Short Answer:

A raft foundation (also called mat foundation) is a large concrete slab that spreads over the entire area of a structure to support multiple columns and walls. It distributes the building load uniformly over a large area, reducing pressure on the soil.

Raft foundations are used when the soil has low bearing capacity, when columns are closely spaced, or when settlement must be minimized. It is ideal for basements, heavy structures, and weak soil conditions, providing stability and reducing differential settlement across the structure.

Detailed Explanation:

Raft foundation

A raft foundation is a type of shallow foundation that consists of a thick, reinforced concrete slab covering a large area, often the entire footprint of the building. The main purpose of a raft foundation is to distribute the load of the structure evenly across the soil beneath, especially in places where the soil’s bearing capacity is low or the building loads are heavy.

Unlike isolated footings that support individual columns, raft foundations act like a “single mat” or platform, supporting all structural elements together. This method greatly reduces uneven settlement and increases the stability of the entire structure.

When Raft Foundations Are Used

Raft foundations are selected when certain site and structural conditions make other types of foundations unsuitable or uneconomical:

1. Low Soil Bearing Capacity

• If the soil is soft or weak (e.g., clay, loose sand), isolated footings may not be sufficient.
• Raft foundations spread the load over a larger area, reducing stress on the soil.

2. Closely Spaced Columns

• In buildings where columns are close together, individual footings would overlap.
• A continuous raft avoids this issue and supports all columns uniformly.

3. Heavy Structural Loads

• Rafts are used when loads from the building are very high, such as in multi-storey buildings, factories, or commercial complexes.

4. Basement Construction

• In buildings with basements, a raft acts as both the foundation and floor slab, saving time and material.

5. Waterlogged or Submerged Areas

• Rafts perform better in areas with high water tables, offering resistance to uplift and providing a sealed surface.

Types of Raft Foundations

There are different types of raft foundations based on design and loading:

• Flat Plate Raft: Simple slab used when loads are light and soil is uniform.
• Slab with Beams: Reinforced beams under the slab for better load transfer.
• Cellular Raft (Box Foundation): Hollow rafts used for very heavy structures.
• Piled Raft: Combination of piles and raft slab, used when even the raft needs extra support due to very poor soil.

Construction Process

1. Excavation of the foundation area to the required depth.
2. Placing of waterproofing layers (if needed).
3. Laying reinforcement steel as per design.
4. Pouring concrete in one or more stages.
5. Curing and finishing the top surface.

Proper care is taken to ensure uniform thickness, quality materials, and good workmanship to prevent cracking or settlement issues.

Advantages of Raft Foundations

• Reduces differential settlement.
• Simple and faster to construct for large buildings.
• Combines foundation and floor slab in one.
• Economical in poor soil conditions when compared to deep foundations.

Conclusion:

A raft foundation is a thick concrete base that supports the entire structure by distributing loads evenly, especially useful in weak soils or where loads are heavy. It is commonly used in high-rise buildings, basements, and areas with close column spacing. Raft foundations ensure structural stability, prevent excessive settlement, and are cost-effective in the right conditions.