Estimating Sheet

# Mat Foundation - Types and Design

Construction Software

Mat foundation alias raft foundation stands for a continuous solid concrete slab that is provided on the soil to expand the whole footprint of the building and enhance the bearing strength of the soil.

Mat foundation mainly provides support to bear the whole building loads as well as securely transmit it to the ground.

Raft/Mat foundation is suitable for those areas where the soil contains less bearing strength. The purpose of using mat foundation is to allocate the entire loads of the structure to the soil (If the area of the footing is raised, the bearing strength of the soil is also enhanced) to minimize the stress on the soil at the similar area.

Mat foundation defines the stress in civil engineering. To find out the stress, divide weight with per unit area. Suppose, a building measure 10 m x 7 m and weight on it is 130 Tons so the result will be as follow :-

Stress = Weight / Area = 130 / 70 = 1.85 tons weight per m2.

This foundation is ideal for basement structures. There should be a water-resistant membrane sheet over the ground surface and then 4″ PCC is poured to develop a level base for the Mat/raft foundation.

Once the PCC is completed, reinforcement steel bars are attached (cage) in place and then the required thickness of the concrete(According to the design) is poured.

REASON FOR USING MAT FOUNDATION

It is used for the following purposes :-

1. The bearing strength of the soil is weak and fails to transmit the load of building to the ground.
2. A column is provided adjacent to the property line and walls are very close that lead to the overlapping of the individual footing.
3. When the cost of a deep foundation (Pile foundation) exceeds the raft foundation in use to make the structure cost-effective.
4. If a spread footing column has the ability to cover up to 50% of the foundation area.

### Also Read: Construction Sequence followed in Raft or Mat Foundation

TYPES OF MAT/RAFT FOUNDATION

The mat foundation is categorized as follow :-

1. Flat plate raft
2. Plate thickened under columns
3. Two-way slab and beam
4. Piled mat
5. Rigid frame raft
6. Cellular mat foundation

FLAT PLATE RAFT: Flat plate raft is mainly found in the small and lightweight load structure. It is applicable if the soil is not compressible. The reinforcement bars are placed in both direction top and bottom as a mesh (cage).

1. RCC slab with least thickness of 6 inches should be applied in this foundation.

THICKENED PLATE UNDER COLUMNS: The thickness of the slab should be raised if the upcoming column loads are very heavy. The flat plate foundation should not be used in those structures where the loads of the column are extreme then the thickened flat plate is useful.

1. The heavy loads develop the diagonal shear in the slab as well as produce a negative bending moment on columns, so the thickness of RCC slab under the column should be condensed.

TWO-WAY SLAB AND BEAM: Under this type of mat foundation, beams are provided in perpendicular directions and all the beams are attached with RCC slab. The columns are provided on the intersection of beams.

1. Two-way slab and beam raft foundation can be used if the columns are bearing the different loads and the spacing is huge among them.

PILED RAFT FOUNDATION: Piles provide the support to this foundation in the soil. The piled raft foundation is best suited for the soil of high compressibility and where the water table is high. Piled raft foundation is mainly recommended for the high rise buildings.

1. The piles can minimize the amount of soil settlement (With the passage of time) and enhance the load-bearing strength of the soil.

RIGID FRAME RAFT: Rigid frame mat is applicable if there are high loads on the columns. In rigid frame mat design, the basement RCC wall functions as a deep beam or ribs.

1. If the depth of the foundation remains higher than 90 cm then the rigid frame raft should be utilized.

CELLULAR MAT FOUNDATION: It is also known as a box mat foundation. Under this type of foundation, the structures of boxes are created and the walls of each box function as beams. The walls are attached by slabs at the top and bottom.

1. This type of foundation is ideal for loose soil.