Experimental study of the dehydration-shrinkage characteristics of red-bedded mudstone in Central Sichuan under different humidity gradients

To study the problem of long-term uplift of high-speed railway subgrade in a red mudstone area, dehydration shrinkage under different relative humidity gradients was tested. The combined analysis of mineral composition and micro-fissure structure was made to assess the influences of the humidity gradient on the dehydration-shrinkage characteristics and residual deformation of mudstone. Compared with the process of gas moisture absorption-expansion, the range of humidity gradient in which irreversible residual deformation occurs significantly was determined. The results show that the dehydration-shrinkage characteristic of mudstone is time-dependent, but the dehydration process lags the shrinkage process; the dehydration rate and shrinkage rate increase with the increase of humidity gradient. Unlike the gas moisture absorption-expansion, the rate of moisture migration is higher in the dehydration process, and the shrinkage rate is much higher than the expansion rate. This sharp shrinkage is more likely to lead to residual deformation of mudstone. The larger the humidity gradient experienced by the absorption-dehydration cycle, the stronger the water-holding capacity of mudstone, indicating that the change of humidity has an irreversible effect on the structure of mudstone. The measured humidity range of irreversible residual deformation of mudstone is RH85% to RH99%. It is under the action of repeated changes in environmental humidity that the continuous accumulation of irreversible residual deformation causes the slow uplift of mudstone subgrade. The results provide a new perspective for revealing the internal mechanism of long-term uplifting of red mudstone in central Sichuan, China.