Mat Foundation | Types Of Mat Foundation


A mat foundation is a continuous foundation with a thick slab that supports a whole structure. The foundation is shallow and poured over reinforcing mats. They are sometimes referred to as raft foundations.

What is the difference between raft and mat foundation?

Mat foundation is otherwise called as Raft Foundation and its working principle is similar to shallow foundation. Mat foundation is preferred for large space where separate footings are not possible to construct due to overlapping condition. Generally, Mat foundation’s footing will extend over a large area. The main reason why the Mat adopted by most Engineers is Mat will distribute loads equally to prohibit differential settlement. Mat is considered when the soil condition is so poor & weak.
Mat foundations are selected when:

    • The area covered by the individual footings exceeds 50% of the structural plan area. This is usually the case for buildings higher than 10-stories, and/or on relatively weak soils where q < 3 ksf = 150 kPa;

 

    • The building requires a deep basement, below the phreatic surface. For example, to build
      several levels of parking, for mechanical systems, access to subway stations,

 

    • The Engineer wishes to minimize the differential settlement in variable (that is, heterogeneous) soils, or if pockets of extremely weak soils are known to be present

 

  • The Engineer wishes to take full advantage of the soil’s increasing bearing capacity with depth by excavating basements, and thereby seek a fully or a partially compensated foundation.

There are several types of Mat Foundation:

  • Flat plate mat
  • Plate thickened under columns
  • Two-way beam and slab
  • Plate with pedestal
  • Rigid frame mat
  • Piled raft
Flat Plate Mat:
A flat plate mat is used for fairly small and uniform column spacing and relatively light loads. A flat plate type of mat is suitable when the soil is not too compressible. A line sketch of this type of mat is shown in Fig 1.
Flat plate mat
Plate Thickened under Columns:
For columns subjected to very heavy loads usually the flat plate is thickened under columns as shown in Fig 2 to guard against diagonal shear and negative moments.
Two-way Beam and Slab:
When the column spacing is large and carries unequal loads it would be more economical if a two-way beam and slab raft as shown in Fig 3 is used. This type of mat is particularly suitable when underlying soil is too compressible.
Plates with Pedestals:
The function of this mat is same as that of flat plate thickened under columns. In this mat pedestals are provided at the base of the columns.
Rigid Frame Mat:
This type of mat is used when columns carry extremely heavy loads. In such design, basement walls act as ribs or deep beam. When the depth of beam exceeds 90 cm in simple beam and slab mat, a rigid frame mat is referred. Fig 4 shows a typical rigid frame mat.
Piled Raft:
In this type of construction, the mat is supported on piles as shown in fig 5. This type of mat is used where the soil is highly compressible and the water table is high. This type reduces settlement and control buoyancy.

Raft Foundation- Reinforcement Details: