Shallow Tunnel Research Articles

Stability Analyses of Shallow Rectangular Tunnels in Anisotropic and Nonhomogeneous Soils Using a Kinematic Approach

Stability Analyses of Shallow Rectangular Tunnels in Anisotropic and Nonhomogeneous Soils Using a Kinematic Approach

Dynamic response of a non-circular shallow tunnel with visco-elastic slip interface

Dynamic response of a non-circular shallow tunnel with visco-elastic slip interface

Effect of tunnel-building separation distance on the dynamic soil-structure interaction response

Understanding how buried structures respond to cyclic loading is crucial for designing earthquake-resistant underground infrastructure. The complexity of the dynamic response of these underground structures is accentuated when accounting for their interaction with adjacent buildings. This study focuses on the impact of the lateral distance between a shallow rectangular tunnel and an adjacent residential structure on dynamic soil-structure interaction, employing two dynamic centrifuge tests. Particle Image Velocimetry technique was adopted to observe the developed mechanisms of liquefied soil deformation, tunnel uplift, and foundation settlement. Results show the pronounced effects on soil deformation above the tunnel crown with a reduction in the tunnel-building separation distance. The distribution of excess pore pressure along the tunnel lining will be presented. Tunnel manifested additional lateral movement and reduced uplift when the adjacent building was in close proximity. A decrease in the tunnel-building distance contributed to a reduction of building rotation, primarily due to the significant mitigation of non-uniform settlement under the footing. Overall, the soil deformation mechanism around the tunnel and the building will be shown to change depending on the separation distance between them.

Numerical simulation and parametric analysis on a shallow tunnel in liquefiable ground subject to multiple shakings

In active earthquake zones, it is very likely that the structures would experience multiple shakings in major seismic events. For underground structures, the soil-structure interactions and the resultant dynamic responses could vary significantly according to the characteristics of the shakings. In this paper, the scenario of a shallow tunnel in liquefiable ground subject to multiple shakings is studied. The OpenSees platform and the PM4Sand constitutive model are employed to construct a plane-strain numerical model, where a rectangular tunnel is buried in saturated Hostun sand. Its validity is verified by the data from a centrifuge test of four consecutive shakings. The numerical soil-structure system is then configured for further investigation on the subject matter. Firstly, the liquefaction-induced uplift is identified as the main threat to the integrity of the tunnel in a seismic event of multiple shakings. Then, the pertinent soil-structure interactions are intensely discussed in a parametric analysis considering the influences of the permeability coefficient, the tunnel self-weight, and the shaking sequence.

Calculation of support forces at the excavation face of a shallow buried shield tunnel in dense sandy soil considering construction disturbances

ABSTRACT Too little support force at the palm surface during shield excavation can lead to destabilization of soil before the excavation face and cause ground subsidence. Construction disturbances during excavation can affect the mechanical properties of the surrounding soil, which cannot be ignored when determining the needed support force at an excavation face. Based on the Mohr – Coulomb yield criterion, the collapse of dense sandy soils before the excavation face of shallowly buried shield tunnels is simulated using PLAXIS 3D to determine the collapse mode. Furthermore, the prismatic body in the traditional 3D wedge model is modified to an inverted elliptical-truncated cone with a certain inclination so that the collapse zone is closer to real sliding soils, and the disturbance ratio r is introduced as an index to consider a construction disturbance. The expression of the active limit support force with respect to wedge inclination is derived, and the maximum support force is determined by trial and error to be the minimum support force needed. The results of the modified 3D curved model are in good agreement with the numerical simulation results, as well as the results of theoretical methods and model tests.

Study on dynamic and failure characteristics of sand and anti-floating safety of tunnel structures under long-term vibration loads of the Beijing subway

Most research into sand liquefaction has predominantly focused on the effects of earthquake ground motion, with a limited exploration into the possibility of sand liquefaction induced under long-term train vibration loads. Therefore, in order to explore the possibility of saturated sand liquefaction under long-term vibration loads of the Beijing subway, this article selected Tongzhou sand to conduct dynamic triaxial tests and obtained the dynamic and failure characteristics of saturated sand under different confining pressures and dynamic stresses. The test results showed that the confining pressure has a greater impact on sand liquefaction. When the confining pressure exceeds 100 kPa, the possibility of liquefaction of Tongzhou sand is fairly limited. Furthermore, a numerical model of an operating tunnel under the influence of long-term train vibration loads is established, and the dynamic response characteristics of the saturated sand and tunnel structure are studied. Numerical simulation results show that the local liquefaction of sand caused by train vibration will not cause the tunnel structure to float. Under local liquefaction conditions at the extremely shallow tunnel base, the soil did not completely lose its bearing capacity, and the displacement of the soil and structure was still dominated by settlement. Changes in tunnel burial depth have a greater impact on segment structure than train vibration loads. Tunnels with a more than 10 m burial depth will not liquefy in saturated sand within one year, and the segment structure is safe. The research results can provide a reference for the anti-floating safety of tunnel structures in similar saturated sand strata.

Optimizing the Support System of a Shallow Buried Tunnel under Unsymmetrical Pressure

In the construction process of tunnel inlet sections, the rock mass can sustain unsymmetrical pressure due to asymmetrical terrain on the two sides of the tunnel. The fact that the inlet sections are usually under shallow buried conditions with strongly weathered rock mass exacerbates the issue. This paper discusses optimization strategies of the initial support of a shallow buried tunnel based on the analytical results of asymmetrical loading characteristics. Numerical simulation is performed with particle flow code (PFC) using the Jianshanji tunnel project as an example. The simulation results show that the bench excavation has slightly less total deformation than the full-section excavation but the deformation range is wider, especially in the tunnel arch. Both lining support and slope reduction treatments can effectively improve rock deformation, with lining support demonstrating better performance in controlling deformation and adjusting stress distribution. Based on the simulation results, the bench excavation and lining support are used in the actual project, and the corresponding optimization control measures were adopted to address deformation issues, including crushed-stone backfilling for compression resistance, advanced grouting reinforcement, and grouting. The field data show that the tunnel stability is effectively improved by adopting the optimization schemes, which further validates the effectiveness of the proposed unsymmetrical control method.

Support mechanical response analysis and surrounding rock pressure calculation method for a shallow buried super large section tunnel in weak surrounding rock

At present, China's demand for high-speed railway construction is constantly increasing, and the construction of Multi line high-speed railway tunnels has been put on the agenda. The design and construction issues of super-large-sections tunnels urgently need to be addressed. The Xiabei mountain No. 1 and No. 2 tunnels in the Hangzhou-Taizhou Railway are typical shallow-buried super-large-section-tunnels in weak surrounding rock, and their design and construction issues are representative. Eleven monitoring sections were set up in the tunnel, including tunnel deformation, surrounding rock, shotcrete, steel frames, bolts and temporary support mechanical responses. Taking the monitoring data of the most typical cross-section as an example, the mechanical response of the support structure of a shallow-buried super-large-section tunnel was analyzed in detail. Based on previous research results, this paper discusses and summarizes the common construction problems of this type of tunnel, and puts forward corresponding suggestions. The existing formula for calculating surrounding rock pressure has poor applicability to super-large-section tunnels constructed by step excavation, resulting in conservative support parameters. Therefore, based on the monitoring values of surrounding rock pressure at 10 monitoring sections in Xiabei mountain No. 1 and No. 2 tunnels, empirical parameters reflecting the impact of step excavation were summarized. Based on the Wang formula and combined with the step excavation empirical parameters, an empirical formula for the surrounding rock pressure of shallow-buried super-large-section tunnels considering step excavation was constructed. The calculated results are in good agreement with the on-site monitoring data. This study can provide a good reference for similar projects.

Multifractal Characteristics and Displacement Prediction of Deformation on Tunnel Portal Slope of Shallow Buried Tunnel Adjacent to Important Structures

The tunnel portal section is often in extremely weak and fragmented strata, and the deformation of the portal side and slope will affect the stability of the surrounding rock and the tunnel-supporting structure. However, the deformation characteristics and displacement development patterns of slopes in the tunnel portal section are not clear. In this paper, the multifractal characteristics and displacement prediction of the deformation sequence of the tunnel portal slope at of a weak and water-rich shallow buried tunnel adjacent to an important structure are studied in depth. Combined with the deformation characteristics of the tunnel portal slope, a suitable slope monitoring and measurement scheme is designed to analyze the deformation pattern of the tunnel portal slope. Based on the multifractal detrended fluctuation analysis (MF-DFA) method, the multifractal characteristics of the deformation monitoring sequences at each monitoring point of the tunnel portal slope are analyzed. The multifractal characteristics of displacement sequences at different monitoring points of the tunnel portal slope are consistent with the actual monitoring results. Furthermore, the Long Short-Term Memory (LSTM) model is optimized using the Particle Swarm Optimization (PSO) algorithm to predict the deformation of the tunnel portal slope. The results show that the maximum mean square error (MSE) of the horizontal displacement test set prediction results is 0.142, and the coefficient of determination (R2) is higher than 91%. The maximum value of MSE for vertical displacement test set prediction is 0.069, and the R2 are higher than 91%. The study shows that the performance of the PSO-LSTM prediction model can meet the requirements for predicting the displacement of the tunnel portal slope. Based on the MF-DFA method and PSO-LSTM prediction model, the fluctuation characteristics of the displacement value of the tunnel portal section can be accurately identified and the displacement development pattern can be effectively predicted. The conclusions of the study are of great practical significance for the safe construction of the tunnel portal section.

Predictive modeling of shallow tunnel behavior: Leveraging machine learning for maximum convergence displacement estimation

Accurate prediction of maximum convergence in unsupported, shallow tunnel construction is crucial for optimizing the lining and ensuring tunnel safety. Machine learning (ML) algorithms, especially through boosting techniques, enable effective solution of complex engineering problems and demonstrate their capabilities in problem solving and optimization. In this study, the FLAC 3D package was used to create a robust and validated database of 954 datasets. Five tree-based ML algorithms, including extreme gradient boosting (XGBoost), adaptive boosting (AdaBoost), gradient boosting machine (GBM), histogram-based gradient boosting (HGB) and categorical boosting (CatBoost), were used to predict the maximum convergence displacement for unsupported shallow tunnels. For the test dataset, XGBoost outperformed the other models with an excellent coefficient of determination of 0.9633, a minimum mean absolute error of 0.0021 and a low root mean squared error of 0.00725. HGB followed closely behind, and GBM and CatBoost showed strong performances, while Adaboost was less effective. The superior performance of XGBoost highlights its effectiveness in predicting maximum convergence in shallow tunnels. An in-depth sensitivity analysis within the XGBoost model showed the significant influence of soil elastic modulus on the maximum convergence displacement in unsupported tunnels. The remarkable results achieved by the XGBoost algorithm on our complex tunnel convergence predictions illustrate the profound ability of ML to tackle complicated geotechnical challenges. This interdisciplinary collaboration demonstrates the potential of advanced algorithms to improve safety and efficiency in construction, underlining the crucial role of technology in tackling complex problems and establishing a new paradigm for innovation in the field.

Deformation of Existing Shield Tunnel Adjacent to Deep Excavations: Simulation and Monitoring Analysis

Deep excavations near subway tunnels can induce deformation, necessitating a comprehensive investigation into causal factors and mitigation strategies. Field measurements were conducted to assess both vertical and horizontal displacements of existing tunnels near a deep excavation in Shenzhen. Utilizing a validated three-dimensional finite element model that considers structure−strata interactions, this study analyzes tunnel displacements, ground movements, diaphragm wall impacts and the sensitivity of enclosure structure parameters. The results indicate that tunnel deformation correlates with enclosure structure deformation, particularly near the center of the pit. Moreover, shallow soil excavation significantly affects the vertical displacement of shallow-buried tunnels. However, the design parameters of the existing enclosure structures inadequately limit tunnel displacement. Therefore, it is crucial to intensify vertical displacement monitoring in shallow tunnels during early excavation stages and to enhance horizontal displacement monitoring during later phases. Implementing measures such as optimizing central support design or retaining soil at the pit bottom helps control maximum horizontal displacement. While support stiffness plays a greater role than retaining wall thickness, its impact on deep excavation projects is limited.

Shallow tunneling's impact on surface settlements

The research described in this paper deals with the impact of shallow tunneling on surface settlements in urban areas. The study focuses on carrying out a numerical analysis and a parametric geotechnical study to investigate the effects of tunnel excavation on the surface ground, particularly with regard to distortion and potential damage to structures above the tunnel. To carry out the study, the researcher used a finite element method (FEM) software package called Plaxis. This software is commonly used for geotechnical analysis and allows simulating and analyzing the subsurface conditions in Algiers underground in the context of excavating a shallow tunnel. The New Austrian Tunnelling Method (NATM) excavation method was chosen for the study. The results of the numerical analysis are presented in terms of displacements. The results indicate that ground flow has a significant influence on ground movement, probably contributing to surface settlement. However, the effect of tunnel excavation on the lowering of the water table and Young's modulus (a measure of soil stiffness) is relatively small in comparison. The research provides valuable information on the potential impacts of shallow tunneling in urban areas and the factors that play a crucial role in determining the extent of ground movement and surface settlement. By using numerical analysis and conducting a parametric geotechnical study, and gives valuable insights into the potential impacts of shallow tunnelling in urban areas and the factors that play a crucial role in determining the extent of ground movements and surface settlements. By using numerical analysis and conducting a geotechnical parametric study, and contributes to a better understanding of the behavior of the ground during tunnelling operations in Algiers. The findings can be helpful in designing future tunnelling projects to mitigate potential risks to structures and infrastructure located above the tunnels.

Deformation behavior and damage characteristics of surface buildings induced by undercrossing of shallow large-section loess tunnels

The ground movement during the construction of shallow loess tunnels can easily cause deformation damage to surface buildings. Most the current studies focus on the damage soft soil and rock tunnels to independent buildings, and there are few studies on the case of building groups in loess areas. Using the new Xi’Yan Railway Luochuan Tunnel as a case study, we conducted on-site testing to study building settlement and crack development characteristics. Three-dimensional numerical simulations were carried out to analyze settlement, flexure deformation, and main tensile strain distribution characteristics of the buildings at different buried depths. The study determines the extent of damage resulting from differential settlement and tension cracks. The results show that construction during the upper, middle, and lower bench stages results in significant ground volume loss, leading to a ’wide and steep’ settlement pattern with a maximum settlement value of 567 mm. Building cracks exhibit positive and inverted splayed shapes, with lengths ranging from 0.5 to 6.0 m and widths between 0 to 170 mm. As buried depth increases, maximum settlement, flexure deformation, and main tensile strain of buildings also increase. The severe damage range of buildings initially increases and then stabilizes, with the maximum range caused by differential settlement and tensile cracks being 34 m and 29 m from the tunnel axis, respectively. Based on the analysis of building damage characteristics, it was determined that a combination of surface measures and measures within the tunnel should be used to control building damage caused by tunnel construction. These research findings can serve as valuable references for similar projects.

Anatomic Double-Bundle Transtibial Anterior Cruciate Ligament Reconstruction With Ligament Advanced Reinforcement System

It has been reported that anterior cruciate ligament reconstruction (ACLR) with the Ligament Advanced Reinforcement System (LARS) could obtain similar clinical outcomes to ACLR with autograft. However, in most related reports, single-bundle ACLR was performed. Given that double-bundle ACLR is more favorable than single-bundle ACLR biomechanically, it is reasonable to try double-bundle ACLR with the LARS clinically. Thus, we introduce an anatomic double-bundle transtibial ACLR technique with the LARS, in which the most critical step is to create a shallow tibial tunnel for the anteromedial bundle to further create the corresponding femoral tunnel in a transtibial manner, as well as to fix both bundles in full extension of the knee.

Elastic-plastic response of tunnel in GZZ-based constitutive model

Investigating the stress redistribution characteristics after tunneling is critical for designing and constructing deeply buried tunnels. Three-dimensional stresses are more prominent in deep than in shallow tunnels, resulting in nonlinear strength. The generalized Zhang–Zhu (GZZ) criterion captures the nonlinear strength well. Therefore, this study established a 2D plane-strain numerical model using a GZZ-based constitutive model. The effects of the axial stress and rock properties on the stress redistribution and plastic zone are discussed. The initial axial stresses changed the axial stress distribution after excavation and had little effect on the radial and tangential stress distributions. The size of the plastic zone decreases with decreasing geological strength index and increasing rock mass mi . However, with increasing dilation coefficient, the stress distributions remained almost unchanged, whereas the radial deformation gradually increased.

Prediction of ground surface settlements by analytical and finite element methods

The soil surrounding the excavation of shallow tunnels may experience destructive deformation near the tunnels. Particularly in densely populated areas where land resources are scarce, ground subsidence caused by tunnel excavation, which in turn affects surrounding structures, deserves more attention. Over the past few decades, various research methods for predicting the maximum vertical ground surface settlement have been proposed. In this study, an empirical estimation method, an analytical technique for evaluating ground movements, and a numerical simulation method using Plaxis 2D, a finite element model software, were selected as prediction approaches for ground surface settlements. Subsidence measurements were obtained from a metro tunnel excavated using an earth pressure balance shield tunnel boring machine and constructed in a dense sandy gravel layer. Comparison of the monitored deformations with the estimation results calculated by different prediction approaches showed that the maximum vertical settlement of the ground surface evaluated by the analytical technique and finite element model agreed well with the actual measured values. This suggests that these two prediction methods have the potential for conserving nearby structures during the construction of underground infrastructures.

Mechanical Properties of Super-large and Shallow-buried Loess Tunnel with the Prefabricated Temporary Support

Background: At present, there are few studies on the mechanical properties of tunnels with prefabricated temporary support. Based on the project of the Xichengshan Tunnel on the highway from Zhuanglang to Tianshui, this paper studies the mechanical properties of tunnels with prefabricated temporary support The utility model patent for this article is a temporary steel support device for loess tunnel excavation. The patent number is ZL 2022 2 0141276. 1. Objective: The study aims to show the mechanical properties of a super large section of prefabricated temporary support shallow tunnel. Methods: The mechanical properties of the tunnel surrounding rock, lining, steel arch, anchor, and temporary support during construction were analyzed by combining numerical simulation and theoretical analysis. The maximum and minimum stresses of the tunnel surrounding rock, lining, steel arch, and anchor, and the maximum and minimum displacements of prefabricated temporary support were found through the analysis results. Results: The stress and displacement fields of surrounding rock, lining, steel arch, bolt, and temporary support of loess tunnel were analyzed. Results showed that the maximum principal stress of the surrounding rock is 0.293MPa, and the maximum horizontal displacement is 0.359cm. The maximum horizontal displacement of the lining is 0.413cm, the maximum vertical displacement of the steel arch is 1.538 cm, and the maximum displacement of the temporary support structure is 1.459 cm. The above data verify the safety of the proposed combined temporary support structure. Conclusion: Through the analysis results, the maximum and minimum stresses of the tunnel surrounding rock, lining, steel arch, and anchor, and the maximum and minimum displacements of assembled temporary support are found, which can guide subsequent construction.

Study on progressive failure mode of surrounding rock of shallow buried bias tunnel considering strain-softening characteristics

Mountain tunnels portal often have to pass through slope terrain unavoidably, thus forming a shallow buried bias tunnel. During the construction of shallow buried bias tunnel, disasters such as slope sliding and tunnel collapse frequently occur. The failure mode of surrounding rock obtained by current research is based on the limit equilibrium theory, which cannot reflect the progressive failure characteristics of the surrounding rock of shallow buried bias tunnel. In order to reveal the failure mechanism of the gradual instability of surrounding rock of shallow buried bias tunnel, the problem of gradual failure of the surrounding rock is reduced to an elastic–plastic analysis problem for surrounding rock considering the strain-softening characteristics. Based on the elastic–plastic analysis of the failure process of shallow buried bias tunnel, MATLAB was used to compile a program to read the finite-difference calculation result file, extract the effective information such as shear strain and tensile strain at the center point of each unit, and establish the analysis method of the progressive failure mode of shallow buried bias tunnel. The reliability of the method proposed was verified by comparing the failure process of the model test with the development process of shear strain increment. Under the condition of no support, the formation mechanism of failure plane of surrounding rock on both sides of shallow buried bias tunnel is different. The shallow buried side is the shear failure plane formed by the collapse of surrounding rock, while the deep buried side of the tunnel is the shear failure plane formed by the collapse of surrounding rock and slope sliding. Under the conditions of excavation and support, the failure plane of the shallow buried bias tunnel can be divided into three parts according to the formation sequence and reasons. The part I is the failure plane, which is formed by active shear under the influence of tunnel excavation. The part II is the failure plane formed by tensile crack of slope top. The part III is the failure plane formed by passive shear under the push of the soil in the upper part of the slope.

Assessment for shallow and large tunnel construction in weak ground conditions: Application of tunnel boring machines

AbstractWith recent technological advancements, tunnel boring machines (TBM) have developed and exhibited high performance in large diameters and weak ground conditions. Tunnels are crucial structures that significantly influence the timelines of highway and railway projects. Therefore, the construction of tunnels with TBMs becomes a preferred option. In this study, a comparative analysis between TBM and the New Austrian Tunneling Method (NATM) for tunnel construction is performed in the construction of the T1 tunnel with a diameter of 13 m, which is the longest tunnel in the Eşme‐Salihli section of Ankara‐İzmir High‐Speed Railway Project (Türkiye). The selection of TBM type, measures taken in problematic sections, and application issues of TBM are discussed. The impact of correct description of geological and geotechnical conditions on both selection and performance of TBM is presented. An earth pressure balanced type TBM is chosen for the construction of the T1 tunnel. Because of the additional engineering measures taken before excavation in problematic areas, the tunnel was completed with great success within the initially planned timeframe. From this point of view, this study is an important case and may contribute to worldwide tunneling literature.

Investigation on the natural smoke exhaust performance by vertical shaft in tunnel fires under different ambient pressures

In high-altitude tunnels, due to the ambient pressure and density of air being different from those under normal pressure, the convection and heat radiation in tunnels are also different when fires occur. As a result, the smoke exhaust capacity varies under different pressures. This work aimed to numerically explore the impact of ambient pressure on the smoke exhaust efficiency at high-altitude shallow tunnels with natural ventilation by shaft. The numerical results demonstrated that the smoke exhaust capacity is enhanced with increasing ambient pressure. This is because of the increase in the air entrainment coefficient and air density, and it causes plug-holing more easily to occur under higher ambient pressures. Once the plug-holing has taken place, the fresh air can be directly exhausted through the shaft, which results in a poor smoke exhaust capacity. By accounting for the factors of the smoke layer thickness, ambient pressure, the exhausted smoke temperature and heat release rate, the Richardson number was introduced as the criterion for determining whether plug-holing occurs. As ambient pressure was increased, the critical Richardson number for plug-holing was observed to decrease, a finding supported by existing research.