Types of Raft Foundations

Types of raft foundations, their advantages and features are a critical choice in construction. Engineers select them based on bearing capacity, applicable loads, specific site conditions, and overall cost-effectiveness. A raft foundation is a continuous concrete slab; it rests directly on the soil, covering the entire footprint of the proposed structure. Consequently, this is a very commonly used foundation type. We classify raft foundation types primarily by their application.

But what defines a raft foundation? It is a large concrete slab that efficiently spreads the building’s total load over a wide area. This crucial feature helps prevent uneven settling. Various types of raft foundations exist, each boasting unique features. Understanding these specific raft foundation types is essential for selecting the right one for your project. Fundamentally, raft foundations are versatile, cost-effective, and highly efficient. This blog will explore the different raft foundation types, detailing their key advantages and important features. We will cover mat foundation variations and raft slab classifications to help you make an informed decision for your construction needs.

What is a raft foundation?

A raft foundation/mat foundation is a solid slab that is placed at a specific depth and spreads across the entire structure. Raft foundations have shear walls and columns to transfer loads from the structure to the ground. These foundations are typically used when the soil’s bearing capacity is low and it becomes challenging for individual footings to handle the loads. The raft foundation aids in transferring the entire load of the structure to a larger area. This type of foundation helps prevent uneven settling.

There are different types of raft foundations, each designed for specific needs. Knowing the various raft foundation types is essential for choosing the right one for your project. Raft foundations are cost-effective and versatile, making them a popular choice in construction. By understanding what a raft foundation is, you can make informed decisions for your building’s foundation needs.

Types of raft foundation – Principle

The raft foundation distributes the total loads from the structure over the entire area of the structure. When compared to other types of foundations used in civil construction, they can reduce soil stress. Raft foundations differ from other foundations due to this mechanism of stress distribution.

Soil Stress Calculation

stress = total load coming on the structure + self-weight of raft/ Area of raft foundation

Consider a total load is 300 T and a foundation size

Size : 20 m x 10 m

Stress on the soil = 300/200 = 1.5 t/sqm

The same structure supported with 8 individual footing

Size : 2m x 2 m

Total area = 8 x 4 = 32 sqm

Stress on soil = 300/32 = 9.375 t/sqm

This shows that same load we are getting stresses of 1.5 T/sqm for raft and 9.375 T/sqm for individual foundations.

As the contact area of the raft is more the load is distributed over a larger area and hence stresses coming on the soil are very less.

Why choose Raft Foundations?

Raft foundations are typically preferred over other foundations when one of the following situations arises.

Individual footing design and pile foundation construction can be expensive when the soil’s bearing capacity is very low.
When the soil’s bearing capacity is less and it is essential to minimise stresses that have been induced into the soil.
The columns, shear walls, and so on are so close to each other that individual footings may overlap.
Any other type of foundation may cover more than 50% of the total ground area beneath the structure.
When a possibility of unequal settlement exists.
Preferred for complex equipment foundations.
Used when the proposed structure includes basements.

Designers use raft foundations for basement buildings where live loads directly act on the foundation slabs, depending on the building’s utility. They prefer mat foundations because light excavators can complete excavations in areas with poor soil conditions and limited access to heavy excavation equipment.

Engineers choose the types of raft foundations based on various factors, including bearing capacity, applications, cost-effectiveness, and more. They broadly categorize raft foundations as follows.

Solid Slab Raft Foundation
Slab Beam Raft Foundation
Piled Raft foundation
Cellular Raft Foundation
Balancing or Floating raft foundation

Types of raft foundations: Solid slab raft foundation

In a solid slab raft foundation, engineers equally space the columns and walls, and the load distributes evenly. They design these raft foundations as slabs of uniform thickness, which is why they call them solid slab raft foundations. Builders reinforce these foundations with a bottom layer and a top layer.

Solid slab raft foundations are classified into four types.

Flat raft mat foundation
Wide toe raft
Blanket raft foundation
Slip plane rafts

Flat raft mat foundation

Flat raft mats are used for small buildings with uniform column spacing and a foundation that covers the entire structure. These foundations have bottom and top reinforcements.

Flat mat — ***Types of raft foundation – Flat raft mat***

Wide-toe raft

A wide-toe type of raft foundation is used when the structure needs to be economical. A full-size solid slab mat foundation may not be required to support the structure’s loads. In that case, a heavily reinforced toe is provided on both sides, as shown in the figure, to handle the loads.

Blanket raft foundation

Blanket rafts are used when the surface has unequal settlements or nonuniform strata. In this type of situation, stone blankets will be laid as shown in the figure on a compacted surface. The stone blankets and raft shoes help to distribute the load on the structure.

Slip plane rafts

The slip plane raft foundation has a fully compacted sand bed beneath the raft. To facilitate the transfer of loads, the sand bed size should be slightly larger than the raft size. The sides of the foundation can be filled with any compressible material.

Slab beam-type raft foundation

Slab beam-type raft foundations are used when the loads are unequally distributed and the foundation is prone to distortions. Beams included with the slabs serve as stiffeners. The raft is reinforced with two layers of mesh, one at the bottom and one at the top. The beams can offer additional stiffness and guard against distortion.

Piled raft foundation

Pile foundations support rafts in this type of mat foundation, as illustrated in the figure. Engineers use these foundations when the structure’s loads are extremely high, the soil bearing capacity is very low, and the water table is very high. Piled raft foundations suit high-rise buildings and heavy industrial structures such as RCC chimneys, silos, and storage tanks, which typically rely on a single foundation element. Because of their high cost, builders rarely use them in residential applications. Piled raft foundations eliminate the need to design a very heavy raft foundation or a very conservative pile foundation with larger depths.

Instead, they opt for a combination of an optimised raft foundation and a pile foundation capable to share the loads. Over the pile foundation, the raft foundation floats. Typically used in structures such as chimneys, silos, bunkers, and overhead storage tanks where even minor soil settlement may cause the structure to fail.

Cellular raft foundation

Builders make a cellular raft from two-way foundation beams with a solid slab on the ground below and a suspended slab on top. Intermediate beams join the upper and lower slabs, transforming the foundation into an I-beam structure.

For covering the top slab, precast soffits can be used. The top slab is cast using precast soffits or other types of permanent formwork or sacrificial formwork, and it is filled with lightweight infill blocks.

Typically used in areas subjected to heavy mining activity and with poor soil-bearing capacities. The foundations must withstand massive bending moments. They are the preferred option in these cases. Cellular rafts are used when removing overburdens resulting in increased bearing capacity. Cellular rafts can be used to control soil uplift pressure.

Balancing or floating raft foundation

Balancing rafts or floating foundations are used in areas where the soil’s bearing capacity is very low and the soil settlements must be kept within an acceptable range.

The floating foundation operates on the principle that the total weight of the soil and water removed from the excavated area must equal the weight of the proposed structure.

Advantages of the Raft foundation

Raft foundations are a safe and cost-effective alternative to other shallow and deep foundation types.
Raft foundations are preferred in areas with low soil bearing capacity, uneven settlement, and mixed soil types. The load-bearing capacity of these foundations is achieved by distributing stresses over a larger area.
In densely populated urban areas, access to the sites is frequently restricted, making it impossible to mobilise heavy equipment for foundation construction using other techniques. Raft foundations can be built with very little equipment because of their low heights.
Raft foundations, when compared to other isolated foundations, provide a much-needed option for designers in terms of limiting settlement limits within the codal provisions.
When deciding on settlement values, the designers have the option to choose higher values when compared to standard foundations. The raft foundation prevents uneven settlement.
Raft foundations are a very flexible design option that can be customised to the soil conditions and workability.
The execution of a raft foundation is simpler than that of individual footings. This, in turn, can speed up the project.

Disadvantages of raft foundations

Most of the time, raft design is not considered economically when the soil conditions are extremely poor. Complex raft foundations consume a large amount of concrete and steel and necessitate precise professional/technical supervision and workmanship. As a result, the structure is more expensive than any other alternative foundation. The soil beneath the foundation, especially near the edges, must be preserved.

Key Takeaways

Understanding the types of raft foundations is crucial for making informed decisions in construction. Raft foundations are continuous slabs of concrete that distribute loads across a wide area, preventing uneven settling and providing stability in poor soil conditions. They are versatile and cost-effective, making them popular in various construction projects. Different types of raft foundations, such as solid slab, slab beam, piled, and cellular rafts, each offer unique advantages tailored to specific structural and soil requirements. By choosing the appropriate raft foundation type, you can ensure the stability and durability of your building while optimizing construction costs and efficiency.

Conclusion

Raft foundations are an essential element in modern construction, particularly in areas with challenging soil conditions. They provide a robust solution for distributing loads evenly, preventing differential settling, and ensuring the structural integrity of buildings. Understanding the various types of raft foundations, from solid slab to piled and cellular designs, allows for tailored applications that meet specific project needs. While they offer numerous advantages, including cost-effectiveness and versatility, it is important to consider site-specific conditions and professional expertise in their design and implementation. By doing so, you can achieve a durable, stable foundation that supports your building efficiently and effectively

Types of Raft Foundations – Advantages and features

What is a raft foundation?