Shallow Tunnel Research Articles

Excavation method optimization and mechanical responses investigating of a shallow buried super large section tunnels: a case study in Zhejiang

The construction of super large section (SLS) shallow buried tunnels involves challenges related to their large span, high flat rate, and complex construction process. Selecting an appropriate excavation method is crucial for ensuring stability, controlling costs, and managing the construction timeline. This study focuses on the selection of excavation methods and the mechanical responses of SLS tunnels in different types of surrounding rock. The research is based on the Yangjiashan tunnel project in Zhejiang Province, China, which is a four-line highway tunnel with a span of 21.3 m. Three sequential excavation methods were proposed and simulated using the three-dimensional finite difference method: the “upper first and lower later” side drift (SD) method, the central diaphragm method, and the top heading and bench (HB) method. The mechanical response characteristics of tunnel construction under these methods were investigated, including rock deformation, rock pressure, and the internal forces acting on the primary support. By comparing the performance of the three construction methods in rock masses of Grades III to V, the study aimed to determine the optimal construction method for SLS tunnels considering factors such as safety, cost, and schedule. Field tests were conducted to evaluate the effectiveness of the optimized construction scheme. The results of the field monitoring indicated that the “upper first and lower later” SD method in Grade V rock mass and the HB method in Grade III to IV rock mass are feasible and cost-effective under certain conditions. The research findings provide valuable insights for the design and construction of SLS tunnels in complex conditions, serving as a reference for engineers and project managers.

Seismic isolation technology of shallow buried large section utility tunnel with soft soils in seismically vulnerable area

Shallow utility tunnels with exceptionally large cross-sections in weak soil can face significantly more severe seismic risks compared to conventional deep tunnels in seismic hazard areas. This study investigates the seismic response and isolation technologies applied to a large section utility tunnel with 4 compartments in one layer, employing seismic simulations. The engineering context, dynamic motions, and measuring points of the numerical simulation were introduced. Subsequently, the finite element method was employed to explore the seismic behaviors of the large section utility tunnel when subjected to strong earthquake excitations under four conditions. The study explored and comparatively evaluated the seismic isolation effectiveness of three proposed schemes: the grouting scheme, the buffer layer, and the assembly of three-quarters buffer layer, cushion and grouting. The analysis included various indexes such as the deformation, the principal and shear stress, and the safety factor. The finding reveals that the assembly seismic scheme exhibits the most significant seismic effect (95.39%), followed by the buffer layer (38.82%), and the grouting scheme (25.66%). The assembly seismic scheme is recommended for the seismic design in the current large-section utility tunnels. These conclusions provide valuable scientific guidance for the seismic design of large-section utility tunnels, aiding in enhancing earthquake resilience.

The influence of geometry on the heat exchange potential of embedded planar retaining walls with seasonally-balanced thermal load

A number of operational cases exist where embedded retaining walls, used in the construction of underground spaces such as basements and shallow tunnels, have also been utilised as ground-heat exchangers in shallow geothermal energy systems. These are complex structures in terms of their geometry, the surrounding temperature field and boundary conditions, and there are currently no methods to assess their heat exchange capacity in a simple and expedient manner. This contribution uses the finite element method to validate the use of the method in predicting heat flow for this application and then, to assess the influence of wall and excavation geometry in the heat exchange process. The influence of the soil and wall thermal conductivity is shown to be quasi-linear with the latter showing the greatest influence on peak heat exchange. The work identifies a geometric parameter - the ratio of excavation depth to total wall panel depth, H/L which in combination with the wall thickness (D), provide a consistent and simple means by which the heat exchange potential can be estimated, for a given set of wall and soil thermal properties and boundary conditions.

Numerical Analysis of a High-Velocity Projectile’s Impact on Shallow Steel Tunnels in Soft Sandstone

Tunnels are underground infrastructures intended for diverse community applications as well as military applications. During impact loading due to high-velocity projectiles such as ballistic missiles, materials experience a high strain rate. Moreover, there is a superficial augmentation of the dynamic strength when geomaterials such as rock are subjected to a high strain rate. Despite this strength enhancement, tunnels can get damaged by the impact load of a projectile hitting at a high velocity if they are present at a shallow depth. The present study is an effort to comprehend the response of a shallow tunnel in soft sandstone due to the impact load by a ballistic projectile using the FEM-based software ABAQUS/CAE 6.11. The Drucker–Prager damage model and the Johnson–Cook damage model were used to define the properties of the rock mass and steel tunnel lining, respectively. The crown of the 3 m diameter tunnel was kept at different depths from 1 m to 5 m from the surface. A striking velocity of 1000 m/s at a normal position to the target was given to the projectile. The projectile caused noticeable damage to the tunnel lining up to 3 m crown depth. Increasing the crown depth had a positive effect on the maximum depth of the projectile penetration up to 4 m tunnel crown depth, after which the effect reversed, making the tunnel safer. The maximum von Mises stress on the tunnel lining reduced in a logarithmic trend with an increase in the crown depth, gradually lowering to an impact load lesser than the yield stress of the tunnel lining material after a crown depth of 4.5 m.

Research on spatiotemporal patterns of ground deformation caused by shallow buried tunnel construction in urban areas based on three-dimensional numerical simulation

According to incomplete statistics, since 2018, there have been as many as 15 geological safety accidents caused by urban subway construction in Guangzhou, resulting in significant loss of people’s lives and property and social impact. The main reason for this is insufficient research on the spatiotemporal patterns of ground deformation caused by shallow buried and underground excavation tunnel construction in complex disaster-prone environments and a lack of targeted geological risk prevention measures. This study combines various methods such as statistical analysis, on-site investigation and monitoring, and numerical simulation to reveal the spatiotemporal deformation laws of the ground caused by the construction of shallow buried and underground tunnels in cities. The results obtained can provide a reference for reducing or preventing ground accidents caused by urban subway construction, thereby better ensuring the safety of people’s lives and property.

Tunnel construction in shallow soft rock using the pipe shed support

In order to clarify the impact mechanism of shallow buried soft rock tunnel excavation on the upper existing highway, as well as the mechanism of pipe shed reinforcement, a combination of theoretical analysis and on-site monitoring was used to conduct in-depth research on the Diantou Tunnel Crossing the existing highway project of Dayong Expressway. The impact of shallow buried soft rock tunnel crossing construction on the existing highway and safety control issues were studied, a new deformation control index, namely deformation difference rate, was introduced. The results show that the main lateral area of influence of rock deformation is within a distance of one diameter, and the overall area of influence is within a distance of twice the diameter. From the perspective of axial tunnel deformation, the deformation of surrounding rock tends to be stable when the excavation face passes through the monitoring section about twice the tunnel diameter. Effectively controlling rock deformation depends on the total amount of settlement deformation, the change rate of deformation, and the difference rate of deformation. For V-class shallow buried tunnel surrounding rock, in order to prevent cracks on the surface, the cumulative deformation of the surrounding rock needs to be less than 50 mm, the change rate needs to be less than 3 mm/day, and the difference rate needs to be less than 5 mm/m. Finally, the proposed control standard for surface subsidence is given for similar engineering reference.

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.

Analysis of dynamic behavior and damage mode of shallow buried tunnel due to underwater explosions

In order to explore the dynamic behavior and damage mode of shallow buried tunnel induced by underwater explosions (UWEPs), a fully coupled numerical model of a shallow buried tunnel based on Arbitrary Lagrangian and Eulerian (ALE) method was established. The strain rate effects of concrete and steel bar under explosion load, the explosion wave propagation, the interaction between fluids and solids, and the nonlinear response of the structure were considered. The reliability of the numerical method was verified by comparing the experimental and analytical results. The damage process and damage mechanism of submerged shallow buried tunnel due to UWEPs were investigated. The effects of saturated soil covering, explosive weight, buried depth and water depth on the dynamic behavior and damage mode of the tunnel were discussed. Finally, the dimensional analysis was used to obtain the functional relationship between the peak displacement, charge weight, buried depth and water depth. The results show that the saturated soil covering can effectively reduce the impact of explosion load on the tunnel. Increasing the buried depth can mitigate the blast effects on the structure, and the failure modes switch from local damage to global failure. The response of the tunnel increases with the increasing water depth, but the influence of water depth decreases gradually. The damage modes of shallow buried tunnel can be classified into local punching or spalling damage, global bending failure accompanied by spalling damage, and global bending failure.

Molecular Graphene Nanoribbon Junctions.

One of the challenges for the realization of molecular electronics is the design of nanoscale molecular wires displaying long-range charge transport. Graphene nanoribbons are an attractive platform for the development of molecular wires with long-range conductance owing to their unique electrical properties. Despite their potential, the charge transport properties of single nanoribbons remain underexplored. Herein, we report a synthetic approach to prepare N-doped pyrene-pyrazinoquinoxaline molecular graphene nanoribbons terminated with diamino anchoring groups at each end. These terminal groups allow for the formation of stable molecular graphene nanoribbon junctions between two metal electrodes that were investigated by scanning tunneling microscope-based break-junction measurements. The experimental and computational results provide evidence of long-range tunneling charge transport in these systems characterized by a shallow conductance length dependence and electron tunneling through >6 nm molecular backbone.

Experimental study of face passive failure features of a shallow shield tunnel in coastal backfill sand

Experimental study of face passive failure features of a shallow shield tunnel in coastal backfill sand

FEM modeling requirements for accurate analysis of highly nonlinear shallow tunnels

Modern tunnel design in urban areas heavily relies on numerical modeling to assess excavation stability and predict ground movement. Recent advancements in soil modeling, hardware, and software have facilitated the development of sophisticated 3D models within tight schedules. Urban tunnels are often shallow and excavated in challenging ground conditions, with proximity to existing structures and infrastructure. Consequently, numerical modeling of such tunnels involves highly nonlinear analyses with complex boundary conditions. Despite the widespread use of numerical modeling in tunnel research and design, there is a lack of publications addressing modeling procedures to ensure accurate and reliable results for highly nonlinear shallow tunnel analyses. This paper investigates the requirements for accurate results for highly nonlinear shallow tunnel analyses. The Finite Element Method (FEM) is employed with different mesh refinements and element types. The study focuses on the hypothetical excavation stability scenario explored by Carranza-Torres et al. (2013). Tunnel stability is assessed using Caquot’s analytical solution based on the lower bound theorem of plasticity, as well as FEM modeling with the strength reduction method. The FEM numerical solution, which approaches the exact solution for the problem, provided a factor of safety slightly larger (2.3%) than Caquot’s lower-bound solution. The results of the FEM modeling indicate that a significantly less refined mesh is required to achieve accurate results for highly nonlinear shallow tunnel analyses when adopting 2nd-order elements (i.e., quadratic interpolation) instead of 1st-order elements (i.e., linear interpolation). This study improves our understanding of FEM modeling requirements and provides practical insights regarding the numerical modeling of highly nonlinear shallow tunnels in urban areas.

Analysis of the Blow-out Occurrence When Tunnelling an Urban Shallow Tunnel: a Case Study of Ho Chi Minh Metro Line 1 Project

Analysis of the Blow-out Occurrence When Tunnelling an Urban Shallow Tunnel: a Case Study of Ho Chi Minh Metro Line 1 Project

Solving Conformal Mapping Issues in Tunnel Engineering

The calculation of conformal mapping for irregular domains is a crucial step in deriving analytical and semi-analytical solutions for irregularly shaped tunnels in rock masses using complex theory. The optimization methods, iteration methods, and the extended Melentiev’s method have been developed and adopted to calculate the conformal mapping function in tunnel engineering. According to the strict definition and theorems of conformal mapping, it is proven that these three methods only map boundaries and do not guarantee the mapping’s conformal properties due to inherent limitations. Notably, there are other challenges in applying conformal mapping to tunnel engineering. To tackle these issues, a practical procedure is proposed for the conformal mapping of common tunnels in rock masses. The procedure is based on the extended SC transformation formulas and corresponding numerical methods. The discretization codes for polygonal, multi-arc, smooth curve, and mixed boundaries are programmed and embedded into the procedure, catering to both simply and multiply connected domains. Six cases of conformal mapping for typical tunnel cross sections, including rectangular tunnels, multi-arc tunnels, horseshoe-shaped tunnels, and symmetric and asymmetric multiple tunnels at depth, are performed and illustrated. Furthermore, this article also illustrates the use of the conformal mapping method for shallow tunnels, which aligns with the symmetry principle of conformal mapping. Finally, the discussion highlights the use of an explicit power function as an approximation method for symmetric tunnels, outlining its key points.

Study on mechanical characteristics of pipe umbrella support in shallow buried tunnels

Considering the micro-arching effect between pipe umbrella, the paper investigates the mechanical response of pipe umbrellas during tunnel excavation by simulating pipe umbrellas as Timoshenko beams on a Pasternak foundation. The finite difference method is employed to solve the deflection, bending moment, and shear force of pipe umbrellas. By analyzing and contrasting with real-world engineering monitoring data, finite element computation results, and existing theories of other researchers, the reliability of the proposed method is verified, and good agreements are observed. A new parameter δp is introduced in our method to represent the proportion of overlying load transmitted through the pipe umbrella, and influences of stiffness of the surrounding rock in front of the tunnel face, initial support stiffness, pipe umbrella design schemes, and excavation depth on the load transfer capacity of the pipe umbrella are subsequently investigated. The conclusions reveal that increases of surrounding rock stiffness, initial support stiffness, and diameter of pipe umbrellas would all enhance the stability of both the tunnel and pipe umbrellas. However, such enhancements have certain limitations. Particularly, when the excavation footage c <1 m, the pipe umbrella diameter should be specifically selected based on cases.

Time-series prediction of settlement deformation in shallow buried tunnels based on EMD-SSA-GRNN model

Tunnel settlement deformation monitoring is a complex task and can result in nonlinear dynamic changes. To overcome the disturbances caused by historical data and the difficulty in selecting input parameters during deformation prediction, a decomposition, reconstruction and optimization method for tunnel settlement deformation prediction is proposed. First, empirical mode decomposition (EMD) is used to decompose the in-situ monitoring data and reduce the interactions among information at different scales in sequences. Then, the monitoring data are decomposed into intrinsic mode functions (IMFs). Secondly, the smoothing factor of the generalized regression neural network (GRNN) is optimized by using the sparse search algorithm (SSA). An EMD-SSA-GRNN deformation prediction model is developed using the optimized GRNN algorithm and is used to predict the changes in the decomposed IMFs. Finally, using the measured deformation data from a shallowly buried tunnel along the Kaizhou-Yunyang Highway in Chongqing, China, the reliability and accuracy of different models are analysed. The results show that tunnel settlement deformation exhibited a trend and a slow change in the early stage, a rapid change in the middle stage and a slow change in the late stage, and the rate of change was significantly influenced by the excavation time and the upper and lower geological layers. The prediction accuracy of the EMD-SSA-GRNN model after EMD improved from 19.2 to 59.4% relative to that of the SSA-GRNN and single GRNN models. Moreover, we find that the three error evaluation indicators of the EMD-SSA-GRNN model are lower than those of the other models and that the results of the proposed model and are more strongly correlated with measured data.

Reverse fault rupture propagation through dense sand and interaction with a shallow RC tunnel

Reverse fault rupture propagation through dense sand and interaction with a shallow RC tunnel

Bayesian-based seismic risk assessment of shallow tunnels in soft soils

Bayesian-based seismic risk assessment of shallow tunnels in soft soils

Analysis of face stability for shallow shield tunnels in sand

The stability of the tunnel face is the key problem in shield tunnel construction. This paper focuses on the face stability of a shallow tunnel in sand. Numerical simulation and theoretical analysis are combined to study the limit support pressure and failure zone. Firstly, numerical simulation is employed to study the collapse of the tunnel face, obtaining the limit support pressure and collapse zone. A new failure model suitable for shallow tunnels is constructed based on these numerical simulations. Then, an analytic solution for the limit support pressure is derived using limit analysis upper bound theory. The accuracy and applicability of this proposed model are verified by comparing it with numerical results and classical analytical models. Through this research, it is found that the proposed model provides a more accurate description of situations where soil arches cannot be formed for shallow tunnels in sand, leading to higher accuracy in calculating the limit support pressure. The influence of various factors on stability of the tunnel face is analyzed, revealing mechanisms of tunnel face collapse.

Reasonable mechanical model on shallow tunnel excavation to eliminate displacement singularity caused by unbalanced resultant

When considering initial stress field in geomaterial, nonzero resultant of shallow tunnel excavation exists, which produces logarithmic items in complex potentials, and would further lead to a unique displacement singularity at infinity to violate geo-engineering fact in real world. The mechanical and mathematical reasons of such a unique displacement singularity in the existing mechanical models are elaborated, and a new mechanical model is subsequently proposed to eliminate this singularity by constraining far-field ground surface displacement, and the original unbalanced resultant problem is converted into an equilibrium one with mixed boundary conditions. To solve stress and displacement in the new model, the analytic continuation is applied to transform the mixed boundary conditions into a homogenerous Riemann-Hilbert problem with extra constraints, which is then solved using an approximate and iterative method with good numerical stability. The Lanczos filtering is applied to the stress and displacement solution to reduce the Gibbs phenomena caused by abrupt change of the boundary conditions along ground surface. Several numerical cases are conducted to verify the proposed mechanical model and the results strongly validate that the proposed mechanical model successfully eliminates the displacement singularity caused by unbalanced resultant with good convergence and accuracy to obtain stress and displacement for shallow tunnel excavation. A parametric investigation is subsequently conducted to study the influence of tunnel depth, lateral coefficient, and free surface range on stress and displacement distribution in geomaterial.

A new complex variable solution on noncircular shallow tunnelling with reasonable far-field displacement

A new static equilibrium mechanical model on noncircular shallow tunnelling considering initial stress field is proposed in this paper by constraining far-field ground surface, and a mixed boundary value problem thus forms with elimination of displacement singularity at infinity. By applying analytic continuation, the mixed boundaries are transformed to a homogeneous Riemann–Hilbert problem, which is subsequently solved via an efficient and accurate iterative method with boundary conditions of static equilibrium, displacement single-valuedness, and traction along tunnel periphery. The Lanczos filtering technique is used in the final stress and displacement solution to reduce the Gibbs phenomena caused by the constrained far-field ground surface to obtain more reasonable results. Several numerical cases are conducted to intensively verify the proposed solution by examining the utilization of Lanczos filtering, the elimination of displacement singularity, and solution convergence of constraining arc and truncation number, and comparisons with existing solutions, and all the results are in good agreements. Finally, more numerical cases are conducted to investigate the stress and deformation distribution along ground surface and tunnel periphery considering different tunnel geometries, lateral coefficients, and tunnel depths. Further discussions on the defects of the proposed solution are also conducted for objectivity.