Extrusion 3D printing circular and horseshoe tunnel physical models: A comparative study of deformation and brittle failure

The instability of high-stress hard rock tunnels mainly presented with the deformation and brittle failure, the tunnel brittle failure's location and range are dissimilar under different stress and shape conditions. In this study, similar 3D printing materials are developed according to the brittleness index of the hard rock in an underground cavern. The physical models of circular and horseshoe tunnel with the equal area are printed, and overloading tests are carried out. The test results reveal that the horseshoe tunnel's peak strength and residual strength are decreased by 17.2% and 6.6% compared with that of the circular tunnel, the circular tunnel's vertical displacement is greater than the horseshoe tunnel's, and the depth and range of brittle failure in the circular tunnel are bigger than the horseshoe tunnel's. The circular tunnel brittle failure is mainly in the middle of both sides' walls, while the horseshoe tunnel brittle failure is mainly in the bottom of both sides' walls. The location and range of the tunnels' brittle failure can be well simulated by using the rock mass deterioration model (RDM) combined with the rock fracture degree (RFD) index, and the numerical simulation results are consistent with the physical simulation experiment results. According to the results of laboratory physical model experiment and numerical simulation, the brittle failure evolution model of hard rock tunnel is proposed, which can provide a reference for the excavation and support design of hard rock tunnel under high stress.