Cinder gravel, a porous, lightweight, and durable volcanic byproduct, has the potential to be a sustainable and cost-effective alternative to conventional stone columns for ground improvement applications. Its use in soft soils, however, requires sufficient confining pressure to prevent bulging and thus performance degradation. Geotextile-encased cinder gravel (GECG) columns are therefore an innovate method to overcome this, however their bearing response and pressure-deformation characteristics have received limited study. This paper presents a comprehensive numerical analysis for GECG columns using a coupled discrete element and finite difference method (DEM-FDM). The hybrid DEM-FDM framework enables the simulation of individual particle behavior while maintaining efficiency in modeling continuous, homogeneous materials. The key novelties are examining the macro and mesoscopic behavior of GECG columns under triaxial compression. To do so, the development of the numerical model is introduced, followed by its validation and calibration against triaxial test results. Subsequently, a parametric analysis of GECG columns investigates the influence of relative density and gradation on the compression behavior and load capacity. Upon triaxial compression, the findings reveal a significant radial expansion near the column top, with stress and deformation fields aligning with the column's bearing capacity. The relative density exerts limited influence on the geotextile's radial deformation, and the higher content of coarse particles in the gradation enhanced the bearing capacity of the GECG columns.

The present paper aims to compare the behaviour of dense granular soils inside and outside the Shear Band (SB) during the Plane Strain Compression (PSC) test using the Discrete Element Method (DEM). The flexible membrane in the PSC test was modelled by adopting a discrete element-finite difference (DEM-FDM) hybrid approach. In contrast with previous 2D studies, the present study employed 3D simulations to consider the effects of out-of-plane characteristics such as the intermediate principal stress. According to the findings, the minimum and intermediate principal stresses outside the SB were twice as high as those inside at the residual state. It was also found that mechanical coordination numbers and redundancy indexes both inside and outside the SB decreased up to the onset of the residual phase, with a higher reduction rate inside the SB. As far as fabric anisotropy is concerned, the degree of anisotropy increased both in and out the SB until the peak state, with a somewhat higher rate inside the SB. A further finding was that the distribution of normal contact forces outside the SB remained symmetrical around the vertical axis after shear banding, however, it became asymmetrical inside the SB.