The foundation is the ultimate part of every construction project. One cannot rise to a high-rise building without a strong foundation. Strong foundations help to bear the heavy loads above the ground. Different types of foundations are used for various construction works.
Shallow foundations can be used for midrise buildings and small-scale construction projects. On the other hand, deep foundations are required for building tall structures with heavy load-bearing capacity. In this article, we will explore the benefits, pros, and cons of piled raft foundations in detail.
What is the Piled Raft Foundation?
To support superstructure loads and effectively distribute them over competent strata, the piled raft foundation is a hybrid structure that combines piles and reinforced concrete rafts.
The raft is a suspended slab that transfers the axial and lateral loads of the superstructure to the piles, which support the loads via end bearing and shaft friction. However, a conventional raft foundation doesn’t provide support for the structure; its functionality is enhanced by the piles inserted into the soil or ground, thus resulting in a pile raft foundation.
Uses of Piled Raft Foundation
Pile raft foundations are essential for supporting heavy structures in locations where building structures are difficult to establish. The below points will discuss various benefits provided by pile raft foundations.
- Piling raft foundations support large structures and conditions where the soil is not stable or insufficient to bear the heavy load of the structure.
- Tall buildings are always constructed using piled raft foundations.
- The ideal number, placement, diameter, reinforcement, and length of piles are all established during the design phase to guarantee the stability of the structure, which is cost-effective.
How Piled Raft Foundations Work?
When the capacity of a raft alone is insufficient to meet design requirements, a mixed-piled raft foundation is an effective foundation for medium-rise buildings. Through a process of interaction between the pile, soil, and raft, a load-sharing mechanism between the pile and raft transfers the applied load.
Both the raft’s and the piles’ capacities are used for load-bearing capacities, in contrast to a traditional pile foundation design where the piles are intended to support the majority of the weight. The majority of piled raft foundations come with piles to reduce settlement.
Advantages of Piled Raft Foundation
- Piled raft foundations are an economical and cost-effective solution when the ground condition doesn’t support the bearing of raft and strip foundations that are less than 1.8 meters.
- Highly concentrated loads can be supported by suspended raft foundations, which are normally shallow reinforced concrete structures that are 225-300 mm thick.
- Usually, a piled raft foundation supports the external balcony or canopy posts, along with the entire building’s footprint.
Disadvantages of Piled Raft Foundation
- Pile raft foundations are costly compared to traditional foundations due to the involvement of more piles, reinforcement, and a large raft structure.
- Their complex design and structure require skilled labour and engineers to perform this process.
- Piled raft foundations may not be beneficial for high-rise buildings resting on soft and clayey soils.
- Seismic effects can negate the beneficial effects of piled-raft foundation on the superstructure’s response.
- If any issues arise during the construction process of piled raft foundations, additional costs and time may be needed to repair the structure.
What is the Difference Between a Pile and a Piled Raft?
Their purpose is similar when compared, but you may find some differences while understanding them much better.
| S.No | Aspect | Piled Raft Foundation | Pile |
| 1. | Foundation Structure | A piled raft foundation includes at least four piles per column to support a building’s weight. The piles are grouped under each column and secured by a pile cap. | Piles are vertical structural elements that transfer the load of the building to deeper, more stable soil layers. |
| 2. | Pile Cap | The pile cap, although touching the ground, does not bear ground pressure. It is a thick, rigid slab designed primarily to resist shear forces and is reinforced with horizontal links. | The pile cap connects the tops of the piles and distributes the load from the structure above. |
| 3. | Raft Role | The raft supports part of the building’s load and resists bending forces. It is reinforced with horizontal bars on the top and bottom but does not include links. | Not applicable |
| 4. | High Shear Force Areas | Where shear forces are high, beams with vertical links are added between columns to strengthen the foundation. | Not applicable |
| 5. | Pile Function | Piles in a piled raft reduce settlement and may increase the load-bearing capacity of the foundation. | Piles transfer loads from the structure to the soil, providing stability and minimising settlement. |
| 6. | Soil Compatibility | Friction piles are unsuitable for clay, as they may cause greater settlement. End-bearing piles, which reach gravel or rock layers, are preferred for clay to avoid excessive settlement. | Piles can be designed as friction piles or end-bearing piles, depending on soil conditions. |
| 7. | Load-Bearing Capacity | Pile groups enhance the soil’s ability to carry the load when a single footing cannot provide sufficient bearing capacity. | Piles increase the overall load-bearing capacity by transferring loads to deeper, more competent soil. |
| 8. | Primary Reinforcement | Horizontal bars reinforce the raft at the top and bottom. | Not applicable |
| 9. | Connection of Piles and Raft | The piles are grouped under each column, with the pile cap holding the group securely. | Not applicable |
| 10. | Comparison of Serving | Although both serve a similar purpose in supporting buildings, each component (pile, cap, and raft) has a unique role and may function differently depending on the soil conditions and load requirements. | Not applicable |
Design Considerations for Piled Raft Foundations
Traditional Design Approach:
To safely support the vertical portion of the total load transferred by the superstructure, the traditional design technique for piled-raft foundations involves modifying the diameter, length, and number of piles. The raft-soil contact and load-bearing capacity is considered.
A Creative Approach:
The new modern design approach involves both the raft and piles in the piled raft foundation process. This design approach’s primary goal is to balance the raft’s and pile’s physical properties of being stiff and resisting bending. The load sharing between the raft and pile is determined by their respective stiffness.
Conclusion
In conclusion, the combined performance of both the raft and piles in giving stiffness and resisting bending enhances the structural integrity of the tall-rise building. So, a pile raft foundation is a suitable combination of the raft and pile to achieve the stability of heavy-load-bearing structures.
FAQ’s
The primary purpose of piled raft foundations is to support heavy-load-bearing structures and reduce soil settlement.