CHARACTERISTICS OF RAFT FOUNDATION .

A raft or mat is a combined footing thai covers the entire area beneath a structure and supports all the walls and columns. When the allowable soil pressure is low, or the building loads are heavy, the use of spread footings would cover more than one-half of the area and it may prove more economical to use mat or raft foundation. They are also used where the soil mass contains compressible lenses or the soil is sufficiently erratic so that the differential settlement would be difficult to control. The mat or raft tends to bridge over the erratic deposits and eliminates the differential settlement. Raft foundation is also needed to reduce settlement on highly compressible soils, by making the weight of structure and raft approximately equal to the weight of the soil excavated. Fig. 3.26 shows different types of raft or mat foundation.

A true raft or mat is a flat concrete slab with uniform thickness throughout the area, as shown in Fig. 3.26 (a). This is adopted only when the column spacing is small and column loads are also relatively small. If the column loads are heavy, the slab under the columns is thickened, as shown in Figs. 3.26 (b) and (c). If the column spacing is large, and/or the column loads are heavy, thickened bands may be provided along the column lines in both the directions. These bands are cal1ed main and secondary beams. If the loads are extremely heavy, two way grid structure made of cellular construction [Fig. 3.26 (e)] may be used. Where basements are to be provided, the basement walls may be used a ribs or deep beams [Fig. 3.25 (f)]. A raft often rests directly on soil or rock. However, it may also rest on piles.

Ordinarily, rafts are designed as reinforced concrete flat slabs. If the C.G. of loads coincide with the centroid of the rift, the upward load is regarded as a uniform pressure equal to the downward load divided by the area of the raft. The weight of the raft is not considered in the structural design because it is assumed to be carried directly by the subsoil. Since this method does not take into account moments and shears caused by differential settlements, it is customary to reinforce the raft more heavily than required according to the analysis.

 FIG. 3.26 COMMON TYPES OF RAFT FOUNDATION.
(a) Flat plate type (b) Flat plate thickened under columns (c) Flat plate with padestals (d) Two way beam and slab type (e) Cellular construction (f) Basement walls as rigid frame.

A raft may undego large settlements without causing harmful differential settlement. For this reason, almost double the settlement of that premitted for footings is acceptable for raft. Ia a maximum settlement of 5 cm is permitted for a raft, the differential settlement is not likely to excedd 2 cm. Based on this consideration, the net pressure can be calculated from the following equation for rafts having width greater than 6 m:


The penetration resistance N value should be taken at 75 cm intervals for depths equal to width of the raft, below the base of the raft. The minimum average value of N for the holes should be used in the above equations. If N is less than 5, sand should be compacted by artificial means to raise N above 10, or else piles or piers should be used.

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