Investigation of the Performance of Granular Columns in Soft Clay Soil Using Discrete Element Method

Abstract

Ground improvement techniques are essential for enhancing poor ground conditions in construction, particularly in soft soils where low bearing capacity and high compressibility occur. Common methods include reinforcement, material replacement, and chemical stabilization, with Vibro stone columns (VSC) being the most common due to their versatility and ease of construction. VSC design methods were established in the 1970s, but research continues to enhance predictions of the improvements achievable in bearing capacity and settlement behaviour. However, the complexity of soil-column interactions and the behaviour of column materials remain a significant challenge, particularly at particle level. Primary aggregates used in VSC construction are becoming scarce, prompting the use of secondary and recycled aggregates. However, their behaviour within VSCs is not fully understood, necessitating further particle-level research. Numerical continuum modelling, such as the finite element method (FEM), have limitations in capturing particle level behaviour, thus restricting the use of alternative aggregates. This research focuses on the impact of column particle size and shape on VSC performance using the Discrete Element Method (DEM). The study evaluated the effect of particle size distribution and shape on performance after validating the DEM stone column model with published experimental data. Parametric studies were conducted on particle size, particle size distribution (PSD), and shape for different scenarios. In total, 36 DEM scenarios were examined, covering particle size/PSD and particle shape. All results were compared against the validated model to ensure consistent and reliable assessment. The results show that particle size is key for stone column performance, and PSD is also an important factor. PSD affects how the column material behaves during construction and how the improved ground responds under loading. In general, larger particles and a wider PSD improve confinement and increase the overall load-bearing response. Uniform grading can help with constructability and drainage but tends to reduce compaction effectiveness and stability. The PSD study also identified the best non-uniform grading with a higher proportion of larger particles, and this gave better performance than the validated reference case. The layered-column results further show that placing larger particles in the bulging zone can improve load resistance. Thus, this suggests that a mixed columns consisting of two PSD along the length of the column will improve overall performance. The work on Particle shape proved to be the most influential parameter, with a greater effect on performance than particle size. Angular particles develop strong interlock, producing peak axial stresses approximately 67% higher than the validated baseline. Rounded particles, by contrast, suppress interlock and reduce the mechanical response, with peak axial stress falling approximately 13% below the baseline. Rounded aggregates should therefore be avoided in applications requiring high stiffness and effective load transfer. Analysis of idealised particle shapes confirmed that interlock capacity is the dominant performance-controlling mechanism. Blended (mixed-shape) columns responded differently from single-shape configurations which demonstrates the importance of mixture effects and why shape should be assessed rather than assumed from one type. Thus, the effectiveness of stone columns is associated with combinations of particle size, size distribution, and shape. By adjusting the size and shape of these particles, stone columns can be constructed with enhanced effectiveness, stability, and durability. The findings provide a basis for more evidence-led specification and quality control of column aggregates, particularly where secondary or recycled sources are considered. This research advances understanding of particlelevel mechanisms in VSCs and supports more informed, reliable, and efficient ground improvement practice.