Abstract: Water saturation in rock significantly affects its mechanical parameters. However, the existing methods for preparing rock specimens with different water saturation levels have some common problems, such as low accuracy, long time, and uneven distribution of water. This paper proposed a rapid and efficient procedure to prepare rock samples with different water saturation degrees based on chemical thermodynamic principles. This method can control water saturation of rock specimens precisely by adjusting the humidity of environment with a negative pressure, which governs the fugacity of water in rock samples. Since the fugacity of water vapour in constant humidity environment is identical, the water vapour and the content water in rock samples would form a fugacity difference when the fugacity of the water vapour and the content water are different. Content water in rock samples would escape from rock samples when the fugacity of water vapour in the environment is less than that of content water in rock specimens. Otherwise, water vapour would infiltrate into rock samples. Eventually, rock specimens with certain water saturation levels are obtained when the fugacity of content water in rock specimens and water vapour in the environment are equal. Rock specimens were prepared in a negative pressure environment. Air in the porosity of rock specimens would decrease and the seepage velocity of water vapour in rock samples increases in this kind of environment, which can shorten the time of rock specimens to reach the steady state. The fugacity of content water in every component of rock specimen is equal when rock specimen reaches the steady state, according to vapour-liquid equilibrium principles. Therefore, the distribution of water in rock specimens is even.

Key words: water saturation, fugacity, constant humidity environment, negative pressure, gas-liquid equilibrium, chemical thermodynamics