Abstract: This paper aims to study the softening characteristics of gypsum surrounding rock in the construction process of the tunnel. The multi-factor orthogonal softening experiments were carried out on gypsum rock from Lower Triassic Jialing River Formation. Then we analysed the effects of immersion time, solution temperature and concentration on mechanical parameters of gypsum rock after softening, respectively. The results showed that both the softening coefficient and the elastic modulus decreased with the increase of immersion time, furthermore their decreasing rates showed the same patterns. It was also found that they linearly decreased with increasing the solution temperature. Among these three influencing factors, the immersion time played the most critical role in softening experiments, and its weight was up to 80%. However, the effect of temperature ranked second with its weight of about 16%, and the effect of concentration was very slight. The prediction formulas were established for the elastic modulus and the uniaxial compressive strength of gypsum rock under the coupling action of the immersion time and the solution temperature. By analysing the experimental results, the construction methods and suggestions for gypsum rock tunnels were put forward from the aspects of the division of surrounding rock hardness, prevention and drainage measures and the supporting measures.

Key words: gypsum rock, softening characteristics, orthogonal softening experiment, effect weight