Mat Foundations: Types, Uses, and Construction Techniques

What Is Mat Foundation?

Mat foundation is also called a raft or mat foundation is also called a raft or mat foundation. Generally, it is a continuous slab laying on the soil on which all the columns and beam of the structure are constructed.

By the mat foundation, all load of the building or structure is transferred to the soil. Where the soil condition is very weak there the mat foundation is used. Mat foundations are designed to distribute the weight of the building evenly over the soil, preventing differential settlement.

It is distributed all the load simultaneously to the soil that’s coming from the superstructure. For this, the particular zone of the soil area under the mat foundation is not collapsing.

Because the stress under the soil area for all zone is equal. For the basement, a mat foundation is very good. Where one column is overlapping another column there we used a mat foundation.

Mat Slab:

Mat slab, sometimes referred to as matt slab, is a part of the shallow foundation.
Mat slab is a hard surface on which all the column or beam are constructed.
Where they have heavily loaded columns or building the mat slab is used.
Mat slab is a common base for many columns.
To ensure a relatively uniform load transfer in high rise building mat slab foundation is used.
Near the ground surface, mat slabs are used.
Mat slab reduced the stress of the soil on the particular zone under the slab to do distribute equally the load that’s coming from the superstructure.
To construct a mat slab at first, the ground should be excavated at a uniform flat level.
After that, a waterproof sheet is laid over the excavated ground then a thin layer of plane cement concrete is poured to obtain a perfectly flat surface for the mat slab.

Mat Building:

The term matt foundation or mat building expressed that has a type of building in which has a layout, daylighting, and ventilation solved for a plan that’s repeated when needed.

Mat Footing:

When the soil conditions under the foundation have not adequate strength to bear the load of the superstructure then the matt footing or mat footing is provided.
It provides a perfect grip on the structure of the soil and prevents the tendency of overturning the building.
It is provided when a large number of the floor are exited.
The mat footing prevents the sudden settlement of the structure.
There are two types of mat footing – one is to a square shape and another is a rectangular shape.
The square mat footing is constructed to provide stability to the building.
When a column is too close to the building or structure where mat footing is required.
When two columns are so close to each other or they overlap each other where mat footing is necessary to provide.
The mat footing is constructed where there the strip footing or spread footing is not enough to bear the load that’s coming from the superstructure.
To distribute a uniform load from the superstructure to the subsoil mat footing is constructed.

Mat Slab Foundation:

It is a thick layer of concrete slab on the soil on which all the footprint of the building is resting.
When the construction cost of depth foundation is more than from the mat foundation to make the structure economical mat slab foundation is used.
The mat foundation resting the entire load of the building and transfer safely to the subsoil.

Advantage and Disadvantage of Mat Slab Foundation:

It has many advantages and disadvantages, or in other words, mat foundation vs slab on grade, are as follows.

Advantage of Mat Slab Foundation:

It has good resistance for this the building can not slide during the flood.
The mat slab foundation can help transfer the load of the building to a verse area.
To make an economical structure like when the cost of depth foundation is more than from the shallow foundation then we constructed a mat slab foundation.
In this mat slab foundation, we can reduce the materials cost because when we construct the foundation at the same time the ground floor slab is poured for this reason the time and materials both are safe.
The mat slab is a shallow foundation for this the depth of the foundation is less so the excavation for the foundation is less.
The mat slab foundation can take more than load from the other type of foundation.
In another advantage is that the mat slab foundation can construct in poor soil.

Disadvantage of Mat Slab Foundation:

It has some disadvantages are as follows.

The number of quantity materials like steel or concrete required more for the mat foundation.
The mat slab foundation is not suitable for domestic uses.
In the case of concentrated loads, sometimes a special measurement is needed.
For the mat slab, the area of the foundation is large so the cost of the structure will increase.

Mat Foundations Design:

There have two types of mat foundation design one is a conventional design that uses charts and manual calculation and another is a finite element analysis method that design solved by a computer program.

Conventional Design Method:
Finite Analysis of the Method:

#1. Conventional Design Method:

The total applied load is calculated into the conventional design method. The shear force and bending moment are determined. The reinforcement area per unit width is calculated.

The depth of the footing determines. Determine the positive or negative bending moment per unit area from the bending moment diagram.

The eccentricity of loading for the calculated pressure under each column. To determine the effect depth of the footing.

#2. Finite Analysis of the Method:

In the structural analysis, the finite analysis method is a consideration. Into the analysis and design, the soil is model and its behaviour is incorporated.

Types of Mat Foundation:

There are six types of mat foundation that are-

Flat Plate Mat
Plate Thickened Under Columns
Two-Way Slab and Beam
Rigid Frame Mat
Piled Mat
Cellular Mat Foundation

#1. Flat Plate Mat Foundation:

A flat plate mat is used where the structure is lightweight and small. When the soil is not compressible where this type is suitable. In the form of the cage, the reinforcement bar is provided on both the top and bottom sides. At least 6 inches of RCC thick slab is provided in this type of foundation.

#2. Plate Thickened Under Columns:

When the upcoming column load is very heavy the slab thickness should be increased. Where the flat plate mat foundation is not enough to carry the heavy load that comes from the superstructure then the thickened plate foundation is connected.

The heavy load creates a negative bending moment on the column and diagonal shear in the slab. So the thickness of the RCC slab under the column should be thickened.

#3. Two-Way Slab and Beam:

In a two-way beam and slab foundation, all beams are placed in a perpendicular direction, and each beam is connected to the slab foundation.

And the intersection of the beam all columns are placed. This type of foundation suitable when the column is carrying an unequal load and between the column, the spacing is more.

#4. Rigid Frame Mat Foundation:

When the column has a heavy load then the mat foundation is used. In this type of foundation design, the basement RCC wall acts as a deep beam. When the foundation depth is required more than 90 cm then the rigid frame mat foundation is used.

#5. Piled Mat Foundation:

This type of foundation constructs in the soil by piles. Where the water table is near the ground level and the soil compressibility is high the piled mat foundation is suitable for those places.

Where the high-rise building is constructed the piled mat foundation is used. To reduce the chance of soil settlement and increase the soil bearing capacity piled mat foundation is used.

#6. Cellular Mat Foundation:

The cellular mat foundation has also termed a box mat foundation. A box type of structure is formed whose wall acts as a beam.

The walls are both sides top and bottom are connected by the slabs. At loose soil this type of foundation is suitable.

Mud Mat Concrete:

A structural floor or footing over soft, wet soil the layer of concrete beneath is called mud mat concrete.
It provides a plane or smooths surface at all-weather.

Mat Construction:

The mat construction process is very important in which extra care should be needed. The few processes of mat foundation are described below:

Soil Excavation for Mat Foundation.
Waterproofing for Mat Foundation.
Mat Foundation Reinforcement.

#1. Soil Excavation for Mat Foundation:

The excavation of soil for the foundation is more important because it provides a plane surface for the foundation. The excavation depth depends on the bearing capacity of the soil.

#2. Waterproofing for Mat Foundation:

All the mat foundation are generally waterproofed. When it is construed at the ground level waterproof, the membrane is added. That makes it waterproofed. For this, the foundation is always protected from wetness or dampness.

#3. Mat Foundation Reinforcement:

Generally, there are two types of reinforcement is provided in the mat foundation one is bending reinforcement and another is shear reinforcement.

The shear reinforcement is provided in the column and bending reinforcement provides as usual. The shear link is provided as the requirements of the design.

Where Mat Foundation Is Used?

The uses of mat foundation –

Where the soil bearing is very poor and the soil does not bear the load of the structure then mat foundation is used. When a column is near the building or property line then mat foundation is used.

When the distance of the two columns is very less than the mat foundation used. When the depth foundation cost is more than that of the shallow foundation then the mat foundation is used to reduce the cost of the structure.

FAQ: Understanding Mat Foundations

What is a mat foundation?

A mat foundation, also known as a raft foundation, is a continuous slab of concrete laid on the soil, supporting all the columns and beams of a structure. It distributes the weight of the building evenly over the soil, preventing differential settlement.

When is a mat foundation used?

Mat foundations are used in areas where soil conditions are weak and unable to bear the load of the structure. They are also used when columns are close together or overlap, and when the cost of a deep foundation is higher than a shallow foundation.

What are the advantages of a mat foundation?

Good resistance to sliding during floods.
Efficient load distribution over a large area.
Economical for reducing excavation depth.
Suitable for poor soil conditions.
Reduced material costs as the ground floor slab is poured simultaneously.

What are the disadvantages of a mat foundation?

Requires more materials like steel and concrete.
Not suitable for domestic uses.
Special measures needed for concentrated loads.
Larger foundation area increases the overall cost.

What is a mat slab?

A mat slab is a thick, hard surface on which all columns and beams are constructed. It ensures a uniform load transfer, especially in high-rise buildings, and is used near the ground surface.

What are the types of mat foundations?

Flat Plate Mat Foundation: For lightweight structures and incompressible soil.
Plate Thickened Under Columns: For heavy column loads.
Two-Way Slab and Beam: For unequal column loads with more spacing.
Rigid Frame Mat Foundation: For heavy loads, with basement walls acting as deep beams.
Piled Mat Foundation: For high water tables and compressible soil.
Cellular Mat Foundation: For loose soil, forming a box-like structure.

How is a mat foundation constructed?

Soil Excavation: Ensuring a plane surface for the foundation.
Waterproofing: Adding a membrane to protect from wetness and dampness.
Reinforcement: Providing bending and shear reinforcement as required by design.

What is the difference between conventional design and finite analysis method for mat foundation design?

Conventional Design Method: Uses charts and manual calculations to determine load, shear force, bending moment, and reinforcement area.
Finite Analysis Method: Utilizes computer programs to model soil behavior and incorporate it into structural analysis and design.

Where is a mat foundation typically used?

Mat foundations are used in areas with poor soil bearing capacity, near building property lines, when columns are closely spaced, and when the cost of deep foundations is higher than shallow foundations.