Numerical Simulation of Geogrid Encased Stone Column beneath the Oil Storage Tank Using Plaxis-3D

Abstract

Geosynthetic Encased Stone Columns (GESCs) represent a significant advancement over traditional stone columns (OSCs), particularly in improving the performance of ground support beneath circular oil storage tanks. Using PLAXIS 3D simulations, it was observed that GESCs provide enhanced settlement control and lateral restraint due to the increased lateral confinement offered by the geosynthetic encasement. The study varied key design parameters including column length-to-diameter ratio (L/D), spacing ratio (S/D), area replacement ratio, and encasement length. Results showed that longer columns, especially when paired with high geogrid stiffness, significantly reduced both long-term settlements and lateral spreading of the ground. Among the spacing ratios tested, reducing S/D from 4 to 2 led to a 45.9% reduction in settlement for short columns (L/D = 2), a 63.5% reduction for medium (L/D = 4), and up to 82.5% for the longest columns (L/D = 6), emphasizing the stronger effect of spacing with increased column length. Similarly, increasing area replacement ratio improved overall ground stability and load distribution. Regarding encasement length, extending it from 2D to 8D resulted in a 32.1% reduction in settlement, with the most substantial improvement noted between 2D and 4D; further increases beyond 4D provided only minor benefits. Therefore, a 4D encasement is considered optimal, providing effective settlement control while remaining cost-effective. Settlement profiles across the tank’s radial distance confirmed GESCs’ superior performance over OSCs in reducing ground movement. Analyzing the effect of stone column by different area replacement ratios along with improvement factor and stress concentration ratio. Overall, GESCs designed with optimal parameters of column length, spacing, area ratio, and encasement length offer increased efficiency, stability, and cost savings for oil tank foundations on soft clay.

Introduction

The growing construction demands require effective ground improvement techniques to stabilize weak soils, with granular columns being a practical and economical solution. However, Ordinary Stone Columns (OSCs) face limitations in very soft soils like marine clays due to poor lateral confinement leading to bulging and reduced capacity. To overcome these issues, Geosynthetic-Reinforced Stone Columns (GRSCs), particularly Geosynthetic Encasement Stone Columns (GESCs), have been developed. GESCs enhance lateral confinement through hoop tension while maintaining drainage and preventing soil intrusion, improving soil consolidation, reducing liquefaction risk, and minimizing settlement.

Three main numerical modelling approaches exist for stone columns: artificially increasing earth pressure coefficients, preloading with elastic assumptions, and prescribing radial displacement to simulate lateral expansion. The radial expansion approach most accurately reflects real soil-column interactions, especially lateral stress redistribution.

This study uses PLAXIS 3D finite element modeling to simulate soil behavior beneath a storage tank foundation reinforced with stone columns and granular blankets. The soil and materials are modeled using the Mohr-Coulomb failure criterion. Results show that GESCs significantly reduce settlement (up to 45%) and lateral displacement (60%) compared to OSCs and untreated soil.

Further findings indicate that closer column spacing (lower spacing-to-diameter ratios) greatly decreases settlement and increases load-bearing capacity. Higher slenderness ratios (length-to-diameter) improve stress capacity at a fixed settlement. Increasing the length of geosynthetic encasement reduces lateral displacement, with encasement lengths around 4 times the column diameter being cost-effective.

Different reinforcement configurations combining fully and partially encased columns demonstrate varying performance improvements, guiding optimal design for better ground stabilization in soft soil conditions.

Conclusion

1) Geosynthetic Encasement Stone Columns (GESCs) outperform Ordinary Stone Columns (OSCs) in settlement control, load-bearing capacity, and lateral stability beneath oil storage tanks. 2) Untreated soil shows the poorest performance, while OSCs improve settlement moderately. GESCs provide the highest improvement, sustaining greater loads while significantly reducing deformations. 3) GESCs also minimize lateral displacement, especially at mid-depths, enhancing overall structural stability. 4) Column spacing (S/D) strongly influences performance — closer spacing delivers better settlement control and more efficient load transfer. Increasing the length-to-diameter ratio (L/D) increases stress-carrying capacity, with longer columns mobilizing higher shaft resistance and end-bearing. 5) Partial encasement provides improved performance compared to OSCs, but full-length encasement offers the best results. However, partial encasement can be more cost-effective when material savings are considered. 6) Beyond a certain encasement length, improvements in displacement control become marginal, making moderate encasement depth the most practical solution. 7) Overall, full-length geogrid encasement at closer spacing with higher aspect ratios delivers the best performance, while partial encasement at optimized configuration serves as a balance between effectiveness and cost. 8) Model 2 provides the most effective settlement control, significantly reducing settlement compared to others. Models 2 and 3 both improve performance, with Model 3 balancing cost and effectiveness well. Settlement decreases with depth and radial distance across all models. 9) As the length increases, IF value increases with increase in L/D ratio and Ar, where settlement decreases. As the area ratio (Ar) increases, the stress concentration ratio rises gradually at first and then sharply, indicating that larger stone columns share more load and greatly improve soil reinforcement efficiency.

References

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Copyright

Copyright © 2025 B. Shirisha, G.V. Narsimha Reddy. 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.