Abstract
Soft soil poses more challenges to the construction of large-diameter shallow-buried tunnels than other soils: the surrounding rock mass is more easily disturbed, and the tunnel stability is more influenced by various factors. This paper focuses on the excavation of a large-diameter shallow-buried twin-tunnel in soft ground using the CRD (Cross Diagram) method and the four-step method. Based on field monitoring results, it analyzes the effects of both construction factors, such as tunneling methods and construction speed, and environmental factors, such as soil conditions and continuous rainfall, on the ground surface and tunnel vault settlements. For construction factors, it is indicated that the ground surface settlement caused by the four-step method is 332% of that caused by the CRD method in argillaceous siltstone. When the twin-tunnel is excavated from opposite directions, the shape of the ground surface settlement curve of the second tunnel excavation is affected by whether or not the surface settlement caused by the first tunnel excavation is stable. For environmental factors, it is indicated that the ground surface settlement is generally large (most of the monitoring values are larger than 100 mm) with a long duration under poor soil conditions. In muddy silty clay, the range of the longitudinal surface settlement affected by the tunnel excavation face is between −0.5D and 4D, and the ratio of the vault settlement to the surface settlement is less than 1. That ratio obtained when the CRD method was used in muddy silty clay is less than that obtained when the four-step method was used in the weathered argillaceous siltstone. In other words, the lower the strength of the soil is, the smaller the ratio of the vault settlement to the surface settlement becomes. The CRD method-induced ground surface settlement in muddy silty clay is 324% of that in weathered argillaceous siltstone. Continuous rainfall increases the load on the upper soil while reducing the strength of the upper soil and surrounding rocks, which causes significant deformation of the surface settlement within a short period of time.
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