Physical Model Test and Evaluation for the Tunnel Stability Influenced by Magnitude and Path of Loading

The stability of an underground tunnel is closely related to its stress path. Here, a total of twelve load levels with three load paths are designed to analyze the influence of magnitude and path of stress on the stability of an underground tunnel from the perspective of tunnel displacement characteristics, stress distribution, and crack propagation by physical model testing and numerical simulation. The results show that the cracks are generated from the corner of the sidewall first by the vertical maximum principal stress; the larger the initial loading, the higher the initial crack position in the sidewalls of the tunnel, and the tendency of the cracking to converge to the arch foot is also more obvious. As the applied pressure increases, the average length and depth of cracking of the sidewall, the radial strain, and the circumferential strain all undergo an exponential increase; the average length grows faster than the depth, and the growth in radial strain is faster than that of the circumferential strain. When tunnels with different loading paths are subjected to the same applied pressure, and if the load rate is low because of progressive damage, it leads to damage at greater depths, but the displacements are relatively small. On the contrary, when the load rate is rapid, under the effect of a certain dynamic load, the damage is more severe, and the displacements are larger; but, because the load is borne by the entire model, the depth of the damage is relatively small. It is important to note that the effect of the stress path applies only within certain stress magnitudes. With increasing stress, the effect of the stress path diminishes.