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Railway infrastructure earthwork is subjected to traffic-induced cyclic loads, which can be particularly challenging when constructed on weak or problematic soils. To enhance the engineering properties of such soils, various soil improvement techniques, including the use of cement and lime, have been widely applied. However, these conventional methods raise environmental concerns due to their contribution to greenhouse gas emissions. As a result, there is a growing need to explore eco-friendly ground improvement alternatives. This study investigates the use of biochar as a sustainable soil reinforcement material. The shear behaviour of both unreinforced and biochar-amended soil was assessed through direct shear tests and triaxial (static and cyclic) tests. Direct shear test results showed that adding 5% biochar notably improved the soil's cohesion and internal friction angle, optimising the balance between particle interlocking and porosity. However, increasing the biochar content to 10% led to a 32.4% reduction in the internal friction angle. Static triaxial tests demonstrated that biochar reinforcement improved deviator stress and accelerated pore water dissipation compared to unreinforced soil. Meanwhile, cyclic triaxial tests revealed that higher biochar content resulted in increased accumulated axial strain and excess pore water pressure. Additionally, scanning electron microscopy analyses examined the interaction between soil and biochar particles. The findings highlight biochar's potential as a sustainable, eco-friendly soil reinforcement for enhancing ground stability in railway constructions and other geotechnical applications.