Raft Foundation – When to Use, Types, Construction

A raft foundation is one of the most popular choices in the world of structural engineering, and many people consider it a single footing since it distributes all the load of the building onto a single ‘mat’ which increases the building’s stability.

Raft foundations have been used in construction for a long time and have served their purpose well, in this article, we will be discussing slab-on-grade mat foundations, their working principles, and their uses.

What is Raft Foundation?

A raft foundation is also known as a “mat” foundation. These types of foundations can be constructed in various shapes and dimensions such as a cross-section of a slab or a rectangular block. Like any mat or slab foundation, a raft foundation is used to support a building in lifting the aggregate load in a distributed manner.

The soil below the raft is reinforced with steel which greatly reduces the strength of columns or walls in conjunction with transferring load sideways reaching the structure’s internal area. In typical situations, mat foundations cover the complete frame area of the structure that it maintains.

These foundations are helpful in constructing warehouses, homes, and skyscrapers when the soil condition is inadequate.

Working Principle of Raft Foundation

It is helpful to know when to use a raft foundation, hence it is equally important to review how a raft works. Let’s do a recap on this as well.

Raft foundation is a type of foundation where the entire building load is transferred to the entire area of the ground floor. Stress distribution mechanism of raft foundation is a bit straightforward.

This is arrived at by summing up the weight of the structure and the self-weight of the mat and dividing this value by the total area of the foundation it encompasses, in order to arrive at the stress exerted on the soil.

Like that of raft foundation where the contact area of the foundation with soil is large, there potrays a fact where there is necessitated a lesser stress on soil bearing and thus decreased possibility of shear failure of soil.

When to Choose Raft Foundation

For structure design, choosing the suitable foundation type is one of the salient points. Raft foundation is most appropriate when:

• The Soil has Low Bearing Capacity
• The structure load is intended to be spread over a significantly larger area.
• Any individual’s or other footing area would be expected to approximately equal to 50 percent of the total area of the bottom floor of the structure.
• Where columns or walls are spaced so close that their individual footings would be compulsorily dislocated.
• A need to lessen the stress on the soil.
• There is a possibility of differential settlement in case individual footing is used.
• Whereas soil layers are heterogenous and contain lenticular deposits of compressible materials.’
• Basement is to be constructed.
• None other type of footing can be employed effectively.

Types of Raft Foundation

Depending on the soil properties and the load applied on the foundation, there several types of Raft foundations that may be used.

The following are the other types raft foundations utilized in construction practices.

1. Flat mat
2. Plate thickened under the column
3. Beam and slab Raft
4. Plate raft on pedestals
5. Raft with poles

Rigid Frame Mat or Cellular Raft Foundation

The various types of mat foundations are discussed briefly here after.1 Flat Plate Mat This is the most basic form of raft foundation. This type of mat is used where columns and walls are adequately spaced and the applied forces are moderately low.

Reinforcement is placed in two directions under the slabs with columns and load-bearing walls needing more. The thickness of this types of raft foundation is usually required to be limited to 300mms for reasons of economy. A thicker slab would not be economically feasible.

Plate Thickened under Columns

In the event that the columns and load-bearing walls are overloaded, the standard practice is to thicken the slabs and walls located underneath the columns and walls and provide additional reinforcement to protect against diagonal shear and negative reinforcement.

Two-way Beam and Slab

In this type of raft, beams are cast monolithically with the raft slab connecting the columns and walls. This type of raft is suitable when a wider center distance of the columns is used and loads above the columns vary.

Plates with Pedestals

In this type of mat, a pedestal is provided at the base of the columns. Purpose of this type of foundation is the same as flat plate thicknessed under columns.

Piled Raft

This type of raft foundation is carried or constructed on piles. The condition of the soil which is located at relatively shallow depth is very compressible and the level of the water table is high a piled raft is applied. The Piles under raft assist in limiting the amount of settlement that will take place and help to counteract buoyancy.

Rigid Frame Mat/Cellular Raft Foundation

In this category of foundation, the foundation walls function as a deep beam. A rigid frame mat is referred to when columns carry huge weight and the beams connecting the columns are more than 90cm in depth.

Here two concrete slabs are placed one above the other and connected to opposing foundation walls in both vertical and horizontal directions, thus forming a cellular raft foundation. This type of raft is very rigid and is cost effective when there is a need for very thick slab.

Materials Used for Raft Foundation Construction. Here are some materials used for the erection of Raft foundation-

• Formwork
• Spacer
• Reinforcement
• Concrete.
• Construction Process of Raft foundation

The most important key steps of construction of a raft foundation are noted below.

Identify and set the depth of the foundation that one would want and the thickness that should be provided.

• Dig the soil to the recommended level.
• Compress the soil.
• Put a waterproof layer.
• Pour a layer of cement sand paste which is 3 inches in thickness.
• Arrange the reinforcement according to the spacing required using the spacers.
• Dump the concrete upto the specified thickness.
• Curing.

To conclude, mat is one of the easiest dominant and widely used types of foundation systems owing to its easy construction technique to satisfy the need where soil availability is shallow.

The soil condition and the loads applied on the building should be analyzed to make better use of the foundation, and precautions for safe construction should be taken.