Raft foundation is recommended when the bearing capacity of soil is low when huge loads are imposed.Advantages:• Economic if used as floor slab• Reduces differential settlementLimitations:• Edge erosion if not treated• Highly subjected to uplift presure

1. Introduction

Raft foundations vary in design, however the main are classified into three types:

(a) Solid raft foundation:

Solid raft is commonly used for slightly loaded types of buildings and consists of reinforced concrete slab where the reinforcements is provided in a form of mesh on top and bottom of the slab faces.

2. Types of raft

(b) Ribbed raft foundation:

Suitable in the case of variations in ground stiffness that may cause differential settlement. Ribbed raft is a solid raft but reinforced with ground beams. If a ribbed raft has the same geometric shape of a solid raft, than it is called raft foundation with hidden beams.

(C) Cellular Raft foundation:

Consists of two slab linked with walls which divide the void into cells. Cellular raft foundation provides great stiffness against differential settlement.

Raft foundation fail due to soil failure.

4. Failure

Design of Raft FoundationAhmed Al-Aamri and Toufic Khoury

University of Birmingham – Msc Civil Engineering and Management

3. Design methods(a) Conventional design method: Computations are carried using hand calculations the following steps:

• Check max soil pressure to its net allowable bearing capacity

• Compute and draw the shear force and bending moment diagram

• Design for flexure• Design for punching shear• Reinforcement arrangement

(b) Finite element method:

Computer programs are used for the analysis of raft regardless of columns arrangements, and loading conditions.

6. References

5. Conclusion

A raft is a large spread footing that encompasses the entire footprint of the building. It is also known as mat foundation. Raft foundation is used when the bearing capacity of the soil is low.

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