Design of Raft Foundations - Methods and CalculationsAccording 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 25mm 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.
- Conventional Method
- Soil Line Method.
1. Conventional Method of Raft Foundation Design
Assumptions:1. The soil pressure is assumed to be plane such that the centroid of the soil pressure coincides with the line of action of the resultant force of all the loads acting on the foundation. 2. The foundation is infinitely rigid and therefore, the actual deflection of the raft does not influence the pressure distribution below the raft. In this method, allowable bearing pressure can be calculated by the following formulae:
2) Soil line Method (Elastic Method) of Raft Foundation DesignA 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 (Ks) 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 given in table-1 for cohesionless soils and table-2 for cohesive soils. Table -1: Modulus of subgrade reaction Ks for cohesionless soils
|Relative Density||Values of N||Dry or moist state||Submerged state|
|1. Medium||10 to <30||4.7||2.9|
|3. Dense||30 and over||18||10.8|
|Consistency||Unconfined compressive Strength (Kg/cm2))||-|
|1. Stiff||1 to <2||2.7|
|1. Very Stiff||2 to <4||5.4|
|3. Hard||4 and over||10.8|