Twin Tunnels Research Articles

Displacement and Deformation of the First Tunnel Lining During the Second Tunnel Construction

A three-dimensional twin tunnels scale model was established utilizing the discrete element method (DEM) with PFC3D. This model aims to investigate the displacement (in horizontal and vertical directions) and deformation of the first tunnel lining in four different cases which the clear distance of twin tunnels are 5, 10, 15 and 20 m during the second tunnel construction process. The numerical results indicate that the clear distance between twin tunnels and the distance between the measurement points of the first tunnel and the excavation area of the second tunnel are two most critical factors that influence the displacement and deformation of the first tunnel lining. Meanwhile, the soil arching effect, gravity, water pressure and lateral pressure also have an impact on the behavior of the first tunnel. The maximum disturbance of horizontal and vertical displacements occurred in the time points of finishing of the second tunnel. However, the horizontal displacement of the first tunnel is much more sensitive to the vertical displacement. The first tunnel turns to the right and down in direction while having an anticlockwise rotation (φ) during the process of construction of the second tunnel. In addition, the displacement and deformation of the lining of the first tunnel are critical to monitor, and the necessary precautions should be taken to decrease the risk of craze. In conclusion, the influence of the second tunnel excavation on the first tunnel lining could be neglected when their distance is more than 15 m.

Optimization of the outlet portal of Heybat Sultan twin tunnels based on the value engineering methodology

Due to uncertainties in determining the geotechnical parameters, it is inevitable to change the supporting system and excavation method during the tunnel construction projects. Such changes will in many cases lead to an increase in the foreseen time and cost. Since the beginning of the 1960s, value engineering (V.E.) was raised based on three principles of safety, cost optimization and sustainable design. Iraqi Kurdistan has had an increasing trend in design and implementation of development projects since 2008, but high safety factors considered to compensate for uncertainties in geotechnical parameters has imposed high construction costs to these projects. The design of more than 70 km of tunnels, 212 dams and more than 500 km of highways under construction in this region necessitates performing applicable case studies on the implementation of V.E. in these development projects. The Heybat Sultan twin tunnels project is being constructed as a part of the Erbil–Koya–Sulaymaniyah highway in Iraqi Kurdistan. Preliminary studies showed that the outlet portals of these tunnels had been designed with a high factor of safety. Hence, the decision was made to make changes based on V.E. principles prior to the commencement of the excavations and at the time of site mobilization. For this purpose, the most optimal method statement was selected for the outlet portal, and then the slope stability was evaluated using numerical analysis. By implementing the optimized design, the factor of safety was increased from 1.27 to 2.48 and the corresponding construction costs was decreased for 2.4 million USD compared to the original plan.

An analytical model for calculating vibrations from twin tunnels in a saturated poroelastic half-space

In this paper a semi-analytical solution is proposed for modelling vibrations from a twin tunnel in a saturated poroelastic half-space. The twin tunnels are simulated as two hollow cylinders made of a viscoelastic material. The half-space containing the two circular cavities is described as a two-phase poroelastic material. The wave field in the half-space with two cylindrical cavities is taken as the combination of down-going plane waves from the ground surface and outgoing cylindrical waves from each tunnel. With the help of transformation and translation properties of the plane and cylindrical waves the boundary condition at each scattering surface can be expressed with the appropriate coordinates. Numerical results suggest that the reflection effect of the ground surface has limited influence on the response of the tunnels, but has a greater influence on the free-field response near the surface. Compared with the single tunnel solution, the wave field above the twin tunnels is greatly distorted.

Three-dimensional numerical simulation of pile-twin tunnels interaction – Case of the Shiraz subway line

The use of an underground transportation network to solve urban traffic problems is a practical solution. Shiraz, one of the most famous touristic cities of Iran, is developing its own urban railway network using mechanized twin tunnels. It is almost inevitable for tunnels to pass nearby an existing pile foundations. In this study, a three-dimensional numerical model was developed to investigate the pile-tunnel interaction. All the mechanized excavation phases of an EPB-TBM (Earth pressure balance, Tunnel boring machine) and the segmental lining were modeled using a finite difference code. The role of the Constitutive soil’s model is also investigated by using both a linear elastic-perfectly plastic one (failure criteria of Mohr-Coulomb type) named as MC and a nonlinear perfectly plastic one the Cap-Yield (CYsoil) constitutive model. Two different scenarios were considered: the influence of tunneling on existing piles and the impact of installing bridge piles nearby existing twin tunnels. The influence of the soil/structure interaction is presented in terms of surface settlements, forces induced in the tunnel lining and forces along the piles. It should be noted that on the base of the obtained results, the MC model cannot accurately estimate the ground settlements and the forces in tunnel and piles, therefore it is necessary to use advanced constitutive models like CYsoil for design purposes.

The scattering of plane P, SV waves by twin lining tunnels with imperfect interfaces embedded in an elastic half-space

A viscous-slip interface model is employed to simulate the contact between the tunnels lining and the surrounding rock, and the scattering of P, SV waves by twin shallowly buried lining tunnels is investigated with the indirect boundary integral equation method (IBIEM). The amplification effect of the dynamic stress concentration of the lining and the surface displacement near the tunnels is examined. It is evident that the slipping-stiffness coefficient and viscosity coefficient at the lining-surrounding rock interface have a significant influence on the dynamic stress distribution and the nearby surface displacement response of the lining tunnel, while the influence characteristics strongly depend on the incident wave type, frequency and angle. Under the incidence of low frequency wave, as a whole, with the increase of the sliding stiffness, the hoop stress increases gradually for plane P and SV waves; while in the resonance frequency (the incident wave frequency is consistent with the natural frequency of the soil column above the tunnels), specially for high-frequency band, the dynamic stress concentration effect is more significant for smaller sliding stiffness. With the increase of viscosity coefficient, the dynamic stress concentration factor inside the lining gradually decreases. Also, the tunnels with viscous-slip interfaces have a more significant amplification effect on the nearby surface displacement amplitude. Moreover, the hoop stress of the twin tunnels may be obviously larger than that of single tunnel in most cases. The dynamic analysis of the underground structure under the actual strong dynamic loading should consider the influence of the slip effect between the lining and surrounding rock interface.

Three-dimensional analytical model for the dynamic interaction of twin tunnels in a homogeneous half-space

Ground vibration and noise induced by underground railways have become a major environmental issue. Significant research has been carried out to investigate vibrations generated by a single tunnel, with less attention been paid to the dynamic interaction of twin tunnels. This paper presents a three-dimensional analytical model for the dynamic interaction of twin tunnels in a homogeneous half-space. The two tunnels are modeled using the cylindrical thin-shell theory. The soil, a homogenous half-space with two cylindrical cavities, is simulated using the elastic continuum theory. The translation between two cylindrical waves and the transformation between plane waves and cylindrical waves are introduced to satisfy the boundary conditions on the ground surface and the twin tunnel–soil interfaces. The accuracy of the proposed model is verified by comparison with numerical models. A numerical example is presented to investigate the influence of the dynamic interaction between twin tunnels on the ground vibrations.

Estimating the Effects of Tunnelling on Preexisting Jointed Pipelines

Tunnel excavation inevitably results in ground movements and changes in soil stress, leading to additional stress on and settlement of nearby buried pipelines. This article focuses on the response of jointed pipelines to twin tunnelling. The relationship between the relative pipe‐soil displacement and the relative pipe‐soil stiffness was first determined. Based on this analysis, a series of numerical parametric studies encompassing 7776 conditions were performed to investigate the responses of a jointed pipeline to twin tunnelling. The results are used to estimate a regression equation for the relationship between the relative pipe‐soil stiffness and the normalized maximum joint rotation angle. This equation can be used for the direct calculation of the maximum joint rotation angle that will result from single or twin tunnelling and for the assessment of the tunnelling‐induced risk to jointed pipelines. The applicability and reliability of the regression equation are validated by comparing the calculated values with the results of earlier centrifuge tests.

Ground Settlements due to Construction of Triplet Tunnels with Different Construction Arrangements

Combining two or three shield tunnels that are drilled consecutively at small intermediate distances into a single open cross section creates a larger useful tunnel area, while keeping the total construction area rather compact. However, the combination of several bored tunnel tubes into a singular cross section results in a complicated construction procedure. Current methodologies address the evaluation of the settlement trough for one single tunnel or two parallel tunnels only. This study aims at expanding the current methodologies to the multiple‐tunnel geometry. Various construction orders and intermediate distances between the tunnels are evaluated using finite element analyses. The grout pressure method is applied in combination with the hardening strain with the small strain stiffness (HS small) constitutive model to investigate the relative magnitude of the settlements due to shield tunneling. It is concluded that the configuration with a Top‐Left‐Right construction sequence is the optimal arrangement. It results in 22.4% and 36.9% lesser settlements when compared to the equivalent horizontal twin tunnels spaced at 3D and the Left‐Right‐Top sequence, respectively.

Explanation for twin tunnelling-induced surface settlements by changes in soil stiffness on account of stress history

In this article, a group of representative centrifuge tests were selected for numerical modelling to explain the surface settlements induced by sequential twin tunnelling. Both Modified Cam Clay model (MCC) and Three-Surface Kinematic Hardening model (3-SKH) were adopted in the simulation, which indicated the use of 3-SKH model conduced to mimicking more closely centrifuge model response. Via performing more contrastive numerical analyses with 3-SKH model, the influence of the first tunnel event on the stiffness of the soil around the second tunnel was quantitatively investigated, whereby the mechanism behind the observed surface settlements was finally made clear.

Dynamic Responses of a Twin-Tunnel Subjected to Moving Loads in a Saturated Half-Space

With the fast development of rail transit, the environmental vibration problems caused by subways have received increasing attention. A 3D finite element model was built in this study to investigate the ground vibrations induced by the moving load operating in the parallel twin tunnels. Compared to the model consisting of a single tunnel that was commonly adopted in the past studies, a pair of tunnels is considered and the surrounding medium of the tunnels is taken as a saturated porous medium. The governing equations of the 3D finite element method modeling of the saturated poroelastic soil have been derived according to Biot’s theory. Computed results showed that the dynamic response of the twin-tunnel model is greater than that of the single tunnel model. And the spacing between two tunnels, tunnel buried depth, and load moving speed are the essential parameters to determine the dynamic response of the tunnel and soil.

Analytical solution for calculating vibrations from twin circular tunnels

The calculation of vibrations from a single tunnel embedded in a full- or half-space due to underground moving trains has been well studied in the literature. A common assumption made in the majority of vibration prediction models is to neglect the presence of a neighboring tunnel, however, twin tunnels are a particularly preferred construction for urban underground railways with one as the inbound tunnel and the other as the outbound tunnel. The neglect of the interaction between twin tunnels may lead to inaccuracies of the calculation of vibrations from underground railways. The present paper proposes a novel analytical solution, which takes the multiple scattering effects between the two tunnels into account. The two tunnels are modelled as elastic hollow cylinders and the soil surrounding the tunnels as an elastic, homogeneous full-space containing two cylindrical cavities. The wave field in the full-space with two cavities consists of outgoing waves from each tunnel outside the two scattering surfaces while the wave field in the tunnel wall is a combination of outgoing and regular cylindrical waves. The translation of the outgoing cylindrical wave functions is required to satisfy the boundary conditions at the other tunnel. Numerical results show that there are two critical velocities for a twin tunnel in a full-space system, both of which are around the shear wave velocity of the soil. Through comparison with the single tunnel solution, the addition of a second tunnel at different separation distances and angles has a significant influence on the soil response, especially in the high frequency range, but the response at the tunnel with the source is not much affected.

A new computational tool for the follow up of mechanized excavation of tunnels: the case study of the subway line 5-LILAC in São Paulo, Brazil

Abstract The management and monitoring of mechanized tunnel excavation parameters are crucial for ensuring the safety, quality and project's time/costs, since these parameters are the only existing elements that enable the evaluation of the performance of the excavation itself. In this study, will be presented the computational tool christened as SAPE (Support System and Monitoring of Underground Excavation Parameters), an innovative tool meant to assist the monitoring of ongoing or completed mechanized tunneling projects. The tool was designed to be a flexible and simple management tool, which means that it can be customized as a function of the characteristics of each project. The default version comprises the most important excavation parameters (a total of 14). Besides, it has important functions, such as geolocation (GoogleEarth program), historical production data and basic excavation statistics. The software was tested in the recently completed twin tunnels of the Subway Line 5 - Lilac in the city of Sao Paulo, Brazil.

Elastic analytical solution of shallow tunnel owing to twin tunnelling based on a unified displacement function

• Unified displacement function of cross-section of each tunnel owing to twin tunnelling. • Complex variable method and Schwartz alternating method. • Elastic analytical solution for ground displacement of twin tunnels • The laws of ground displacement field for twin tunnels. A unified displacement function is used as the displacement boundary condition of the cross-section of each tunnel, which can be used to capture the asymmetrical deformation behaviors about the horizontal and vertical center line . Complex variable method is adopted to obtain the analytical solution of a single tunnel. Based on the Schwartz alternating method, the analytical solution of twin tunnels is obtained and the correctness of the analytical solution of twin tunnels is verified. A series of analyses are carried out to investigate the effects of the displacement boundary condition of the tunnel cross-section and the distance of twin tunnels on the ground surface settlement trough curve and the ground displacement field. Based on the proposed method, the parameters of displacement mode of the cross-section of twin tunnels are calibrated through field monitoring data of the Bangkok Subway Tunnel project, and the displacement field of the whole ground are analyzed. The proposed method can provides guidance in the conceptual stage of the design process of twin tunnels.

Investigation into the influence of excavation of twin-bored tunnels within weak rock masses adjacent to slopes

The focus of this investigation is twin tunnelling projects designed and constructed adjacent to slopes. To this end, several state-of-the-art two-dimensional (2-D) and three-dimensional (3-D) numerical models for multiple scenarios were utilized. The novelty of this research (further highlighted by two case studies) explicitly demonstrates that for 2-D and 3-D elastic and plastic analyses for weak rock masses, the interaction between the two tunnel branches for various conditions show how the pillar width minimally influences such interaction past two diameter widths of separation. As well, the 3-D sequential longitudinal tunnel excavation of the twin tunnel does not significantly influence the crown displacements of the two tunnels prior to construction of the mid-span of the twin tunnel. Slope stability analysis was conducted by employing the novel shear strength reduction (SSR) method. These findings confirm that changes in the location of the twin tunnels with respect to the slope and one another in combination with the natural angle of the slope can improve the overall stability without any other special measures taken. As well, by having the twin tunnel pillar width constant, the vertical translation of the tunnels (i.e., increase in the overburden) is more beneficial than that of a horizontal translation, resulting in a higher factor of safety.

Deformation and stress characteristics of existing twin tunnels induced by close-distance EPBS under-crossing

This paper presented a case study on the deformation and stress characteristics of twin tunnels induced by close distance earth pressure balance shield (EPBS) under-crossing in sandy soil stratum, located in Changsha, China. The horizontal columns constructed by Metro Jet System (MJS) method were used to stabilize the sandy soil below the existing twin tunnels. The deformation and the stress of existing tunnels which affected by the construction of new tunnels were systematically monitored. The settlement development of the existing tunnels was found to experience four stages: (i) subsidence, (ii) heave, (iii) second subsidence, and (iv) steady state, respectively. The settlement profiles of the existing tunnels induced by the second tunnel under-crossing were found to be asymmetric with respect to the second tunnel centerline, and the location of the maximum settlement point deviated toward the twin tunnels center. The settlement caused by the second shield under-crossing was found significantly larger than that caused by the first tunnel under-crossing. After the two shields passing, the final settlement profile of the existing tunnels displayed a “U” shape. The induced hoop stress of the existing tunnels changed sharply during the shield under-crossing. The cross-section of the existing tunnels changed into a skewed oval shape. The final rotation direction of the existing left and right tunnels above the first and second tunnels centerline was opposite. Both the existing tunnels rotated toward the location of the large volume loss. The shifted Gaussian distribution curve was adopted to model the settlement profiles of the existing tunnels. The differences of the settlement profiles caused by the first and second tunnels excavation were also explained.

Twin neighboring tunnel construction under an operating airport runway

Beijing Capital International Airport, as one of the busiest airports in the world, has three terminals and three runways. The middle runway lies between Terminal 2 and Terminal 3, and underground transportation is necessary to alleviate the congested surface traffic and reduce the impact of the surface traffic on the middle runway operations. As a result, twin neighboring tunnels, one each for passenger transportation and automobiles, passing underneath the operating middle runway were constructed. The section of the twin tunnels that was approximately 6 m under the operating runway was excavated using the sequential excavation method under the protection of a pipe-screen system. The elaborate installation of the pipe-screen system was performed using slurry pipe-jacking machines with an excavation diameter of 1020 mm. The annular gap between the excavation diameter and the outer diameter of the pipe was filled in time to reduce surface settlements. To conduct an information-oriented construction project, the real-time monitoring of the runway pavement surface with high-precision automatic total stations was employed during and after construction. With the monitored results, in-pipe grouting was performed when necessary to decrease the runway settlements. The undercrossing construction was completed as scheduled, and the final settlements of the runway were controlled within the predefined allowable settlement. The airport continued all operations without any interruptions. The success of crossing under the operational runway of the twin tunnels lies in the use of the large-diameter pipe screen, the well-controlled excavation of the twin tunnels, the installation of the high-precision and automatic runway movement measurement system and, most importantly, the employment of the in-pipe grouting system.

Numerical Research on the Mixture Mechanism of Polluted and Fresh Air at the Staggered Tunnel Portals

In longitudinal ventilation, circulating air is formed in portals for closely spaced twin tunnels, which causes mixing between the polluted air exhausted from one tunnel and the fresh air flow of another tunnel, thus leading to the rising costs of ventilation system construction and operation. In this study, for the closely spaced tunnel with staggered inlet and outlet, the computational fluid dynamics (CFD) numerical simulation method was adopted to reveal flow characteristics of the circulating air as well as variation rules of the circulating air mixing ratio φc with tunnel structure and operation parameters. Results show that both reducing inlet air velocity and increasing outlet air velocity and lateral distance can reduce the impact of the negative-pressure zone at the tunnel entrance on the jet flow structure at the tunnel exit, thus weakening the circulating air. When the inlet is placed behind or aligned with the outlet (staggered distance ∆l ≤ 0), φc will increase linearly along with the increase of staggered distance; when the inlet is placed before the outlet (∆l > 0), φc will first increase and then decrease with the increase of staggered distance. An expression to predict circulating air mixing ratio was created by sections. The predictions show a good correlation with the measurements and indicate that the front slope gradient of the tunnel portal is also one of the factors affecting the circulating air mixing ratio.

Grouting-based treatment of tunnel settlement: Practice in Shanghai

Excessive tunnel settlement, especially differential settlement, will deteriorate the operational serviceability and safety of a metro system. This paper presents grouting-based treatment technology for tunnel settlement in the soft deposits of Shanghai, China. The aim of the grouting treatment is to control the development of the tunnel settlement. The mechanism of settlement control through grouting is presented. A dual-fluid system of cement and sodium silicate is adopted in the proposed grouting-based treatment technology. A new front-end device is designed specially to avoid blocking the grouting holes. The grouting parameters are determined based on field tests and engineering practices. Engineering cases of a double-O-tube (DOT) tunnel and twin tunnels are used to present the feasibility and efficiency of the grouting treatment. For the DOT tunnel, the grouting treatment was carried out in multiple stages; the tunnel settlement finally ceased, from an initial settlement rate of 0.4 mm/d, and the maximum horizontal differential settlement decreased from 20 mm to 10 mm. For the twin tunnels, the tunnel settlement recovered after grouting; the recovery of the maximum tunnel settlement was 8 and 11 mm for the upper- and lower-line tunnels, respectively, and the settlement finally ceased, from an initial rate of 0.027 mm/d. Moreover, in situ measurements of the surroundings indicate that the proposed grouting-based treatment technology creates minimal disturbance to the surrounding soil. This grouting-based treatment provides an effective way to control the settlements of tunnels in soft deposits.

Effect of Closely Spaced Twin Tunnel Construction beneath an Existing Subway Station: A Case Study

Abstract Closely spaced twin tunnels were excavated vertically beneath an adjacent subway station in service in Beijing, China. Because of restrictions on the settlements of the existing subway station in the area, a special protection methodology was adopted, and an analysis was performed using real-time monitoring data to investigate the response of the overlying subway station to twin tunnelling and surface settlement characteristics. The results indicated that advanced grouting realizes regional integral uplifts within the range of a reinforced area. The lifting work with jacks is critical to restraining the trend of settlements. The final settlement trough of the columns displays a double-trough “W” shape that is induced by nonuniform lifting loads, and the total ground surface settlement profile displays an asymmetric single-trough “U” shape. The moment when temporary inverts and middle diaphragms are demolished is important, as it causes potential excessive settlements. With monitoring, this tunnel engineering was completed without interrupting the subway traffic, and the safety of the operating subway station and ground surfaces was successfully guaranteed. This is a valuable project case for establishing criteria to guide the construction of closely spaced twin tunnels in similar conditions.

Settlement and Load Transfer Mechanism of Pipeline Due to Twin Stacked Tunneling with Different Construction Sequences

The tunnel-pipeline-soil interaction is attracting more and more wide range of scholars and was treated as a plane strain problem, with little attention given to the responses of existing pipeline to twin-tunneling process with different construction sequences. Moreover, there are rare reports available on the influences of tunnel construction sequences on an existing pipeline. In this study, a three-dimensional centrifuge model test and numerical simulations using an advanced hypoplasticity constitutive model were conducted to investigate the influences of twin stacked tunneling with different construction sequences on an existing pipeline. The results shows that the maximum values of greenfield ground settlement, pipeline settlement and additional pipeline bending strain due to twin stacked tunneling when the upper tunnel was excavated first were 25%, 18% and 11% larger, respectively, than those when the lower tunnel was excavated first. The maximum values of greenfield ground settlement, pipeline settlement and additional pipeline bending strain due to twin stacked tunneling when the lower tunnel was excavated first were slightly smaller than those when the twin tunnels were excavated simultaneously. After the twin-tunneling process, the sagging regions were located within ± 1.5DT (tunnel diameter) in these three conditions. The maximum pipeline bending strain within the hogging regions was approximately half of that within the sagging regions. Because of shielding effects from the upper tunnel, the region where vertical stress of the soil reduced was transferred to both sides of the tunnel due to the second tunneling process when the upper tunnel was excavated first. As a result, the extended sagging regions due to the second tunneling process when the upper tunnel was excavated first were two times than that when the lower tunnel was excavated first.