Optimization of piled raft foundations with superstructure interaction in a clayey soil using numerical modeling

Piled raft foundation systems represent a departure from conventional piled foundation solutions by primarily aiming to mitigate differential settlements, thereby achieving a more uniform and stable foundation response. In such systems, the contribution of the raft to the overall load-bearing capacity is explicitly considered, with the primary objective being the reduction of total settlements, particularly through the control of differential movements. Consequently, these systems also ensure the required level of safety in terms of bearing capacity. The literature identifies soil conditions containing stiff clays or dense sands as the most favorable environments for the implementation of such foundation systems. This thesis presents a numerical investigation into the behavior of piled raft foundations, focusing on key parameters influencing performance, including pile spacing, diameter, length, and the mode of superstructure loading. To address these complexities, the study utilized Plaxis 3D finite element software to develop two detailed numerical models representing different loading scenarios. The first model applied a uniformly distributed load directly on the raft foundation, simulating a simplified load transfer mechanism. The second model explicitly incorporated the superstructure's structural components and their interaction with the foundation, thereby capturing a more realistic behavior of the entire soil-structure system. This comparative modeling approach allowed for a critical assessment of the influence of superstructure interaction on piled raft performance. Key geometric and mechanical parameters, including pile spacing, pile diameter, and pile length, were systematically varied in the superstructure interaction model to investigate their effects on settlement, bending moments, and overall foundation performance. The pile diameter was examined across a wide range of values, while pile lengths spanned from relatively short to extended depths to simulate engagement with different soil strata. Similarly, pile spacing was varied to understand the trade-offs between settlement mitigation and economic feasibility. The numerical results demonstrated that the superstructure interaction model yielded significantly different displacement patterns compared to the uniform load model. While the uniformly loaded model exhibited maximum settlement concentrated at the center of the raft, the superstructure-interactive model showed pronounced settlements around the foundation. This shift in settlement distribution underscores the importance of considering structural effects in design analyses, as ignoring these interactions may underestimate critical displacement zones and stress concentrations. Furthermore, bending moments in the piles were notably reduced in the superstructure interaction model, approximately half of those observed in the uniform loading scenario. This suggests that the structural frame effectively redistributes loads, thereby relieving pile stresses. Such findings highlight that conventional approaches applying uniform loads directly on the raft can lead to conservative designs with potentially unnecessary pile reinforcement. Examining pile spacing revealed a nonlinear relationship with foundation settlement. Decreasing pile spacing led to reduced settlements, confirming the beneficial role of closely spaced piles in enhancing foundation stiffness and load transfer. However, the marginal gains diminished with very tight spacing, indicating an optimal spacing beyond which further reduction is economically unjustifiable. These results emphasize that pile layout optimization is crucial to balance performance and cost, avoiding over-conservative designs with excessive pile counts. In terms of pile diameter, the study found that increasing the diameter reduces settlements due to enhanced cross-sectional stiffness and load-bearing capacity. Nonetheless, beyond a certain diameter threshold, the incremental benefits plateaued, revealing diminishing returns on settlement control. This observation supports the adoption of an economically optimized pile diameter rather than indiscriminate enlargement, which would increase material costs and construction complexity without proportionate performance improvements. Interestingly, bending moments within piles were relatively insensitive to diameter variations, suggesting that diameter influences vertical load transfer more than flexural demands. The effect of pile length was also significant. Longer piles demonstrated superior performance by penetrating deeper soil layers, engaging firmer strata, and facilitating load dissipation over a greater depth, thereby reducing both settlements and bending moments. However, similar to spacing and diameter, the improvements decreased beyond a certain pile length, reflecting practical limits imposed by construction constraints and diminishing mechanical returns. This finding highlights the necessity for comprehensive pile length optimization during design to avoid excessive depths that do not justify their added cost. A noteworthy component of this study was the pile placement optimization, comparing uniform pile distributions with localized arrangements focused on zones exhibiting maximum settlement in raft-only models. Localized pile layouts achieved similar displacement control using approximately 40% fewer piles than uniform distributions. This substantial reduction in pile count demonstrates that targeted reinforcement based on displacement contours can deliver cost-effective and structurally efficient designs. The findings encourage the integration of site-specific deformation analyses in pile layout planning, promoting sustainability and economic viability in foundation engineering. In addition to geometric parameters, the study considered the mechanical behavior of the clayey soil, which exhibits complex stress-strain characteristics including consolidation, creep, and rate-dependent strength properties. The numerical model incorporated appropriate constitutive soil models calibrated against typical clay behavior to ensure realistic simulation of soil-structure interaction phenomena. This inclusion allowed for capturing non-linear soil response under loading and the redistribution of stresses around piles and raft, enhancing the reliability of the results. Moreover, the study emphasized the critical role of load path and distribution in piled raft foundations. The superstructure interaction model accounted for stiffness compatibility between the frame and foundation, which influences load sharing between raft and piles. The numerical findings confirmed that neglecting these factors could result in either overestimating or underestimating pile loads, leading to inefficient design. Hence, the integration of superstructure characteristics in foundation analysis is paramount for accurate prediction of settlements and internal forces. The research methodology also highlighted the importance of mesh refinement and element selection in finite element modeling of piled raft systems. A careful mesh sensitivity analysis was performed to balance computational cost with solution accuracy. The results demonstrated that an adequately refined mesh around pile shafts and raft edges is essential to capture stress concentrations and displacement gradients. This practice ensures the generation of robust and convergent solutions, further strengthening the validity of the numerical study Furthermore, the study addressed the influence of soil layering and heterogeneity, typical of natural clay deposits, on piled raft behavior. While the main analyses assumed homogenous clay properties, additional sensitivity checks indicated that stratification and variable stiffness within the soil profile can significantly affect load transfer mechanisms. Incorporating such variability into the numerical model can further refine settlement predictions and pile design, suggesting an avenue for future research and practical consideration. Additionally, the long-term performance of piled raft foundations in clayey soils is affected by time-dependent phenomena such as consolidation settlement and creep deformation. Although the present study primarily focused on immediate load responses, recognizing these effects is essential for realistic serviceability assessments. Advanced constitutive models and time-dependent analyses could be integrated into future works to evaluate the evolution of settlements and internal stresses over the lifespan of the structure. Overall, the research underscores the importance of incorporating superstructure interaction effects in numerical modeling for piled raft foundations, particularly in clayey soils where soil-structure behavior is complex and nonlinear. The optimized design parameters-including pile spacing, diameter, and length-must be judiciously selected to balance mechanical performance and cost efficiency. The presented methodology and results offer valuable guidance for geotechnical engineers aiming to enhance foundation designs, ensuring both serviceability and economic practicality. Future research avenues could explore dynamic loading scenarios such as seismic effects and wind loads, which impose additional demands on pile foundations. The inclusion of time-dependent soil behaviors, such as consolidation and creep in clayey soils, would provide a more realistic assessment of long-term foundation performance. Moreover, probabilistic design frameworks could be integrated to quantify uncertainties inherent in soil properties and loading conditions, thereby advancing robust optimization strategies. Finally, experimental validation through field monitoring and model testing would further consolidate the numerical findings and facilitate practical adoption. This study contributes to the growing body of knowledge on piled raft foundations by demonstrating that detailed numerical modeling and optimization can significantly improve foundation efficiency and reduce construction costs. By embracing superstructure interaction and targeted pile placement, geotechnical designs can evolve beyond conservative empirical approaches toward data-driven, site-specific solutions tailored to the complex soil-structure systems encountered in practice. Such advancements support the development of sustainable infrastructure, optimizing resource use while maintaining safety and performance standards.