| With the increasing scale of urban rail transit construction in China,the number of shield tunneling projects in karst areas is increasing.The influence of hidden soil caves on the stress and deformation of tunnels in operation period and the safety of tunnel foundation settlement under the cyclic dynamic load of trains have become a very concerned and urgent problem for geotechnical engineers.At present,there are many studies on the influence of karst soil caves on the stability of tunnel construction in Karst areas.However,there are few studies on the dynamic response of shield tunnels during operation,the deformation of tunnel structure and the uneven settlement of tunnel basement when there are concealed soil caves near shield tunnels in Karst areas.In fact,the limitation of the survey technology before shield tunnel construction in karst area can not effectively detect all karst soil caves along urban rail transit in Karst area.In addition,the buried characteristics of soil caves in Karst Geology and the development of soil caves in karst geology within 100 years of the design period of tunnel operation still need to consider the potential risks of surrounding soil caves to tunnel operation after the completion of shield tunnels.Based on the karst geology of the north extension section of Guangzhou Metro Line 2 and the possible undiscovered karst caves in the karst area before the construction of the tunnel,a detailed numerical test scheme for the existence of Soil Caves at the bottom and side of the tunnel is formulated in this paper.The size of the hidden soil caves around the shield tunnel under the vibration of the subway train during the operation period and the size of the hidden soil caves around the shield tunnel are simulated by three-dimensional finite element method.Based on the analysis of the dynamic response of tunnel segment and stratum,and the Bayesian risk probability model,the existence of soil cave development is predicted.Then,the settlement value caused by soil loss is deduced by using Peck formula,and the prediction formula of uneven settlement in karst area is deduced.Combined with the settlement data of monitoring points,the results are as follows:(1)A three-dimensional finite element numerical model is established to simulate the train dynamic action on the rail by using the artificial excitation function.Based on the deformation and stress of the segment structure of shield tunnel in operation period,the dynamic response of the segment under train vibration is analyzed.The calculation results show that the stress under dead weight stress and train load meets the strength requirements of segment design,but when the earth tunnel is located at the bottom of shield tunnel,the vertical deformation of tunnel structure is larger.Based on the results of three-dimensional finite element numerical model,this paper summarizes the size,location of soil cave and the safe distance between soil cave and tunnel.(2)The Bayesian risk probability model is used to identify the historical settlement data.After updating the new settlement data,a new probability value is given.By comparing the probability value with the settlement data after many years,it shows that the model has strong ability of judging and predicting.It can provide reference value for risk control of uneven settlement in Karst area,achieve pre-judgment and pre-elimination of risk,and provide reference for the operation safety of urban rail transit.(3)Combining with the determined development interval of soil cave,the empirical fitting formula is used to obtain the soil deformation and drainage consolidation under undrained conditions.Then the Peck formula is used to deform and establish the time-history analysis function of soil loss in the uneven settlement interval.The soil loss volume is obtained by integral method,and the soil loss volume in the interval is fitted with the variation of the operation time of urban rail transit to determine a certain year.In order to predict the maximum settlement caused by soil layer absence in the non-uniform zone,the expansion coefficient determined by three-dimensional finite element numerical simulation is used to correct the deformation and consolidation settlement.The settlement caused by soil layer absence is combined with the consolidation settlement to obtain the final settlement value.The comparison of the monitoring data proves that the prediction model is effective. |
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