The swelling of rock is a key issue in engineering. With large infrastructure construction trending towards complex geological conditions, an increasing number of projects could suffer from soft-rock swelling. To solve this problem, an understanding of the evolution of damage occurring during the swelling process is essential. Argillaceous slate is a typical metamorphic rock and shows strong swelling characteristics in a water-rich environment due to its abundance of hydrophilic minerals. In this paper, argillaceous slate was selected as the research object. The evolution of damage in argillaceous slate during swelling was explored via laboratory tests that included swelling characteristic tests, soft-rock needle penetration tests and scanning electron microscope tests, and damage theory. The research findings reveal that the slate swelling process in a water-rich environment can be divided into three stages, with the swelling rate being highest in the early stage of the tests, then slowing over time and finally stabilizing. However, the strength of the argillaceous slate decreased rapidly in the early stage, and the degradation rate was generally slow in the later stage. After water absorption, the clay mineral particles begin to expand and the microscopic structure of the argillaceous slate becomes complicated, with the pore area increasing and the pore diameter decreasing. A low degree of microscopic damage was found to induce a high degree of macroscopic mechanical degradation. In addition, the damage mechanism that occurs during argillaceous slate swelling is discussed in this paper from both a microscopic and a macroscopic perspective. Indeed, a relationship between macroscopic mechanical degradation and microscopic damage to argillaceous slate was established that can provide theoretical support for further research on soft-rock swelling characteristics: the microscopic damage drives the macroscopic mechanical degradation.

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