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Raft Foundation – Understand Building Construction

Raft foundation support columns or walls and transmit loads from the building to the earth by way of a thick concrete slab sitting on a sizable area of steel-reinforced soil. The area of the structure that a mat foundation supports is often covered with it.

In areas with poor soil conditions, raft foundations are frequently used to support structures like homes, businesses, storage tanks, silos, the foundations for large industrial equipment, etc.

Operational Principle of Raft Foundation

It’s critical to comprehend how raft foundation functions in order to have a better understanding of when to employ it. Let’s quickly go through its guiding ideas.

The whole ground floor area receives the full weight of the structure through the raft foundation. The raft foundation’s stress distribution system is fairly straightforward. Calculating the stress on the soil involves dividing the combined weight of the structure and the mat by the entire area of the foundation it is covering. Because raft foundations have a greater amount of soil contact than any other style of foundation, the load is dispersed over a broader area, resulting in less stress on the soil and a decreased risk of shear failure.

How to Select a Raft Foundation

Choosing the appropriate foundation type is one of the most crucial elements in foundation design. It is best to use a raft foundation when

  • The Soil can support little weight.
  • The structure’s load must be dispersed across a big region.
  • About half of the entire ground surface beneath the structure would be covered by each and every foundation area.
  • The various footings would overlap if the columns or walls weren’t positioned so tightly together.
  • Reduced soil stress is necessary.
  • If individual footing is employed, a differential settling is a possibility.
  • when pockets of compressible soil are present in soil layers and the strata are unexpected.
  • It is necessary to build a basement.
  • Any other footing cannot be utilised profitably.

Raft Foundation Types

Depending on the soil’s state and the weight placed on the foundation, many types of raft foundations may be employed.

The many forms of raft foundation utilised in building are as follows:



This is the most straightforward raft foundation design. When the weights that must be supported are minimal and the columns and walls are equally placed at close intervals, this kind of mat is used. There is reinforcement in both directions, and extra reinforcement is needed at the positions of the load-bearing walls and columns. For financial reasons, the thickness of these sorts of raft foundations is often limited to 300mm. A slab that was thicker would not be practical.


The slab is enlarged under the columns and load-bearing walls and further reinforcement is added to resist diagonal shear and negative reinforcement when the columns and load-bearing walls are subjected to larger loads.


The raft slab joining the columns and walls is made of a monolithic cast of beams. When the columns are spaced farther apart and the stresses on the columns are changeable, this sort of raft is appropriate.


At the foot of the columns in this kind of mat is a pedestal. Similar to thicker flat plates under columns, this sort of foundation serves the same purpose.


The walls of the foundation serve as a deep beam in this kind of raft. When columns support really enormous loads and the connecting beams are deeper than 90 cm, a rigid frame mat is used. Here, a cellular raft foundation is created by stacking two concrete slabs one on top of the other and connecting them in both directions with foundation walls. When a very thick slab is needed, this form of raft is both robust and cost-effective.


The base of this kind of raft is supported by piles. When the water table is high and the soil is very compressible at a shallow depth, a stacked raft is employed. Pile support under the raft reduces settling and offers buoyancy resistance.

Materials Used to Build Raft Foundations

The following materials were utilised to build the Raft foundation:





Building Mat or Raft Foundations

  • Excavate the soil until it is uniformly flat by removing dirt.
  • After that, ramming is used to compact the foundation bed.
  • The dirt is then covered with a piece of waterproof plastic.
  • After that, pour a layer of normal cement concrete about 7 cm thick to provide a basis for the foundation that is completely smooth and level.
  • Over the bed of the foundation, place reinforcement on spacers. Steel mesh is utilised as reinforcement in both directions. To balance upward and downward bending pressures, the top and bottom of the foundation are strengthened with two meshes.
  • After all of the steel is in place, the concrete is poured to the necessary thickness, which for small structures is typically between 200 and 300 millimetres thick, but it may be considerably thicker if high weights are to be handled. Maintaining a 50mm minimum rebar cover is recommended.
  • To ensure that concrete reaches the desired compression strength, a proper curing regime should be applied.

In conclusion, because of their ease of construction and efficacy in areas with poor soil conditions at shallow depths, mats are among the most often used and well-liked foundation systems. For the best use of the foundation, one must examine the soil’s state and the building’s loading situation, and essential safety measures must be followed during construction.

Read Also:

Bearing Capacity of Soil – Types and Calculations

What Is a Quantity Surveyor? A Complete Guide

What Is Total Station? Operation, Uses and Advantages

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