A raft foundation is a reinforced concrete slab of uniform thickness which is capable of bearing the total load of the building. It is particularly suitable for a structure where the structural loads are heavy and the allowable soil pressure is low. It is also used where the site has a high water table and where differential settlements are difficult to control. In structures like chimneys, silos, cooling towers or buildings with basements, a raft is preferred to a spread footing.
It is important to consider the design of a raft foundation before it is built, to ensure that damp coursing or cold bridging does not occur. It is also vital to be aware of any issues with land contamination which may impact on the construction of a raft foundation. If you have any concerns, then it is best to speak with the local authority building control surveyor for the area where you are planning to build, to obtain their advice before you start constructing.
Various types of raft foundations are used depending on the nature of the ground and the size of the structure. Raft foundations are often constructed in situations where the structure’s footprint would be very large if it was designed on a spread footing, or where it is necessary to limit the number of columns such as on a floating raft. The raft foundation also tends to be adopted where the soil mass contains compressible layers.
The design of a raft foundation starts with excavating the required depth and preparing a base which is usually made up of compacted fill. It is a good idea to place a sand “cushion” on the bottom of the pit, which will help to prevent movement of the soil during excavation. This is followed by a layer of waterproof plastic sheet and a thin layer of concrete, which is typically 3″ thick for small buildings. Reinforcement steel for the raft slab is then placed and a formwork is installed. Once the formwork is in place, concrete is poured to the required thickness.
In some cases, a raft foundation is supported on piles. This is especially useful where the soil at shallow depth is highly compressible and a high water table is encountered. Piles under the raft reduce settlement, control buoyancy effects and increase the ultimate load capacity of the foundation.
The load sharing ratio of a raft with piles can be varied significantly by altering the value of the stiffness modification factor, which is the ratio of pile and raft stiffnesses. Using this method of calculating the load sharing ratio, the interaction between pile and raft is modelled, and it is shown that a more realistic distribution of the pile force is obtained than if perfect bonding is assumed, notably at the pile tip level. This is an important step towards achieving more accurate models of pile-raft interactions for the design of floating rafts. Further, the influence of the number of piles on the load-sharing ratio is also investigated.
