Effect of unloading rate on the mechanical and permeability characteristics of shale in Mazui tunnel: an experimental study

In order to in-depth understanding of the influence law of the unloading rate on the mechanical and permeability properties, the triaxial unloading tests were conducted on the shale from Mazui tunnel under different unloading rates by using Geotechnical Consulting & Testing Systems, and a prediction model of unloading stress-strain relationship based on PSO-RF algorithms was proposed. The results showed that the maintenance time of the stress platform stage decreased as a power function with the unloading rate. The strain and confining pressure followed a logarithmic distribution, where ε 1 increases slowly and is insensitive to the variation of unloading rate, ε v sharply increases after reaching the yield point, with the magnitude of increase is positively correlated with the unloading rate. Meanwhile, the k first rises slightly and then undergoes a abrupt change, whit the abrupt change point always lags behind the yield point; The k max exhibits a quadratic relationship with the unloading rate, while the relationship between k and ε v shows a quadratic increase. Furthermore, the goodness of fit (R 2 ) of the PSO-RF model was above 0.9988, and the maximum MSE was only 8.2682; Moreover, the fitting effect of PSO-RF model was better than that of the SVM model and the BP neural network model.