Whether it is a residential structure or an industrial one, choosing an appropriate type of footing is one of the most serious matters in constructing a building. The foundation of any structure is the part that bears the weight of the entire structure and its load. It distribute these loads to the ground safely. Raft and Slab foundations are two conventional types of foundations, each having its own merits and demerits. This Brick & Bolt blog examines the nature of structural design and load distribution of slab and raft foundations. In addition, it gives a detailed view of their differences, uses, and scopes, construction costs, soil and site conditions. This blog article provides clarity on the scope of work required to ascertain the most appropriate foundation for the project.
What is Raft Foundation
Raft foundation, also known as a Mat foundation is a commonly used type of foundation. It is a thick concrete slab reinforced with steel, supporting columns or walls and transferring loads from the structure to the soil. This concrete slab rests on a large area of soil. In most cases, the mat foundation is spread across the entire area of the structure it is supporting. This type of foundation is a large area foundation built either solidly or in cellular form. Its tendency to tolerate pressure makes it apt for lightly loaded structures on soft natural ground or other ground where a weak zone exists.
What is Slab Foundation
Slab foundation, also known as slab-on-grade foundation is a type of foundation made up of a flat poured concrete surface. This creates the floor of the structure it supports by settling directly on prepared soil. This foundation is mostly preferred in places with warm climates and low soil moisture. The slab is often reinforced with steel bars to increase its strength and stability. The thickness of the slab ranges from around 4 to 6 inches in the centre, whereas the edges are thicker, about 2 inches wide. This allows for an additional strength around the perimeter. The slab foundation is widely used in areas with flat ground, fewer freeze-thaw cycles, and high-water tables.
Key Difference Between Raft and Slab Foundations
As discussed above, the raft foundation is a reinforced concrete slab covering the entire footprint. In contrast, a slab foundation is a reinforced concrete slab that directly supports the structure. Raft foundation is widely used in the construction of heavy structures such as warehouses, and industrial buildings. In contrast to that, slab foundation is used in residential and commercial buildings. The slab foundation is well compacted and water flows to a suitable substrate. The liquefaction effect of the raft foundation allows it to float when shaken.
Structural Design and Load Distribution
In general, the structural design for both raft and slab foundations is the same. The structural design of these two foundations includes soil investigation, structural analysis, foundation design, and settlement analysis. However, the inherent characteristics of these two types of foundations leads to differences in load distribution, settlement, and reinforcement. When it comes to load distribution, raft foundation reduces stress on the soil by spreading out over a large area. The slab foundation concentrates much of its load on a smaller area. The larger footprint of the raft foundation provides an advantage over differential settlement. In contrast, slab foundation needs additional measures to mitigate differential settlement. In terms of reinforcement, the raft foundation requires extensive reinforcement due to the larger area. On the other hand, a slab foundation requires simpler reinforcements.
Application and Usage
Raft foundations, which were once used in the construction of commercial buildings such as warehouses or supermarkets are now becoming popular in domestic construction projects. In simple terms, raft foundation is commonly used in various construction projects including large buildings, weak soils, areas with high water tables, and seismic zones. Whereas a slab foundation is preferred to provide direct support for the superstructure and its reinforcement capability provides some degree of lateral resistance. They are widely preferred in areas with stable soil conditions and limited space.
Construction and Cost Implications
Slab foundation construction begins after site preparation, minimal excavation, and formwork is installed to define the structure and shape of the entire slab. Following this, reinforcements are placed using bars and other structures. After the sand layer or gravel is compacted, plumbing, water lines, and electrical are laid down. When all these preparations are set, concrete is poured, finished and cured. Frameworks are removed after the concrete is hardened.
Raft foundation construction includes identifying the desired depth at which the foundation is to be placed. The soil is then excavated up to the required depth and compacted. Following this, a waterproofing membrane is provided and reinforcements are laid using spacers. Finally, the concrete is poured to the desired depth and cured.
In terms of cost implications, larger structures will require larger foundations which will naturally cost more. Raft foundation in poor soil need special attention, such as deeper foundations or specialised reinforcement, which can further increase the cost. A slab foundation is more cost-effective than a raft foundation as it includes a small structure. This is because of its simpler design and construction process.
Soil Type and Ground Conditions
Raft foundation is often preferred for cohesive soils. Its ability to distribute loads and reduce the risk of differential settlement makes it appropriate for cohesive soil. While slab foundation can be used in cohesive soils, careful consideration must be given to drainage and swelling. In granular soil, a slab foundation is often preferred. The raft foundation is suitable for areas with low bearing capacity, whereas a slab foundation is used in areas with moderate to high bearing capacity. Additional waterproofing and drainage measures are required in slab foundations in comparison to raft foundations.
Which Foundation Type to choose
While no single factor would be sufficient to determine which type is better, requirements such as bearing capacity, settlement, and expansive soils may provide an answer. For large structures, a raft foundation is better than a slab foundation. In terms of cost-effectiveness, the slab foundation is less expensive. Besides, factors such as groundwater level and lateral loads need to be considered before making a choice. One could conclude that the factors mentioned above and the type of project determine the most appropriate foundation type.
Conclusion
Finally, as discussed above, preference between a raft and slab foundation depends on various factors including soil conditions, structure size, budget, and local building codes. For the structural integrity and long-term performance of any building, a well-designed foundation is essential. While a raft foundation is well-suited for structures on unstable ground, a slab foundation is suitable for stable ground. With careful consideration of various factors involved in the project and consultation with a structural engineer, one can select the most appropriate foundation type for the project.