Design effects of the use of double raft in piled foundation

Mat/raft foundations are one of the most widely adopted shallow foundation systems in structural engineering, primarily due to their ability to distribute applied loads over a relatively large soil contact area. This load distribution capability significantly reduces stress concentrations in the underlying soil, thereby lowering the likelihood of excessive settlement and differential settlement between different parts of a structure. By providing a broad bearing surface, mat/raft foundations can minimize non-uniform deformation in the subgrade and enhance the long-term serviceability of structures. For high-rise buildings and heavy structures, this method offers notable benefits in terms of stability and economy when soil conditions are favorable. However, the performance of mat/raft foundations is highly dependent on the mechanical properties of the foundation soil, including its shear strength, compressibility, and consolidation characteristics. In situations where the near-surface soils possess low bearing capacity or are prone to large settlements, mat/raft foundations alone may not provide adequate performance. In such problematic ground conditions, deep foundation systems, particularly pile foundations, become a preferred solution. Pile foundations function by transferring structural loads through weak upper layers into deeper strata that possess higher strength and stiffness. This load transfer can occur through end-bearing resistance at the pile tip, skin friction along the pile shaft, or a combination of both. The primary advantage of piles lies in their ability to bypass weak layers and engage more competent soil or rock, thus reducing settlement magnitudes and enhancing structural stability. In addition, pile foundations provide resistance against lateral loads and overturning moments, making them suitable for sites subjected to significant horizontal forces from wind, seismic activity, or earth pressure. Despite these advantages, pile foundations can be cost-intensive, and their design requires careful consideration of soil-structure interaction effects. A hybrid approach that combines the benefits of shallow and deep foundations is the piled raft foundation system. In this configuration, a raft footing is used in conjunction with a strategically arranged group of piles. The raft and piles act together to share the applied loads, with the piles reducing the settlement of the raft and the raft providing additional load distribution. This method allows for a reduction in the total number of piles compared to a fully piled foundation, resulting in cost savings while maintaining performance. The raft in a piled raft system can also bridge between piles, redistributing loads in case of differential pile settlement. By combining both systems, designers can achieve improved bearing capacity, reduced settlements, and more uniform load sharing between foundation elements. In the present study, the performance of piled raft foundations was examined through numerical simulations, focusing on the influence of raft embedment depth and pile geometry on load transfer and deformation behavior. Two raft thickness configurations were considered: 20 m × 20 m × 1.0 m and 20 m × 20 m × (1+0.4) m. Each raft was supported by 49 piles, each with a length of 10 m, installed at a constant center-to-center spacing of s = 3 m. Three pile diameters-D60 cm, D80 cm, and D100 cm-were evaluated. To investigate the role of raft embedment, two embedment depths were modeled: Df = 0 m, representing a surface-mounted raft, and Df = 1 m, representing a raft partially embedded into the ground. The analysis focused on four selected piles representative of different positions within the foundation layout. Performance metrics included axial compression forces (N, kN), shear forces (Q13, kN), bending moments (M3, kNm), skin friction forces (Tskin, kN/m), vertical displacements (Uz, mm), and horizontal displacements (Ux, mm). The numerical modeling was performed using the finite element software PLAXIS 3D, which is widely recognized in geotechnical engineering for its ability to capture complex soil-structure interaction effects. The simulation results demonstrated a clear influence of embedment depth on foundation performance. Increasing the raft embedment depth from 0 m to 1 m yielded significant reductions in vertical displacement, with Uz decreasing by approximately 18-52% depending on pile diameter and loading scenario. The increased embedment provided additional confinement to the soil beneath and around the raft, enhancing stiffness and reducing deformation. Shear forces (Q13) also decreased by 7-12% with greater embedment, indicating a beneficial reduction in lateral load effects transmitted to the piles. These findings highlight the importance of considering partial embedment in raft design, particularly in cases where controlling settlement is a priority. The use of piles in conjunction with the raft proved highly effective in controlling both vertical and horizontal movements. Compared to an unpiled raft foundation, the piled raft reduced vertical settlements by approximately 75-80% and horizontal displacements by around 15%. The analysis also revealed differences between single and double raft configurations. In double raft systems, horizontal displacements (Ux) were observed to be 8-21% smaller than in single raft systems under equivalent loading. This suggests that double raft systems offer enhanced stiffness and load distribution characteristics, making them advantageous in applications where lateral stability is critical. A notable finding from the analysis was the concentration of force effects near the pile heads. Axial forces, bending moments, and shear forces all reached peak values at or near the pile-raft interface. This concentration underscores the need for detailed structural design in the pile head region to ensure adequate strength and durability under service loads. Neglecting this aspect could lead to overstressing and potential structural deficiencies in the long term. Overall, the results of this study confirm that both raft embedment depth and the use of a hybrid piled raft configuration significantly influence foundation performance. Deeper raft embedment improves settlement control and reduces shear demands, while the inclusion of piles enhances overall stiffness and load-carrying capacity. The hybrid piled raft system offers an efficient solution for challenging soil conditions, allowing for performance improvements without the full cost burden of a fully piled foundation. These insights can guide the optimization of foundation designs for high-rise buildings and other structures in geotechnically complex environments, promoting both safety and cost-effectiveness in structural engineering practice.