Model test to investigate the failure mechanisms and lining stress characteristics of shallow buried tunnels under unsymmetrical loading

The similitude criterion for the model used in this study was derived from similarity theory and elasticity mechanics equations. An experimental program was designed and used to simulate the excavation of three unsymmetrical loading model tunnels with different bias angles. The change laws and distribution forms of structural stress and surrounding rock pressure, and the failure mechanism of lining and surrounding rock on shallow buried tunnels under unsymmetrical loading were then studied systematically. Results show that the surrounding rock pressure and structural stress both change constantly in the process of tunnel excavation, and the surrounding rock stress releases obviously near the excavation face, which presents a noteworthy biased feature and time-space effect. As bias angle increases, the difference of surrounding rock pressure between the shallow and deep side of tunnel increases, and compared with the Code method, the difference obtained from the model test and field test results are increasing at a faster pace, especially, the vertical surrounding rock pressure. Thus it can be seen that the solution of Code method underrates the bias feature of surrounding rock pressure, causing the tunnel to be underdesigned. The existence of unsymmetrical load transforms the state of structural stress, and the failure form differs in location. Moreover, owing to the effect of bias angle, the development of the morphologic traits of the lining structure should be closely monitored during tunnel construction, so as to take specific measurements. The failure process over the entire construction process can be described as follows: local movement → superficial crack in tensile area of deep side → shear slip develops in deep ground. The failure mode presents as an inverted cone, with the tunnel as the top and the slope as the bottom. And owing to the effect of bias angle, the rupture angle of shallow side is smaller than that calculated by the Code method, whereas the reverse is true for the deep side.