Multiscale study on the shearing behaviour of unsaturated basalt fibre modified red mudstone as subgrade fill material

High-speed railway development in Southwest China often involves traversing vast areas of red mudstone, a material with poor mechanical properties unsuitable for subgrade applications. This study proposes the use of basalt fibre to enhance the shearing performance of the unsaturated red mudstone fill material over a broad range of degree of saturation. A series of direct shear tests were conducted to analyse the influence of degree of saturation and basalt fibre content, accompanied by scanning electron microscopy to investigate the underlying microscale mechanisms. The results show that the cohesion of untreated red mudstone initially increases but subsequently decreases with rising degree of saturation, while the internal friction angle consistently diminishes. An increased degree of saturation also leads to a reduced brittleness and a lower dilatancy angle. The observed shearing behaviours are classified into three distinct types: softening-dilation, hardening-dilation, and hardening-contraction. The maximum cohesion occurs at the optimal degree of saturation and fibre content, although the internal friction angle decreases as fibre content and degree of saturation increase. Furthermore, the brittleness index reduces considerably with a higher fibre content, and the hardening-dilation stage occurs earlier compared to untreated red mudstone. The improvements are attributed to the formation of 3D-grid structures within the basalt fibre modified red mudstone, enhancing particle connectivity. This study provides essential insights into addressing subgrade stability challenges in red bed regions through basalt fibre modification, while also presenting a sustainable approach to repurposing red mudstone construction waste in engineering applications.