundation.

6. In truly elastic foundation, the soil is assumed to be obey ____________
a) Terzaghi’s theory
b) Hooke’s law
c) Skempton’s theory
d) All of the mentioned

Explanation: The soil in a simplified elastic foundation is assumed to be continuous elastic medium obeying Hooke’s law.

7. Both conventional and flexible method can be used only in the case when _________
a) Foundation is laid on cohesive soil
b) Soil pressure is low
c) Foundation is flexible
d) Load is concentrated on larger area

Explanation: Conventional and flexible methods are to be used in case the foundation is comparatively flexible and the loads tend to concentrate over small areas.

8. The modulus of subgrade reaction is applicable only when the load is applied through _________
a) Plate of size 30 ×30 cm and Beam 30 cm wide on soil area
b) Plate size is 10 × 10 cm
c) None of the mentioned
d) All of the mentioned

Explanation: The modulus of subgrade reaction (Ks) is applicable to the case of load through a plate of size 30 × 30 cm or beam 30 cm wide on soil area.

9. The value of Ks(modulus of subgrade reaction) can be found out, based on the effect of ___________
a) Size and Shape
b) Properties
c) None of the mentioned
d) All of the mentioned

Explanation: The value of Ks can be found corresponding to a different size and shape for both cohesive and cohesionless soils.

10. The effect of shape is based on ____________principal.
a) Principal of superposition
b) System of forces
c) Hooke’s law
d) Principal of center of gravity

Explanation: The effect of shape utilizes the principal of superposition.

11. In effect of shape method, the columns loads and bearing pressure distribution are divided in to ___________ system of forces.
a) Two
b) Four
c) Three
d) Five

Explanation: The columns loads and bearing pressure distribution are divided in three system of forces such that each system balance within itself so that the sum of forces is zero and the sum of moments of these forces about any point is zero.

12. The first system of forces consist of _____________
a) Difference between actual column loads
b) Varying distributed load acting downwards
c) Column support reaction acting downwards
d) All of the mentioned

Explanation: The first system of forces consists of i) uniformly distributed bearing pressure w acting upwards along the entire area of the raft ii) the column support reaction acting vertically downwards obtained by treating the raft as an inverted floor carrying the uniformly distributed load w with the reaction at the point of loading.

13. The resultant earth pressure at any point of a raft is considered to be ___________
a) w + mw
b) (1-m)w
c) w-mw
d) (1-w)m

Explanation: The earth pressure varies from point to point of a raft, and hence the resultant pressure at any point is considered to be w + mw, where w is constant and m varies throughout the raft.

14. The third system of forces is composed of ___________
a) Varying upward pressure and Varying downward pressure
c) None of the mentioned
d) All of the mentioned

Explanation: The varying distributed load pressure in raft, acting both upward and downward forms the third system of forces.

Sanfoundry Global Education & Learning Series – Geotechnical Engineering.

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