Failure mechanism and damage constitutive model of rectangular tunnels under water-rich condition

To investigate groundwater influence on stability and rockburst mechanism of deep hard-rock rectangular tunnels, water-immersed treatment and uniaxial compressive acoustic emission (AE) experiments were conducted on rectangular tunnel specimens. Energy dissipation characteristics, AE evolution characteristics and damage evolution characteristics of rectangular tunnels were analysed under water-immersed condition. Under water-immersed condition, tunnel specimens were quite sensitive to water. Average peak stress and average peak strain energy exhibited negative exponential decay with water-immersed time. Among them, after 12 d of water immersion, average peak stress of specimens decreased by 28%. Average total strain energy decreased by 70%. Average elastic strain energy decreased by 71% and average dissipated strain energy decreased by 68%. After 62 d of water immersion, average peak stress of specimens decreased by 34%. Average total strain energy decreased by 78%. Average elastic strain energy decreased by 79% and average dissipated strain energy decreased by 75%. Water weakened bonding among mineral particles. Moreover, it undermined load-bearing capacity and diminished energy-storage properties. Under high stress, massive releasable elastic strain energy stored in natural specimens within pre-peak stage may abruptly release after peak stress. This caused rapid crack development and connection in specimens. During accumulation and release of elastic strain energy, initial failure typically occurred at sidewalls. This failure location was not affected by water. Compared with natural specimens, Specimens immersed in water for 62 d had the lowest peak values of cumulative amplitude, cumulative AE energy and cumulative AE count. After 62 d of water immersion, peak values of cumulative amplitude, cumulative AE energy and cumulative AE count of specimens decreased by 84%, 97% and 99%. Compared with AE damage model, fitting degree of energy damage model was higher. For natural specimens, fitting degree of energy damage model was 0.96. For specimens immersed in water for 12 d, fitting degree of energy damage model was 0.96. For specimens immersed in water for 62 d, fitting degree of energy damage model was 0.72. Therefore, an energy damage model had more remarkable applicability and reliability. By establishing dynamic mapping relationship between energy and damage in the model, accuracy of rockburst early warning has been significantly improved. This provided scientific basis for support structure design of rectangular tunnels and regulation of high strain energy.