LOAD DEFORMATION BEHAVIOUR OF STONE COLUMN IN EXPANSIVE SOIL REINFORCED WITH GEOSYNTHETICS

Abstract

The present study investigates the improvement of weak soils using granular columns, with emphasis on the role of geosynthetic encasement and iron dust inclusion in enhancing load-bearing capacity and reducing settlement. Soft soils often exhibit excessive compressibility and inadequate strength, making them unsuitable for direct foundation support. To address these challenges, granular columns are widely employed; however, their efficiency depends on factors such as column arrangement, diameter, and confinement. This research systematically explores the behavior of both single and group columns under different conditions to identify optimum configurations for practical applications. The primary objectives of the study were to evaluate the load–deformation behaviour of ordinary and geosynthetic-encased stone columns in expansive soil, to investigate the influence of column configuration, encasement material, and iron dust stabilization on bearing capacity and settlement characteristics, and to identify the most efficient ground improvement system for expansive soils. The study employed both experimental and numerical methods. Laboratory model tests were conducted on single columns of diameters 50 mm and 70 mm, as well as group columns arranged in triangular, square, and hexagonal patterns with varying spacing-to-diameter (s/d) ratios. Both ordinary and encased stone columns were examined, using geotextile and geogrid as encasement materials. The stone column mix was prepared using stone dust, fly ash, cement, and iron dust to enhance column strength. Tests were performed under monotonic vertical loading, and settlements were measured up to 50 mm. The experimental results were further validated using numerical modeling in PLAXIS 3D. The findings revealed that untreated clay beds had very low load capacity (5.9 kN at 50 mm settlement). Single ordinary stone columns improved the load resistance, with 70 mm end-bearing columns carrying 7.2 kN and floating columns 6.5 kN. Encased single columns showed further gains, with geotextile-encased end-bearing viii columns sustaining up to 8.15 kN, about 38% higher than untreated clay. In group configurations, the triangular pattern consistently gave the best performance, followed by square and then hexagonal arrangements. Geosynthetic encasement enhanced group behavior significantly, with geotextile generally outperforming geogrid due to better fines retention and hoop stress mobilization. The inclusion of iron dust in the column mix further reduced settlement and improved strength, particularly in group columns. Overall, the study concludes that the triangular arrangement of group stone columns, encased with geotextile or geogrid and stabilized with iron dust, provides the most efficient configuration. This system achieves the highest load-bearing capacity and the least settlement, demonstrating its practical applicability for soft soil improvement. The research contributes valuable insights into the design and optimization of granular column foundations and establishes the benefits of combining geosynthetic encasement with stabilizing additives. Additionally, the experimental program was conducted using single columns of diameters 20 mm, 30 mm, and 50 mm, along with group columns arranged as double columns along the mould diameter and triangular columns with a spacing to-diameter ratio of 1. Tests were performed with and without encasement using geotextile and geogrid to assess their relative performance. Additionally, iron dust was introduced as a stabilizing additive to study its effect on settlement and strength. Load–settlement responses were recorded and analyzed to evaluate the comparative performance of each configuration. The results revealed that single columns performed best when encased with geotextile, with the 30 mm diameter column providing maximum load capacity. Group column behavior was influenced more by arrangement, with triangular patterns offering superior settlement resistance compared to double columns. Among the encasement materials, geogrid provided better confinement in group columns, whereas geotextile was more effective for single columns. The inclusion ix of iron dust consistently enhanced overall performance, lowering settlements and increasing load capacity, particularly in group column arrangements. It is concluded that the most efficient configuration for ground improvement in weak soils is the triangular group arrangement of granular columns encased with geogrid and stabilized with iron dust. This combination delivers the highest load bearing efficiency and the least settlement, offering a practical and effective solution for foundation support in soft soils. The findings provide valuable insights into the design and application of granular columns, contributing to the advancement of ground improvement techniques in geotechnical engineering.