Construction method & types of raft foundation

The raft foundation alias mat foundation is a type of foundation system that is utilized frequently.

Definition of Raft Foundation: Raft foundation stands for a solid concrete slab that is built on an extensive area of soil reinforced with steel. It provides support to the columns or walls and transmits loads from the structure to the soil. Normally, mat foundation is expanded across the whole area of the structure it is supporting.

Raft foundation usually provides support to the structures like residential or commercial buildings where the nature of soil is weak as well as storage tanks, silos, foundations for weighty industrial equipment etc.

Working Concept of Raft Foundation: The main purpose of raft foundation is to transfer the total load from the building to the complete ground floor area. The raft foundation comes with a very easy stress dispersion system. Total weight of the structure and self-weight of the mat is measured and divided with the total area of the foundation being covered for working out the stress on the soil.

For raft foundation, the contact area of the foundation with soil is greater than any other type of foundation, as a result the load is dispersed across a greater area and thus the stress on soil becomes low and the chance of shear failure of soil is also minimized.

When to select Raft Foundation - Raft foundation should be constructed in the following situations :

1. The soil contains a low bearing strength.
2. Load of the structure should be dispersed across a greater area.

3. Individual or any other foundation area would roughly cover 50% of the total ground area underneath the structure.
4. The columns or walls should be arranged narrowly in order that the individual footings can overlap.
5. Stress on soil should be minimized.
6. Differential settlement may happen when individual footing is utilized.
7. When soil strata are unforeseeable and comprise of pockets of compressible soil.
8. The construction for basement should be started.
9. Any other type of footing is not applied expediently.

Types of Raft Foundation - Different types of Raft foundations are found based on the soil condition and the load enforced on the foundation.

1. Flate plate mat
2. Plate thickened under the column
3. Two-way beam and slab Raft
4. Plate raft with pedestals
5. Plied raft
6. Rigid Frame Mat or Cellular Raft Foundation

Materials Used for Raft Foundation Construction: The following materials are commonly used for Raft foundation

? Formwork
? Spacer
? Reinforcement
? Concrete

Construction method of raft foundation - The following steps are involved in Raft foundation construction :

1. Specify the required depth at which foundation should be provided.
2. Excavate soil up to the desired depth.
3. Condense the soil.
4. Arrange for a waterproofing membrane.
5. Pour 3? of plane cement-sand paste.
6. Place reinforcement retaining the desired spacing with spacers.
7. Pour concrete to the required depth.
8. Curing.