Study on the failure mechanism of the shield excavation face in the sandy cobble stratum under different buried depths

Abstract One of the significant challenges faced in shield tunnel construction is the risk of collapse resulting from abrupt changes in soil conditions in advance of the working face, particularly within the sandy cobble stratum. This study aims to effectively manage these sudden variations and to further investigate the failure mechanisms associated with shield tunneling. The limit support pressure, instability patterns, and soil arch ranges at the excavation face under varying burial depths were analyzed using PFC 3D discrete element software, specifically in relation to the shield section of the Beijing Subway New Airport Line 06 cigezhuang~1# wind shaft project. The findings of this research indicate the following: (1) The limit support pressure at the shield excavation face in sandy cobble strata increases with the burial depth ratio; however, the ratio of the limit support pressure decreases as the burial depth ratio increases. (2) A burial depth ratio threshold of 1.0–1.5 suggests that no soil arch forms in the stratum ahead of the excavation face when the burial depth ratio is below this range. Conversely, a soil arch develops when the burial depth ratio exceeds this threshold. (3) When the burial depth ratio is less than the threshold range of 1.0–1.5, the longitudinal instability zone transitions from a wedge shape to a barrel shape, extending to the surface. Conversely, when the burial depth ratio surpasses this range, the longitudinal instability zone changes from a wedge shape to a bulb shape and does not extend to the surface. (4) This paper proposes a calculation model for determining the limit support pressure of the excavation face under both shallow and deep burial conditions in the sandy cobble stratum, providing a calculation formula for the limit support pressure and establishing a reference range for the calculation parameters.