A raft foundation, also called a mat foundation, is essentially a continuous slab resting on the soil that extends over the entire footprint of the building, thereby supporting the building and transferring its weight to the ground.
A raft foundation is often used when the soil is weak, as it distributes the weight of the building over the entire area of the building, and not over smaller zones (like individual footings) or at individual points (like pile foundations). This reduces the stress on the soil. The concept of stress is very basic to civil engineering. Stress is simply weight divided by area. For example, if a building measuring 5 x 5 weighs 50 tons, and has a raft foundation, then the stress on the soil is weight / area = 50/25 = 2 tons per square meter. If the same building were supported by say 4 individual footings, each of 1 x 1m, then the total area of the foundation would be 4 m2, and the stress on the soil would be 50/16, which is about 12.5 tons per square meter. So increasing the total area of the foundation can dramatically lower the stress on the soil, which is nothing but weight per square meter.
A raft foundation is also very good for basem*nts. Foundations are created by excavating soil in order to find strong, compact, undisturbed natural soil that is at least a few feet below ground level. This soil is much stronger than the loose soil at the surface. If we construct a raft foundation at say 10 feet below ground, and build concrete walls around the periphery, this makes an excellent basem*nt. Therefore, an engineer designing a building with a basem*nt will tend to choose a raft foundation over other types of foundations.
how to construct a raft or mat foundation
A raft foundation is constructed by first excavating the ground to a uniform, flat level. Then, a waterproof plastic sheet is laid over the earth, and a thin 3" layer of plain cement concrete (PCC) is poured just to create a perfectly flat and level base for the foundation. After this, a waterproofing layer is installed, and then reinforcement steel for the raft slab is tied in place. After all the steel has been put in place, concrete is poured to the desired thickness, which is usually in the range of 200mm (8") to 300mm (12") thick for small buildings: this can be much thicker if heavy loads are to be carried.
waterproofing of basem*ntsBasem*nts often extend several stories below ground. The soil or rock around the basem*nt can easily be saturated with water. This water will seep into the building if the building is not waterproofed properly, as concrete is not impermeable to water (it allows water to pass through). So engineers pay a great deal of attention to the waterproofing design of basem*nts, as once this is done, there is no way to repair it, so it must be done perfectly during construction. |
the concept of uplift
When working with basem*nts, one needs to understand the concept of uplift. Uplift happens when the soil outside a basem*nt gets saturated with water, say during a period of very heavy rain. Then, the basem*nt begins to act like a ship that floats in a sea of water. The water will exert a strongupwards force on the basment. In case this upwards force is balanced by a strong downwards force, such as the weight of many floors above the basem*nt, there is no problem. If, however, there is no balancing force, as may be the case if the basem*nt has just been constructed, and there is nothing above, then the upwards force may be strong enough to lift the basem*nt out of the earth, ruining it completely and causing great damage.
ground improvement mats
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Introduction to Foundations
