Experimental study on the influence of organic matter on the soil mechanics and permeability of the base soil of open-pit coal mine

To explore the influence of organic matter content in the basement soil of open-pit coal mines on its mechanical properties，this study focuses on the Yimin open-pit coal mine in Inner Mongolia. Various tests，including bounded water content，unconfined compressive strength，direct shear，repeated shear，penetration，and SEM，were conducted on dump basement soil samples with different organic matter contents. The effects on mechanical properties，permeability，and microstructure were analyzed at both macroscopic and microscopic scales. Results indicate that the plasticity index is positively correlated with organic matter content. The peak unconfined compressive strength increases with higher organic matter，changing the failure mode from plastic to brittle. Peak shear strength also rises linearly with organic matter content，more prominently under high consolidation stress. As organic matter content increases，cohesion increases while the internal friction angle decreases. Under various normal consolidation stresses，the shear strength of plain soil samples significantly decreases after three shear cycles (16.88% to 21.24%). Adding organic matter significantly reduces the shear strength attenuation rate(2.17% to 7.25%)，which remains stable as organic matter content further increases. The permeability coefficient decreases sharply from plain soil to 5% organic matter content，significantly affected by the presence of organic matter. SEM analysis shows soil particles evolving from point-to-point and point-to-surface contact to face-to-surface contact with increasing organic matter，filling pores and making soil denser. Quantitative analysis using PCAS software reveals that microscopic porosity，fractal dimension of porosity，orientation probability entropy，and fractal dimension increase with organic matter content，while the average pore shape coefficient initially increases then decreases，peaking at 15% organic matter.