The expansion mechanism of expansive surrounding rock in Guiyang City, China.

Expansive surrounding rock, such as mudstone, poses significant risks to supporting structures like tunnel linings due to its tendency to expand. To explore the expansion mechanism of expansive surrounding rock and address underground engineering challenges associated with it, this study, based on the first phase of Guiyang Metro Line S1, samples expansive surrounding rock from the section and conducts basic physical tests, X-ray diffraction (XRD), and scanning electron microscopy (SEM) analyses on the samples. The influence of water content (0%, 2.93%, 6.84%, and 8.20%) on the mechanical properties of mudstone, as well as its expansion characteristics under natural conditions, were examined. Results indicate that the microscopic surface of mudstone is rough, with cracks distributed around pores. These elements interconnect to form natural water channels, with clay minerals and non-clay minerals comprising 47.2% and 52.8% of the sample, respectively. Mudstone contains multiple pores that gradually develop as water content increases. Mudstone expansion significantly reduces its compressive strength. When lateral constraints are applied, mudstone exhibits significantly greater expansion compared to axial and radial changes. When mudstone encounters water, the expansion pressure fluctuates significantly. Upon water absorption following dehydration, cracks in the rock sample develop rapidly, accelerating collapse. These findings offer valuable insights for the construction and design of tunnels in expansive surrounding rock formations within karst areas.