Raft Foundation — Design Requirements and Application

A raft or mat foundation is a large concrete slab or slab-and-beam system that supports all superstructure loads via walls or columns in two or more rows and rests on a soil layer or rock. A raft foundation can be rectangular (Figure 1) or circular (Figure 3). The term “piled raft foundation” refers to a mat foundation that is supported by columns rather than underlying materials.

Mat or raft foundations are classified as shallow foundations because their depth is significantly less than deep foundations but greater than other types of shallow foundations. It is useful for controlling differential settlement and transferring loads that do not exceed the soil’s bearing capacity due to the integral action of the raft foundation.

Mat foundations are an ideal type of foundation in a variety of conditions, including low bearing capacity soil, spread footings covering approximately 70% of the structure, high structure loads, soft pockets or cavities in the soil to an unknown extent raft, and highly compressible soil that extends to great depths.

Certain requirements must be considered during raft foundation construction in order to produce an adequate design, such as a minimum depth of 50cm, a required excavation depth of approximately 2.5m, and a rebar cover of 50mm.

Raft Foundation Design Requirements

1. Mat foundation design calculations must adhere to current codes.
2. It should not be placed on topsoil.
3. Mat foundations must have a minimum depth of 50 cm according to IS 1080 standards. This is required to ensure that the soil has a safe bearing capacity, as assumed in the design.
4. The mat foundation’s depth must meet shear requirements.
5. For raft foundations with evenly spaced columns and light loads, a uniform thickness can be used.
6. Mat foundations must have a minimum cover of 50 mm per ACI 318-14, British standard, Eurocode 7, and IS 456 guidelines.
7. The reinforcement cover may need to be increased due to soil contaminants, water table fluctuations, and proximity to the foundation.
8. Place the mat foundation below the soil level to prevent swelling and shrinkage caused by seasonal weather changes. In addition, frost can endanger the foundation if it is placed at a shallow depth.
9. For mat foundations on sand, the foundation should be at least 2.5 m below the ground surface. If a shallower depth is considered, the raft’s edges settle significantly more than the interior due to the lack of sand confinement.
10. British standards require a minimum depth of 0.6m below the surrounding ground surface.
    The design of a raft foundation on sand is influenced by differential settlement, which is difficult to assess due to the raft structure’s strength and stiffness.
11. Accurate estimations of loads, moments, and forces are necessary for both current and future expansion. This is critical because once the foundation is built and settles deeply into the soil, it will be difficult to strengthen it in the future.
12. Foundation structures must support loads, moments, forces, and reactions within the soil’s safe bearing capacity.
13. Ensure the structure settles uniformly and within acceptable limits.
14. Mat foundations should ensure structural stability against overturning and sliding.
15. Foundation structures interact with the surrounding soil. As a result, the behavior of foundation structures is determined by the properties of structural materials and the soil. That is why soil investigation is required to specify strata-specific soil properties as well as settlement criteria.
16. When building a raft on saturated clay, it’s important to consider both bearing capacity and settlement to ensure proper design.
17. The structural design does not account for the weight of the raft, as it is assumed to be carried by the subsoil.
18. Ribbed rafts can be used for irregular column spacing or to save money by using a thin slab for the majority of the area.
19. For cost-effectiveness, thicken rafts at column locations and ensure sufficient depth to resist shear.

Applicability of Raft (Mat) Foundations

• Soil with low bearing capacity • 70% footing coverage
• High structure loads
• Suitable for structures such as chimneys, silos, tanks, and large machines.
• Equipment and structures prone to differential settlement.
• Soft pockets or cavities in the soil to an unknown extent.
• Watertight construction under basements below groundwater table.
• Highly compressible soil that extends to great depths.