To get a better idea about when to use raft foundation, it is important to understand how raft foundation works. Let’s get a quick review of its working principle.

Raft foundation transmits the total load from the building to the entire ground floor area. Stress distribution mechanism of raft foundation is very simple. Total weight of the structure and self-weight of the mat is calculated and is divided by the total area of the foundation it is covering to calculate the stress on the soil.

As in case of raft foundation the contact area of the foundation with soil is much more than any other type of foundation, so the load is distributed over a larger area and thus the stress on soil is lesser and the possibility of shear failure of soil is also reduced.

When to Choose Raft Foundation

For foundation design, one of the most important aspects is choosing the right type of foundation. Raft foundation is preferred when-

  • The soil has a low bearing capacity.
  • Load of the structure has to be distributed over a large area.
  • Individual or any other foundation area would approximately cover 50{cfe35340c81e4299fa689fe359b97718a46b004390759cec6d75fcacb6718f28} of the total ground area beneath the structure.
  • The columns or walls are placed so closely that the individual footings would overlap.
  • Stress on soil needs to be reduced.
  • There is a possibility of differential settlement in case individual footing is used.
  • When soil strata are unpredictable and contain pockets of compressible soil.
  • Basement is to be constructed.
  • Any other type of footing cannot be used advantageously.

Types of Raft Foundation

Several types of Raft foundation may be used depending on the condition of soil and the load imposed on the foundation.
Followings are the different types of raft foundation used in construction:

  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