A STUDY ON OPTIMISATION OF PILE RAFT CONFIGURATION FOR PERFORMANCE EVALUATION UNDER SEISMIC LOADING

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A STUDY ON OPTIMISATION OF PILE RAFT CONFIGURATION FOR PERFORMANCE EVALUATION UNDER SEISMIC LOADING Arkachari Prasad1, Gopalakrishnan N2, H C Muddaraju3 1Post graduate student, Dept. of Civil Engineering UVCE college (University) Bangalore, Karnataka, India.

2Assistant Professor Department of Civil Engineering Presidency University, Bengaluru.

3Associate Professor, Dept. of Civil Engineering, UVCE college Bangalore, Karnataka, India.

---------------------------------------------------------------------***--------------------------------------------------------------------Compared to fully piled foundations, piled rafts offer a Abstract - This study investigates the performance cost-effective solution by strategically using fewer piles to optimization of Piled Raft Foundations (CPRF) using threecontrol settlement without compromising safety. This dimensional numerical simulations in PLAXIS 3D. The aim is makes them particularly useful for high-rise and offshore to understand the effects of varying raft thickness, pile structures. Combined piled raft systems (CPRS), with rigid diameter, and pile length on total and differential pile-to-raft connections, further enhance performance settlement behavior, which are critical for the design of under vertical and lateral loads, including seismic efficient and stable foundation systems in complex soil conditions. conditions. The substructure was modeled with detailed soil stratification and appropriate constitutive model.

Depending on geometry, piled rafts are categorized as Structural loads were derived from prior structural analysis small or large, each with different design priorities— and applied to the foundation model. The numerical bearing safety in small systems and settlement reduction simulations captured soil-structure interaction through in large ones. The piled raft approach integrates structural interface elements and enabled a comprehensive evaluation and geotechnical design, offering an optimized balance of load transfer mechanisms, settlement behavior, and between performance and economy. foundation stiffness. The results reveal that increasing raft thickness from 0.5 m to 0.75 m consistently reduces total Supporting this concept, Bhaduri et al. (2020) [2] settlement by 5.67% to 7.28%, while also significantly highlighted that CPRF systems exhibit superior seismic minimizing differential settlement by up to 38.54%, performance in liquefiable soils, with shear resistance especially for smaller pile diameters. Additionally, improvements of up to 70% compared to traditional pile increasing pile diameter from 0.35 m to 0.50 m and pile groups. Dang Dinh Chung Nguyen et al. (2014) [3] length from 10 m to 14 m yielded improvements in conducted parametric studies using PLAXIS 3D and differential settlement up to 11.59% and 10.99%, demonstrated that optimized pile arrangements respectively. Among the three parameters studied, raft significantly reduce total and differential settlements and thickness showed the most pronounced impact on overall bending moments in large piled rafts. Similarly, Rabiei et foundation performance. The findings underscore the al. (2015) [7] utilized ELPLA software and showed that a importance of optimizing raft stiffness and pile non-uniform pile layout concentrated under maximum configurations for improved structural safety, serviceability, loading zones improves settlement control while and cost-effectiveness. This research offers practical design enhancing economic efficiency. insights for implementing CPRF systems in seismic and settlement-sensitive environments through an integrated 2. METHODOLOGY geotechnical-structural approach.

A three-dimensional numerical analysis was conducted Key Words: Piled Raft, PLAXIS 3D, Settlement, using PLAXIS 3D to investigate the performance of a pileDifferential settlement, Raft Thickness (Rt), Pile length raft foundation system under various loading conditions.

(Lp), Pile diameter (Pd), Foundation Optimization.

The substructure model incorporated detailed soil stratification, with material properties defined using the 1. INTRODUCTION Mohr-Coulomb model for soil and Hoek-Brown model for rock to simulate realistic nonlinear soil behaviour. The Piled raft foundations are composite systems combining a properties used in the analysis are given in Table 1. The raft and piles to support heavy structures, especially in foundation loads were obtained from a detailed structural challenging soil conditions. In such systems, both the raft analysis of the superstructure, incorporating seismic and piles contribute to load sharing, improving bearing effects as part of the load assessment to ensure realistic capacity and reducing total and differential settlements. load transfer from the superstructure to the substructure.

The effectiveness of load distribution depends on the The pile and raft elements were modeled with appropriate relative stiffness of the soil, raft, and piles. geometric and material specifications and is given in the

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3. RESULTS AND DISCUSSION

Table 2. Interface elements were introduced at critical contact zones to accurately capture the soil–structure interaction, including slip and shear transfer mechanisms.

Subsurface soil data was obtained through a detailed geotechnical investigation aimed at determining soil stratification and relevant engineering properties. Based on the findings, a representative soil profile was developed, incorporating layers of clayey sand, weathered rock, and soft rock. These parameters were used to model the ground conditions accurately for foundation analysis.

3.1. INFLUENCE OF SETTLEMENT

CPRF

ON

OVERALL

3.1.1. For a given pile length and diameter varying raft thickness i.

Pd=0.35m

The Fig 1, Fig 2 and Fig 3 illustrates the load vs. settlement behavior for a foundation system with constant pile diameter (Pd=0.35m) and pile length (Lp=10m,12m,14m), while varying raft thickness (Rt) between 0.5m and 0.75m.

This modeling approach enabled the evaluation of settlement profiles, stress distributions, and load-sharing behavior within the foundation system. By simulating multiple design scenarios, the analysis facilitated the optimization of pile-raft configurations for enhanced structural performance.

Table 1: Soil and rock parameters used in the analysis.

Parameter

Clayey sand

Weathered rock

Soft rock

Model type

MohrColumb

Hoek-Brown

Hoek Brown

Dry density (kN/m3)

18.3

23

24

Saturated density (kN/m3)

20.0

24

25

Specific gravity

2.61

-

-

Poisson's ratio (ν')

0.3

0.4

0.4

Cohesion (kN/m2)

5

-

-

Angle of internal friction

26°

-

-

Fig 1: Load vs Displacement for Different Raft Thicknesses (Pd = 0.35 m Lp = 10m).

Table 2: Material properties Parameter

Raft

Pile

Material used

Plate

Embedded beam

Unit weight (kN/m3)

24

24

Young's modulus (kN/m2)

27.39 x 106

27.39 x 106

Poisson's ratio (ν')

0.15

0.15

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From the curves (Fig 1), it is evident that increasing the raft thickness has a positive influence in decreasing the settlement at all load levels however it is not pronounced.

The maximum settlement observed for Rt=0.5m is 72.64 mm, whereas for Rt=0.75m, it is reduced to 68.52mm. This translates to a 5.67% reduction in settlement, which, indicates a clear improvement in performance.

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Pd=0.45m

The Fig 4, Fig 5, and Fig 6 illustrates the load vs. settlement behavior for a foundation system with constant pile diameter (Pd=0.45m) and pile length (Lp=10m,12m,14m), while varying raft thickness (Rt) between 0.5m and 0.75m.

Fig 2: Load vs Displacement for Different Raft Thicknesses (Pd = 0.35 m Lp = 12m) From the plotted data (Fig 2), it is clear that increasing the raft thickness reduces settlement across the full range of applied loads. Specifically, the maximum settlement for Rt=0.5m is 68.78mm, while for Rt=0.75m, it reduces to 64.75 mm. This indicates a 5.85% reduction in settlement, which highlights the beneficial impact of increasing raft thickness.

Fig 4: Load vs Displacement for Different Raft Thicknesses (Pd = 0.45 m Lp = 10m) From the curves (Fig 4), it is evident that increasing the raft thickness results in a noticeable decrease in settlement at all load levels. The maximum settlement observed for Rt=0.5m is 68.93mm, whereas for Rt=0.75m, it is reduced to 64.92mm. This translates to a 5.82% reduction in settlement, which, indicates a clear improvement in performance.

Fig 3: Load vs Displacement for Different Raft Thicknesses (Pd = 0.35 m Lp = 14m) From the plot (Fig 3), it is evident that increasing the raft thickness leads to reduced settlement. The maximum observed settlement for Rt=0.5m is 64.32mm, while for Rt=0.75m, the settlement decreases to 60.33mm. This yields a 6.21% reduction in settlement, indicating that a thicker raft contributes to improved foundation performance.

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Fig 5: Load vs Displacement for Different Raft Thicknesses (Pd = 0.45 m Lp = 12m)

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Fig 6: Load vs Displacement for Different Raft Thicknesses (Pd = 0.45 m Lp = 14m)

Fig 7: Load vs Displacement for Different Raft Thicknesses (Pd = 0.50 m Lp = 10m)

From the plotted data (Fig 5), it is clear that increasing the raft thickness reduces settlement across the full range of applied loads. Specifically, the maximum settlement for Rt=0.5m is 63.64mm, while for Rt=0.75m, it reduces to 59.72mm. This indicates a 6.16% reduction in settlement, which highlights the beneficial impact of increasing raft thickness.

From the plot (Fig 6), it is evident that increasing the raft thickness leads to reduced settlement. The maximum observed settlement for Rt=0.5m is 57.34mm, while for Rt=0.75m, the settlement decreases to 53.46mm. This yields a 6.76% reduction in settlement, indicating that a thicker raft contributes to improved foundation performance. iii.

Pd=0.50m

The Fig 7, Fig 8 and Fig 9 illustrates the load vs. settlement behavior for a foundation system with constant pile diameter (Pd=0.45m) and pile length (Lp=10m,12m,14m), while varying raft thickness (Rt) between 0.5m and 0.75m.

Fig 8: Load vs Displacement for Different Raft Thicknesses (Pd = 0.50 m Lp = 12m)

From the curves (Fig 7), it is evident that increasing the raft thickness results in a noticeable decrease in settlement at all load levels. The maximum settlement observed for Rt=0.5m is 66.81mm, whereas for Rt=0.75m, it is reduced to 62.87mm. This translates to a 5.90% reduction in settlement, which, indicates a clear improvement in performance.

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From the plotted data (Fig 8), it is clear that increasing the raft thickness reduces settlement across the full range of applied loads. Specifically, the maximum settlement for Rt=0.5m is 60.66mm, while for Rt=0.75m, it reduces to 56.89mm. This indicates a 6.21% reduction in settlement, which highlights the beneficial impact of increasing raft thickness.

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improved structural integrity. These findings affirm that increasing raft thickness enhances the performance of piled raft foundations by promoting better interaction among the raft, piles, and soil, and should therefore be considered a beneficial design strategy in geotechnical engineering practice.

3.3. INFLUENCE OF SETTLEMENT i.

CPRF

ON

DIFFRENTIAL

For a given pile diameter & raft thickness, varying pile length

Fig 9: Load vs Displacement for Different Raft Thicknesses (Pd = 0.50 m Lp = 14m) From the plot (Fig 9), it is evident that increasing the raft thickness leads to reduced settlement. The maximum observed settlement for Rt=0.5m is 53.69mm, while for Rt=0.75m, the settlement decreases to 49.78mm. This yields a 7.28% reduction in settlement, indicating that a thicker raft contributes to improved foundation performance.

3.2. KEY OBSERVATIONS The analysis of the load-settlement behavior across various configurations of piled raft foundations clearly reveals that increasing the raft thickness from 0.5 m to 0.75 m leads to consistent and measurable reductions in settlement for all combinations of pile diameter (Pd = 0.35 m, 0.45 m, 0.50 m) and pile length (10 m, 12 m, and 14 m). The observed reduction in maximum settlement ranges between 5.67% and 7.28%, indicating that a thicker raft significantly enhances the load-distribution capacity of the system. Across all applied load levels, settlements were uniformly lower for thicker rafts, regardless of the pile geometry, demonstrating a clear performance benefit.

Furthermore, the influence of raft thickness becomes more pronounced with increasing pile diameter and length, suggesting an interaction effect between raft and pile stiffness. The improved behavior can be attributed to the increased stiffness of the thicker raft, which allows it to distribute loads more evenly over a larger area and engage more effectively with the surrounding soil. This reduces stress concentrations and localized settlements, while also enabling the raft to carry a greater portion of the load before transferring it to the piles. As a result, the overall foundation system experiences reduced settlement and

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Fig 10: Influence of Pile Length and Pile diameter on Differential Settlement (Rt=0.5m) The bar graph (Fig.10) for a constant raft thickness of 0.5m illustrates the influence of increasing pile length on differential settlement across different pile diameter. At a pile diameter of 0.35m, settlement reduced by 2.07% and 3.97% for pile length of 12m and 14m, respectively. At 0.45m, reductions of 2.91% and 6.88% were recorded. The highest improvement occurred at 0.5m pile diameter, with reductions of 3.52% and 10.99%, respectively.

The bar graph (Fig 11) for a constant raft thickness of 0.75m illustrates the effect of increasing pile length on differential settlement across various pile diameters. A marginal reduction in differential settlement is observed as the pile length increases from 10m to 12m and then to 14m. At a pile diameter of 0.35m, the differential settlement decreased by 0.77% and 0.55% for pile length of 12m and 14m, respectively. For a 0.45m pile diameter, the reductions were 1.94% and 3.07%, while for 0.5m pile diameter a reduction of 2.18% and 4.81% was observed.

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settlement is critical for structural performance and longterm stability.

3. CONCLUSIONS This study presents a detailed investigation into the performance of piled raft foundations (CPRF) using threedimensional numerical simulations in PLAXIS 3D, with emphasis on optimizing raft thickness, pile diameter, and pile length under various loading conditions. The primary objective was to understand the influence of these parameters on total and differential settlement, thereby aiding in the design of an efficient and cost-effective piled raft foundation system.

The results demonstrate that raft thickness plays a dominant role in controlling both overall and differential settlements. Increasing the raft thickness from 0.5 m to 0.75 m resulted in a consistent reduction in maximum settlement across all pile configurations, with improvements ranging from 5.67% to 7.28%. This enhancement is attributed to the increased flexural stiffness of the raft, which allows for more uniform stress distribution and greater load-sharing capacity with the piles.

Fig 11: Influence of Pile Length and Pile diameter on Differential Settlement (Rt=0.75m)

3.4. KEY OBSERVATIONS

Similarly, differential settlement showed a marked decrease with increased raft thickness, reaching a maximum reduction of 38.54% for smaller pile diameters and shorter lengths. This demonstrates the raft’s increased ability to span non-uniform soil support and redistribute stresses more evenly. This effect was especially significant in cases where piles were short or of smaller diameter, highlighting the raft's compensating role.

The comparative analysis of bar graphs (Fig. 10 and Fig. 11) highlights that differential settlement in piled raft foundations is significantly influenced by three primary parameters: raft thickness, pile diameter, and pile length.

Among these, increasing the raft thickness from 0.5 m to 0.75 m emerges as the most effective strategy, resulting in the highest percentage reductions in differential settlement—up to 38.54%, particularly for configurations with smaller pile diameters and shorter pile lengths. This substantial improvement is attributed to the increased stiffness of the thicker raft, which enhances its ability to distribute loads more uniformly and control settlement variations across the foundation. The influence of pile diameter is relatively less pronounced but still beneficial; for example, at a pile length of 14 m, increasing the diameter from 0.35 m to 0.50 m yields a reduction of up to 11.59% in differential settlement, reflecting an improvement in load-bearing capacity and a reduction in stress concentrations on the raft. Meanwhile, the impact of pile length shows moderate effectiveness, with reductions in differential settlement reaching up to 10.99% for larger diameters, indicating improved end-bearing and skin friction contributions. Overall, the findings suggest that while all three parameters positively affect the control of differential settlement, raft thickness plays the most dominant role, followed by pile diameter and pile length.

These insights are essential for optimizing the design of piled raft foundations, particularly in challenging geotechnical conditions where controlling differential

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The study also analyzed the effects of pile diameter and pile length. An increase in pile diameter from 0.35 m to 0.50 m led to differential settlement reductions of up to 11.59%, owing to the higher axial stiffness and loadcarrying capacity of the piles. Increasing pile length from 10 m to 14 m showed a moderate improvement, with differential settlement reductions reaching 10.99%, primarily due to enhanced end-bearing resistance and increased shaft friction.

An important observation from the results is that the combined optimization of all three parameters i.e raft thickness, pile diameter, and pile length, yields the most favorable outcomes in minimizing settlements. However, the influence hierarchy among the parameters is evident: raft thickness has the most significant effect, followed by pile diameter, and then pile length.

The outcomes of this study provide valuable insights into the design of piled raft systems, particularly in seismic and settlement-sensitive zones. By strategically enhancing raft

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stiffness and appropriately selecting pile dimensions, engineers can achieve efficient load distribution, improved structural performance, and economic savings in foundation design. The methodology and results also emphasize the importance of integrated soil-structure interaction modeling in capturing realistic foundation behavior.

8018/© 2024 Elsevier Ltd. https://doi.org/10.1016/j.oceaneng.2024.117839 [10] Shivanand Mali

Baleshwar Singh “Behavior of large piled raft foundation on different soil profiles for different loadings and different pile raft configurations.” © Springer Nature Switzerland AG 2019 https://doi.org/10.1007/s41062-018-0193-9

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