# Role of Crushable Biochar in the Micro and Macro Mechanical Behaviour of Biochar-Amended Soil: A DEM Study

## TL;DR

This study uses simulations and experiments to show how adding biochar to soil affects its compressibility and mechanical behavior at both micro and macro levels.

## Contribution

The study introduces a DEM model that captures biochar particle crushing and its impact on soil mechanics, revealing stress-dependent fragmentation stages.

## Key findings

- Moderate biochar content reduces soil compressibility by forming stable force chains with larger contact areas and higher stiffness.
- Exceeding 40% biochar content increases compressibility due to intensified particle crushing under high void ratios.
- Biochar fragmentation occurs in three stress-dependent stages, with larger particles showing higher fragmentation rates and weaker soil skeletons.

## Abstract

This study investigates the microscale mechanisms underlying the compressibility of biochar-amended soils through combined discrete element method (DEM) simulations and laboratory consolidation tests. A three-dimensional discrete element model was established based on the MatDEM platform, accounting for the particle crushing process of biochar particles and its impact on soil mechanical properties. The biochar agglomerate particles generated in the simulation exhibit irregular morphology, and particles within different size ranges were selected for investigation. According to the model and experimental results, the average relative error is about 7%. Results demonstrate that moderate biochar content effectively reduces soil compressibility by enhancing load transfer through stable force chains formed by biochar particles, which exhibit larger contact areas and higher stiffness compared to native soil particles. However, when the biochar content exceeds approximately 40%, particle crushing intensifies, particularly under high initial void ratios, leading to increased soil compressibility. Furthermore, a larger initial void ratio weakens interparticle confinement, promotes microcrack propagation, and thereby exacerbates compressive deformation. Biochar fragmentation progresses through three stress-dependent stages: initial compaction (<100 kPa), skeletal damage (100–800 kPa), and crushing saturation (>800 kPa). Increased biochar particle size correlates with higher fragmentation rates, refined particle gradation, and reduced coordination numbers, collectively weakening the soil skeleton and promoting deformation. These findings underscore the importance of optimizing biochar content and applying graded loading strategies to balance enhanced soil performance with material integrity. These findings emphasize the necessity of optimizing biochar application rates to balance enhanced soil performance with resource efficiency, providing critical insights for sustainable geotechnical practices.