21-12-2012, 12:20 PM
RAFT FOUNDATION
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INTRODUCTION
If the loads transmitted by the columns in a structure are so heavy or the allowable soil pressure so small that individual footings would cover more than about one-half of the area, it may be better to provide a continuous footing under all columns and walls.
Such a footing is called a raft or mat foundation. Raft foundations are also used to reduce the settlement of structures located above highly compressible deposits.
METHODS OF RAFT FOOTING DESIGN
According to IS – 2950:1965, the design criteria of raft footings are given below:
The maximum differential settlement in foundation on clayey soils and sandy soils should not exceed 40mm and 25 mm respectively. The maximum settlement should generally be limited to the following values:
Raft foundation on clay – 65 to 100 mm.
Raft foundation on sand – 40 to 65 mm
SOIL LINE METHOD
A number of methods have been proposed based on primarily on two approaches of simplified and truly elastic foundations.
i. Simplified elastic foundation: The soil in this method is replaced by an infinite number of isolated springs.
ii. Truly elastic foundation: The soil is assumed to be continuous elastic medium obeying Hooke’s law.
In the case of foundation which is comparatively flexible and where loads tend to concentrate over small areas these methods are to be used.
The method assumes in addition to other factors that the modulus of subgrade reaction, determined from tests is known. The modulus of subgrade reaction (k) as applicable to the case of load through a plate of size 30 cm x 30 cm or beams 20 cm wide on soil area is known.