Experimental study on the collapse evolution law of unlined tunnel in Boulder-Cobble mixed formation

The construction of shallow tunnels has become increasingly challenging due to the complex geological condi-tions encountered, particularly in boulder-cobble mixed (BCM) formations found in river terraces. To understand the deformation characteristics of these formations, it is necessary to study the instability and failure evolution process of unlined tunnels in BCM formations. This research systematically investigates the failure evolution process and the variation laws of surrounding rock stress and displacement through a physical model test under 1 g conditions, focusing on the two most common external disturbance factors: excavation and rainfall. The study findings reveal a grouped pattern of movement in the collapsing movement. During the evolution process of instability, the surrounding rocks formed pressure arches multiple times, resulting in a clear development of staged deformation. The instability of the pressure arch zone leads to an expansion of the loose zone in the surrounding rocks, particularly in the tunnel vault area. To prevent the collapse of surrounding rocks effectively, it is suggested to control the stability of the first-formed pressure arch. Based on the instability evolution characteristics of the surrounding rock, strategies for preventing and controlling collapse are summarized and refined. Finally, a practical project is presented to demonstrate the positive impact of optimizing the excavation method and support system. The conclusions of this research have guiding significance for the prevention and control of tunnel collapse in BCM formations.

Automated summary

This study focuses on the instability and failure evolution process of unlined tunnels in boulder-cobble mixed (BCM) formations found in river terraces. Through a physical model test, the research investigates the variation laws of surrounding rock stress and displacement under excavation and rainfall conditions. The findings reveal a staged deformation process with the formation of pressure arches and expansion of the loose zone in the tunnel vault area. The strategies for preventing and controlling collapse are summarized based on the instability evolution characteristics of the surrounding rock.