Impact of parallel construction schemes on the stability of surrounding rock in closely spaced multi-line tunnels
Objectives Closely spaced multi-line tunnels are widely used in rail transit engineering but pose challenges due to complex construction procedures and numerous influencing factors during excavation.Therefore,it is crucial to design a rational parallel excavation scheme for closely spaced multi-line tunnels.Methods This study evaluates two parallel excavation schemes with different excavation sequences by estab-lishing a three-dimensional finite element numerical simulation model.The stress and displacement evolu-tion patterns of the surrounding rock are compared,and the simulation results are validated with field moni-toring data to assess model accuracy and scheme feasibility.Results The excavation sequence significantly impacts the interaction between the surrounding rock,with the middle tunnel experiencing the greatest stress and deformation.The"middle tunnel first"method effectively mitigates stress concentration,with de-formation values under both methods remaining within safety limits.For horizontal stress at the arch haunch,the variation patterns of the sequential and"middle tunnel first"methods are generally consistent,while the vertical stress at the arch crown in the sequential method is approximately 40%higher.Compared to the sequential method,the"middle tunnel first"method exhibits a 23%increase in horizontal deforma-tion at the arch haunch and a 26%increase in settlement at the arch crown but shows lower initial settle-ment.The"middle tunnel first"method progresses faster and demonstrates superior stress and settlement control.Numerical simulations closely reflect actual construction conditions,and field monitoring confirms the suitability of the"middle tunnel first"method for the entrance sections of closely spaced tunnels.Con-clusions Considering the stress,deformation of lining structures,and construction timeline,the"middle tunnel first"method is recommended for on-site implementation.These findings offer valuable guidance for the design and construction of similar closely spaced multi-line parallel tunnel engineering.
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