A 'raft (mat) foundation' is used when:
Choose the correct answer
Rock is available at shallow depth
Soil bearing capacity is low or differential settlement risk is high — raft covers entire footprint distributing load uniformly
Only for one-storey light buildings
Only for industrial chimneys
Correct Answer
B. Soil bearing capacity is low or differential settlement risk is high — raft covers entire footprint distributing load uniformly
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Raft/mat foundation: single slab covering entire building footprint. Used when: (1) Soil BC is low and individual footings would cover > 50% of plan area; (2) Differential settlement risk is high (raft distributes load, reduces differential); (3) Basement is needed (raft = basement floor slab); (4) Heavy column loads close together. Raft distributes load over entire building footprint — reduces soil pressure.
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