Longitudinal Settlement Research Articles

Longitudinal structural resilience of shield tunnel: Characterization and field application

The longitudinal structural resilience performance of shield tunnels is an important concern given the dramatic development of underground systems and the increasing demand for maintenance work. In this paper, a new model using longitudinal relative differential settlement as the index of tunnel structural resilience performance is proposed. The resilience metric (Re) is defined as the ratio of the area integrated by the residual performance during environmental disruptions to the area integrated by the normal performance for the corresponding duration. Then, the proposed resilience analysis model is applied to a well-documented case in Shanghai, where the existing metro tunnel is disrupted by a newly constructed large-diameter shield tunnel undercrossing and subsequently repaired by soil grouting. The variations of tunnel settlement concerning construction parameters and driving distance of the shield machine are analyzed. The performance degradation characteristics of the tunnel during disruption and recovery are effectively captured. The results show that 32.3 % of the performance loss is attributed to the new tunnel undercrossing in the first 38 days. After the completion of the grouting reinforcement, the tunnel performance is improved from 0.677 to 0.868, accounting for approximately 59.1 % of the performance loss during the crossing period. Moreover, the resilience metric (Re) is calculated as 0.764, indicating a high level of resilience for the existing metro tunnel in this case. In addition, other performance indexes based on tunnel longitudinal settlement are discussed, demonstrating the good rationality and applicability of the proposed index.

Deformation analysis of ground and existing tunnel induced by overlapped curved shield tunneling

The deformation caused by tunnel excavation is quite important for safety, especially when it is adjacent to the existing tunnel. Nevertheless, the investigation of deformation characteristics in overlapped curved shield tunneling remains inadequate. The analytical solution for calculating the deformation of the ground and existing tunnel induced by overlapped curved shield tunneling is derived by the Mirror theory, Mindlin solution and Euler-Bernoulli-Pasternak model, subsequently validated through both finite element simulation and field monitoring. It is determined that the overcutting plays a crucial role in the ground settlement resulting from curved shield tunneling compared to straight shield tunneling. The longitudinal settlement distribution can be categorized into five areas, with the area near the tunnel surface experiencing the most dramatic settlement changes. The deformation of the existing tunnel varies most significantly with turning radius compared to tunnel clearance and grouting pressure, especially when the turning radius is less than 30 times the tunnel diameter. The tunnel crown exhibits larger displacement than the tunnel bottom, resulting in a distinctive 'vertical egg' shape. Furthermore, an optimized overcutting mode is proposed, involving precise control of the extension speed and angular velocity of the overcutting cutter, which effectively mitigates ground deformation, ensuring the protection of the existing tunnel during the construction.

Mechanical behavior of immersed tunnel under the influence of differential settlements: A case study of GZ tunnel

As a typical marine and underwater infrastructure, the joints stress state of immersed tunnels is complicated due to the differential settlement during operation. Based on the current attitude of the segmental elements, this study takes GZ Tunnel as an example to preliminarily discuss the influence of differential settlement on the mechanical behavior of immersed tunnels through field monitoring, on-site detection and numerical simulation. The result shows that: The differential settlements of immersed tunnels can be divided into longitudinal and transverse, specifically manifested as bending or torsion of elements. The maximum longitudinal settlement and maximum transverse settlement of immersed elements is 108.7 mm and 27.9 mm, respectively. Under the influence of differential settlement in different directions, and the shear keys of the central wall and the sidewall with larger settlements are subjected to greater forces. Numerical simulation shows that the maximum principal stress and minimum principal stress of GZ Tunnel can reach 20.87 MPa and 22.21 MPa, respectively, which is located at the joint between E2 element and the northern buried element. The failure mode of the shear keys is cracking near the inner corners and local concrete crushing. The failure reason can be summarized as the compressive or tensile shear caused by the misalignment and torsion of the elements. The reason of differential settlement of GZ tunnel is caused by construction and long-term operation. In studying underwater facilities such as immersed tunnels under the influence of differential settlement, it is suggested to consider the three-dimensional attitude change of the structure comprehensively.

Prediction interaction responses between railway subgrade and shield tunnelling using machine learning with sparrow search algorithm

Tunnelling-induced uneven ground structure settlement is a hot research topic involving various interrelated factors. This paper employs hybrid algorithms to establish the predictive model for the interaction responses, including maximum settlements, the longitudinal settlement curve, and the shield operational parameters. We choose four machine learning (ML) models: back-propagation neural network (BPNN), long short-term memory neural network (LSTM), least squares support vector machine (LS-SVM), and deep extreme learning machine (DELM). The sparrow search algorithm (SSA) searches for optimal hyperparameter combinations to improve prediction performance. We comprehensively compare the above models' accuracy and generalization ability for different predicting objects. The database used in this study is collected from a subway project in Beijing, China, where the excavation of twin shield tunnels caused subgrade differential settlements on four national railway lines. The in-situ data from the right line of twin shield tunnels is used to train and test the models, while that from the left line is applied to verify the generalization ability of the models. The DELM-SSA model performs well in predicting maximum settlement, while the LSTM-SSA model excels at predicting shield operational parameters. The LS-SVM-SSA model accurately predicts the monitoring points' longitudinal settlement curve. According to the results, different models are recommended for predicting the interaction responses. The analysis of the Pearson correlation coefficient also reveals that shield operational parameters, such as shield driving speed (Sds) and cutterhead rotational speed (Crs), correlate relatively strongly with the settlement.

Experimental and theoretical study on longitudinal deformation and internal force of shield tunnel under surcharge

The longitudinal behavior of a shield tunnel is essential for analyzing its deformation and internal forces under surcharge. This study involved the design and construction of a reduced-size indoor model of a tunnel to investigate its performance with various initial ovalities when subjected to different surcharges. Furthermore, a model was proposed to investigate the longitudinal deformation of the tunnel caused by rotation and dislocation, which was verified and compared. The results reveal that the longitudinal settlement of the tunnel follows a normal distribution. The maximum settlement occurs at the central ring and increases linearly with the applied load. The zone of strong influence extends approximately 1.6 times the load width, and an additional load of 20 kPa can be employed as a control value for the surcharge. Additionally, the study found that stress concentration typically occurs on the side of the tunnel waist under surcharge. As a result, the horizontal convergence at the central ring increases at a faster rate, leading to transverse elliptical deformation of the entire structure. The longitudinal curves of curvature and dislocation visually reflect that the dangerous sections of the tunnel structure are concentrated at the center and edge of the load. Moreover, the rate of deformation development and final deformation values differ among tunnels with varying initial ovalities, and there is no simple linear relationship between them. The oval shape of the tunnel exacerbates the progression of longitudinal deformation while inducing a three-dimensional coupling effect between adjacent segments. Ultimately, the plausibility of the experimental results is confirmed based on the proposed longitudinal model of the tunnel, thereby establishing a connection with practical engineering applications.

The Optimization of Secondary Lining Construction Time for Shield Tunnels Based on Longitudinal Mechanical Properties

In the field of shield tunnels, the occurrence of uneven longitudinal settlement in segment linings has presented persistent challenges, including heightened risks of localized damage and water leakage. While the adoption of a secondary lining has been proposed as a viable solution to these issues, the question of how to select an appropriate construction time for the secondary lining, one that enables it to fully harness its load-bearing capacity while optimizing the tunnel’s overall stress and deformation characteristics, continues to be a pressing concern. To address this issue, this study established a three-dimensional longitudinal refined numerical model of double-layer-lined shield tunnel. In addition, the deformation degree of the segment lining was used as a time indicator to define the construction time for the secondary lining. Subsequently, an analysis of the impact of the construction time of the secondary lining on the longitudinal mechanical properties of the double-layer-lined shield tunnel is conducted through an assessment of tunnel longitudinal deformation and structural stress. The research findings indicated that the construction of the secondary lining improved the longitudinal deformation resistance of shield tunnels. Simultaneously, it led to a significant increase in the longitudinal shear forces within the segment lining and a notable reduction in longitudinal bending moments. Moreover, the construction time of the secondary lining played a pivotal role in these alterations. Considering the longitudinal force situations and load-bearing characteristics of the double-layer lining structure, it was determined that the optimal construction time for the secondary lining fell within the range of 20% to 40% of the total construction duration. In this scenario, the deformation and internal forces within the segment lining remained within permissible limits. Additionally, both the segment lining and the secondary lining were able to fully utilize their load-bearing capacities, ensuring the economic and safety aspects of the tunnel.

SBD-K-medoids-based long-term settlement analysis of shield tunnel

Long-term settlement is essential to operational tunnel safety. In this study, a systematic method for analysing shield tunnel settlement during the operation period is proposed. First, a new time-series clustering algorithm, namely shape-based distance (SBD)-K-medoids, is proposed to mitigate the limitation of low efficiency. The clustering precision and efficiency of the new algorithm were validated through hypothesis testing. Subsequently, the long-term settlement analysis framework is illustrated in detail, consisting of four parts: data pre-processing, section division and settlement characterisation, similarity clustering, and attribution analysis. A new index, called the incremental tunnel settlement state (ITSS), is proposed to depict a shield tunnel’s long-term longitudinal settlement during the operation period. Finally, a case study of the Shanghai Metro Line 10 (ML10) was conducted to validate the method. The results indicate that the uplink of ML10 is divided into four clusters, which correspond exactly to four situations a shield tunnel section undergoes during operation. Through an attribution analysis, the formation conditions of each cluster are summarised. Corresponding settlement patterns are concluded and engineering countermeasures are proposed for rehabilition. In general, this case exemplifies the rationality and engineering practice value of the study’s approach, and the systematic method offers a new approach for analysing the long-term settlement of shield tunnels.

Dynamic Prediction of Longitudinal Settlement of Existing Tunnel Using ConvRes-DLinear Model with Integration of Undercrossing Construction Process Information

Undercrossing construction can cause severe structural deformation of the above existing tunnel in operation. The induced longitudinal differential settlement between the segments can pose a huge risk to running subways, hence it is of great importance to monitor and predict the settlement. Within this study, a Wireless Sensor Network (WSN) system was implemented to obtain hourly monitoring data of settlement from the very beginning of undercrossing to post construction period. An improved direct multi-step (DMS) forecasting model called ConvRes-DLinear is proposed, which fuses monitoring data with time and process encoding bias to deeply extract and learn temporal correlation of time series. A residual LSTM model is also constructed to compare the accuracy of the improved DLinear model. The training and testing experiment on the monitoring data of longitudinal settlement obtained by WSN system shows that the ConvRes-DLinear model with time and process encoding bias performs surprisingly well with a minimum prediction error. The features of the proposed model are discussed to make the results explainable. The monitoring system and time series forecasting model proposed in this study have a guiding significance for the monitoring and prediction of longitudinal differential settlement of tunnels under environmental disturbance.

Wireless tilt sensor based monitoring for tunnel longitudinal Settlement: Development and application

The monitoring of longitudinal settlement of tunnel linings plays a great role in the preventive maintenance due to the significant effect of settlement on degradation of the serviceability of tunnel. This paper introduces a novel tilt sensor based vertical deformation monitoring system (VDMS) for the longitudinal settlement of tunnel linings. The VDMS utilizes a Micro-Electronic-Mechanical-System based wireless tilt sensor that is embedded in a rigid tube, making it easy to assemble and disassemble. The joint design between successive tubes is well-designed to ensure accurate monitoring of the vertical deformation of linear structures. A calibration test is carried out, and a compensation algorithm is proposed to reduce systematic error. The VDMS achieves a purely random relative closure error of about 2.22%. The monitoring accuracy is compared to the robotic total station, which is more expensive and only used for emergence monitoring. The mean discrepancy between the VDMS and the total station is limited to 6.15%, demonstrating the high accuracy of the VDMS. Overall, the VDMS offers high precision, ease of assembly and disassembly, and friendly installation, making it a valuable tool in preventive maintenance for tunnel serviceability.

Longitudinal settlements during tunneling in soft Clay, using transparent soil models

The purpose of this study was to measure internal longitudinal displacements taking place during tunnel excavation and compare them to the displacements predicted using available analytical and empirical formulations. A physical model made with transparent soil permitted exploring the internal soil deformations, within the supporting soils. A soil surrogate made of Magnesium Lithium Phyllosilicate (MLPS) was used to represent soft clay. Tunneling was simulated using a moveable trap door arrangement for different face losses ranging from 5 to 30% D. Internal soil movements in the longitudinal direction were captured for three cover-to-diameter (C/D) ratios ranging from 1.5 to 3.5. Shear as well as volumetric strains were computed using the captured spatial displacements. The results demonstrate that the shape of the longitudinal profile is more dependent on the overburden depth of the tunnel in comparison to face losses, in particular: (i) subsurface settlement is approximately 2–4 times the observed surface settlement for the same C/D ratio; (ii) a longitudinal soil arch is likely to form with the increase of C/D ratio which limits the observed settlement; and (iii) shear strain fields showed that a cascading failure may occur with the increase in face movements, especially for shallow tunnels. Finally, the computed and measured surface and subsurface settlement profiles were generally in agreement especially as C/D increases, thus validating methods used to predict subsurface settlements where measurements are sparse.

Tunnel depth effect to pile in Tunnel's influence zone

The increased population in urban areas resulted in high demand for public facilities; hence, tunnels became popular, especially in developed countries. However, tunnel activity may alter the original ground equilibrium, leading to a ground stress release. These may result in an adverse pile response and potential hazard to a nearby building structure. Previous research focuses on the tunnel-pile interaction but solely investigates in homogeneous soil formation. Therefore, this paper presents the tunnel-pile-ground interaction with various tunnel depths in multiple soil formations. A three-dimensional model was developed by using the finite element method of ABAQUS based on Circle Line Stage 3 (C852) projects in Singapore. The models were carried out in two stages: first, by investigating the tunnel-induced ground movements (free-field condition) and second, with the presence of a pile. The tunnel model depth varied at respective ratios of C/D of 1, 2, 3, and 4. The results show, with a low tunnel depth ratio, a high maximum ground settlement is obtained. The longitudinal settlement trough presents a smooth surface settlement curve for free-field conditions, but a chimney-like transition at the pile periphery for conditions with the pile. The ground settlement trough for tunnelling near to adjacent pile leads to a skewed maximum settlement towards the pile. A higher C/D ratio produced higher lateral stress induced on the pile but the opposite for a lower C/D ratio. Pile was significantly affected and shown by extensive lateral stress for the pile body that lies parallel to the tunnel depth.

A closed form shape function describing 3D settlement field around a deep excavation in sand

Soil movements produced around deep excavations are one of the main issues to be addressed during design and construction of underground structures in urban environment. Several methods to predict ground movements are currently available. Semi-empirical methods correlate displacements and simple geometrical features of the excavation; these methods predict separately transversal and longitudinal settlement troughs or at least provide a conservative envelope of them and they are mainly based on empirical data of excavations in clay. Numerical methods based on the solutions of FEM or DFM models usually provide prediction of the green field subsidence. In this paper existing 2D semi-empirical methods are first shortly reviewed. Some of these methods are then modified and combined into a new 3D analytical description of the subsidence trough around a deep excavation. The proposed two variable subsidence function depends on several parameters: four independent parameters defining the overall shape and the maximum settlement acting as a scale parameter. Settlement fields reproduced by a 3D FEM nonlinear parametric study of deep excavations inspired by real case studies are thus presented and discussed. The newly proposed 3D analytical description is applied to fit the FEM results and some fundamental relationships among geometrical features of the excavation pit and the 3D shape function parameters are identified. These relationships are validated via the application to three case studies of deep excavations in sand where the subsidence was controlled by the deformation of the retaining structures finding a satisfactory and encouraging agreement.

A Novel Detection and Assessment Method for Operational Defects of Pipe Jacking Tunnel Based on 3D Longitudinal Deformation Curve: A Case Study.

Adjacent tunnel construction and environmental disturbances can lead to longitudinal deformation in pipe-jacking tunnels. The longitudinal deformation of the tunnel is closely related to the occurrence of joint dislocation, joint opening, and other defects. In view of the difficulty of obtaining 3D longitudinal deformation curves, a method is proposed to obtain 3D longitudinal deformation curves based on a large number of 3D point cloud data with high spatial resolution and large spatial dimensions. Combined with the mechanism of defects occurrence, a theoretical basis for tunnel defects assessment based on tunnel longitudinal deformation is proposed. Taking one pipe jacking tunnel as an example, the longitudinal settlement curve and the 3D longitudinal deformation curve are compared. The correlation between the 3D longitudinal deformation curve and defects such as mud leakage, cracks, and differential deformation is illustrated from the perspective of three indexes: deformation amount, bending deformation, and shearing deformation. The accuracy and reliability of the 3D longitudinal deformation curve in tunnel defects detection and assessment are verified.

A Three-Dimensional Model of Shield Tunnel considering Interaction of Soil and Lining

The interaction between soil and tunnel lining is an important factor affecting the performance of the tunnel longitudinal structure. In this paper, a three-dimensional model (Model 1) considering the interaction between soil and lining by interface element is built to analyze the response of shield tunnel lining to local surcharge loading. In order to do a comparison, the other model (Model 2) is established without an interface element too. The parametric study of the interface element in Model 1 is carried out, in which parameters such as cohesion, friction angle, normal stiffness, and tangential stiffness are considered. The numerical models are calibrated with experimental results of the model test in the lab obtained from the literature. It is concluded that, among the parameters of the interface element, the changes of cohesion and friction angle have little influence on soil-lining interaction, and the values of normal stiffness and tangential stiffness are more important in simulating soil-lining interaction. The maximum settlement of the tunnel is generally located in the center of the load, and the settlement is symmetric about the center of the load. With the increase of stiffness of soil, the trend of the longitudinal settlement of tunnel is similar, and the maximum settlement of tunnel decreases gradually. The result of Model 1 is much closer to experiment results than Model 2. The general deformation in Model 1 is smaller than the one in Model 2, and the slippage between the soil and lining can be found clearly in Model 1. Because of the interface element in Model 1, the restrain of soil around the lining can be simulated better. Through an engineering project, it can be found that the tunnel deformation can be predicted by the three-dimensional model considering the interaction between soil and lining by interface element well.

Three-dimensional coupled hydromechanical analysis of localized joint leakage in segmental tunnel linings

Water leakage has become the main defect that affects the safe operation of shield tunnels in the saturated stratum. Long-term leakage is proven to change the load distribution and stress state of the tunnel lining, which hampers the structural performance of the tunnel. This paper proposes a novel numerical approach to simulate the localized leakage of segmental linings. The mechanical response of the longitudinal/circumferential joints is obtained by using a developed joint connection element, while the seepage response of the joints is simulated by implementing the DFLOW subroutine in Abaqus. The hydraulic boundary conditions are set at both longitudinal joints between segments and circumferential joints between rings. The hydromechanical behavior of the surrounding soil is incorporated based on the stress-pore water pressure interaction. A series of parametric numerical analyses is performed to explore the influences of partial leakage at the circumferential joints on the lining forces, lining deformation, and ground settlement. The simulation results demonstrate that the internal forces and maximum settlement of the tunnel structure and the magnitude of the ground settlement are substantially affected by the seepage rate, while the shear stiffness of the circumferential joints has only a slight effect. The longitudinal settlement of the tunnel and ground surface increase with the increase in the seepage rate of the circumferential joints, and a leakage-location-related distribution pattern is observed. The shear stiffness of circumferential joints mainly affects the pattern of the differential settlement of segmental linings in the longitudinal direction. These analyses indicate the appropriateness of such detailed three-dimensional simulations for capturing the realistic hydraulic and mechanical interactions of joint-leaked segmental linings.

Resilient analysis on tunnel structural serviceability based on lifetime dynamic prediction model

In order to ensure tunnels safety and serviceability of under whole life cycle, it is indispensable to predict tunnel performance and appropriate maintenance time under multi-defects. In this study, the performance of tunnel system is regarded as the composition of long-term aging deterioration and a series of short-term events to define its development with time. With the subsequent events updating the residual performance, an innovative method for dynamic analysis of tunnel performance is proposed. According to failure mode and effects analysis (FMEA) method and hierarchical model, an initial life performance model is established. Divided into the failure, evolution and recovery stages, the system evaluates the performance variation of specific defect under short-term events. In the case study, an additional excavation event is put into consideration to analyze the performance loss and the new pattern of lifetime performance model. The key issues of the tunnel are extracted into longitudinal settlement, differential settlement and leakage. With the theory of key defects, the thresholds conditions are classified into four stages and the damage-recovery process are evaluated by the concept of resilience. The process of dynamic prediction model to tunnel system is employed to update the variation of maintenance schedule and the serviceable time among defects, meanwhile the new performance curve of tunnel system can be modified through the evaluation of structural equation. Finally, if all thresholds are not exceeded by short-term events, the original life-cycle service curve and short-term event are superimposed as the update of life cycle. The result implies that despite the short-term event only costs minor damage to the system, the consequence may cast a critical loss on the residual of operation time.

An Investigation on Longitudinal Surface Subsidence Deformation of Large-Span Undercut Metro Tunnel

Underground excavation techniques are frequently used in subway stations during the construction of urban rail transit. The current study analyzes and investigates the surface settlement and deformation laws of large-span shallow buried underground excavated stations depending on a vast quantity of measured data from the Qingdao Metro Line 3. According to the comprehensive study, obtained results show that: 1) the excavation of pilot tunnels on both sides will result in the longitudinal settlement at the ground surface; however, the settlement will be quite minor, within −10 mm. During the excavation of the middle part of the pilot tunnel, large surface settlement may occur, and the settlement may reach −30 mm, while during the excavation of the lower soil and construction of the station structure, the surface settlement changes insignificantly, and some monitoring points show a sinking trend. 2) Data fitting reveals that regression analysis with a one variable cubic polynomial produces excellent fitting results. In addition, the analysis based on statistical methods reveals that the conditions of the surrounding rock are poor (Class that the arch, and the overall buried depth of the station is not larger than 10 m. 3) Since the surface settlement control standard for large-span boring tunnels is −60 mm, using −48 mm as the warning value is more reasonable. The test conducted in the current study has helped accumulate a large amount of ground settlement monitoring data, which can provide a particular reference for the subsequent design of similar lines.

Deformation Characteristics of Existing Twin Tunnels Induced by Double Shield Undercrossing with Prereinforcement: A Case Study in Hangzhou

This paper is based on the case of the earth pressure balance (EPB) shield tunnelling project of the new Metro Line 2 undercrossing the existing Metro Line 1 in the soft soil urban area of Hangzhou. Because the EPB shield must break through a plain concrete wall before undercrossing the existing tunnels, the pipe roof prereinforcement was adopted to stabilize the soil between the existing tunnels and the new shield tunnel. The deformation characteristics of the existing tunnels in the process of double shield undercrossing were discussed. According to the variation of shield position, the settlement development could be divided into three stages: shield approaching subsidence, shield crossing heave, and shield leaving subsidence. The horizontal displacement shows a back and forth variation characteristic consistent with the direction of shield tunnelling. At the junction of tunnel and station, the shield undercrossing caused considerable differential settlement between the existing tunnel and the station. The construction of pipe roof prereinforcement will lead to the presettlement of the existing tunnels. The settlement of the existing tunnels caused by the attitude deviation of pipe roof and grouting disturbance should be reduced in reasonable ranges. In addition, the maximum longitudinal settlement of the existing tunnel during the shield second undercrossing was also discussed. It was considered that the influence of wall breaking is greater than the sequence of shield undercrossing. The driving parameters of shield tunnelling should be optimized before the second crossing.

Three-Dimensional Finite Difference Analysis on the Ground-Sequential Tunneling-Superstructure Interaction

In this paper, a three-dimensional finite difference analysis is presented to investigate the interactive effects of sequential tunneling and the superstructure on the settlement profile of the ground. To simulate the practical sequential tunneling procedure, tunnel excavation is conducted in a step-by-step framework; tunnel excavation starts from the beginning of the model and is updated in a continuous manner, and the installation of the tunnel support system is done with a delay step compared to tunnel excavation. The numerical modeling accuracy is validated using the available analytical and numerical solutions for both two-dimensional and three-dimensional simplified cases. The well-validated modeling procedure is adopted to investigate effects of tunnel diameter, depth of tunneling, and number of superstructure stories on the profile of occurring settlements. Two cases of free-field and three-dimensional superstructural modeling are compared with regard to the effect of tunneling. In addition, the effect of tunneling advancement on the generation of excess structural forces and moments are studied as another important factor in the soil-tunneling-superstructure interaction problem. It is observed that, in the free-field case, with advancing the tunnel face, the longitudinal settlement profiles approach the steady-state condition and the maximum ground settlement tends to converge to a specific value, whereas as the tunnel passes under a structure, the settlement increases steadily as the tunnel progresses. There is a direct relationship between the depth and diameter of the tunnel and the settlement. In addition, the effect of the number of superstructure stories on the maximum settlement is more considerable compared to the free-field condition. According to the results, when the tunnel passes under 8-story and 12-story structures, the maximum settlement increases by 40% and 70%, respectively, compared to the free-field condition. It is also shown that tunneling-induced settlements result in the regeneration of structural forces.

Subsurface settlements of shield tunneling predicted by 2D and 3D constitutive models considering non-coaxiality and soil anisotropy: a case study

Appropriate constitutive models and reliable excavation and support sequences are believed to be the major concern in using finite element (FE) analysis to simulate shield tunnel excavation. This paper presents systematic two-dimensional (2D) and three-dimensional (3D) FE analyses employing a number of constitutive models accounting for initial soil anisotropy and non-coaxial plasticity, as evidenced within site investigations from the Tsinghuayuan Tunnel of the Jing-Zhang high-speed railway in China. The aim is to assess the effects of both the initial soil anisotropy and non-coaxiality on longitudinal and transverse tunneling-induced surface settlements. It is shown that the excavation procedures combined with the degree of cross-anisotropy are key towards the accurate prediction of maximum vertical displacements from tunneling, matching field data. Knowledge of the initial soil strength anisotropy can further improve the shape prediction of the transverse tunneling-induced surface settlement troughs. When considering n = 0.6 and β = 0° in simulations, the transverse surface settlement trough obtained is almost coincident with monitored field data. Initial stiffness anisotropy used in the prediction of shield tunnel-induced surface settlements in sandy pebble soils does improve realism of results significantly. The maximum longitudinal settlement predicted by considering cross-anisotropy is larger than that predicted by its isotropic counterpart.