A raft foundation, also called a mat foundation, is essentially a continuous slab resting on the soil that extends over the entire footprint of the building, supporting the building and transferring its weight to the ground which reduces stress on the soil.
It consists of thick reinforced concrete slabs that cover all the area of the ground of the structure like a floor.
Here we will learn about mat foundation, types of mat foundation & much more.
Introduction to mat foundation:
A mat foundation is a thick reinforced concrete slab arranged in a row or rows supporting columns or walls and carrying the load to the soil.
This slab is reinforced with bars running at proper angles to one another and each close to the bottom and the top face.
Why it is used:
- The spread footings are large, covering more than 50% of the foundation area.
- Soils with low bearing capacity are softer.
- When the deep foundation has cost more than fleet foundations.
- The walls of the structure are so close that individual footings will overlap.
Types of mat foundation:
1.Flat Plate Raft Foundation:
Flat plate raft is used in small and lightweight structures, this type of foundation is appropriate when the soil is not compressed.
Reinforcement bars are provided in both up and down directions in the form of a mesh (cage).
Minimum 6 inch thick RCC slabs are used in this foundation.
2.Thickened Plate raft foundation:
The thickness of the slab should be increased when the subsequent column load is too heavy.
Flat plate foundations are not suitable in structures where the load of the column is very high, then thicker flat plates are used.
Heavy loads create diagonal shear in the slab and negative bending moment at the columns, so the thickness of the RCC slab at the bottom of the column should be thicker.
3.Two-Way Slab and Beam mat foundation:
In Two wat slab and bean mat foundation, the beams are positioned in vertical directions, all beams are connected by RCC slabs.
The columns are positioned on the intersection of the beam.
This foundation is suitable when columns are carrying uneven loads and the spacing between them is large.
4.Piled Raft Foundation:
Pile raft foundation is supported by piles in the soil.
This foundations are suitable for high compressive soils, where the water table is high in tall buildings.
Piles are used to reduce the amount of soil disposal (with the passage of time) and to increase the load-carrying capacity of the soil.
5.Rigid Frame Raft Foundation:
Rigid frame mats are used when there is high much load on the columns.
In the rigid frame mat design, the basement RCC wall serves as a deep beam or rib, if the depth of the foundation is more than 90 cm then a rigid frame raft is used.
6.Cellular Mat Foundation:
In the cellular mat foundation, the structures of the boxes are formed, the walls of each box act as beams.
The walls are connected to the top and bottom slabs, it is suitable for loose soil.
It is also known as box mat foundation.
Advantages of mat foundation:
- The foundation and surface slab are inserted at the same time, reduces construction time and materials.
- This requires less digging.
- This is provided where shallow foundations are possible but soil conditions are poor.
- Reduces the cost of construction of floor slabs (but not completely economical).
- This helps in transfer of load over a wide area.
- It shows good resistance and cannot slide during floods.
- We can handle heavier loads than other types of foundations.
Disadvantages of mat foundation:
- Raft foundation requires large quantities of steel and concrete.
- This foundation is expensive (footing volume was increasing).
- It is not suitable for domestic home construction.
- Particular measurements are required within the case of concentrated loads.
- The mat foundation requires expert laborers.
Uses of raft foundation:
A raft foundation is most popular under various circumstances:
- It is used for big loads like industrial building a lot larger than in-home and subsequently heavier.
- The soil has a low bearing capability so the load of the building must be spread over a big space to form a stable foundation.
- The ratio of individual footings to complete flooring space is excessive.
- Normally the raft foundation will likely be used if the footing will cover greater than half of the development area.
- If the walls of the building are so shut that it could cause different footings, then the raft basis must be used.
Also read: Floating Foundation, Pile Foundation & Deep Foundation
A mat foundation system in which the entire building is placed on a big steady footing used as the foundation for silos, chimneys, large machinery.
It is a flat concrete slab reinforced with metal, which carries loads beneath individual columns or walls.