Numerical Analysis of Pile-Raft Foundations on Soft Soils Improvement Using PLAXIS 3D

Abstract

Soft soil deposits present serious challenges to foundation design due to excessive compressibility and low load-carrying capacity. To overcome these limitations, deep foundation systems such as piles and pile–raft foundations are commonly adopted to enhance stiffness and control settlement. The present study evaluates the performance of raft, pile, and pile–raft foundation systems resting on soft soil through three-dimensional finite element modelling. Numerical simulations were performed using PLAXIS 3D to analyse settlement behaviour under an applied pressure of 500 kN/m². A parametric investigation was conducted by varying pile diameters (0.8 m, 1.0 m, and 1.5 m) and embedment depths (10 m, 20 m, and 30 m). The findings demonstrate that pile–raft foundations provide an effective and practical solution for mitigating excessive settlement in soft soil conditions.

Introduction

Soft soils such as clay and silt, commonly found in coastal and floodplain regions, have low bearing capacity and high compressibility, leading to excessive settlement under structural loads. While deep pile foundations are often used in such conditions, designing piles to carry the entire load can be uneconomical. A pile–raft foundation offers a more efficient alternative, where both the raft and piles share the load, improving stiffness and reducing total and differential settlement.

Objectives

The study aims to:

• Model and analyze raft, pile, and pile–raft systems using PLAXIS 3D.

• Investigate the effects of pile diameter (0.8 m, 1.0 m, 1.5 m), pile length (10 m, 20 m, 30 m), and pile configuration (4×4 group).

• Optimize pile dimensions for economical and efficient load distribution.

• Compare performance under identical loading conditions.

Literature Review

Previous research highlights that:

• Pile–raft foundations effectively reduce settlement without requiring piles to carry the full load.

• Performance depends on pile number, length, diameter, spacing, and soil–structure interaction.

• The system is cost-effective compared to conventional pile foundations.

• Finite Element Method (FEM) analysis is widely used for settlement evaluation.

• Piles primarily act as settlement reducers rather than full load-bearing elements.

Numerical Modelling

A 3D finite element analysis was conducted using PLAXIS 3D:

• Soil domain: 60 m × 60 m × 60 m.

• Raft size: 15 m × 15 m.

• Pile group: 4 × 4 configuration with spacing of 3D.

• Load applied up to 500 kN/m².

• Pile diameters: 0.8 m, 1.0 m, 1.5 m.

• Pile lengths: 10 m, 20 m, 30 m.

• Plastic analysis considered soil yielding and stress redistribution.

Results

• Raft foundation alone showed excessive settlement in soft soil (≈280 mm at 500 kN/m²).

• Pile foundations reduced settlement, and performance improved with increasing pile length and diameter.

  • For 0.8 m diameter piles at 500 kN/m²: settlement reduced from 180 mm (10 m) to 140 mm (30 m).

  • For 1.0 m diameter piles: settlement reduced to 125 mm (30 m).

  • For 1.5 m diameter piles: settlement further reduced to 86 mm (30 m).

• Pile–raft foundation demonstrated the best performance due to load sharing between raft and piles, improved stiffness, and greater shaft resistance mobilization with longer piles.

Conclusion

1) The pile foundation significantly improved performance through mobilization of shaft friction and end- bearing resistance. • For 0.8 m diameter piles, settlement reduction increased from 35.71% (10 m length) to 50.00% (30 m length). • For 1.0 m diameter piles, reduction ranged from 48.21% to 55.36%. • For 1.5 m diameter piles, reduction further improved from 58.93% to 69.29%. 2) The pile foundation demonstrated superior composite action compared to pile-only systems. The interaction between raft and piles resulted in efficient stress redistribution and improved stiffness mobilization. • For 0.8 m diameter, settlement reduction ranged from 44.64% to 57.14%. • For 1.0 m diameter, reduction increased from 50.00% to 64.29%. • For 1.5 m diameter, the improvement reached a maximum of 75.0% for 30m length. 3) The rate of settlement reduction increases not only by the pile diameter, while the increase of length of the pile becomes comparatively moderate beyond 20 m. This suggests that diameter plays a dominant role in stiffness enhancement at higher depths. 4) Overall, the comparative analysis confirms that the pile–raft foundation system provides the most efficient and reliable solution for soft soil conditions, offering enhanced bearing capacity, reduced settlement, and improved structural stability when compared to isolated raft or pile foundations.

References

[1] IS 2911 (Part 1/Sec 2) : 2010 - DESIGN AND CONSTRUCTION OF PILE FOUNDATIONS — CODE OF PRACTICE. [2] IS: 2950 (Part I) -1981 (Reaffirmed 2008) - CODE OF PRACTICE FOR DESIGN AND CONSTRUCTION of raft foundation. [3] Vijaykumar, Dr. S K Prasad “Parametric Study of Pile-Raft foundation System Using PLAXIS-3D “ISSN : 2349-5162 l ESTD Year : 2014. [4] Akshay Wadhwa, Er. Mohd Aamir “A Study on Piled Raft Foundation: a complete Reviewing” e-ISSN: 2395 -0056, p-ISSN: 2395-0072 [5] Thasleena Haris and Niranjana K, “Finite Element Analysis of Pile-Raft System Using PLAXIS 3D” Thejus Engineering College Vellarakkad, Thrissur, Kerala, India– 680 584. [6] Phung Duc Long Prediction of Piled Raft Foundation Settlement – A Case Study. [7] Amey R. Khedikar, Saurabh M. toned Dynamic Analysis of Multistoried Building Resting on a Combined Pile Raft Foundation [8] Vijaykumar, S K Prasad and Chethan L Amey R. “STUDY ON MECHANISM OF LOAD TRANSFER IN PILE-RAFT FOUNDATION”

Copyright

Copyright © 2026 P. Devisri, Dr. K. Ramu. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.