Experimental Investigation of Stress Distribution Characteristics in Surrounding Rock during Unloading in Oblique Upper Crossing Tunnel

The construction technique of oblique upper crossing tunnels with small clear distance is difficult and dangerous. Relying on the existing oblique upper crossing tunnel project of Xinfucun tunnel, an assembly 3D-printed tunnel model was developed, and similar model tests were designed and conducted to examine the stress distribution of the surrounding rock during the unloading process, through varying both tunnel clearances and unloading methods. The results show that the surrounding rock constraint of the lower existing tunnel was reduced when the oblique upper crossing tunnel was unloading. As the clear distance between two tunnels (CDT) increased, the stress diffusion along the outer edge of the tunnel diminished. When CDT exceeded 1.5B, the arch roof stress of the existing tunnel stabilized. The stress growth rate at the bottom of the new tunnel decreased when CDT is greater tahn 2.0B. During the unloading process of the three-step and six-guide-hole method (3S-6HM), the stress growth rate of the surrounding rock decreased, and the stress diffusion range of 3S-6HM was about 31.54% of the diffusion range observed of the three-step method(3SM) and the coordination ability of the new tunnel-middle rock mass-existing tunnel system was improved. The spatial distance ratio coefficient (λ) can better reflect the unloading effect of the oblique upper crossing tunnel(OUCT) on the existing lower tunnel. When λ > 2.0, the stress at the bottom of the new tunnel increased as λ decreased, and the arch roof stress of the existing tunnel decreased when λ > 3.0. The study of the stress distribution characteristics of the surrounding rock of OUCT during the unloading process revealed the effects of CDT and excavation methods on stress diffusion within the structural system of OUCT and surrounding rock and existing tunnels. Therefore, this study is beneficial for establishing the failure mechanism of surrounding rock in oblique upper crossing tunnels.