Test: Mat or Raft Footing - Civil Engineering (CE) MCQ

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.

Raft Foundation:
Raft foundation is generally a thick concrete slab resting on a large area of soil reinforced with steel rods and is used to support columns or walls and transfer loads from the structure to the soil.
Raft foundation may be used under the following conditions:

• The soil has a low bearing capacity
• The load of the structure has to be distributed over a large area
• Individual or any other foundation area would approximately cover 50% of the total ground area beneath the structure.
• The columns or walls are placed so closely that the individual footings would overlap
• Stress on soil needs to be reduced
• There is a possibility of differential settlement in case individual footing is used
• When soil strata are unpredictable and contain pockets of compressible soil
• The basement is to be constructed
• Any other type of footing cannot be used advantageously

Ordinarily, rafts are designed as reinforced concrete flat slabs using heavy beams.

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).

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.

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.

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.

The weight of the raft is not considered in the structural design because it is assumed to be carried by directly by the subsoil.

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.

The raft is subdivided into a series of continuous beams (strips) centered on the appropriate column rows to establish shear failure and moment diagram.