Raft foundation, also known as mat foundation, is a widely used type of foundation system in construction, particularly for structures built on poor soil conditions. In this article, we will explore the concept of raft foundations, their working principles, when to use them, various types, and the construction process.
What is a Raft Foundation?
A raft foundation is a large, thick concrete slab that supports the entire structure by distributing the building load evenly over a wide area. Reinforced with steel, it transfers the structural load to the soil beneath, minimizing the risk of uneven settlement. Typically, raft foundations cover the entire footprint of a building, making them ideal for both residential and commercial structures, as well as heavy industrial equipment.
Working Principle of Raft Foundation
The working principle of a raft foundation is based on spreading the building load across a large surface area. Here’s how it works:
- The total load from the building, combined with the self-weight of the raft, is calculated.
- This total weight is then divided by the area of the raft, resulting in the load distributed over the entire foundation area.
- Since the foundation covers a large area, the stress on the soil is significantly reduced, minimizing the chances of shear failure and settlement.
The key benefit is that raft foundations help to stabilize structures, especially on weak or uneven soil, by distributing the load evenly and reducing differential settlement.
When to Use Raft Foundation
Raft foundations are chosen in specific circumstances. Here’s when they are typically preferred:
- Low Soil Bearing Capacity: When the soil has a low capacity to bear weight.
- Large Load Distribution: When the structure’s load needs to be spread across a large area.
- Overlapping Footings: If individual footings would cover more than 50% of the area beneath the structure.
- Closely Spaced Columns: When columns or walls are placed too close to use individual footings without overlapping.
- Soil Stress Reduction: To reduce the stress exerted on the soil.
- Differential Settlement Risk: When there is a risk of uneven settlement using individual footings.
- Unpredictable Soil Conditions: When soil layers are inconsistent or contain compressible pockets.
- Basement Construction: When a basement is being constructed below the structure.
- Other Footing Types Are Infeasible: When alternative foundations are not suitable or economical.
Types of Raft Foundation
Several types of raft foundations are used depending on soil conditions, load requirements, and structural considerations. Below are the main types:
| Type of Raft Foundation | Description |
|---|---|
| Flat Plate Mat | Simple slab foundation, used when loads are relatively small, and columns are uniformly spaced. |
| Plate Thickened Under Columns | Slab thickened at column locations for added strength when columns are subjected to heavier loads. |
| Two-Way Beam and Slab | Beams are cast with the slab to connect columns, suitable for larger column distances and variable loads. |
| Plates with Pedestals | Similar to thickened plates but includes pedestals at column bases for additional support. |
| Piled Raft | Raft supported on piles, used for compressible soil and high water table areas to reduce settlement. |
| Rigid Frame Mat | Extremely rigid foundation, used for heavy loads with deep beams and two connected concrete slabs. |
Detailed Explanation of Raft Foundation Types
- Flat Plate Mat
- Simplest type of raft foundation, used for buildings with small loads and uniformly spaced columns.
- Reinforced concrete slab, generally up to 300mm thick for cost efficiency.
- Plate Thickened Under Columns
- Thickened areas of the slab under heavy columns and walls to resist diagonal shear forces.
- Extra reinforcement is added to withstand the increased stress.
- Two-Way Beam and Slab
- Beams are integrated with the slab to connect columns. This type is useful for larger column spacing.
- Effective for variable loads, providing strength and flexibility.
- Plates with Pedestals
- Pedestals are provided under columns for additional stability, especially when columns are subjected to heavy loads.
- Piled Raft
- Combines a raft foundation with piles, especially useful when soil is highly compressible or when dealing with high water tables.
- Piles reduce settlement and provide resistance to buoyancy.
- Rigid Frame Mat/Cellular Raft Foundation
- Uses two concrete slabs connected by deep beams, forming a cellular structure for extreme load-bearing situations.
- Offers high rigidity and is economical when very thick slabs are needed.
Materials Used for Raft Foundation Construction
Here are the essential materials used in the construction of a raft foundation:
- Formwork: Temporary structures to mold the concrete until it hardens.
- Spacer: Keeps the reinforcement at the desired spacing.
- Reinforcement: Steel bars or mesh to reinforce the concrete.
- Concrete: The primary construction material, poured to form the slab.
Construction Process of Raft Foundation
Constructing a raft foundation involves several critical steps:
- Site Preparation:
- Identify the desired depth for the foundation.
- Excavate the soil to the required depth.
- Soil Compaction:
- Compact the soil to provide a stable base for the foundation.
- Waterproofing Membrane:
- Lay a waterproofing membrane to protect the foundation from moisture.
- Pouring Concrete Bed:
- Pour a 3-inch layer of cement-sand mix as the base.
- Reinforcement Placement:
- Place steel reinforcement bars according to the design, ensuring proper spacing using spacers.
- Concrete Pouring:
- Pour the concrete to the desired thickness, ensuring the entire area is covered.
- Curing:
- After the concrete is poured, allow proper curing to ensure the foundation gains strength over time.
Conclusion
Raft foundations are an essential component of construction, particularly when dealing with poor soil conditions or when the structure requires a wide area of load distribution. They provide stability, minimize settlement, and ensure that the building remains secure over time. By understanding the types, when to use them, and the construction process, you can ensure the foundation is both efficient and economical for your project.
