Segmental Tunnel Linings Research Articles

Research on mechanical performance of longitudinal joints in segmental tunnel linings strengthened by fiber-reinforced plastic grid with polymer-cement-mortar method

Research on mechanical performance of longitudinal joints in segmental tunnel linings strengthened by fiber-reinforced plastic grid with polymer-cement-mortar method

An analytical solution for shear bearing capacity of circumferential joints of precast concrete segmental tunnel linings considering dowel action

AbstractThe capacity of the circumferential joint of the precast concrete segmental tunnel lining (PCTL) structure in terms of shear performance principally includes the dowel action by connector/bolt as well as friction force, and it is a vital parameter to assess the mechanical response of the circumferential joint. Further, as there was no analytical model available for precise estimation of a circumferential joint in terms of the shear‐bearing capacity considering the dowel action of the bolt in the presence of axial/normal force, therefore in this investigation, an analytical model has been proposed to estimate the shear‐bearing capacity of the circumferential joint. Furthermore, the analytical model's precision and accuracy were validated via large‐scale experimental investigation on a circumferential joint of PCTL. Upon comparing analytical model outcomes with experimental results, absolute error varied between +5% and −9%, with an average value of the coefficient of friction at the yield point of the bolt. Moreover, the formation of hinges on the side of the bolt within the segment was considered a failure of the circumferential joint. Additionally, the parametric investigation revealed that with just a 1% change in axial force, the diameter, pre‐tightening, and yield strength of the bolt and concrete strength improved by 2.85%, 1%, 0.27%, 0.26% and 0.17% shear‐bearing capacity of the circumferential joint respectively. Axial force variation greatly influences the shear‐bearing capacity of circumferential joint followed by diameter, pre‐tightening, yield strength of the bolt, and concrete strength. Conclusively, with analysis of axial force distribution around the ring for the tunnel, the proposed model has been applied to a full ring to estimate shear bearing capacity.

Lok-Test and Capo-Test pullout for in-situ concrete strength

Among the test systems for in-place concrete strength available today, two measure the in-place physical strength, pullout, and cores. Both systems are dealt with in detail in this paper, the pullout systems named LOK-TEST/CAPO-TEST (ASTM C900-19) and coring (ASTM C42/42M-18). Testing in-situ with accurate test systems will reveal effects on the final strength from a potential mix’s over transportation, pumping, consolidation, compaction, and curing. With the LOK-TEST system testing of the pre-installed inserts takes 4–5 minutes each, easily and with only one small suitcase brought along. The CAPO-TEST, originally designed to supplement the LOK-TEST, takes 15–20 minutes for each test to be performed anywhere on a structure without pre-installed inserts. No large holes are left in the structure from coring and thinner elements may be tested without weakening them structurally. The pullout test provides accurately the In-Place Strength without testing cores and the duration is about 15 minutes compared to 3–4 days for coring correctly cured. General robust correlations to strength of standard specimens exist no matter what parameter is considered for normal concrete, even for carbonation of the surface layer. With the systems the cover layer protecting the reinforcement may be checked efficiently and quickly, not at least in areas with dense reinforcement or on slim structures. Bad curing conditions are revealed and the consequences in terms of reduced service life for presence of chlorides or carbonation may be estimated swiftly. This paper is benchmarking 50 years of successful in-situ concrete strength measurements, from studies of the failure mechanism and laboratory/on-site correlations to full scale testing of structures in Europe, Scandinavia, and Canada. Six testing cases with emphasis on pullout and cores are illustrating different applications: Case 1. Production testing at the Great Belt Link, Denmark. Case 2. Service life of bridge pier, Great Belt Link, Denmark. Case 3. Curing of the cover layer evaluated by pullout and conductivity, Denmark. Case 4. Strength testing with CAPO-TEST for further loading of old bridges, Poland. Case 5. In-Situ compressive strength testing of quarantined precast concrete tunnel lining segments using CAPO-TEST, UK. Case 6. Safe and early loading with LOK-TEST, Canada. Other cases are given on www.NDTitans.com

Simulation-based Optimal Sensor Placement in Tunnel Structures considering Uncertain in-situ Conditions and Multiple Loading Scenarios

The expansion of the underground infrastructure and the necessity to maintain the full functionality of the existing tunnels highlights the role of structural health monitoring in keeping track of the behaviour of the structure over time. In the study, the focus is on segmental tunnel lining for deep and long tunnels. The aim of this research is to investigate the best position for sensor placement based on the potential loading scenarios that might insist on the tunnel structure. Since for the evaluation of the lining safety we are interested in the maximum stresses reached in the structure, which might lead to durability critical cracking or crushing of concrete, a method is presented to suggest optimal positions for strain gauges. Due to uncertain geological conditions, multiple loading configurations acting on the tunnel structure are simulated using a finite element model. The results are employed to better identify the sensor locations, on the basis of where damage is prone to occur and where maximum information about the stress state in the structure can be inferred. The results obtained by the framework are discussed in light of a practical engineering perspective and the advantages as well as the limitations of the proposed approach are expounded.

Convergence-Related Serviceability Limit States of Segmental Tunnel Rings: Lessons Learned from Structural Analysis of Real-Scale Tests

An extended hybrid method for the analysis of the symmetric structural behavior of segmental tunnel linings is proposed. Their deformations cannot be determined by the traditional hybrid methods when the convergences are close to the serviceability limit states (SLSs). Two real-scale tests are employed for the validation of the proposed method. It involves structural analysis that is based on transfer relations. They represent analytical solutions of the linear theory of thin circular arches, and they contain the symmetric mode of rigid-body displacements. The method is termed hybrid because it is based on two types of input, namely the external loading and experimental data of displacements monitored during the test. It is termed extended because, additionally, the vertical and horizontal convergences are employed to produce more accurate structural deformations than obtained by the traditional hybrid method. The latter is found to be unsuitable for structural analysis after sudden failure has occurred in the vicinity of segmental interfaces. At the respective load steps, the structures were in convergence-related serviceability class C, referring to endangered serviceability. The local failures occurring at these load steps resulted in a rapid increase in the structural deformations and fast attainment of the SLS. If, in convergence-related serviceability class C, local failures at segmental interfaces are detected, the strengthening of the structure is necessary. This cannot be delayed to the attainment of serviceability class D, i.e., to reaching the SLS.

Contribution of Closed Ties to Shear Strength of GFRP-Reinforced Precast Concrete Tunnel Lining Segments: Experimental and Analytical Study

Contribution of Closed Ties to Shear Strength of GFRP-Reinforced Precast Concrete Tunnel Lining Segments: Experimental and Analytical Study

Passive confinement of reinforced concrete members revisited

AbstractPassive confinement provided by transverse reinforcement is taken into account in the design of reinforced concrete structures subjected to high compressive loading such as columns, piers, or tunnel lining segments. In current guidelines, several models exist to account for this beneficial effect. However, these approaches lack mechanical consistency regarding the three‐dimensional load dispersion of the confining forces. The article addresses this knowledge gap by introducing a new model based on a lower‐bound solution according to the theory of plasticity, along with a simplification regarding the governing confined concrete area. The proposed model and its simplified version are compared to the design approaches proposed by current guidelines, and both are successfully validated against experimental data. The simplified new model is a valuable alternative to existing models in current guidelines, as (i) it is mechanically more consistent, (ii) its application is more straightforward, and (iii) it allows the comprehensive treatment of rectangular and circular cross‐sections using the same equations.

Interpretation method for laser-scanning point clouds of segmental tunnel linings based on hybrid analysis

Owing to its convenience and precision, laser scanning has been applied in inspecting segmental tunnel linings. The point clouds generated by laser scanning provide valuable geometric information regarding the lining profile. Numerous studies have identified defects including cracks, leaks, and concrete spalling from these point clouds. However, they primarily relied on computer vision and neglected the mechanical information inherent in point clouds. This paper proposes a method for interpreting laser-scanning point clouds related to segmental tunnel linings based on analytical back analysis, known as hybrid analysis. The objective involves interpreting the mechanical behavior of segmental tunnel linings derived from point clouds. Through hybrid analysis, the deformation of the segmental tunnel linings was broken down into deflection and rigid-body motion. This process revealed the contribution of joints to the overall deformation of the linings. The strain of the segmental tunnel linings can be further quantified using the elastic theory of an arch slender. To validate the hybrid analysis, results were compared with experimental monitoring results. Further, potential application scenarios for the proposed method were explored. This method holds promise in engineering practice because it requires limited knowledge of the external loads, rendering it suitable for in-operation tunnels buried in uncertain strata.

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

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

An enhanced polar-based GPGPU-parallelized contact detection algorithm for 3D FDEM and its application to cracking analysis of shield tunnel segmental linings

This study introduces the three-dimensional combined finite-discrete element method (3D FDEM) to perform cracking analysis of segmental linings under extreme conditions. Considering the complex contact interactions of segments, this study first proposes a polar-based GPGPU-parallelized contact detection algorithm to handle memory issues confronted by existing FDEM algorithms. Initially, a spatial decomposition approach based on the polar coordinate system is implemented during the broad search phase. Each tetrahedral element is positioned within suitable search cells according to the axis-aligned bounding box. Subsequently, element pairs within each search cell are iteratively traversed, and potential contact pairs are identified using the judge cell criteria. After the broad search, a narrow search phase is executed to determine all real contacts. Following the above implementation, a load-structure model encompassing soil spring and external pressure calculations is then proposed. Based on the shape functions and coordinate transformation, the formulations of soil springs and external loads are derived and parallelized in the 3D FDEM framework. Three numerical tests are presented to validate the effectiveness of the proposed approach. Simulation results confirm the suitability of the proposed method for cracking analysis of segmental linings. Compared to the existing methods, it reduces GPU memory usage by 56 ∼ 76 % without extending time, which enhances the computational scale of 3D FDEM simulations. Furthermore, the proposed method is applied to two engineering scenarios, i.e., straight and curved segmental linings, both considering the absence and presence of contact defects between segments. The results show that contact defects can significantly reduce the resistance of the structural system comprising bolts and concrete segments for straight and curved scenarios.

Response and Damage Characteristics of Segmental Tunnel Lining under Various Dynamic Load Conditions

Response and Damage Characteristics of Segmental Tunnel Lining under Various Dynamic Load Conditions

Seg2Tunnel: A hierarchical point cloud dataset and benchmarks for segmentation of segmental tunnel linings

Point clouds provide a novel and effective alternative to understanding the structural behaviours of segmental tunnel linings. 3D deep learning (DL) has emerged as a promising technology capable of automatically deriving point-wise semantic and instance labels from point clouds. The utilisation of 3D DL in segment segmentation of tunnel point clouds has not been explored and the development of tailored 3D DL networks has been hindered by the absence of specialised datasets and benchmarks. To bridge this gap, this paper introduces a richly annotated hierarchical dataset: ‘Seg2Tunnel’, acquired from five tunnels and including 1,300 tunnel rings. Using the Seg2Tunnel dataset, the feasibility of applying 3D DL to the segment segmentation is demonstrated for the first time. Experiments are conducted to investigate the influences of training set size, data augmentation strategy, input size, and hyperparameter on the performance of trained 3D DL models and to provide benchmarks and insights for future uses of the Seg2Tunnel dataset. The 3D DL models trained by the Seg2Tunnel dataset outperform currently existing image- and voxel-based DL methods. The Seg2Tunnel dataset and benchmarks are fundamental in shaping the design of 3D DL networks tailored for tunnel point clouds. The study provides a novel paradigm for automatically understanding the tunnel structural elements in the point clouds, paving the way for unmanned construction and intelligent evaluation of segmental tunnel linings.

Precast CSA-based concrete tunnel lining segments reinforced with GFRP bars: Challenges and opportunities

This research study investigates the potential of using Calcium Sulfoaluminate (CSA) binder as a sustainable alternative to ordinary Portland cement (OPC) in the production of precast concrete tunnel lining (PCTL) segments. The effectiveness of CSA-based concrete, reinforced with Glass Fiber-Reinforced Polymer (GFRP) bars, was examined through an experimental program. Comparative analyses, including analytical, experimental, and Life Cycle Assessment (LCA) comparisons, were conducted with OPC-based segments reinforced with traditional steel. The results indicate that the proposed CSA/GFRP solution has the potential to improve sustainability in mechanized tunnelling and prefabrication. A case study is also presented to support these findings.

Experimental and Theoretical Investigations of the Mechanical Behavior of Column-Free Quasi-Rectangular Segmental Tunnel Linings

To investigate the mechanical behavior and design methodology of column-free QRST (quasi-rectangular segmental tunnel) structures, a theoretical analysis based on prototype experiments and simulation models is conducted. Initially, a prototype experimental investigation is conducted to reveal the structural behavior at the service stage. Subsequently, the ESHR model (Equivalent Stiffness Homogeneous Ring), the BS model (Beam Spring), and the MBS model (Modified Beam Spring) are used to simulate structural behavior. For design purposes, the design methodology is explored based on the ESHR model, followed by a sensitivity analysis of several key load parameters. Based on the experimental results, weak parts of the column-free QRST structure are found to include several joints (Joint 1, Joint 5, Joint 3, and Joint 8), and corresponding optimization measures are proposed. By comparing the test results, the above-mentioned three models demonstrate their applicability in structural simulation, with the ESHR model having sufficient design accuracy. A model-based deformation mechanism analysis found that joints contribute approximately 2/3 of the structural deformation. For the structural design of the column-free QRST using the ESHR model, amplifying the calculated results of structures directly subjected to the service stage by 10% suffices to meet engineering requirements. Based on the test and study, special attention should be paid to the negative bending moment regions at the waists of the structure during both the design and service stages.

Seepage propagation simulation of a tunnel gasketed joint using the cohesive zone model

The gasketed joint plays a critical role in the waterproof or water-leakage resistance performance of tunnel segmental linings. This study presents a novel numerical simulation method based on the cohesive zone model (CZM) to accurately analyze the seepage phenomena along the interface of elastic sealing gaskets in shield tunnel joints. While traditional finite element methods are widely used, they cannot simulate interface opening, limiting their applicability. To overcome this challenge, the proposed method leverages the concept of hydraulic fracturing and redefines the seepage problem as a seepage problem of elastic impermeable media. By establishing a two-dimensional hydraulic fracturing model using the CZM, this study conducts a comprehensive sensitivity analysis of the cohesive element parameters and validates the accuracy and effectiveness of simulating seepage along the interface of elastic impermeable media. Based on the insights gained from the sensitivity analysis, a seepage calculation model is successfully developed, considering the unique characteristics of elastic sealing gaskets. The model is applied to three cross-sectional shapes of elastic sealing gaskets, and their seepage characteristics are analyzed based on the interface opening displacement and liquid pressure distributions during seepage. The results highlight the SC3 cross-sectional shape, which exhibits minimal opening displacement and the highest net water pressure at the moment of seepage occurrence, demonstrating superior waterproof performance compared to other forms. Through the introduction of the CZM, comprehensive parameter analysis, validation, and practical application, this study overcomes the limitations of traditional finite element methods in simulating interface opening. The proposed model, which more accurately captures seepage characteristics than other methods, provides an optimized simulation method for analyzing hydraulic coupling issues related to seepage at shield tunnel joints and is a significant contribution to the field, enhancing our understanding and enabling more accurate analyses of seepage phenomena in shield tunnel joints using elastic sealing gaskets.

Implementation of an enhanced fiber optic sensing network for structural integrity monitoring at the Brenner Base Tunnel

The reliable assessment of deformation characteristics along tunnel linings is essential to understand ongoing structural processes during construction and operation. Conventional monitoring techniques may involve limitations, either in the spatial or the temporal resolution and do not deliver the overall deformation behavior along the entire lining. Distributed fiber optic sensing (DFOS) has significantly evolved in recent years to monitor large-scale civil infrastructure, with scientific sensing designs being realized within various research projects. The technology can be advantageous for in-situ tunnel monitoring since the distributed strain and temperature sensing feature delivers a more complete picture of the linings’ structural deformation behavior.This paper introduces the design and realization of an enhanced distributed fiber optic sensing network inside concrete tunnel lining segments, currently being implemented at the Brenner Base Tunnel. The construction is one of the largest civil infrastructure projects world-wide and will be globally the longest underground railway connection with a total length of about 64 km once completed. The designed DFOS system, consisting of more than 35 km sensing cable overall, provides distributed strain and temperature information along numerous tunnel cross-sections, spread over more than 30 km tunnel drive and two different construction lots. After installation, measurements are being continuously performed, autonomously evaluated, and transferred to the ACI online visualization dashboard in real time. It can be shown that fiber optic sensors have considerably developed from research into innovative practice and are capable to extend or even to replace conventional, geotechnical sensors.

Evaluation of environmental and economic compatibility of different types of segment linings based on the structural performance with some aggravating factors

The performance of supporting systems can be significantly affected by aggravating factors that contribute to segmental tunnel lining damage. Based on previous studies and statistical data, the excessive thrust force and defect in support conditions are the crucial factors that can aggravate the damage of segmental linings during the installation stage. This study aims to evaluate the effect of influential factors, including the excessive thrust force, the incomplete contact of loading shoes, configuration of loading shoes, defect gap thickness, and segment type. In addition, a series of numerical simulation are performed to investigate the impact of the influential factors on the performance of three different segmental lining types, including conventional reinforced concrete segment, recycled steel fiber reinforced concrete segment, and new hybrid design recycled steel fiber reinforced concrete segment. The results indicated that numerical modeling can accurately simulate stress concentration and damage patterns of segmental lining in different conditions. This method can provide useful information about the performance of segmental tunnel linings, considering essential phenomena that are neglected in traditional design methods, such as the effect of incomplete contact of the support area, and the thickness of circumferential gap. Numerical analysis of segmental tunnel lining behavior by considering different conditions can help reduce undesirable consequences and construction costs. The results indicated that the increase in the thickness of the gap significantly increase in crack opening and destructive damages. While, the configuration of the loading shoes affects the amount of crack openings, the number and position of these cracks; the effect of this parameter is very lower compared to the presence of defects in the supporting area. Finally, it was concluded that the hybrid design showed high efficiency for all investigated conditions, in terms of structural performance, environmental impact and economic compatibility.

Performance of Precast FRC Tunnel Lining Segments Reinforced with GFRP Bars under Quasi-Static Cyclic Flexural Loading

Performance of Precast FRC Tunnel Lining Segments Reinforced with GFRP Bars under Quasi-Static Cyclic Flexural Loading

Tunnel lining segments with enhanced bearing capacity using hybrid concrete concepts

In general, the design of tunnel lining segments used in mechanized tunneling is dominated by the joints. In fact, the effects of partial area loading dominate segmental failure and, consequently, the concrete bearing capacity of the central region remains underutilized. This paper presents two types of new hybrid segments with enhanced utilization ratios throughout their bodies. The first type comes with strengthened longitudinal joints using high performance steel fiber reinforced concrete, in addition to conventional reinforcement. The second incorporates one-sided barrel-shaped recesses in the central region, which yield volume savings of up to 23.8%. The performance of the new designs is experimentally evaluated on a full-size testing rig made from two steel frames with a capacity of 5MN each. It captures the on-site conditions in tunnels during service (final state). The first type of segments shows a 74.3% higher loading capacity than a conventional specimen for reference. The second performs even better and possesses 97% higher capacity. In both cases, the failure occurs in the central region. This increases structural safety and simplifies the design. Especially the design of the central region subjected to bending and axial forces gains more relevance against the controversially discussed design of the longitudinal joints.

Experimental Studies on the Seismic Performance of Underwater Concrete Piers Strengthened by Self-Stressed Anti-Washout Concrete and Segments

Given that the existing drainage strengthening methods for underwater damaged piers are expensive, inefficient, and cause shipping traffic disruptions, an urgent need exists to explore undrained strengthening methods, such as the precast concrete segment assembly method (PCSAM). However, the PCSAM has certain limitations, including a considerable strength loss of filled concrete, poor accuracy, poor connection performance of the segment sleeves, etc. Hence, this study developed an improved PCSAM (IPCSAM) by adopting self-stressed anti-washout concrete (SSAWC) as the filling material and developing a lining concrete segment sleeve (LCSS) based on the design principle of shield tunnel lining segments. Subsequently, the seismic performance of the strengthened piers was investigated. First, nine 1/5-scale pier column specimens were designed by considering different influencing factors: the self-stress of the SSAWC, LCSS reinforcement ratio, and initial damage and length–diameter ratio of the pier column. These specimens were tested under low reversed cyclic loading. Second, an extended parameter analysis was performed based on the established numerical models consistent with the quasi-static test’s parameter settings. Finally, a restoring force model of the strengthened piers, including the trilinear skeleton curve model and hysteresis curve model, was established based on the results of the quasi-static test and parameter analysis. The results indicated that the bearing capacity, ductility, and initial stiffness of the specimens strengthened using the IPCSAM increased by approximately 83.5–106.4%, 16.3–50.2%, and 83.9–177.3%, respectively, with the energy dissipation capacity also significantly improved. The self-stress of the SSAWC should not exceed 2.2 MPa, and the recommended ratio of the LCSS thickness to pier column diameter is 1/10. Additionally, the proposed restoring force model is highly accurate and applicable, able to provide a reference for the practical seismic strengthening design of piers.