Pixel-level crack segmentation of tunnel lining segments based on an encoder–decoder network

Regular detection and repair for lining cracks are necessary to guarantee the safety and stability of tunnels.The development of computer vision has greatly promoted structural health monitoring.This study proposes a novel encoder–decoder structure,CrackRecNet,for semantic segmentation of lining segment cracks by integrating improved VGG-19 into the U-Net architecture.An image acquisition equipment is designed based on a camera,3-dimensional printing(3DP)bracket and two laser rangefinders.A tunnel concrete structure crack(TCSC)image data set,containing images collected from a double-shield tunnel boring machines(TBM)tunnel in China,was established.Through data preprocessing operations,such as brightness adjustment,pixel resolution adjustment,flipping,splitting and annotation,2880 image samples with pixel resolution of 448×448 were prepared.The model was implemented by Pytorch in PyCharm processed with 4 NVIDIA TITAN V GPUs.In the experiments,the proposed CrackRecNet showed better prediction performance than U-Net,TernausNet,and ResU-Net.This paper also discusses GPU parallel acceleration effect and the crack maximum width quantification.

Interface failure of segmental tunnel lining strengthened with steel plates based on fracture mechanics

Segmental tunnel lining strengthened with steel plates is widely used worldwide to provide a permanent strengthening method.Most existing studies assume an ideal steel-concrete interface,ignoring discontinuous deformation characteristics,making it difficult to accurately analyze the strengthened structure’s failure mechanism.In this study,interfacial fracture mechanics of composite material was applied to the segmental tunnel lining strengthened with steel plates,and a numerical three-dimensional solid nonlinear model of the lining structure was established,combining the extended finite element method with a cohesive-zone model to account for the discontinuous deformation characteristics of the interface.The results accurately describe the crack propagation process,and are verified by full-scale testing.Next,dynamic simulations based on the calibrated model were conducted to analyze the sliding failure and cracking of the steel-concrete interface.Lastly,detailed location of the interface bonding failure are further verified by model test.The results show that,the cracking failure and bond failure of the interface are the decisive factors determining the instability and failure of the strengthened structure.The proposed numerical analysis is a major step forward in revealing the interface failure mechanism of strengthened composite material structures.

Cohesive zone model-based analyses of localized leakage of segmentally lined tunnels

This paper presents a novel approach for simulating the localized leakage behavior of segmentally lined tunnels based on a cohesive zone model.The proposed approach not only simulates localized leakage at the lining segment,but also captures the hydromechanically coupled seepage behavior at the segmental joints.It is first verified via a tunnel drainage experiment,which reveals its merits over the existing local hydraulic conductivity method.Subsequently,a parametric study is conducted to investigate the effects of the aperture size,stratum permeability,and spatial distribution of drainage holes on the leakage behavior,stratum seepage field,and leakage-induced mechanical response of the tunnel lining.The proposed approach yields more accurate results than the classical local hydraulic conductivity method.Moreover,it is both computationally efficient and stable.Localized leakage leads to reduced local ground pressure,which further induces outward deformation near the leakage point and slight inward deformation at its diametrically opposite side.A localized stress arch spanning across the leakage point is observed,which manifests as the rotation of the principal stresses in the adjacent area.The seepage field depends on both the number and location of the leakage zones.Pseudostatic seepage zones,in which the seepage rate is significantly lower than that of the adjacent area,appear when multiple seepage zones are considered.Finally,the importance of employing the hydromechanical coupled mechanism at the segment joints is highlighted by cases of shallowly buried tunnels subjected to surface loading and pressure tunnels while considering internal water pressure.

A holistic approach for the investigation of lining response to mechanized tunneling induced construction loadings

Design methods for segmental tunnel linings used in mechanized tunnel constructions typically employ numerical bedded beam mod-els and/or classical analytical solutions for the determination of structural forces(i.e.moments and shear and axial forces)and simple load spreading assumptions for the design of the reinforcement in joint areas.However effcient such methods may be,many physical details are often overlooked and/or oversimplified in the process of reducing the actual structure to a structural beam model,e.g.ana-lytically derived loadings are employed,the grouting and ground reactions are reduced to a spring bedding,and the confinement due to grouting at the longitudinal joint is largely not considered in reinforcement design.Such a design process is not able to account for,or predict,the susceptibility of tunnel linings to often observed damages that,although they may not be structurally relevant,lead to ser-viceability or durability issues,such as crack development or chipping at the segment corners.Numerical methods,such as the Finite Element Method,provide an opportunity to model the segmental tunnel lining and its response to the entire TBM construction process and to explicitly model the crack development within individual segments using modern methods to model the discontinuities in struc-tures.In this contribution,a holistic modeling procedure for the representation of the tunnel lining within the tunneling process is pro-posed and compared to traditional lining models.A 3D process oriented Finite Element model is used to calculate the predicted forces on the tunnel lining and the obtained results are compared with those generated by traditional methods.Subsequently,the predicted defor-mations are then transferred to a detailed segment model in which the nonlinear response of the segment at the longitudinal joint is mod-eled using an interface element based approach to simulate concrete cracking.