Raft foundation: Know uses, types, benefits and downsides

Raft foundations are massive, flat concrete slabs that run the length of the whole footprint of a building atop a level, stable soil bed. A raft foundation is one in which the weight of the building is distributed evenly throughout the whole space beneath the structure. In doing so, the pressure is relieved of the subterranean environment, laying a firm basis that is flexible enough to withstand the effects of ground movement without compromising its structural integrity.

These may be shallow foundations, but they may distribute loads more evenly than deeper ones. The tension operating on the soil underneath is substantially decreased since the weight of the structure is distributed across a much broader surface area than with conventional trench footings or pile footings.

Source: Pinterest

Raft foundation: Uses

Generally speaking, raft foundations are appropriate when the footprint of a structure is modest and the structural load requirements are light. They may also be used in basements, provided that the foundation slab can support direct living loads for the intended use of the structure. Raft foundations may once again prove to be a good choice in locations with poor soil conditions or when access to heavy excavation machinery is restricted.

Raft foundation: Functioning theory

The whole building’s weight is transferred to the ground level through the raft foundation. The process by which the stresses are distributed in a raft foundation is rather elementary. Soil stress is determined by dividing the sum of the structure’s weight plus the mat’s weight by the area of the foundation the mat is covering. Raft foundations have a wider contact surface with the soil than any other form of foundation, meaning the weight is distributed more evenly and the soil is subject to less stress and less risk of shear collapse.

Source: Pinterest

Raft foundation: Types

The five most common raft bases are:

01. Flat plate type raft foundation

The bearing area of a raft foundation with a flat plate design is covered by a single, uniformly thick slab of reinforced concrete. When the column loads are minimal and consistent, this arrangement works well.

02. Thickened flat plate type raft foundation

In cases when the weights on the columns are very heavy, the flat plate type is unsuitable. The thickness of the slab has to be raised so that it can be used. Negative bending moments and diagonal shear are introduced into the slab because of the strong loading from the columns.

03. Beam and slab type raft foundation

For a raft foundation of the beam and slab kind, the beams are put at right angles to one another, and the raft slab connects all of the beams. The raft’s beams meet at right angles, and that’s where the columns go.

04. Piled raft foundation

In a piled raft foundation, piles are driven into the ground to provide support for the raft slab. When the soil has a high water table and low compressibility, this form of foundation is recommended. This works well for skyscrapers. Pile provision beneath the raft mitigates settling and allows for regulation of buoyancy impact.

05. Cellular raft foundation

Some other names for cellular raft foundations include box-type raft foundations and rigid-frame raft foundations. The walls of the individual boxes serve as the beams in this foundational arrangement, which is held together at the top and bottom by slabs.

Raft foundation: Benefits

• The soil’s tendency towards differential settling may be mitigated.
• Given that raft foundations are built to support structures at just shallow depths, they are particularly well suited for weak soils.
• The superstructure’s loads are spread out more widely.

Raft foundation: Downsides

• Expert labour is needed to build the raft’s skeleton.
• Large amounts of reinforcement may be needed for the raft’s base, which would raise the overall cost of the project.
• In the event of localised loads, raft foundations will need special care.
• Edge erosion may occur if they are not properly cared for.

FAQs