Geomechanical model test for analysis of surrounding rock behaviours in composite strata

Due to the large differences in physico-mechanical properties of composite strata, jamming, head sinking and other serious consequences occur frequently during tunnel boring machine (TBM) excavation. To analyse the stability of surrounding rocks in composite strata under the disturbance of TBM excavation, a geomechanical model test was carried out based on the Lanzhou water supply project. The evolution patterns and distribution characteristics of the strain, stress, and tunnel deformation and fracturing were analysed. The results showed that during TBM excavation in the horizontal composite formations (with upper soft and lower hard layers and with upper hard and lower soft layers), a significant difference in response to the surrounding rocks can be observed. As the strength ratio of the surrounding rocks decreases, the ratio of the maximum strain of the hard rock mass to that of the relatively soft rock mass gradually decreases. The radial stress of the relatively soft rock mass is smaller than that of the hard rock mass in both types of composite strata, indicating that the weak rock mass in the composite formation results in the difference in the mechanical behaviours of the surrounding rocks. The displacement field of the surrounding rocks obtained by the digital speckle correlation method (DSCM) and the macro-fracture morphology after tunnel excavation visually reflected the deformation difference of the composite rock mass. Finally, some suggestions and measures were provided for TBM excavation in composite strata, such as advance geological forecasting and effective monitoring of weak rock masses.