Abstract
Urban subway construction inevitably causes disturbances to the rock strata. It can even cause ground collapse accidents when construction encounters a bad geological body in the stratum. To verify the influence of cavities on surface settlement and the mechanism of formation instability, the instability mechanism of cavity-bearing strata under tunnel excavation disturbance was herein studied by tests using self-designed indoor models and numerical simulations. This study was based on the concrete project case of Qingdao Metro. Two groups of experiments (with and without cavities) were designed to simulate the four-step excavation by staged unloading of an airbag. The results show that the settlement value in the cavity state was about twice as much as that in the nonvoid state at the same stage. Besides the first step of excavation, in the numerical simulation, the settlement value of the same stage is about 1.3 times that of the model test. Simulating the deformation process of the surrounding rock in model tests shows that, in the excavation of the tunnel, the collapse surface of the tunnel arch roof will be connected with the sliding surface of the formation cavity. The cavity will gradually change from a regular circle to an ellipse or may even close, resulting in a sudden increase in land subsidence or even ground collapse. This indicates that disturbance during tunnel excavation would cause greater instability in a stratum when bad geological bodies, such as cavities, exist in the stratum.
Highlights
As an important part of the development of urban underground space, the subway has greatly reduced the congestion of ground traffic
The stratum is inevitably disturbed during the construction of the subway, which further increases the possibility of stratum instability
Using the Qingdao Metro as an example, this study evaluates a shallow buried subway tunnel in a cavity-containing stratum and uses plane strain model test and numerical simulation to investigate the mechanism of tunnel instability under tunnel excavation disturbance. e process of excavation, which takes place in four steps, is simulated by the uniform unloading of an airbag. e instability of the formation cavity during tunnel excavation is observed from a two-dimensional plane. e surface settlement data during this process is recorded and compared with results of numerical simulation
Summary
As an important part of the development of urban underground space, the subway has greatly reduced the congestion of ground traffic. The current research mainly focuses on the instability of cavity-bearing strata in karst and goaf areas, and there is not much research on the development law of cavity-bearing stratum caused by urban subway construction. Using the Qingdao Metro as an example, this study evaluates a shallow buried subway tunnel in a cavity-containing stratum and uses plane strain model test and numerical simulation to investigate the mechanism of tunnel instability under tunnel excavation disturbance. The evolution process of ground damage under the influence of cavities and the law of surface settlement change are analyzed, and the instability mechanism of cavity-bearing formations under the disturbance of tunnel excavation is studied. E tunnel length is about 851.6 m and the distance between the left and right lines is between 13 and 15 m. e buried depths of the roof and floor of the experimental interzone tunnel structure are 10.5–12 m and 17.5–18 m, respectively. e cross section is almost circular; its height is 6 m and its width is 6.2 m
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