Biochar and Matric Suction: Modulators of Soil Resistance and Resilience Under Uniaxial Compression Loading Test

Intensive agricultural mechanization in Northeast China has exacerbated soil compaction and degraded water retention. Although biochar modifies soil hydraulics, its combined effect with matric suction on compressive behavior remains unclear. This study investigated the hydraulic and mechanical responses of repacked sandy clay brown soil to biochar (0, 0.5, 1 g kg−1) under varying matric suction (6–1000 kPa). We utilized water retention curves and uniaxial compression tests to assess mechanical properties, including pre-compression stress (σp), penetration resistance (PR), compression index (Cc) and swelling index (Cs). Additionally, an integrated model using the Entropy Weight Method (EWM), the Technique for Order Preference by Similarity to Ideal Solution (TOPSIS), and the Adversarial Interpretive Structure Model (AISM) was developed to evaluate soil resistance and resilience. Results indicated that 1 g kg−1 biochar significantly enhanced field capacity (θFC) and readily extractable water (θMRE) (p < 0.05). While individual factors influenced all mechanical properties, the biochar–suction interaction significantly affected pre-compression stress and the compression index (p < 0.05). The model identified 1 g kg−1 biochar at 1000 kPa suction as the optimal combination for maximizing soil structural stability. These findings highlight the critical role of biochar–matric suction interactions in accurately assessing and managing soil mechanical behavior.