Hygroscopic effect of high clay-content shale under temperature and humidity conditions and its impact on mechanical properties

High clay-content shale, containing hydrophilic clay minerals, is highly sensitive to environmental temperature and humidity. It readily absorbs moisture from the air, leading to increased water content and reduced mechanical strength, which poses challenges for underground structures, such as mining roadways, tunnels, and storage chambers. This study investigates the influence of temperature and humidity on the water content of high clay-content shale during its hygroscopic process and examines the evolution of its mechanical properties under variations in water content, aiming to reveal the effects of environmental temperature and humidity on the mechanical behavior of high clay-content shale. Hygroscopic experiments were conducted using a temperature and humidity chamber, with quartz sand as non-clay mineral control groups, and strength experiments were performed on reconstituted shale samples with varying water content. Results from the hygroscopic experiments showed that the equilibrium water content (EWC) of high clay-content shale decreases with lower humidity and higher temperature. When the humidity decreased from 100% RH to 80% RH, the average EWC dropped from 15.88% to 7.53%. Under high-humidity conditions (100% RH), the EWC decreased to 11.92% only after the temperature increased to 30°C. Within the experimental conditions, reducing humidity was found to be more effective than increasing temperature in reducing EWC. Based on the mechanical test results, reducing humidity can decrease the loss of uniaxial compressive strength (UCS) caused by moisture absorption from approximately 50% to 15.48%. The results indicate that humidity is the primary factor influencing the EWC and mechanical properties of high clay-content shale. Reducing humidity can significantly mitigate strength loss caused by moisture absorption, while increasing temperature plays a supplementary role. These findings provide a scientific basis for controlling temperature and humidity in underground engineering to enhance structural stability.