Abstract
The construction of shallow tunnels has become increasingly challenging due to the complex geological conditions 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.
Published Version
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