A Raft foundation is a type of shallow foundation made of a densely reinforced concrete slab covering the total area of the bottom of a structure. In some instances, the slabs are used at the bottom and the beams at the top. Raft foundations are typically viable when heavy structures are to be constructed on soft grounds. This type of foundation is also used in marshy sites with soils having low bearing capacity. 

This article shall apprise users of all the information related to the raft foundation and its assorted types. This topic is pertinent from an exam point of view, including SSC JE CE and RRB JE Civil.

Raft Foundation

Raft foundation constitutes a thick concrete slab that rests on a large soil area. This foundation supports columns and walls and functions by transferring the loads from the structure to the soil.This foundation is spread over a large area and as such nullifies any differential settlement.

This footing is particularly used when the soil has a low bearing capacity and when the chances of differential soil settlement are high.

Fig 1: Raft Foundation diagram

• In the raft foundation, the load is transmitted from the building to the ground by means of the raft. 
• Its stress distribution mechanism is simple and uncomplicated. For computing the stress on the soil, the total weight of the structure and self-weight of the mat is calculated and divided by the total area of the foundation covered.
• In raft foundation, the contact area of the foundation with soil is significantly large as compared to other types of foundations. The load is distributed over a larger area, and thus the total stress exerted on the soil is lesser, thereby reducing the possibility of shear failure of the soil.

Types of Raft Foundations

Different types of Raft foundations may be employed depending on the condition of the soil and the load imposed on the foundation.

The followings are the different types of raft foundations used in construction:

1. Flate plate mat
2. The plate thickened under column
3. Two-way beam and slab Raft
4. Plate raft with pedestals
5. Piled raft
6. Rigid Frame Mat or Cellular Raft Foundation

1.Flat Plate Mat

Fig 2: Flat Plate Mat Foundation

• This is the simplest type of raft footing used when the loads are small. The columns and walls are uniformly spaced at small intervals in such a raft foundation.
• Reinforcement is placed in both directions, with more raft foundation reinforcement details taken care of at the columns and load-bearing walls.
• The thickness of the Flat Plate Mat type raft foundation section is 300mm. 

2.Plate Thickened under Columns

Fig 3: Plate thickened with Columns

• This type of raft foundation is employed when the columns and the load-bearing walls are subjected to heavy loads.
• Notably, the slab is thickened under the columns and walls in this foundation. Furthermore, some extra reinforcement is required to resist the diagonal shear.

3.Two-way Beam and Slab

Fig 4: Two-way Beam and Slab

• In this type of slab beam raft foundation, beams are cast monolithically with the raft slab connected to the columns and walls.
• Two-way beam and slab type of raft foundation are viable when the columns are placed at a considerable distance, while the loads on the columns are variable.

4.Plates with Pedestals

Fig 5: Plates with Pedestals

• In this type of raft foundation, a pedestal is provided at the base of the columns. 
• The purpose of the plates with pedestal-type raft foundations is the same as a flat plate thickened under columns.

5.Piled Raft Foundation

Fig 5: Piled Raft Foundation

• This footing is supported on different types of piles. 
• Piles placed under the raft allow for a significant reduction of the overall settlement. Besides, they also offer resistance against buoyancy.
• Piled Raft Foundation is typically employed when highly compressible soil lies at a shallow depth.
• Piled Raft Foundation is also adopted if the water table is high. 

6.Rigid Frame Mat/Cellular Raft Foundation

Fig 6: Cellular Raft Foundation

• This type of raft foundation is resorted to in cases of heavy loads carried by the columns. The connecting beams in such a raft foundation section exceed a depth of 90cm. 
• In this raft foundation, the footing walls function as a deep beam. 
• In a typical rigid frame mat footing, two concrete slabs are placed on top of each and connected with walls on both sides. The foundation thus firmed is also called the cellular raft foundation. 
• Rigid frame mat footings are extremely rigid.
• These are economical in cases when the slab thickness is very large.

Materials Used for Raft Foundation Construction

Some of the most commonly used materials for Raft foundations construction are:

• Formwork.
• Spacer.
• Reinforcement.
• Concrete.

Construction Process of Raft Foundation

The various construction steps of the Raft foundation are enumerated below

1. Determination of the depth of raft footing.
2. Excavation of the soil to a suitable depth.
3. Soil Compacting.
4. Laying a waterproofing membrane at the bottom.
5. Pouring 3" of cement-sand mix.
6. Placing the reinforcement and ensuring the predetermined spacing between the reinforcing bars.
7. The next step requires pouring the concrete to the predetermined depth.
8. Setting and Curing of concrete.

Advantages Of Raft Foundation

The various advantages of using the raft foundation are enumerated as follows:

• Raft foundation can be employed under variegated site conditions where ordinary footing isn't suitable. It can be adopted in marshy and soft soils. 
• Raft footing can be used wherein there is a significant chance of differential settlement.
• In a raft foundation, the load is distributed over a wide area. As such, the purpose of raft foundation in such cases is to reduce the overall pressure intensity exerted on the soil.

Disadvantages of Raft Foundation

• High material cost: Raft foundations require a large amount of concrete and steel, which increases the construction cost.

• Not suitable for weak upper soils: If the top soil is too soft or expansive, a raft alone may not be effective without additional piles.

• Chances of uneven settlement: In cases where loads are not distributed uniformly, differential settlement can still occur.

• Complex construction: Requires careful planning, skilled labor, and proper supervision during reinforcement, concrete pouring, and curing.

• Difficult to repair: Once constructed, any faults or damage in the raft foundation are difficult and costly to fix.

• Waterproofing challenges: In areas with a high water table, extra measures are needed to prevent water seepage, increasing cost and complexity.

• Limited depth use: Raft foundations are generally used for shallow depths. For deeper foundations, other methods may be more suitable.

The current piece of writing has enumerated different raft footings and the advantages of such foundations. We suggest our readers enrol in AE/JE Civil Coaching to avail the up-to-date study material on the Testbook App.

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