A raft or mat foundation is a large continuous rectangular or circular concrete slab that carries the entire load of the superstructure and spreads it over the whole area beneath the building. It is considered as one type of shallow foundation and is useful in controlling the differential settlement.

A mat foundation spreads out under the footprint of the building and declines the contact pressure compared to conventional strip or trench footing.

It is a suitable solution for low bearing capacity soil, spread footing cover about 70% of the structure, high structure loads, soft pockets or cavities of in the soil to unknown extent raft, and highly compressible soil that extends to a great depth.

Functions of Mat Foundation

  1. Transfer superstructure loads and spread it over the entire area of the building footprint.
  2. Reduce differential settlement of structures constructed over weak soil.

Uses of Mat Foundation

  1. It is employed for the construction of commercial buildings. In this case, the loads are commonly large. Mat foundations are popular in areas where basements are common.
  2. Mat foundation is also used for low bearing capacity soil in order to spread the load of a building and hence construct a stable foundation.
  3. It is used to reduce differential settlement of buildings.
  4. Raft or mat foundation is used when the soil layer is unstable. In this case, strip foundation would cover more than 70% of the ground area beneath the building. There are also situations usually in mining areas that soil layer may suffer movements.
Mat Foundation
Fig. 1 : Mat Foundation

Construction of Mat or Raft Foundations

  1. Remove dirt and excavate soil to a uniform and flat level.
  2. The foundation bed is then compacted by ramming.
  3. Then, a waterproof plastic sheet is laid over the earth.
  4. After that, pour around 7cm layer of plain cement concrete to create a perfectly flat and level base for the foundation.
  5. Lay reinforcement on spacers over the foundation bed. Reinforcements are provided in both directions in the form of steel mesh. Two meshes are reinforced at the top and bottom of the foundation to balance upward and downward bending forces.
  6. After all the steel has been put in place, concrete is poured to the desired thickness, which is usually in the range of 200mm to 300mm thick for small buildings: this can be much thicker if heavy loads are to be carried. A minimum rebar cover of 50mm should be maintained.
  7. Finally, a suitable curing regime should be used to make sure that concrete achieves the designated compression strength.
Excavation for Raft or Mat Foundation Construction
Fig. 2: Excavation for Raft or Mat Foundation Construction
Installed Reinforcements for Raft or Mat Foundation
Fig. 3: Installed Reinforcements for Raft or Mat Foundation
Concrete Pouring for Raft Foundation
Fig. 4: Concrete Pouring for Raft Foundation

Read More:

Raft Foundation — Design Requirements and Applicability

What are the Foundation Design Requirements?

Safe Bearing Capacity values of Different Soils

Raft Foundations – Types and Advantages

Soil Tests Required for Shallow and Raft Foundations