| With the rapid development of national economy, subway has becomed one of the primary means of solving the urban public transport. But during the long-term operation of the subway, the longitudinal settlement of shield tunnel, especially non-uniform longitudinal subsidence, has been emerging gradually. Because the comfort and safety of the train are affected by longitudinal deformation of tunnel directly, it is urgent and necessary to study the long-term longitudinal deformation of tunnel under loads and set up the monitoring and safety assessment system of tunnel long-term settlement for early prediction and timely elimination of the safety risks.The long-term longitudinal deformation of tunnel mainly results from the inhomogeneous distribution of the soil under cement plies, the change of loads around the tunnel, load from trains and the consolidation and rheology of soil The settlements mainly include the consolidation of soft clay and the deformation of the tunnel. The theoretical analysis, experiments and finite element simulation are used to explore the the causes inducing the longitudinal settlement of tunnel. The cumulative deformation and development trends of tunnel and the soil under cement plies under the long-term cyclic loading are acquired, and then the characteristics of long-term longitudinal deformation of tunnel are revealed. The obtained results will provide a theoretical basis for setting up the scientific and efficient safety monitoring system.The contents of this dissertation are as follows:(1) The longitudinal deformation of tunnel under inhomogenous soil layers was studied. Both the infinite long beam model and the finite long beam model were proposed for inhomogenous elastic foundation, where the differences between underlying soils were considered. Based on the established models, the exact solutions to non-uniform longitudinal settlement of tunnel under loading were derived by superposition principle and singular function method. The numerical examples for the longitudinal settlement under loading were given. The results show that both the longitudinal total settlement and the settlement range of the tunnel increases as the foundation modulus of one side of the foundation decreases. It is further observed that the settlement curve is asymmetric and the asymmetric degree becomes more obvious when the difference of foundation modulus between two sides increases. The settlement curve of the tunnel moves to the weak side of the foundation when varying the foundation modulus. And the rebound of the foundation appears at the hard side of the foundation. The maximum value of the tunnel settlement occures at the weak side of the foundation and decrease in a parabola form. The same variation is also observed for different soft layers.(2) The cumulative deformation of the soil body under cement piles subject to cyclic loads was studied. Using DDS-70Dynamic Triaxial Test System, a series of static triaxial and long life cyclic triaxial experiments for the unsaturated silty clay were carried out to analyze the effects of loading frequency, stress level, confining pressure and moisture content on the cumulative deformation. From the experimental results obtained, it is concluded that:(a) The effect of loading frequency and confining pressure on the cumulative strain is weaker than that of stress level and moisture content;(b) The low frequency cyclic load has a greater influence on the cumulative deformation at higher load levels. Therefore, controlling train speed, the depth of tunnel and seepage will be helpful for reducing longitudinal settlement.(3)The deformation of the tunnel due to the combined action of load and soil was investigated. The three dimensional ADNIA finite element model was set up based on Mohr-Coulomb criterion, where both the local load above tunnel and soil under cement plies were considered. The effects of the elastic modulus, length and width of soil under cement plies and the length and width of local load on the tunnel settlement of were analyzed and discussed. The results show that the maximum value of longitudinal settlement increases linearly with the elastic modulus of soft soil, longitudinal length and load collection increasing. The value of settlement increases with the increase of the length and width of uniform load.(4)The long term deformation of the tunnel was predicted. Based on the artificial neural network, the tunnel settlement coming from several factors was predicted and analyzed. The explicit expression for determining optimum hidden layer nodes of a single hidden layer neural network was given. The relevant normalization, best normalized interval and the range of neuron web in hidden layer were proposed. The BP web model of double hidden layer was constructed and some improvements in pretreatment and normalized range were made, which overcome the limitations of poor learning efficiency, inferior convergence speed and confined prediction accuracy in previous model.(5) According to the properties of deformation. The monitoring scheme of tunnel was proposed. Utilizing the characteristics and technical advantages of long gauge fiber Bragg grating, the real-time monitoring scheme of sectional long gauge fiber Bragg grating along the longitudinal direction was presented. The sectional measurement program, which combines the sensor of fiber grating under static force and the sectional long gauge fiber grating, was conducted at complicated geology and prone to uneven settlement section. While, in relatively better section, the quasi distributed measurement of long gauge fiber Bragg grating was applied. Based on the conjugate beam method, the settlement algorithm was developed and verified by superposition principle. It is indicated that the gauge length and accuracy of measurement play an impotent rule in the computational precision, and monitoring scheme. |
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