Study On The Stability Of The Shield Metro Tunnel During The Operation Period In The Karst Area Underneath The Sediment

With the rapid development of subway construction across the country,shield tunnels will inevitably pass through various complex stratums.In fact,the engineering geology of the karst area lying beneath the sedimentary strata is complex.The special soils such as peaty soil and strawberry soil in the sedimentary strata have special composition,wide distribution,and extremely poor mechanical properties.Severely weaken the stability of the surrounding rock of the shield tunnel.At present,there are relatively few studies on the stability of shield subway tunnels in the karst area underneath the deposits during the operation period.Therefore,the development of related studies is of great value to ensure the long-term safe operation of subway tunnels.This paper takes the typical sedimentary karst section of Kunming Metro Line 4 as the research object.Through theoretical analysis and numerical simulation methods,the effects of shield tunnels on train vibration loads and seismic loads are discussed from the three aspects of displacement,acceleration and stress.Based on the dynamic response of the tunnel,the empirical formula is used to predict and analyze the cumulative settlement of the basement along the tunnel under long-term cyclic loading.The main research contents of the thesis are as follows:（1）Consulted relevant engineering geological and hydrogeological data,investigated the karst survey methods used in the route,based on the results of special karst surveys,ascertained the distribution of karst caves in the engineering section,and used Midas GTS to establish the karst cave distribution of the typical shield tunnel section Three-dimensional model.（2）The artificial excitation function is used to simulate the train load during the operation period,and the influence of the 6 types of tunnel surrounding rock conditions and the 3 types of karst filling level in the study interval on the dynamic response of the tunnel structure were discussed according to the actual tunnel crossing the ground and the distribution of karst caves.And based on the construction design,the improvement effects of4 kinds of soft soil reinforcement schemes in the typical observation surface were discussed.Research shows that the spatial difference of dynamic response of tunnels along the interval is significant,most obvious in weak formations;the karst conditions in the interval have little effect on the stability of the tunnel structure;the selected reinforcement schemes have good vibration damping performance.（3）Based on the dynamic stress of the subway tunnel basement soil under the train load from the numerical simulation,the cumulative settlement experiencal model was established,the cumulative settlement of the tunnel basement under various reinforcement schemes was predicted,and the uneven settlement control of the various schemes along the tunnel was discussed.（4）Two natural waves and one artificial wave were selected to study the dynamic response of the shield tunnel structure at the soft and hard junction in the interval under lateral seismic load.According to the actual situation,the foundation reinforcement scheme adopted in the project was discussed for the tunnel structure.According to the analysis,the tunnel structure is more obviously affected by the earthquake in the weak stratum,the stress of the tunnel structure at the junction of soft and hard increases sharply,and the arch shoulder and arch foot of the tunnel are the most unfavorable positions for earthquake resistance.The reinforcement of the foundation effectively controls the displacement and stress of the tunnel structure under the action of the earthquake load,has a small effect on the acceleration,and has a significant seismic effect.The main innovations of the paper are:（1）According to the actual surrounding rock and karst conditions of the tunnel,the original scale is refined to establish a finite element model,which more truly reflects the influence of the train load and seismic load on the tunnel structure in the underlying karst geology of the Huxiang deposit.（2）Based on the experience of structural damage in the previous tunnel operation stage,combined with the damage characteristics of the two types of loads,the uneven settlement of the tunnel structure in the interval and the influence of the seismic damage in the soft and hard uneven strata on the tunnel structure are specifically discussed.