Types of raft foundation provide a comprehensive support system for structures built on soils with low bearing capacity. Deciding when to use raft foundation is a critical first step, often triggered by poor ground conditions where other foundations would fail. The raft foundation design is a detailed process that dictates the slab’s thickness and the specific raft foundation reinforcement details needed to distribute the building’s load evenly. Many professionals debate the nuances of a mat foundation vs raft foundation, though they are largely synonymous. A significant factor in any project is the final cost of raft foundation, which balances upfront expense against the invaluable long-term structural security it offers, preventing differential settlement and ensuring stability.
But what is a raft foundation? It is a large concrete slab that spreads the load of the building over a wide area. This helps prevent uneven settling. There are various types of raft foundations, each with its unique features. Understanding raft foundation types is crucial for selecting the right one for your project. Raft foundations are versatile, cost-effective, and efficient. This blog will explore the different types of raft foundations, their advantages, and features, helping you make an informed decision for your construction needs.
Table of contents
- What is a raft foundation?
- Types of raft foundation – Principle
- Where to use raft foundation ?
- Types of Raft foundations
- Types of raft foundations: Solid slab raft foundation
- S lab beam-type raft foundation
- Piled raft foundation
- Cellular raft foundation
- Balancing or floating raft foundation
- A dvantages of the Raft foundation
- Disadvantages of raft foundations
- Raft Foundation vs Mat foundation
- Conclusion
What is a raft foundation?
A raft foundation, also known as a mat foundation, is a solid slab that we place at a specific depth to spread across the entire structure. Shear walls and columns on the raft foundation transfer loads from the structure to the ground. Engineers typically use these foundations when the soil’s bearing capacity is low and individual footings cannot 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 foundations – Youtube video
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.
Where to use raft foundation ?
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.
Raft foundations are an excellent choice for basement buildings, as the foundation slab directly supports live loads determined by the building’s use. They are particularly advantageous in areas with poor soil and limited access to heavy machinery because crews can use light excavators to complete the excavation work efficiently.
Types of Raft foundations
We choose the type of raft foundation based on several factors, including soil bearing capacity, the building’s application, and cost-effectiveness. We 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 space the columns and walls equally to distribute the building’s load evenly. We call it a “solid slab” because it is designed as a single slab of uniform thickness. To provide strength, engineers reinforce the foundation with both a top and a bottom layer of steel.
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.
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
They 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
Engineers use slab beam-type raft foundations when loads are unequally distributed and the foundation is prone to distortions. Beams included with the slabs serve as stiffeners. To build this foundation, we reinforce the raft with two layers of mesh, one at the bottom and one at the top. These beams offer additional stiffness and guard against distortion.
Piled raft foundation
In this type of Mat Foundation, pile foundations support the raft, as the figure illustrates. 🏗️
Engineers use these foundations when the structure’s loads are extremely high, the soil’s bearing capacity is very low, and the water table is high. Piled raft foundations are ideal for supporting high-rise buildings and heavy industrial structures—like RCC chimneys, silos, and storage tanks—that typically rely on a single foundation element. Due to their high cost, builders do not commonly 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
A cellular raft consists of two-way foundation beams with both a solid ground slab below and a suspended slab on top. Intermediate beams join these upper and lower slabs, which transforms the entire foundation into a rigid, I-beam-like structure.
To cover the top slab, builders use precast soffits. They cast the slab using this permanent or sacrificial formwork and then fill the structure 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. We use cellular rafts when removing overburden increases the soil’s bearing capacity. These rafts also effectively 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.
- We prefer raft foundations in areas with low soil bearing capacity, uneven settlement, and mixed soil types. These foundations achieve their load-bearing capacity by distributing the building’s stresses over a larger area.
- In densely populated urban areas, restricted site access makes it difficult for crews to mobilize heavy equipment for other foundation techniques. However, because of their low heights, crews can build raft foundations with very little equipment, making them an ideal solution for these challenging sites.
- 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.
Raft Foundation vs Mat foundation
In the construction world, we use the terms mat foundation and raft foundation interchangeably to describe the exact same type of foundation. There is no structural or functional difference between mat foundation vs raft foundation the variation is purely semantic.
The name “raft foundation” comes from the analogy that the large, continuous concrete slab supporting the entire structure “floats” on the weak soil, much like a raft floats on water. The term “mat foundation” is a more technical description, referring to the continuous “mat” of reinforcing steel bars that crisscross throughout the concrete slab to provide tensile strength.
Essentially, both mat foundation vs raft foundation describe a large, monolithic concrete slab that covers the entire footprint of a building to distribute its load over a wide area and prevent differential settlement.
Key Takeaways
- Raft foundations distribute loads over a wide area, making them ideal for weak soil conditions.
- Different types of raft foundations include solid slab, slab beam, piled, and cellular foundations, each with unique features.
- Choosing the right type of raft foundation ensures stability, prevents uneven settling, and optimises construction costs.
- Although raft foundations are cost-effective, complex designs require careful planning and supervision to avoid high expenses.
- Understanding the types of raft foundation is essential for informed decision-making in construction projects.
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.
