Metro Shield Tunnel Research Articles

Experimental Study on Vibration Absorption Performance of Floating Slabs of Vibration Isolation Pads in Metro Shield Tunnels

The monolithic track bed with isolated damping pad and floating slab is a common design of urban subways in recent years.The type of track bed, urban subway lines usually pass through densely populated areas of the city, and subway trains.In the course of driving, it is inevitable that large track vibration noise will be produced. In order to reduce the impact of track vibration noise on the daily life of surrounding residents, noise reduction measures must be considered during track design. Isolated shock absorber floating board The monolithic track bed is one of them. Compared with the steel spring floating slab monolithic track bed, it has the technical characteristics of “low engineering cost, more convenient construction, and good noise reduction effect”. In order to study its damping effect, we selected two working conditions of straight line and curve for comparison. It can be seen from the comparison results with the ordinary track bed: (1) The acceleration level of the shock-absorbing cushion tunnel wall in the straight section is greater than the ordinary track bed section within 1∼10Hz, and is close to the ordinary track bed section within 12.5∼20Hz, at 25 It is smaller than the section of ordinary track bed within ∼1000Hz. (2) The maximum Z-vibration level measured from the tunnel wall is most suitable for evaluating the seismic source intensity of the subway, and the seismic source intensity of the subway line is 65.69 dB.

Dynamic Health Assessment of Shield Tunnel Structures Based on Knowledge Graph

The safety and performance of shield tunnel structures are essential for metro operations. Therefore, a scientific assessment of the health of the tunnel structure is required during its service life. Although the performance of tunnel structures is affected by complex factors, these factors change over time, and so does the health of the tunnel structure. This study introduced a data-driven method based on the Knowledge Graph (KG) to fully assess the structure of a shield tunnel based on the evolution characteristics of the structure and its historical data. First, the method adopted Neo4j as the storage system, thus establishing a KG containing the data of a shield tunnel from its construction to the present. Then, based on the KG, the shield tunnel will be divided dynamically into sections based on their defects characteristics. Finally, the areas with defects will be assessed automatically through an improved tunnel serviceability index. Through KG, more information related to the tunnel structure can be constructed. Simultaneously, the dynamic evolution tendency of the tunnel structure health can be recorded by considering the time effect. Based on the KG, the main factors affecting tunnel structural health can be used to optimize tunnel operation and maintenance decisions. In this study, a framework for structural health assessment of metro shield tunnels based on KG is constructed and applied in Shanghai Metro Line1.

3D reconstruction and automatic leakage defect quantification of metro tunnel based on SfM-Deep learning method

Various structural defects deteriorate tunnel operation status and threaten public safety. Current tunnel inspection methods face problems of low efficiency, high equipment expense, and difficult data management. Combining the deep learning model and the 3D reconstruction method based on structure from motion (SfM), this paper proposes a novel SfM-Deep learning method for tunnel inspection. The high-quality 3D tunnel model is constructed by using images taken every 1 m along the longitudinal direction. The instance segmentation of leakage in longitudinal images is realized using the mask region-based convolutional neural network deep learning model. The SfM-Deep learning method projects the texture of the images after defect recognition to the 3D model and realizes the visualization of leakage defects. By projecting the model to the design cylindrical surface and expanding it, the tunnel leakage area is quantified. Through its practical application in a Shanghai metro shield tunnel, the reliability of the proposed method was verified. The novel SfM-Deep learning method can help engineers efficiently carry out intelligent tunnel detection.

New Damage Identification Method for Operational Metro Tunnel Based on Perturbation Theory and Fuzzy Logic

The structural health of operational metro tunnels is closely related to public safety. Prior research has focused on the locations of structural damage, but few researchers have examined both the location of damage and identifying the degree of damage, especially in metro shield tunnels. This paper proposes a new method for identifying structural damage that entails locating and detecting the degradation of tunnel performance, with a special focus on characterizing the degree of damage. First, the dynamic behaviors (modal frequencies and shapes) of different damage levels are obtained from an analytical model of the original tunnel structure. Second, a modal strain energy damage indicator (MSEDI) is introduced to locate the damage, regardless of size. Once the location of the damage is identified using MSEDI, a fuzzy logic-based damage identification (FLBDI) method is used to determine the actual extent of the damage. Finally, a simplified model of the tunnel is created using the Euler-Bernoulli beam theory and Winkler's foundation, to further test the procedure under an incomplete modal information condition and with differing noise levels. The results reveal that the fuzzy logic- based system can identify the degree of damage and structural degradation with very high accuracy, in which the location of damage and the prediction of performance degradation is satisfactorily confirmed.

Analysis and Prediction of Long-Term Settlement of Metro Shield Tunnel in Saturated Sand

Based on the existing monitoring data, this paper uses the matured data method to analyze, predict the long-term settlement of the subway shield tunnel, and comprehensively analyzes the long-term settlement of the subway shield tunnel in the saturated sand layer of Zhengzhou. Logical curve model, etc. is used to separately predict the long-term settlement of the Zhengzhou metro shield tunnel. Combining the prediction results to summarize the characteristics of the prediction model, a prediction model suitable for the long-term settlement of the subway shield tunnel in the sandy soil area represented by Zhengzhou is obtained, which can be used for the operation of the Zhengzhou subway tunnel. Management and similar engineering projects provide a certain reference basis.

Deep learning based method of longitudinal dislocation detection for metro shield tunnel segment

This paper presents a longitudinal dislocation detection method using an accurate tunnel segment joint labeling algorithm featured by deep CNNs (Convolutional Neural Networks). This method is proposed to be four steps. First, a mobile scanning system is used to acquire 3D point clouds of metro shield tunnels. Then, we use cylinder projection to generate tunnel surface depth images from 3D point clouds for segment joint labeling. Subsequently, two deep CNNs are designed to accurately label the segment joints on the depth images. The first CNN can roughly locate the segment joint positions, and the second precisely label the segment joints. Based on the labeled segment joints, two point data sets are obtained on both sides of each segment joint. By using the RANSAC algorithm, the two point sets can fit into two planes, the equation of which is then calculated to generate the dislocation value of the tunnel segment. Experiment results show that this method can label segment joints integrally and accurately without being affected by nearby tunnel equipment. Compared with traditional image edge detection algorithms (Canny and Sobel with Hough Transform), the CNNs are more powerful in labeling segment joints. When the distance measuring accuracy of scanner is 1.2 mm + 10 ppm, the internal and external accuracy of our detection method are evaluated to be 0.4 mm and 0.9 mm respectively. Compared with the scanning line method, the external accuracy of our method is higher and more reliable when there is tunnel equipment around segment joints.

Research on subway shield tunnel induced by local water immersion of collapsible loess

There are an increasing number of cases wherein the soil around a tunnel is immersed in water, which adversely affects the tunnel. To study the influence mechanism of local immersion of loess stratum on metro shield tunnel in detail, a similarity model test and numerical simulation were used in this study. These were used to investigate the stress and deformation of tunnel lining and stratum and surface settlement after a local soil collapse in the tunnel, and the mechanical mechanism that causes this effect. Further, the influences of different methods of water immersion on the tunnel lining were studied using numerical simulation. The results showed that the larger the collapsing area, the larger the bending moment and the axial force increment of the tunnel lining and the surface settlement; the bending moment and axial force near the side of the collapsing area were larger than those of the non-wet side. Vertical stress in the collapsible area was reduced, while the vertical stress of the non-collapsible soil on both sides of the collapsible area was increased due to the transmission of stress. When the water immersion area was the same, different water immersion methods also made the internal force of the tunnel lining slightly different; however, the difference was not obvious.

Experimental Research on the Vibration Absorption Performance of Medium Damping Fasteners in Metro Shield Tunnel

Medium damping fasteners have the advantages of low cost, easy maintenance, and convenient in gauge adjustment, etc., which are gradually and widely used in metro construction. In order to study the performance of medium damping fasteners in a driving environment, we performed measurement on site about the vertical and lateral displacement of the rail, as well as the vertical acceleration of the rail and the tunnel wall on a straight and curved metro cross-section with medium damping fasteners. By comparing with the results of ordinary fasteners, it can be seen that: (1) The medium damping fastener has a vibration reduction effect on the vertical acceleration level of the tunnel wall in the frequency range of 6.3∼31.5Hz and 50∼80Hz; moreover, it has a vibration reduction effect on the lateral acceleration of the tunnel wall in the frequency range of 5∼31.5 Hz and 50∼80 Hz. (2) The vertical and lateral displacement of the rail and the gauge change with the medium damping fasteners are larger than those of the ordinary fasteners. (3) The maximum Z vibration level measured from the tunnel wall is most suitable for evaluating the source strength of the metro, wherein the source strength of this metro line is 88.42 dB.

Analysis of the Influence of the Unsymmetrical Surface Surcharge on the Structure of the Subway Tunnel

Field case of staggered jointed lining of metro shield tunnel with an unexpected unsymmetrical surface surcharge is introduced in details at first. The finite element model of beam-spring is used to reproduce the surface surcharge process. Results show that the tunnel deviates from the surface surcharge under the action of unsymmetrical surface load. The displacement of adjacent rings A and B is different, which caused tunnel disease such as segment dislocation and water leakage. In the design state, the bending moment and axial force of A and B rings are close to each other, after unsymmetrical surface surcharge, the bending moment of ring B with key segment on the lower surface pressure side is larger than ring A. When the crack width is 0.2 mm, the corresponding horizontal expansion and vertical convergence are 33 mm (5.3D ‰) and 34 mm (5.5D ‰). When the horizontal expansion value is greater than 50 mm (8D ‰), ring B will enter into the stage of rapid development of cracks, which should be closely monitored and treatment measures should be taken when necessary. The bearing capacity of ring A and ring B is different after surface surcharge, which is suggested to be considered in the subsequent structural reinforcement.

Experimental study on mechanical behavior of the skew joints of shield tunnels under large eccentric compressive loading

The existence of joints in shield tunnel lining rings contributes to various deformation problems, such as leakage, joint opening, cracking and dislocation, which will severely impair the safety and durability of tunnel structures. Laborious experiments having tried to work out the mechanical properties of joints, none have drawn attention to two special segmental joints existing in Shanghai metro shield tunnels, which are obliquely intersecting the frontal plane of the lining ring and thus appear askew compared with the rest. The study investigated segmental specimens with the K-shaped top block and its two adjacent segments, including two skew joints connected using straight bolts. Given two groups of experiments, their mechanical behaviors throughout the failure process when subject to a positive and a negative bending moment showed that the bending moment’s impact on bolt strain, joint opening and segmental rotation could be illustrated by two stages when the bending moment was positive and four when negative. Besides, the joint compression height slightly decreased under both conditions and tended to be stable after bolts yielded when subject to a negative moment. The joint opening angle was proved equal to the sum of the segmental rotation angles from both sides of the joint, indicating the individual segment could be regarded as a rigid body. Moreover, the maximum joint opening width increased linearly with the joint opening angle, so did the deflection at the joint position with the segmental rotation. Above all, the skew joints examined behaved similarly to but differently from those of straight joints as there happened strain advance of the bolt and the concrete at one side and strain lag at the other side, which would probably become more obvious as steeper the oblique angle of the skew joints grew.

Case Study of Underground Shield Tunnels in Interchange Piles Foundation Underpinning Construction

Technology of shield tunneling has been widely used in modern city subway system building, however, due to the limit of urban construction land resources, it is inevitable for the new building structures conflicted with existing one. Thus, it is essential to taking correct measures to guarantee the safety of existing structure. In this case, a typical case of metro shield tunnel crossing through city overpass bridge piles underpinning construction project is studied by the numerical method and site field monitoring. According to the existing overpass bridge structure, geological conditions and site operation environment, a suitable construction method of piles foundation underpinning plan for this project is adopted. In order to ensure the structure safety of the existed overpass bridge during the pile foundation underpinning construction, a numerical simulation model is established which takes the whole foundation underpinning structure and construction steps into consideration. The numerical simulation result shows that the stress and the settlement of the piles foundation underpinning structure is reasonable. After the comparing of the site monitoring data with the numerical model simulation results, it is found that the site settlement and stress results are highly consistent with the calculated results, and it proves the feasibility of the piles foundation underpinning construction scheme. Eventually, the site gauging settlement data of the overpass pier and the pile cap indicate that the original overpass structure is barely affected by the shield tunnel construction, the construction method can provide reference and experience for the future similar projects.

Early warning limits for uneven settlement of metro tunnels based on vehicle–track interaction simulations

Metro shield tunnels in soft soils often exhibit uneven settlement, which causes leakage, cracking, dislocation, and bolt failures. Excessively uneven settlement affects the safety and durability of a tunnel structure and aggravates the track irregularity, thereby impeding operational safety, ride comfort, and steady function. As ride safety and comfort are more sensitive than tunnel structure safety to uneven settlement, they can potentially be used as indexes for the early warning of uneven settlement. Accordingly, a 20 m long metro rail car model with 10 degrees of freedom was established, and its behaviors were analyzed under different settlement amplitudes and running speeds. The results indicated that the rail car operational comfort index was more sensitive than the safety index to uneven settlement. An early warning value was then determined on the basis of the comfort index ranges for settlement length and vehicle speed, enabling the development of an early warning strategy for uneven settlement. The proposed strategy was subsequently applied to a case study, which monitored the uneven settlement of an interval of a metro shield tunnel using inclinometers.

The Environmental Effects Induced by a Metro Shield Tunnel Side-Crossing on Adjacent Pile Foundations and Its Impact Partition

With the advantages of fast construction speed and small disturbance to stratum, shield tunneling is becoming the preferred construction scheme for metro construction. However, if the disturbance of stratum cannot be clearly understood, it will affect the safety of adjacent underground structures and aboveground buildings. Based on the construction of the interval tunnel between Shizishan station and Chuanshi station of Chengdu metro line 7 project, this paper proposed a simplified calculation method to analyze the impact of shield excavation on adjacent pile foundation. A two-stage method of modified Peck formula and Winkler elastic foundation model was used to investigate the additional displacement and internal force of pile. A three-dimensional numerical model validated with the field test data was performed to compare with the theoretical results. The impact factors and their impact degree were discussed. The results show that the additional displacement and internal force are at a reasonable and acceptable level. The permanent pile foundation should adopt the small diameter pile as far as possible. When the small diameter pile foundation cannot meet the stress requirements, the design scheme of the pile group should be adopted. The area around shield tunnel is divided into strong impact area of pile foundation (S ≤ 0.75 D), slight impact area (0.75 D ≤ S ≤ 1.5 D), and no impact area (S > 1.5 D). The pile researched in this paper (S = 0.83 D) is in the slight impact area.

Mechanical behavior of sealed waterproof for shield tunnel segment joint under different assembling ellipticity.

During the construction and assembling stages of shield tunnels, the ellipticity of segment assembly, and the angle between segments are the main reasons for joint leakage of shield tunnel. In this paper, taking Chengdu Metro shield tunnel as the research background, the relationship between assembling ellipticity and segment opening angle is obtained by geometric calculation, and the waterproof effect of gasket under different segment joint angle is studied by finite element analysis method. Results of the nonlinear numerical analyses, there is a linear positive relationship between the opening angle of segments and the ellipticity degree. Meanwhile, the different side joint opening angles of the segments have different effects on the waterproof effect of the gasket, which is manifested as the external angle is greater than the internal. Based on the above research results, this article can provide some references for the design, production, processing, and quality inspection of shield tunnel gasket in the future.

Deep learning-based automatic recognition of water leakage area in shield tunnel lining

Difficulties in visual inspection of metro shield tunnels induced by various on-site installations (e.g., lighting instruments and pipelines) have been resolved by two-stage structural damage detection techniques based on deep learning known as the state-of-the-art on surface defects recognition. However, thresholds of the Intersection over Union (IoU) are set extremely low in two-stage segmentation models to obtain more positive samples to suppress overfitting during the training stage. Therefore, the pixel segmentation results generally contain lots of noise and are difficult to meet the requirements of engineering. Accordingly, this study adopted (1) data augmentation to increase the number of positive samples, (2) transfer learning to improve the robustness of convolutional layers, and (3) cascade strategy to enhance the quality of samples. The segmentation results of the improved model on the validation set demonstrated that the above methods achieved high precision pixel segmentation of water leakage (AP0.5 = 0.806), which was greater than the classical deep learning model (i.e., Mask R-CNN with AP0.5 = 0.530). Given the fact that the number of water leakage pixels cannot be regarded as reliable metrics to evaluate the security situation of the shield tunnel, a series of field experiments were conducted to obtain the calibration relationship between the number of target pixels and true areas. The average error rate of the fitted curve was also by only 2.59%, which is within the tolerance of the engineering. Consequently, the proposed method could automatically and accurately calculate the water leakage area from the dataset of images. All the water leakage images used in this study can be downloaded freely from https://doi.org/10.17632/xz2nykszbs.1.

A Risk Assessment Method for Metro Shield Tunnel Construction Based on Interval Number

In recent years, metro construction is flourishing in China. In the construction process of the metro shield tunnel, there are various complex potential risk factors fraught with many uncertainties. Therefore, it is crucial to conduct a risk assessment on urban metro shield construction. To improve the accuracy of the risk assessment results of metro construction projects, a risk assessment model based on interval numbers is proposed from the perspective of uncertainty. The experts’ evaluation information is represented by the interval number, and then the operation rule and membership function of the interval number is used to calculate the risk level of each potential risk factor. Afterward, the weight of each evaluation index is calculated based on the interval number and the C-OWA operator. From the bottom of the evaluation system, the layer-by-layer operation is carried out to determine the risk level of the assessment index of each layer, and the risk grade is divided according to the risk matrix. Taking a section of the shield tunnel of Shenzhen Metro Line 12 as an example, the proposed method is used to evaluate the construction risk, and the potential risk factors that require the implementation of risk prevention measures in the project are obtained. The calculation results show that the assessment model is reasonable and reliable, and has high stability.

Decision method on optimal time of preventive maintenance for metro shield tunnels in soft soils

Abstract A reasonable maintenance time is vital in the metro shield tunnel maintenance to keep good structural performance and reduce maintenance cost. Lacking predictability, the existing maintenance mode cannot effectively resolve the contradiction between the limited maintenance cost and the growing maintenance need in the future. In this study, the degradation model of metro shield tunnels in soft soils was studied to predict the long-term serviceability. Based on that, a method was proposed to decide the optimal time of preventive maintenance for metro shield tunnels. It was found that the tunnel decay process can be expressed by the inverse exponential function-based prediction model. The cost-benefit analysis method was adopted to determine the optimal preventive maintenance time, considering the maintenance time, the maintenance cost and the maintenance measures adopted. The flow chart of this method and an application example were presented. Results can help metro operation owners to develop more scientific and reasonable preventive maintenance plans for shield tunnels in soft soils.

Monitoring System for Circular Deformation in Metro Shield Tunnels in Soft Soils

For in‐service metro shield tunnels in soft soils, large circular deformations are a major concern because they usually lead to various problems, such as water leakage, joint openings, and concrete cracks. However, the monitoring of circular deformation depends mainly on manual surveying, and the automatic monitoring methods developed in recent years generally have low economic applicability and are not widely implemented. In this study, an automatic and cost‐effective system was presented to monitor circular deformation in shield tunnels by using only inclinometers. Experiments were conducted to prove the assumption that each segment can be regarded as a rigid body and to investigate the position of the joint rotation center. Then, a method for monitoring circular deformation based on the rigid body and plane section assumptions was proposed. The joint opening angle, maximum joint opening width, horizontal diameter convergence, and bolt strain were calculated from rotation angles of segments which can be monitored directly by inclinometer. A case study was conducted for a section of a metro shield tunnel with an ongoing pit excavation nearby. The rotation of segments was measured using MEMS inclinometers, and the data were transmitted using ZigBee and general packet radio service (GPRS) wireless communication technology. Results show that the proposed system could be implemented to improve transportation safety in relevant situations and similar conditions.

Deformation Characterisation and Distress Diagnosis of a Metro Shield Tunnel by Adjacent Constructions

This paper presents a case history of the developmental effect of a large‐area excavation, 8 high‐rise main buildings, a series of annex constructions, and ground overloaded building demolition on the deformation characteristics of an existing shield tunnel within Guangzhou Metro Line No. 1 in close proximity to the development. The shield tunnel lies in a sandy layer of the typical upper‐soft and lower‐hard strata in Guangzhou district, and the deformation of the tunnel has been monitored since the tunnel was put into operation. The monitoring results reveal that the adjacent construction induces an excessive tunnel settlement with a maximum of 14.4 mm and an excess tunnel displacement with a maximum of 5.2 mm, which are within the corresponding limitations of the codes for the safe operation of urban rail transit tunnels. While the station expansion project is being conducted beside the tunnels, a series of tunnel distresses, including large‐area water seepage, spalling concrete blocks, and segmental cracks, are recorded. Our field monitoring data indicate that the tunnel is subjected to further vertical contraction and horizontal expansion due to the station expansion project, and a maximum tunnel flattening rate of 36.78% is detected. Furthermore, the tunnel linings are studied numerically and theoretically to obtain the limitations of tunnel deformation and discuss why tunnel distresses of water seepage, concrete spalling, and segmental cracking occur. Finally, on the basis of the analyses and discussions above, counteracting corrective measures, including compensation grouting soil strengthening and bonded steel plates, are adopted as exterior and interior strengthening methods, respectively, to eliminate further tunnel distresses and ensure safe operation. The lessons learned and summarized in this study may help prevent similar tunnel distresses from reoccurring in the future.

Study on the Interaction of Metro Shield Tunnel Construction Under-Crossing the Existing Longhai Railway

Based on the construction of Zhengzhou Metro Line 4 (Huozhan Street Station–Hanghai East Road Station) under-crossing Longhai Railway, this paper investigated the interaction between railway train operation and shield tunneling. A three-dimensional coupled finite element model of track-roadbed-soil, using the finite element software MIDAS/GTS NX, was established to simulate the stages of the shield tunnel construction and analyze the influence on the existing railway. During different stages of the shield tunnel construction, the ground subsidence caused by the passing of single train was analyzed by simplifying the train loads as vibration loads, and the numerical simulation results were compared with the field monitoring data. The results show that: (1) the maximum settlement of the railway is about 3.39 mm without train passing, which is less than the control standard range of 6 mm, and so the shield tunnel construction can ensure the normal operation of the upper railway; (2) vehicle-induced vibration will lead to additional subsidence of the railway, which is only 0.20 mm when the upper train has the speed of 160 km/h, so the total settlement is 3.59 mm, which is also less than the control standard range of 6 mm. In conclusion, the upper train doesn’t need to be limited with velocity in the process of shield tunnel construction.