A raft or mat foundation is a large concrete slab or a slab-and-beam system used to support a structure’s load, distributing the weight of the superstructure through walls or columns over a wide area. This type of foundation is an essential solution for buildings constructed on soil with low load-bearing capacity or where differential settlement could pose problems. In this article, we will explore the fundamental aspects of raft foundations, including their types, applications, design requirements, and considerations to ensure stability and long-term performance.
Introduction to Raft Foundation
A raft foundation is a robust structural solution for distributing heavy loads evenly over a broad area. It can be designed as a solid slab or with a combination of beams and slabs, typically used to support buildings or structures with heavy loads. Raft foundations rest on the underlying soil or rock and are designed to spread the load from walls and columns across a large surface area.
Raft foundations can be of two primary shapes:
- Rectangular (Fig. 1): Commonly used for structures with a more regular shape.
- Circular (Fig. 2): Suitable for structures with a round or irregular footprint.
In some cases, when a raft foundation is supported by piles (columns embedded deep into the ground), it is referred to as a Piled Raft Foundation. This method combines the advantages of a raft foundation with the additional support of piles, making it suitable for weaker soils or deeper foundations.
Classification of Mat Foundation
A mat foundation is typically classified as a shallow foundation, although it is deeper than many other types of shallow foundations like spread footings or slab foundations. This classification is due to the fact that the depth of a raft foundation is still relatively small compared to that of deep foundations (such as caissons or drilled shafts).
The primary role of a raft foundation is to control differential settlement, which refers to the uneven settling of a structure due to variations in the underlying soil. By distributing the load more evenly across the soil or rock beneath, a raft foundation ensures that no single area bears too much weight, thus avoiding the risk of uneven settlement.
Additionally, raft foundations help in transferring the loads of the structure without exceeding the soil’s bearing capacity, which is crucial in preventing foundation failure. The integral action of the raft—its ability to act as a single solid entity—improves the performance of the foundation under varying loads.
Applications of Raft Foundations
Raft foundations are particularly well-suited for specific conditions where other foundation types might not perform as effectively. These include:
- Low Bearing Capacity Soil: When the soil beneath a building is weak or loose, it cannot support heavy loads without excessive settlement. A raft foundation helps distribute the load over a larger area, reducing the risk of failure.
- High Structural Loads: For large buildings, high-rise structures, or industrial buildings with heavy loads, a raft foundation is ideal because it can spread the load more evenly than smaller foundations like isolated footings.
- Soft or Compressible Soil: Soils that are soft, loose, or highly compressible are prone to settling unevenly. A raft foundation can help prevent significant settlement by distributing loads across a large surface area.
- Structures with Extensive Footprint: In some cases, spread footings may cover up to 70% of the structure’s footprint. A raft foundation provides additional support in these cases, particularly when the building requires high stability.
- Sensitive Equipment or Structures: Raft foundations are beneficial for structures and equipment that are sensitive to differential settlement, such as chimneys, silos, tanks, and large machinery.
- Basements Below the Groundwater Table: For buildings with basements, especially those below the water table, raft foundations are often used to create watertight construction that prevents seepage.
- Soil with Unknown Cavities: In areas where soil conditions are uncertain or there are soft pockets and cavities, a raft foundation can help by distributing the load evenly across areas of differing soil strength.
Design Requirements for Raft Foundations
The design of a raft foundation requires careful planning and adherence to specific construction standards to ensure the safety and durability of the structure. Below are the key requirements and considerations when designing a raft foundation.
1. General Considerations
- The design calculations for mat foundations must comply with the latest building codes and standards. These codes are essential to ensure the foundation can bear the expected loads and environmental conditions.
- Raft foundations should not be placed directly on topsoil since the topsoil is often too weak and inconsistent to support large loads.
2. Minimum Depth and Excavation
- According to IS 1080, the minimum depth of a mat foundation should be 50 cm to ensure adequate bearing capacity for the soil. This depth helps to ensure the foundation rests on a stable soil layer, preventing excessive settlement.
- The excavation depth for a raft foundation typically ranges from 2.5 meters, especially when the foundation is constructed on sandy soil. If the foundation is too shallow, it risks uneven settling, particularly around the edges.
3. Reinforcement and Cover
- Reinforcement is a critical component of a raft foundation. As per standards like ACI 318-14, IS 456, and Eurocode 7, a minimum cover of 50 mm for reinforcement should be maintained to protect the steel bars from corrosion.
- In areas where the soil contains harmful chemicals, minerals, or fluctuates significantly in terms of the water table, the cover for the rebar may need to be increased to provide added protection.
4. Soil and Structural Considerations
- The foundation should be placed below the influence of seasonal weather to avoid issues like soil swelling and shrinkage, especially in clay-rich soils. Additionally, frost must not influence the foundation’s stability if it is constructed in colder climates.
- In cases where the foundation is built on sand, a minimum depth of 2.5 meters is recommended to prevent differential settlement, as the edges of the raft tend to settle more than the center in sandy soil.
5. Structural Design and Load Distribution
- The structural design of the raft must account for accurate estimations of loads, moments, and forces both for the current structure and any potential future expansions. Once the foundation is set, it is challenging to make adjustments, so precision is key.
- The foundation should distribute these forces uniformly to avoid excessive stress on any single area. Differential settlement—the uneven settling of the structure—must be minimized to prevent cracking or structural failure.
- Safety against overturning and sliding should be incorporated into the design to maintain structural integrity.
6. Raft Design Variations
- For irregular column spacing or when aiming to use a thinner slab over most of the area, a ribbed design may be adopted, where beams are added to the slab to provide additional strength.
- Alternatively, the raft can be thickened at column locations for better support, ensuring that the thickness is adequate to resist shear forces and maintain structural stability.
Conclusion
Raft (mat) foundations are a critical type of foundation used for structures that require strong load distribution, especially in conditions where the soil is weak or compressible. Their ability to spread loads across a large surface area helps prevent differential settlement and ensures the stability of heavy structures. Proper design and construction are essential for ensuring these foundations perform as expected, taking into account soil conditions, load requirements, and safety standards.