Macro- and Microresearch on Swelling Characteristics and Deformation Mechanism of Red-Bed Mudstone in Central Sichuan, China

Exploring the development of multiscale behaviors of red-bed mudstone is beneficial for in-depth evaluation of swelling capacity and its progressive failure mechanism. In this paper, a macro-micro study was performed on rock samples in central Sichuan, China. The influence of environmental factors such as water content, degree of soaking, and gaseous moisture absorption was discussed. Spatial characterizations were finally described from pore structure and distribution. Experimental results suggested that deformation curve under water immersion had three stages of rapid growth, deceleration growth, and gradual stabilization. It can enter stable deformation within 60 similar to 80h and has short-term swelling effects, while gaseous moisture absorption presented long-term performance during 0 similar to 700 h. The difference in deformation between partial and complete immersion may be the release of swelling potential caused by humidity gradient and capillary suction. The strain-time curves during water immersion all tended to short-term swelling. Moreover, repeated drying and wetting promoted the time of shrinkage adjustment in the second cycle to be significantly shortened, resulting in a large amount of irreversible plastic deformation. Although initial water content influenced the final stable strain, it only changed the degree of deformation at initial stage. For the microscale structures, the closer to the end, the greater the porosity and the smaller the middle. The average porosity after 0, 1, and 2 dry and wet cycles reached 0.227%, 4.027%, and 6.121%, respectively. Geometric parameters such as volume and surface area of the pores enlarged with the increase of the number of cycles. Small pores gradually evolved into microcracks and large fissures. Besides, the pores developed from "tree-like" to "net-like" structures with continuous cycles.