This set of Geotechnical Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Shallow Foundations – Mat or Raft Footing”.
1. The foundation that is used when the soil mass is sufficiently erratic?
a) Strap footing
b) Combined footing
c) Mat footing
d) Rectangular combined footing
View Answer
Explanation: Mat footing is used where the soil contains compressible lenses or the soil is sufficiently erratic so that the differential settlement would be difficult to control.
2. Usually, rafts are designed as _________
a) Reinforced slabs
b) Reinforced concrete flat slabs
c) Ordinary concrete slab
d) Inverted flat slabs
View Answer
Explanation: Ordinarily, rafts are designed as reinforced concrete flat slabs using heavy beams.
3. The weight of the raft is not considered in the structural design, because ___________
a) Weight is carried by subsoil
b) Raft does not remain contact with soil
c) The weight is transferred to column
d) All of the mentioned
View Answer
Explanation: The weight of the raft is not considered in the structural design because it is assumed to be carried by directly by the subsoil.
4. If a maximum settlement of 50 mm is permitted for a raft, the differential settlement must not exceed ___________
a) 30 mm
b) 10 mm
c) 20 mm
d) 25 mm
View Answer
Explanation: A raft may undergo large settlement without causing harmful differential settlement. For this reason, almost double settlement of that permitted for footing is acceptable for raft. Therefore if a maximum settlement of 50 mm (2 in) is permitted for a raft, the differential settlement is not likely to exceed 20 mm (0.75 in).
5. The net ultimate bearing capacity for raft may be determined by __________
a) Skempton’s equation and Terzaghi’s equation
b) Darcy’s equation
c) None of the mentioned
d) All of the mentioned
View Answer
Explanation: The net ultimate bearing capacity may be determined from both Skempton and Terzaghi’s equation as given below:
i) Skempton’s equation: q nf = 5 [1 + 0.2D/B ] [1 + 0.2B/L]c
ii) Terzaghi’s equation: q nf = 5.7[ 1 + 0.3B/L]c.
6. In raft footing, if the C.G of the load coincide with the centroid of the raft, the upward load is considered as ___________
a) Non uniform pressure
b) Uniform pressure
c) Excess pressure
d) None of the mentioned
View Answer
Explanation: If the Center of gravity of the load coincides with the centroid of the raft, the upward load is regarded as a uniform pressure, which will be equal to the downward load divided by the area of the raft.
7. Raft is subdivided in to series of beams to establish __________
a) Shear failure and Moment diagrams
b) Pressure distribution
c) None of the mentioned
d) All of the mentioned
View Answer
Explanation: The raft is subdivided into a series of continuous beams (strips) centered on the appropriate column rows to establish shear failure and moment diagram.
8. The penetration resistance N for designing of raft should be taken at ___________ intervals.
a) 50 cm
b) 60 cm
c) 75 cm
d) 20 cm
View Answer
Explanation: The penetration resistance N values should be taken at 75 cm intervals for a depth equal to width of the raft, below the base of the raft.
9. If the penetration resistance N is less than 5, which of the following measures can be adopted?
a) Using piles and piers and Compacting sand
b) Using inverted flat slab
c) None of the mentioned
d) All of the mentioned
View Answer
Explanation: If N is less than 5, sand should be compacted by artificial means to rise N above 10, or else piles or piers should be used.
10. The net pressure can be calculated for raft having width greater than ___________
a) 6 m
b) 20 cm
c) 30 cm
d) 4 m
View Answer
Explanation: Based on the settlement criterion for raft, the net pressure can be calculated from the following equation having its width greater than 6m.
qp = 20(N-3) Rw2 kN/m2.
Sanfoundry Global Education & Learning Series – Geotechnical Engineering.
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