History of Raft Foundations in structural designing of RCC

Srinivasan

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History of Raft Foundations in structural designing of RCC
In early 70s, PWD, designed raft foundation as RCC solid slab.The additional component of cut off walls on both sides U/s and D/s was considered necessary to take care of seepage and possible undermining of the raft due to seepage and the scour due to floods .

As the design approach was based on beams on elastic foundation, the entire bed was proposed to be provided with the flexible elastic media to ensure the elasticity of the bed. The raft was designed as one way slab spanning between two piers. It was observed that the raft was showing signs of cracks between pier and cut off walls. The arrangement was, therefore, subsequently changed by resting pier on raft over the cut off walls. The cut off walls when constructed monolithic with the raft, it was considered that the raft bed designed as two way slab (cut off walls on U/s & D/s and the piers provide fixity to the raft at all four edges). (Type designs issued in 1973).

The design of raft was then modified as channel type raft ( entirely as a one structural component. In channel type raft, the entire channel formed by cut off and raft acts as a monolithic component. The cut off walls in this case resist most of the shear forces and bending moments and the slab is designed for local conditions. This resulted in reduced thickness of raft slab and also the cut off walls and resultant saving in concrete was found quite substantial.

If heavy de-watering is involved, the quality of concrete for cut off wall becomes doubtful. In case of channel type raft, cut off wall is an important component and its quality has to be excellent. If this is not possible to be achieved then it is desirable to go for detached PCC cut off walls which are supposed to act only for increasing seepage path and keeping soil below foundation raft in tact.

Such type designs are also evolved and issued by PWD in 1996.