Tunnels In Soft Ground Research Articles

Quantitative analysis of construction risks in shallowly buried biased tunnel portal section

To address the prominent problem of collapse instability in shallow buried soft ground tunnels, a non-invasive stochastic finite element method was introduced. Taking Fujian Puyan Wenbishan tunnel as the background project, ABAQUS finite element software was used to analyze the tunnel excavation mechanics and parameter sensitivity. And developed the software interface program based on Python to output explicit limit state equation for the key mechanical indexes of the tunnel, so as to evaluate the tunnel reliability under different excavation methods, quantitatively. Study results show a significant improvement in efficiency and accuracy when calculating the probability of failure in tunnel excavation by the non-invasive stochastic finite element method. The maximum displacement monitoring points for the Wenbishan tunnel portal section were all vault settlement, with displacements of 33.6 mm, 30.2 mm, and 25.3 mm, respectively, using the annular retained core soil method, single sidewall guide pit and double sidewall guide pit method, with probabilities of failure of 36.11%, 28.03%, and 20.02%. It is found that the reliability of the tunnel is mainly determined by the geotechnical weight, elastic modulus and cohesion of the weak sandy soil layer, which can provide ideas for this type of engineering researches.

Characteristics of deformation and defect of shield tunnel in coastal structured soil in China

Shield tunnel is a type of linear underground structure assembled by lining segments, characterized with long joint, weak stiffness, and strict deformation control requirement. The situation of the long-term deformation and defect of the shield tunnel in soft ground in coastal area of China is severe, mainly attributed to the tunneling-induced ground consolidation, frozen cross passage, groundwater pumping, cyclic train load, and nearby construction. Shield tunnel is buried in ground, and the above factors could result in underlying ground settlement, overlying ground loading/unloading, and at-side ground unloading. As a result, the tunnel could suffer from different types of structural deformation and defect. Based upon the aforementioned different reasons, this study investigates the characteristics of the tunnel deformation and defect corresponding to the different types of ground stress change and deformation. It is found that tunneling-induced ground consolidation, frozen cross passage, groundwater pumping, and cyclic train load mainly contribute to the longitudinal differential settlement but negligible transverse convergence, associated with water leakages at circumferential joints. In comparison, surface surcharge and at-side unloading not only cause significant longitudinal differential deformation but also increase transverse lining internal forces, resulting in water leakages at circumferential joints, longitudinal lining concrete cracks and water leakages. Finally, nearby construction could strongly disturb the ground and cause the generation of excess pore-water pressure, making the shield tunnel deformation develops continuously after the nearby construction is completed.

Prediction of adjacent single pile deformation induced by tunnel excavation based on the Pasternak model

The construction of metro tunnels in soft ground would cause significant settlement of the ground surface and deformation of buildings and their foundations in the vicinity of the tunnels. The deformation laws of the pile of the neighbouring structures induced by the tunnel construction have not yet been fully identified. This paper analyzed the impact mechanism of tunnel construction on piles based on the Pasternak model, deduced the calculation methods of additional horizontal displacement and additional internal force of piles, and introduced three solution methods. The computational method results have been validated through numerical simulations using coupling of discrete element method and finite difference method and compared to select laboratory data. The results show that the model accuracy is better when the tunnel axis burial depth is less than the pile bottom burial depth. The analysis shows that the tunnel burial depth significantly influences the displacement distribution of the adjacent single pile, while the impact on the peak displacement is insignificant. The thickness of the elastic layer (C) is inversely related to the maximum horizontal displacement and internal force, but it does not impact the distribution patterns of the displacement and internal force. For thickness of the elastic layer (C), the bending moment’s point is constant at 2 m above and below the tunnel axis, whereas the shear force’s point remains constant at the buried tunnel axis’s depth and 4 m above the tunnel axis.

Ground settlement and tunnel response due to twin-curved shield tunnelling in soft ground with small clear distance

Twin curved tunnels are often encountered in shield tunnelling, where significant complexities in densely exploited underground space are observed. In this study, the ground settlement and tunnel deformation due to twin-curved shield tunnelling in soft ground were investigated using numerical simulation and field monitoring. Different curvature radii of twin curved tunnels and subsequent effects of tunnel construction were considered to reveal the tunnelling effect on ground surface settlement and tunnel deformation. The results show that the settlement trough yields one offset towards inside of curved shield tunnelling. The location of settlement trough and maximum settlement were affected by curvature radius but except for the shape and width of settlement trough. Adjacent parallel twin-curved shield tunnelling could increase the offset of existing settlement trough and maximum settlement. Then, an empirical prediction of surface settlement trough due to twin-curved shield tunnelling with same tunnel diameters in soft clay was proposed, which was applicable to curvature radius less than 800 m. Finally, a minimum radius of 600 m of curvature tunnel was proposed in terms of allowable convergence deformation of tunnel. The result could provide guidance on safety evaluation for twin curved shield tunnelling construction.

Geostress-associated settlements of a raft-foundation building due to shield tunnelling in soft ground

The stress state of soil may affect the building settlements induced by tunnelling, which, however, has not been well understood. In this study, 3D numerical analyses combined with in-situ measurements were performed to investigate the geostress-associated settlements of a raft-foundation building due to tunnelling in soft ground. Basically, two types of geostress fields were investigated: the first type considered the effect of additional stress generated in the foundation soil (FAS) due to building weight; while in the second type, a sequential twin tunnelling was presumed, and the effect of additional soil stress induced by the first tunnel (TAS) on the building response to the second tunnel was considered. The results indicated that FAS may aggravate the stress release of the foundation soil, and thus gave rise to a larger building settlement or inclination. In the sequential tunnelling process, the effect of TAS can be more complex: when the first tunnel lowered the stress of foundation soil, TAS effect of the first tunnel may help reduce the building settlements induced by the second tunnel; otherwise, it may aggravate building settlements. In addition to TAS effect, the sheltering effect was also found to play an important part in twin tunnelling.

Advancing Shallow Tunnel Construction in Soft Ground: The Pipe-Umbrella Box Jacking Method

The development of modern cities has often led to increased traffic congestion and limited usable space. One effective solution to these problems is to construct roadway tunnels, which can expand urban space and alleviate traffic congestion. However, building large underpass tunnels in urban areas, especially in soft ground, presents technical challenges owing to its extensive environmental disturbance. The pipe-umbrella box jacking method offers a promising solution to these challenges as this method does not require breaking the ground from the surface or disrupting surface traffic. This study reviews the key techniques of the pipe-umbrella box jacking method for constructing large rectangular tunnels with small burial depths in soft ground. The study focuses on three main technical issues: pipe umbrella installation, soil face excavation, and box jacking control. Furthermore, two tunnel projects in Shanghai, China, are presented as case studies to showcase the construction challenges and countermeasures during box jacking tunneling with pipe-umbrella. Practical construction experience has revealed that the installation of the pipe umbrella before box jacking serves as an effective method of ground pre-support. However, its performance can be influenced by factors such as pipe installation deviation and inter-pipe connection design. Both open-face steel grid extrusion (SGE) shields and closed-face earth pressure balanced (EPB) shields can be used for soil excavation during box jacking, with different work modes resulting in distinct ground deformation patterns. Additionally, lubrication grouting has been found to effectively reduce friction resistance and affect vertical ground deformation. The study also examines the lessons learned from case studies and proposes optimization measures. This study can serve as a valuable reference for the tunneling industry, providing insights into the pipe-umbrella box jacking method and its application in constructing large rectangular tunnels in soft ground.

Impact of Excavation Phase Shift on Mechanized Twin Tunnels in Soft Ground: A 3D Numerical Study

Impact of Excavation Phase Shift on Mechanized Twin Tunnels in Soft Ground: A 3D Numerical Study

Mathematical modelling for interaction between soft ground and small curvature shield tunneling considering viscoelastic characteristics influences

The current analytical solutions for predicting the ground settlements induced by small curvature tunneling in soft ground are generally conducted on the assumption of linear elastic foundation and provide little attention on the soil rheology. This paper introduces a mathematical model to estimate the small curvature tunneling induced adjacent ground settlement considering the soil viscoelasticity. By introducing the Boltzmann viscoelastic ground model under the Laplace transform, the time domain parameters converted from Poisson's ratio and shear modulus are derived to further obtain the viscoelastic ground loss solution and the Mindlin solution. Then, the proposed viscoelastic solutions are employed for the ground settlement caused by the over-excavation and imbalanced loads for the small curvature tunnel, which accounts for the soil rheology influence. The accuracy of the mathematical model is then verified by comparisons with in-situ observed data and 3D numerical simulation results, as well as good agreement is obtained. Finally, the parametric analyses are performed to estimate the influence for transverse and longitudinal surface settlements, including tunnel curvature radius, shield cutterhead face radius, over-excavation value, creep time and shear modulus ratio of viscoelastic ground.

Numerical investigation on impact of excavations in influence zone of existing MRT tunnels

ABSTRACT Due to a high demand of space utilisation in metropolitan area, the existing tunnels, which are normally used for underground transportation and service utilities, can be suffered from the impacts caused by the adjacent constructions. In general, the protection zone of a tunnel is developed based on the assumption of shear plane of soil, rather than the soil–structure interaction. The assessment of the impacts of new construction nearby the existing tunnel is thus essential for improving the protection zone. 3D finite-element analyses are conducted for the case of the mass rapid transit (MRT) tunnels in soft ground. The impacts on the MRT tunnels from two possible underground construction cases (i.e. open cut and pipe jacking) are investigated in terms of tunnel deformations and changes in tunnel lining moment. The advantages of using 3D analysis are to model the realistic soil–structure interactions and to observe the multidirectional impacts on MRT tunnels. The results show that the bending moments increased by both open cut and pipe jacking construction activities are relatively small. For the tunnel deformations, most cases are sufficiently secure, but only in some cases, the deformations of tunnel lining almost reach the limit of 6 mm. However, it is highly recommended that they are still required to numerically examine in case by case

Optimum spacing and rock breaking efficiency of TBM double disc cutters penetrating in water-soaked mudstone with FDEM

Cutter spacing is one of the key factors affecting the rock breaking efficiency of Tunnel Boring Machine (TBM). In the excavation of deep soft rock tunnels, the water content can change the fragmentation behavior by TBM cutters, and further influence rock breaking efficiency. This study takes mudstone as an example to study the influence mechanism of water content on the optimum cutter spacing and rock breaking efficiency when TBM disc cutters penetrate soft rock. Firstly, the FDEM water migration-fracture coupled model considers the water weakening effect based on existing experimental fitting results. Secondly, several numerical experiments are conducted to calibrate the microscopic parameters input in FDEM. Finally, a series of numerical experiments of TBM double disc cutters penetrating in water-soaked mudstones are carried out using the coupling model. The crushing characteristics, cutter force, acoustic emission (AE), and crack pattern with different water content and cutter spacing are discussed in detail. Moreover, a novel recognition algorithm is proposed to calculate the rock chip distribution and area. The crack number, work done by the cutters, and specific energy (SE) are quantitatively analyzed. The results indicate that the water content of 2%–3% and the cutter spacing of 60–80 mm can lead to the lowest SE and highest rock breaking efficiency. Overall, the optimum cutter spacing decreases with the increase of water content. The modeling results can supply guidance for the optimization of cutter spacing during TBM tunneling in water-rich soft rock ground.

Load-transfer-associated settlements of a piled building during shield tunnelling in soft ground

This study investigates the response of piled buildings to shield tunnelling in soft ground based on both a case history and numerical modelling. The findings indicate that building settlement can be significantly affected by the position of the tunnel relative to the pile foundation. Basically, two different settlement patterns of the pile toe are observed when the tunnel is excavated below and to the side of the pile foundation, implying the presence of different load transfer mechanisms: when the tunnel is excavated on the side of the pile foundation, the loads carried by the raft and pile shaft are transferred to the pile toe. The foundation may settle by compressing the pile-tip soil. In contrast, when the tunnel is excavated below the pile foundation, the loads carried by the pile toe are transferred to the pile shaft and raft. Unloading in the soil above the tunnel crown gives rise to foundation settlement. Notably, the settlement of the foundation in the latter mechanism is shown to be more significant.

Physical modelling of EPB TBM in dry sand and greenfield conditions

This paper presents the results obtained during the first stage of a large experimental project based on the so-called ‘Earth Pressure Balanced Shield’ (EPBS) method adapted to tunnelling in soft ground. In the first part, the details of a 1 g laboratory model of EPBS specially improved within the framework of this project are presented. In particular, the new elements of the experimental setup, its instrumentations, the machine control, the preparation of the soil mass model are detailed. In the second part, the results obtained from this new EPBS model for two excavation tests, carried out in purely frictional soil mass (dry fine silica sand) and greenfield conditions (without neighbouring structures), are presented. The analysis concerns both soil surface and subsurface displacements, vertical and horizontal stress evolution in the ground when the Tunnel Boring Machine (TBM) progresses. It confirms that the highly sophisticated design of the machine allows to simulate the 3D excavation process closely, and to reproduce kinematic phenomena in the ground being in good agreement with those observed in the practice. In the case of the steady state excavation imposed during the tests, the extension of the TBM impact zone could also be identified based on the measured stress and strain variations of the ground.

Analytical approach to estimating the influence of friction slip contact between surrounding rock and concrete lining on mechanical response of deep rheological soft rock tunnels

For a deep-buried tunnel in soft ground, the contact condition between the surrounding rock and lining significantly affects the mechanical response. This is because the slip bond relationship on the contact surface has a direct influence on the tangential slip, resulting in the redistribution of displacement and stress on the contact surface between the surrounding rock and lining. However, previous studies have not paid sufficient attention to this problem, and a well-established method for quantitatively evaluating this influence remains unavailable. Therefore, the influence of the contact behavior between the surrounding rock and lining was examined in this study using an analytical method. First, the concept of friction slip contact and corresponding theoretical equations are proposed based on the real contact behavior between the surrounding rock and lining. Second, considering the friction slip contact between the surrounding rock and lining, a mechanical model of the tunnel excavation-support process was established, and analytical solutions for the stress and displacement of the tunnel within a viscoelastic rock material were realized. Third, the effectiveness and reliability of the proposed solutions were validated. The results could be degenerated to those that do not consider the influence of the mechanical behavior of the contact surface. In addition, there was good agreement between the analytical results and further numerical calculation results. Finally, a comprehensive parametric investigation was conducted, including the influences of the initial release coefficient, friction coefficient, and installation time of the lining. Some useful recommendations for tunnel lining design are provided herein.

Deep learning‐based automatic detection of muck types for earth pressure balance shield tunneling in soft ground

AbstractFor the earth pressure balance shield, the muck can reflect soil information at the tunnel face to track the change in geologic conditions. Thus, this paper presents a general framework for automatic detection of muck types based on the on‐site surveillance camera using deep learning algorithms. A simplified muck classification method and the corresponding muck recognition criteria are proposed for the muck detection task. The muck detection model (MDM) based on You Only Look Once v4, is established on the muck dataset for Shanghai (MSH) after some optimization treatments. The mean average precision value of 97.73% of MDM is twice that of the original model of 48.47%. The MDM is then applied to Metro Line 14 in Shanghai. Results show that The MDM performs well and meets the real‐time requirements with frames per second of 60, and it outperforms other state‐of‐the‐art detection models both in accuracy and speed.

Exploratory application of a novel centre cross diaphragm method for the construction of large cross-section tunnels in soft ground

For tunnelling in urban areas, the current sequential excavation (SE) method usually increases the number of temporary drifts to achieve better control of surface settlements, which may prolong construction duration. In this case study, a four-drift SE method, termed the suspension-type centre cross diaphragm (S-CRD) method, was proposed and applied to the construction of a large cross-section tunnel in soft ground, where normally the six-drift double-side drift (DSD) method would be adopted. Field monitoring results and statistical data indicated that, in comparison with the six-drift DSD method, the four-drift S-CRD method achieved better control of the surface settlements while improving the efficiency of construction by 30%. The advantages of the S-CRD method, i.e., safe and rapid construction, were eminently clear.

Estimating settlements due to tunnel boring machine excavation

Soft-ground tunnel boring machines (TBMs) are the preferred solution for the construction of long tunnels and linear infrastructure assets, especially in urban areas. TBMs allow the control of tunnel-face stability, minimising the effects on surrounding ground. Unfortunately, existing methods for the assessment of ground surface movements due to TBM tunnelling either utilise complex and computationally expensive numerical analyses or rely on simplistic volume-loss theories, which do not consider the characteristics of the ground and TBM operation. This paper presents a simple formulation to estimate the immediate surface settlement due to the applied TBM support pressure, based on an analogy with the hyperbolic behaviour of stress–strain curves of soils. The variables chosen to describe the ground movement were the maximum surface settlement and volume loss, while the TBM face support pressure is used to describe the tunnel internal support pressure. Uncertainties due to the inherent variability of geotechnical parameters were also considered, resulting in the definition of lower and upper boundaries. Data from a series of centrifuge test results, with and without tunnel-face reinforcement by forepoles, and a real-scale TBM case study were used to validate the proposed model. The analyses show that the proposed model adequately represented the observed settlement data.

Risk-based methodology to optimize geotechnical site investigations in tunnel projects

The scope of geotechnical investigations in tunnel projects is generally driven by the allocated resources rather than the expected variability in the ground conditions. Uncertainties in ground conditions may lead to poor decisions in project planning and active risk management. This paper presents a systematic and rational methodology to identify priority locations of additional geotechnical investigations for soft ground tunneling applications based on tunnel risks, site conditions, and project-related constraints. The methodology is applied to the cutter tool wear risk and quantifies the uncertainty in soil abrasivity index (SAI), the relevant geotechnical parameter. Preliminary geotechnical investigation data and site conditions from an actual tunnel project in an urban environment are used to illustrate the proposed methodology and demonstrate its effectiveness. Pluri-Gaussian simulation and sequential Gaussian simulation are used to characterize the 3D spatial variability in soil units and soil abrasivity, respectively, along the tunnel alignment. The study integrates geospatial assessments of SAI uncertainty and consequences of tool wear to develop an R index map that delineates the impact of uncertainty in tool wear rate. Project constraints of drilling accessibility and budget are incorporated in the R index map to find locations of additional geotechnical investigations. The study simulates a virtual sampling of additional boreholes and quantifies the reduction in uncertainty in tool wear rate and tunnel boring machine (TBM) intervention locations. Additional investigation at priority locations is found to reduce cutterhead intervention location uncertainty by approximately 90 rings (160 m). Further, integrating the uncertainty in tool travel distance is found to influence the cutterhead intervention location uncertainty by approximately 40 rings.

Analytical modelling of segmental liner joints for close-proximity tunnelling in soft ground

Close-proximity tunnellng seriously threatens the safety of existing tunnels and, therefore, leads a pressing need of enhancing our understanding of the role and mechanical properties of liner joints in tunnel construction. This study conducted an investigation into the moment-joint opening and load–displacement relationships of the liner joints under the effect of shield approaching, reaching, and passing the existing tunnel using the proposed analytical calculation that takes into account the concrete tensile strength and the real moment of inertia. The most affected liner joints for different tunnelling stages were recognised. The findings of this study indicated that the moment-joint opening relationship of the joints shows a good correspondence with the distance to the new tunnel excavation. The liner joint behaviour can be affected not only by the distance to the new tunnel excavation but also by the jack-arch mechanism. The findings also stand out the importance of the structural capacity available in the liner joints as they may operate completely in different manners, depending on tunnelling stages.

Effects of deep soil mixing on existing shield tunnels in soft soil ground

To mitigate the impact of adjacent construction on existing shield tunnels, deep soil mixing (DSM) has been widely used to reinforce the soft soil ground around shield tunnels. However, the construction of DSM may cause the movement of existing shield tunnels under soft soil and sensitive ground conditions, and reasonable installation parameters will reduce the impact of DSM construction on the existing shield tunnels. Based on the field tests of DSM installation parameters and a program of field measurements of existing shield tunnels during the DSM construction in Suzhou, the reasonable installation parameters of DSM were selected, and the movement of soil behind the soil mixing walls (SMWs) during multirow DSM installation was investigated. The movement of the shield tunnels caused by DSM construction were discussed in detail. The field test results showed that the DSM columns installed at a higher speed and a lower water-cement ratio enlarged the movement of the surrounding soil. The DSM should be installed at a lower speed and a higher water-cement ratio to reduce the movement of the shield tunnels. The field measurement results showed that the displacement of the tunnel lining was primarily caused by the construction of DSM zones beside the shield tunnels, which led to vertical compression and horizontal expansion of the tunnel lining. The construction of DSM immediately above the shield tunnels caused uplift to the tunnels. In addition, the deformed shapes of the two shield tunnels were asymmetric, and the displacement of the spring lining was larger than that of the crown. By taking the reasonable installation parameters of DSM and under the protection of the SMWs, the deformation of the shield tunnels caused by the construction of DSM was effectively controlled, and the maximum displacement was within the control value of the shield tunnels in this study.

Numerical analysis on the bearing mechanism of branch tunnel in upper softer and lower hard ground

It is of great interest to investigate the excavation disturbance mechanism of branch tunnel in the upper soft and lower hard ground. In this paper, a fine 3D numerical simulation, considering the longitudinal gradient of tunnel, was performed to investigate the bearing mechanism of branch tunnel. By comparison of the characterises of stress redistribution, two different bearing mechanisms were obtained: the beam bearing mechanism dominates the capacity of super-large section, and the beam bearing zone and stress concentration zone are identified in the arch zone; In the small interval section, the arch bearing mechanism works, and the stress loosening zone and arch bearing zone appears above the arch. The attributed reasons are related to the thickness of hard layer above the arch and the span of tunnel. Correspondingly, the arch settlement in the super-large section is much larger than that of the small interval section, while the deformation at arch foot of both sections moves outside. Furthermore, through the analysis of the maximum principal stress direction, the determination method of the outer boundary of the pressure arch was discussed.