A Comprehensive Guide for Raft Foundations in Construction

A raft foundation, also known as a mat foundation, is a thick, continuous slab of concrete that covers the entire footprint of the building, acting like a single, large footing.

This is often used when you’re constructing a building on sandy or clay soil.

This guide provides a detailed overview of raft foundations, including the different types, their uses, and the key components involved in their construction, such as formwork and admixtures.

Why Are Raft Foundations Used?

A structural engineer will recommend a raft foundation under specific circumstances where other foundation types would not be safe or practical. The primary reasons include:

• Low Soil Bearing Capacity: The most common reason is when the soil is soft, weak, or has a low capacity to bear weight. A raft foundation spreads the building's load over a much larger area, which reduces the pressure on the soil.

• To Reduce Differential Settlement: In some soils, one part of the building can sink more than another part, a dangerous phenomenon called differential settlement. A raft foundation is used in these cases as it is rigid and helps the entire structure settle uniformly.

• Heavy Structural Loads: For buildings with heavy structural loads, such as high-rise structures or buildings with basements, the individual footings required would be so large that they would overlap. In such cases, it is more economical and practical to construct a single raft foundation.

Common Types of Raft Foundation

The type of raft foundation you should be using depends on the specific load conditions and structural requirements. Here’s a breakdown of the common types of raft foundation.

• Flat Plate Mat: This is the simplest type of raft foundation. It is a solid slab of uniform thickness. It is suitable for buildings with small, evenly distributed loads from columns. This type of raft often functions as a large two-way slab.

• Plate Thickened Under Columns: This is a variation of the flat plate mat. The slab has a uniform thickness, but it is made thicker under the columns to provide extra strength and resist the higher shear forces at these points.

• Two-Way Beam and Slab Raft: This is a very rigid and strong design. It consists of a network of beams running in two directions, connecting the columns. A slab is then cast on top of these beams. This system is suitable for very heavy loads or large spans between columns.

• Cellular Raft Foundation: This type consists of two concrete slabs, one at the top and one at the bottom, connected by internal cell walls. This creates a very stiff, hollow cellular structure, almost like a box. It is used for very heavy loads or on very loose, compressible soils.

• Piled Raft Foundation: In situations with extremely poor soil conditions, a raft foundation is combined with piles. The piles are driven deep into the ground to reach a stronger soil layer, and the raft then rests on these piles. This system provides a very high load-bearing capacity.

Key Components in Raft Construction

Several critical components are involved in the construction of a raft foundation.

• Concrete: A raft foundation requires high-quality and durable concrete. The structural engineer will specify the grade of concrete. This is usually M25 or higher. This ensures it can handle the required pressure.

• Formwork: Formwork is a temporary mould used to hold the fresh concrete. It shapes the concrete into the correct form. Crews build this boundary around the slab's perimeter for a raft foundation.

• Admixtures: Admixtures are special chemicals. They are added to the concrete mix to help to improve its properties. Admixtures can make large concrete mixes easier to work with and also help control the setting time and increase durability.

Frequently Asked Questions (FAQs)

What is a raft foundation?

A raft foundation is a single, large concrete slab. It sits under the entire building. This design spreads the building's weight over a very wide area. It is used when the ground soil is weak. It is also used for buildings with very heavy structures.

What are admixtures?

Admixtures are special chemicals. They are added to the concrete while it is mixing. These chemicals improve the concrete's qualities. They can make it easier to pour. They can also change how fast it sets and improve its final strength.

Is formwork and shuttering the same?

People often use the words formwork and shuttering to mean the same thing. There is a small difference between them. Formwork is the name for the complete temporary support system. Shuttering is the part of the formwork that actually touches the wet concrete. This is usually a sheet of plywood or steel.

What is formwork and scaffolding?

Formwork is the mould for the concrete. It holds the wet concrete in the correct shape until it hardens. Scaffolding is a different kind of temporary structure. It is usually made from steel pipes. It creates a platform for workers to stand on safely. Scaffolding is also used to help support the formwork.