Multiscale investigation on the shear behaviour of lime-basalt fibre modified red mudstone as fill material for high-speed railway

Due to financial and topographical constraints, red mudstone and its lime-modified counterpart have been used as fill materials for the bottom layer of railway subgrades. However, the addition of lime increases the brittleness of the modified red mudstone, potentially leading to subgrade stability issues. This paper proposed the use of basalt fibre to improve the ductility of lime-modified red mudstone fill material. A series of direct shear tests were carried out to study the effect of water contents, modification methods (untreated, lime-basalt fibre modified with different lime and basalt fibre contents etc.) on the shearing behaviour of red mudstone fill material. Then, scanning electron microscopic tests were conducted to clarify the modification mechanism of lime-basalt fibre treatment. Results show that the addition of lime-basalt fibre could significantly improve the water sensitivity and dilation behaviour of red mudstone. The optimal fibre content would be 0.2 % with a lime content of 4 % and the corresponding cohesion is over 14 times than the cohesion of the untreated red mudstone. Meanwhile, with the increase of the basalt fibre content from 0 % to 1.0 %, the ductility of specimens improved, especially under the 25kPa vertical stress condition, evidenced by the decrease in brittleness index and dilatancy angle from 2.31 to 1.93 as well as 23 degrees to 10 degrees, respectively. The improvement is attributed to the formation of 3D- grid structures in lime-basalt fibre modified red mudstone, which enhance the connection between particles. The hydration product of lime adhering to the fibre surface would further enhance the performance of these 3D-grid structures. This study provides key insights into resolving subgrade stability issues via lime-basalt fibre modification in red bed regions of Southwest China, and introduces an innovative approach for the resource utilisation of red mudstone construction solid waste.