| Lacustrine sedimentary soft soil composed of silty soil,peaty soil,clay,silt and silty clay is widely distributed in Kunming area,and its engineering properties are extremely special.In this site,the construction of urban rail transit is increasing year by year,and the shield construction risk is extremely high,which is easy to cause the instability of the end,water and sand gushing disaster.The problem of surface settlement caused by shield construction is more and more prominent,which is difficult to predict accurately by traditional methods.Due to the influence of construction period,multi-mileage short-distance construction of double-line tunnel gradually appears,and the superposition effect of double-line shield excavation is very obvious.The disturbance effect is inevitably transmitted to the adjacent structures,thus endangering the safety and normal use of the existing structures.It has become an urgent problem to explore the different construction effects and key control technologies between the end section and normal excavation section of shield construction in soft soil site.Therefore,on the basis of previous studies and in combination with the actual project of Kunming Rail Transit Line 5,this paper focuses on the key technologies of shield end construction,the formation disturbance effect of shield construction and the construction risks of shield crossing existing Bridges by means of theoretical deduction,process improvement,numerical simulation,machine learning and data statistics.The main work and achievements of this paper are as follows:(1)In view of the high risk problems of shield end construction in soft soil site,the empirical analysis method and orthogonal test were adopted to analyze the end reinforcement process and the range of reinforcement,and the calculation model of the optimal applicable process and the optimal range of reinforcement was obtained.The application suggestions of each process,the ranking of the end stability sensitivity and the theoretical solution of the range of reinforcement were given.A new type of permanent sealing and sealing system composed of three sealing components and two active measures is proposed to solve the problem that the existing sealing and sealing system of the cave door has a high risk of water and sand inrush.Under the condition IV of conventional construction equipment,an innovative construction technology of three walls combined with existing underground anchor cable treatment is put forward.The results and technology are applied to the end of Huibao section of Line 5 of Kunming rail transit.Midas Gts and logarithmic spiral sliding calculation model are used to numerically simulate and calculate the stability of the end soil mass.The displacement nebulae and safety factor obtained are consistent with the actual construction situation,thus achieving the unification of the theoretical solution and the engineering solution of the end reinforcement range.From the construction site,the application effect of the new technology is very ideal,which effectively ensures the safety of shield tunneling in and out of the hole.(2)In order to explore the problem of stratum settlement in shield tunneling construction,based on the idea of unsupervised learning and combined with engineering experience,10 characterization indicators were selected as input sets.According to the ability of principal component analysis algorithm to deal with multiparameter interaction and nonlinear problems,multi-parameter de-coupling dimensional reduction processing of samples was realized,and sensitivity sequence of characterization indicators was obtained to guide the adoption of control measures in the construction.The K-means algorithm was used to search for the optimal initial clustering centers,and a variety of internal evaluation indexes were adopted to optimize the number of fuzzy C-mean clustering centers,in order to shorten the convergence time of the model and improve the prediction accuracy.Based on the optimized clustering algorithm,an unsupervised prediction model of the maximum surface settlement of shield construction was established to measure the spatial similarity of sample data and classify the settlement value,so as to achieve accurate prediction of post-construction settlement grade and adjustment of construction parameters,so as to effectively control the surface settlement.(3)In order to solve the settlement caused by the stacking effect of formation disturbance in close distance construction of double-line shield,based on the random medium theory,the longitudinal excavation degree was introduced to measure the settlement variation rule of successive tunnel excavation,and the dynamic prediction formula of double-line shield settlement based on the Peck formula was obtained.Then,the settlement database of different axis spacing l and excavation fault spacing d was established by numerical simulation to explore the influence of working conditions on the settlement and to invert the prediction formula parameters.The analysis results show that l and d mainly affect the formation loss rate of successive tunnels,which makes the settlement above the front line greater than that of the back line,and has little influence on the width coefficient of settlement trough i.In addition,the influence of l on the settlement curve is more obvious than that of d.As l decreases,the settlement curve transitions from double V shape to W shape,and finally evolves into single V shape.The longitudinal settlement trough width coefficient j is about 2.8times i value.The excavation in front and rear j area will produce 68.2% of the total settlement,and the excavation in 2j area will complete 95.5% settlement.(4)In order to realize accurate assessment and control of construction risks of shield tunnel crossing existing Bridges,the C-V-T model was introduced for the first time to divide the project risks to be evaluated into severity C,vulnerability V of existing bridge system and threat T of new tunnel system,and construct three risk assessment processes of disaster-inducing,disaster-bearing and post-disaster systems.The two-factor catastrophe theory is introduced and the three-level index system is established to obtain the catastrophe order of existing Bridges.An interactive matrix is used to measure the cross-influence between the indicators of the new tunnel,and the index-system weighting method is given.The complex mapping relationship and transformation situation between the quantitative indicators and qualitative concepts are objectively expressed by using the multi-dimensional cloud and forward generator optimization algorithm.The coupling method of risk assessment results and risk control measures of the three systems are given,and the comprehensive risk assessment model and control system of shield crossing bridge construction are constructed.The model is applied to Yixin Bridge crossing project,the evaluation results are consistent with the actual project,and the monitoring data verify the rationality,standardization and applicability of the model.The research results and conclusions effectively ensure the safety and normal use of bridge,and can provide reference for similar engineering. |
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