Compression test and numerical simulation research on improved red beds subgrade fillers in Sichuan–Tibet Railway

The red beds are widely distributed in Yunnan, Guizhou, Sichuan and Southeast Asia. This material is not suitable as subgrade filling due to seriously weathering and its softening characteristic after absorbing water. In the project of the section between Chengdu and Ya'an of Sichuan-Tibet Railway, the graded weakly weathered red-mudstone fragments was added to red beds subgrade fillers with specific proportion to improve size distribution and compactness of the material. The stress-strain characterization of improved red beds subgrade fillings has been obtained from laboratory test. A method of randomly generating fragment geometry model with concave convex polyhedron shape has been proposed. Based on the numerical compression model established by PFC3D, the proportion of different strong force chains during loading and probability density distribution at the peak intensity have been studied. The results show that the improved red beds subgrade fillings with the stone content of 50% performs better mechanically with good deformation characteristics, and its unconfined compressive strength is 421.9 kPa. The fragments form super chains easily by friction under bond strength of red clay and that strengthens the system. The mechanical properties degree of improved red beds subgrade fillers follows the order of the stone content 50%> 40%> 60%. Therefore, it is recommended to add 50% of weakly weathered red-mudstone fragment as the subgrade fillings when improving the red bed clay of the Sichuan-Tibet Railway.