| With the development of shield construction technology in China,the scale and quantity of shield tunnels are increasing.The concrete lining structure of shield tunnel is connected with prefabricated segments by bolts.Under the action of fire and high temperature,the lining and joints of the tunnel will be damaged by spalling,cracks and material deterioration,posing a serious threat to the safety of the structure.How to accurately and quickly evaluate the tunnel structure mechanics performance after fire is of vital importance to the reliability and economy of its emergency response and repair work.However,at present,the evaluation of structural performance after fire mainly focuses on building structures,and there is relatively little research on the mechanical performance of shield tunnel structures after fire.In this thesis,taking shield tunnel as the research object,the mechanical performance evaluation model of shield tunnel structure after fire is constructed based on BP neural network.Firstly,the damage index system of shield tunnel structure after fire is determined by combining analytic hierarchy process and numerical simulation.The influence of existing damage on structural mechanical properties after fire is studied by numerical parameter analysis.Based on the numerical results and BP neural network,the mechanical performance evaluation model of shield tunnel structure after fire is finally established.Based on quantitative data,this model has good reliability and is easy to use.It can provide theoretical and technical support for post fire assessment,repair,and other work in shield tunneling.The main study conclusions and contents of this thesis were as follows:(1)Through the verification of existing experimental results,the material parameters,interaction and boundary conditions of single segment,segment joint and single ring lining were determined,and established a simplified numerical model.The results indicate that: the friction coefficient between the segment and the pad has a significant impact on the ultimate bearing capacity of the segment,and the tangential friction coefficient can be taken as 0.35.The friction between the screw and the screw hole can be ignored,and the tangential friction coefficient between adjacent concrete segments can be between 0.2 and 0.8.The calculation time of the simplified model was only 1/10 of that of the complete model,and the third plastic hinge formed by the simplified model was used as the structural failure criterion.(2)Based on analytic hierarchy process,the evaluation index of mechanical properties of shield tunnel structure after fire was preliminarily established.Through numerical parameter analysis,the evaluation index system of mechanical properties of shield tunnel structure after fire was finally determined.The results show that the seven indexes of concrete spalling depth,spalling area,concrete deterioration depth,concrete strength reduction,concrete deterioration area,bolt strength reduction and bolt deterioration number were the main factors affecting the mechanical properties of shield tunnel structure after fire.(3)Based on the damage model of single-ring tunnel lining structure after fire,considered the five parameters of damage area(the spalling area,concrete deterioration area and the number of bolts were combined into one index),spalling depth,concrete strength reduction,concrete deterioration depth and bolt strength reduction,analyzed the influence of mutual coupling between different parameters on the mechanical performance of lining structures.The results indicate that: 1)With parameters deterioration,structural mechanics performance declines(that was,the structural bearing capacity decreases or the deformation of the top and bottom increases),but the decrease was quite different from the form of expression;2)When the damage area increases to 146°,the bearing capacity decreases by 24.3% and 39.6% respectively under the condition that the deterioration degree of other parameters was in the middle and the most serious at the same time.3)When severe damage occurs from the arch top to the arch bottom,the concrete spalling depth of the sealing block and the standard block increases,and the deformation of the top and bottom of the structure rapidly increases,from 65 mm to 219 mm to 579mm;4)The influence of concrete strength reduction and deterioration depth on the mechanical properties of the structure was basically the same.The concrete deterioration degree of the capping block and the adjacent block increases,and the structural deformation decreases.When serious damage occurs from the vault to the arch bottom,the deterioration degree of the concrete of the capping block and the standard block increases,and the structural deformation increases.5)When the deterioration of other damage parameters was the most serious,the change of ultimate bearing capacity caused by the decrease of bolt strength was less than 5kN.(4)Based on BP neural network,the mechanical performance evaluation model of shield tunnel structure after fire was established.Based on the cluster analysis theory,the mechanical properties of shield tunnel structure after fire were divided into five grades: mild damage,moderate damage,severe damage,extreme damage and damage.This thesis designed sample plans through comprehensive experimental methods and establish sample sets using numerical simulation results,and the basic structure and hyperparameters of the evaluation model were determined.After the training,the average relative error difference between the training set and the validation set was only 0.13%. |
