| In the current context of rapid urban development,urban rail transit has become a favorable approach for solving traffic congestion.However,the coexistence of subway tunnels and surrounding buildings has gradually become a constraint on urban development.Many provinces in China have soft clay layers in the underlying soil,and the phenomenon of foundation settlement during building construction is very severe,which has a very adverse effect on normal use.From the source perspective,the main reason for this effect is that the bearing capacity of the soft clay layer as a foundation is insufficient,and external load influence is one of the main factors causing settlement.Therefore,this paper takes the deep excavation in soft clay layers as the main research object and conducts a series of studies on the dynamic load impact brought by the adjacent subway.First,the physical properties and mechanical behavior of soft clay layers were determined through the study of the structural regularities of special peaty soil in Kunming’s soft clay.Then,the displacement and settlement laws of deep excavations and surrounding ground under dynamic load were explored,and the adaptability of two support structures,isolation piles or underground isolation walls,was analyzed.The main work and research results are as follows:(1)Based on the development law of each parameter in one-dimensional consolidation test,especially the void ratio and yield stress in the e-logp curve,a structural index reflecting the strength and weakness of the structure of Kunming soft soil was proposed,which is the integration of void ratio in the range of zero to yield stress.The compression and structure of soft soil with different water contents in Kunming were studied.The research showed that Kunming soft soil exhibits a consolidation phenomenon under external load,and the higher the water content,the worse its structural characteristics,which may cause large deformation even under small loads.When near the subway line,the strong structural peat soil is more prone to horizontal displacement and settlement under vibration,and the isolation effect of the supporting structure needs to be strengthened.(2)The vibration characteristics of trains on Kunming subway Line 1 were analyzed using the excitation force function,and then applied to a deep excavation project.Simulations were performed using FLAC 3D geotechnical software to investigate the impact of dynamic loads on the surrounding soil displacement and surface settlement during excavation of the soft and structurally complex soil layers in Kunming.Results show that compared to excavation without subway dynamic loads,the horizontal displacement of surrounding soil increased by approximately 15%,and surface settlement reached about 10%.These findings suggest a reduction in the stability of the pit near the subway tunnel.(3)The support design of the original internal support and drilled pile foundation was improved by designing two optimized solutions.It was discovered that adding a row of isolation piles between the retaining structure and the subway tunnel could effectively reduce seismic effects.Additionally,replacing the original support structure with an underground continuous wall support would strengthen water stopping and water reduction measures in high water content soil layers.Through the monitoring of soil and support structure deformation,stress analysis of the support structure,and data analysis and organization,simulations were conducted in the FLAC 3D software.Comparing the simulation data with on-site monitoring data,it was found that adding isolation piles between the excavation and subway tunnel could significantly reduce the displacement of support piles and surrounding ground settlement caused by subway vibration.Setting1000 mm diameter isolation piles at a distance of 5 meters from the pit wall can reduce horizontal displacement and ground settlement by 29% and 14% respectively.Moreover,the use of 0.8m and 1.0m thick continuous walls can mitigate the disturbance of dynamic loads,resulting in a reduction of 24% and 26% in horizontal displacement and 28% and31% in surface settlement near the subway tunnel side.After considering costeffectiveness and construction duration,the optimal solution was determined to be the0.8m continuous wall.Utilizing numerical simulation and actual monitoring data to analyze ground horizontal displacement and settlement,the most feasible optimization of the support structure was obtained,ensuring safe,stable,economical,and efficient construction of the pit. |
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