Abstract: During the operation of the southwestern YD tunnel, heaving appeared in multiple sections at the bottom of the tunnel, which caused the ballastless track slab to crack, resulting in damage and instability to the track structure and affecting the driving safety. The bottom drum of a tunnel is of strong regional characteristics, and the causes of the bottom drum are various. Studies at home and abroad can only discuss some tunnels. In order to find out the cause of the floor structure heaving, the micro-macro-engineering characteristics of the purplish red gently dipping layered silty mudstone in the floor structure heaving section are examined, and its influence on the tunnel floor structure heaving is analyzed. The laboratory experiment method is used to conduct comprehensive experiments on the surrounding rock properties such as the material compositions, microstructure, swelling, shear compressive strength, compression deformation of the silt mudstone, and the ground stress and hydrogeological conditions of the surrounding rock. Experiments and investigations show that the surrounding rocks in the study area can be judged as grade IV surrounding rocks according to rock quality evaluation standards. Its rocks are mainly composed of illite, quartz, chlorite, etc. It does not contain swelling clay minerals such as montmorillonite. There are pores with a size ranging from 2 to 8 μm on the bedding plane of the rock, and there are primary cracks with a length of 3~8 μm inside the rock. The uniaxial saturated compressive strength of the rock ranges from 11.3 to 21.8 MPa, and the rock belong to softer rocks. The physical and mechanical parameters of the rocks are greatly affected by water content. As the water content increases, the cohesion decreases by 50%, the compressive strength decreases by 45%~55%, and the elastic modulus decreases by 50%. The rock mass in the study area does not contain swellable minerals and is not expansive. At the same time, the groundwater level has not changed through on-site monitoring, so groundwater and expansion are not the direct inducements of the tunnel bottom drum. In short, the geological characteristics of the surrounding rocks of the tunnel are complex and special, and the cause of the tunnel bottom drum is not necessarily single. For the analysis of a tunnel bottom drum, multiple angles need to be analyzed.