Twin Tunnels Research Articles

Effect of Closely Spaced, Non-Persistent Ubiquitous Joint on Tunnel Boundary Deformation: A Case Study from Himachal Himalaya

The stability of an underground tunnel excavated in a jointed rock mass is studied using the field investigation and numerical modelling. This research aims to numerically analyze the rockmass behavior as a function of closely spaced non-persistent joints. For this purpose, the Kainchi-mod Nerchowck twin tunnels (Himachal Pradesh, India) is chosen for the in-depth analysis. The host rock encountered is mainly gray sandstone and maroon sandstone with many closely spaced, non-persistent joints, dipping critically into the tunnel. The detailed rockmass properties were collected from the field and intact rock properties were tested in the laboratory. A series of finite element numerical simulations were conducted based on the filed/laboratory data with different values of joint spacing, including the actual values of field joint spacing. It was found that the extent of deformation above the excavation was predominantly controlled by the joint spacing. The results of this study provide an explicit correlation between geometrical features of the rock mass with the total displacement values around the excavation. The study will help the engineers to design an appropriate support system for heavily jointed rocmkass.

Numerical Modeling of the Seismically Induced Uplift Behavior of Twin Tunnels

Abstract During an earthquake, large movements of soil may cause the instability and uplift of buried structures in liquefied soil. Intensive studies on the uplift behavior of underground structure...

Experimental Study on Deformation Law of Soil Layer Induced by Undercut between Existing Twin Tunnels based on 3D Geomechanical Model

The underground excavation of subway station spaces between existing twin tunnels is vital for ensuring the normal flow of ground traffic during excavation, reducing costs, and protecting the environment. However, such excavation would induce the deviation of the existing tunnel segment due to the collapse upon soil layers. Based on model test theory, a large-scale geomechanical model test frame was conducted to investigate the soil layer deformation law. A high-precision multi-point extensometer and an earth pressure cell were embedded in the design position during the manufacture of the model stratum. All of the deformation and earth pressure data at each monitoring point were obtained during the whole test process, and these data were used to analyze the deformation law in three test phases: excavation and support of the station space, transverse tunnel excavation, and overload pressure exertion on the model top surface. Results showed that the deformation induced by the underground excavation space was small, and the excavation of the transverse tunnel and overload pressure on the top surface of the model were the main factors inducing the soil deformation. The maximum vertical displacement and horizontal displacement were 1.29 mm and 0.58 mm, respectively; as indicated by the geometric similarity ratio, the displacements of the corresponding points were 12.9 mm and 5.8 mm, respectively. The test also revealed that a pre-reinforced pile outside the shield tunnel could effectively limit the segment deviation and reduce the deformation of the outside soil mass. This method might be used in the excavation of underground station spaces under good geological conditions.

Complex variable solutions for shallow twin tunnelling in visco-elastic geomaterial considering buoyancy effect and equivalent three-dimensional effects of tunnel faces

Shallow twin tunnels are commonly encountered in urban metro construction. Twin tunnel excavation causes buoyancy effect in geomaterial and the interaction between twin tunnels significantly redistribute stress and displacement around twin tunnels. At the same time, twin, and tunnelling causes three-dimensional effects around tunnel faces. To explore the problems, this paper proposes complex variable solutions for shallow twin tunnelling in visco-elastic geomaterial under the initial stress field, considering the buoyancy effect, as well as the equivalent three-dimensional effect of tunnel faces. The proposed solutions are applicable to two common scenarios: (a) newly constructing twin tunnels, and (b) a newly constructing single tunnel near an existing one. The Schwarz alternating method is applied to the proposed solutions to reduce the half-plane twin tunnel problem into several connected half-plane single tunnel problems. During the alternating procedure, a separation technique is applied to reduce the mixed and complicated numeric-symbolic calculation into a simple numeric one to lower the threshold of the algorithm performed on computers of moderate computation capacity and considerably save the required running time. Additionally, a symmetrical modification method is applied to eliminate the displacement singularity at infinity of shallow twin tunnelling to obtain reasonable displacement in nearby and far-field geomaterial. The proposed analytical solutions are elaborately verified through a comparison with a corresponding numerical solution, which reveals good agreements between both solutions. A subsequent parametric investigation is performed to study the influence of twin tunnel geometry (including twin tunnel spacing, depth, relative size, and relative angle), lateral stress coefficient, preconvergence, and excavation sequence on stress and displacement in geomaterial. The proposed analytical solutions exhibit high accuracy in quickly obtaining the stress and displacement fields, and thus, is a feasible alternative in the preliminary design stage of shallow twin tunnels.

Failure Performance of 3DP Physical Twin-Tunnel Model and Corresponding Safety Factor Evaluation

Quantitative stability evaluation of multi-tunnel structure is an important issue related to the safety assessment and stable construction of geotechnical underground tunnels. In this work, the overall failure process of twin tunnels were exhibited by a physical simulation based on the 3D printing (3DP) sandstone analogues model, and a safety factor method was also presented for evaluating the general safety of multi-tunnel structure. For checking the 3DP material performance of physical model, uniaxial and triaxial compressions for the 3DP cylinder specimens were first tested and showed that their mechanical properties and failure characteristics were similar to natural rock in general. Then, the overloading tests for twin-tunnel model were carried out and have exposed the critical position of overall failure of twin-tunnel structure through visual observation and automatic measurement. Testing results and corresponding numerical back analysis indicated that the connectivity of plastic strain between tunnels can be deemed as the conservative instability criterion (i.e. yielding of material) and the inflection point of tunnels’ displacement can be deemed as overall failure criterion (i.e. structure failure) for twin-tunnel structure. The safety analysis for underground hydraulic caverns indicated that this method can provide a reference for quantitative and reasonable evaluation of the general safety of multi-tunnels or caverns and the local instability zone of surrounding rock.

3D numerical investigation of the effects of driving of the new mechanized tunnel on existing segmental linings and ground surface settlements - a case study: Shiraz metro line 2

ABSTRACT Subway metro systems are one of the most efficient modes of mass transportation, especially in heavily populated areas. A series of 3D Finite Element Analysis (FEA) has been carried out to investigate the interaction between mechanized twin tunnels excavated beneath the highly congested streets of Shiraz (Iran). Firstly, the reference numerical model has been verified using the measured data of surface settlement control pins. The effects of driving of the new closed shield tunnel on the evolution of stress fields in the existing segmental lining as well as the ground surface settlements in the reference numerical model have been then quantified. Lastly, the effects of influencing factors (centre-to-centre distance between twin tunnels and soil types in which tunnels are excavated) on the twin-tunnel interaction have been investigated. It is concluded that with pillar widths almost three times greater than the tunnel diameter, the twin-tunnel interaction effects seem negligible.

Earthquake response analysis of tunnels at a complex saturated site

Only a seismic analysis of tunnels at a single-phase site can be performed using the commonly used finite element software owing to the lack of a dynamic two-phase element. The seismic analysis of tunnels at a saturated site is primarily based on OpenSees or self-compiled programmes, the majority of which are limited to uniform or horizontal-layered sites owing to the related issues of the inconvenience of modelling and the limited computation scale. In this study, based on ABAQUS, a finite element model for the earthquake response analysis of twin tunnels at a complex saturated site is established by calling the user-defined element, which is developed for the dynamic analysis of two-phase media. The wave propagation towards infinity is simulated by the viscous-spring boundary, the seismic wave is inputted in the form of the equivalent nodal force, and the soil nonlinearity is considered via the equivalent linear method. Differences between the earthquake responses of tunnels at a complex saturated site and a complex single-phase site as well as those between tunnels at a complex saturated site and a horizontal-layered saturated site are investigated to emphasise the effects of dynamic solid-fluid coupling and complex topography and geology.

Complex variable solutions for twin tunnelling at great depth in viscoelastic geomaterial considering the three-dimensional effects of tunnel faces

Analytic solutions on twin tunnelling at great depth in viscoelastic geomaterial considering three-dimensional effects of tunnel faces are presented using the complex variable methods. According to the time interval between twin tunnel faces, the proposed solutions can be divided into two cases: A) newly constructing twin tunnels; and B) newly constructing single tunnel influencing an existing one. The analytic solutions can also consider sequential excavations and the mutual influence between twin tunnels. The three-dimensional effects of twin tunnel faces are transformed into equivalent plane-strain problems by introducing dimensionless coefficients. By means of a presented general case, the analytic solutions are verified via comparisons of both stresses and displacements with the numerical solution, in which a good agreement between these two solutions is observed. Subsequently, a parametric investigation is performed to study the influence of several parameters concerning twin tunnel geometry and excavation on stresses and displacements, together with the difference between Cases A and B. On the basis of the results, the analytic solutions provide quantitative insights into the essence of the three-dimensional effects of twin tunnel faces and would have certain values on the preliminary design stage of twin tunnels.

Twin Tunnels Stability Factors Fc, Fs and Fγ

This paper uses tunnel stability factors to estimate the minimum supporting pressure required to maintain the stability of twin circular tunnels drive side-by-side in drained cohesive-frictional soil. Similar to the bearing capacity equation proposed by Terzaghi, the proposed method is based on the three stability factors (Fc, Fs and Fγ) that are functions of the soil internal friction angle and the tunnel depth ratio. Unlike the case of a single tunnel, the centre-to-centre distance between the tunnels appears as a new parameter that must be considered in twin tunnel stability. For various spacing ratios (S/D) between the tunnels, non-dimensional tunnel stability factors (Fc, Fs and Fγ) are established for various depth ratios (C/D) and soil internal friction angles (O) using two-dimensional finite element limit analysis. For practical suitability, the results are presented in the form of dimensionless stability charts with the actual tunnel stability factors closely bracketed from above and below using upper and lower bound methods. As an extra validation, the results are compared with available solutions reported in the literature.

Effect of Considering Seismic Loads on Horizontal Twin Tunnels

Effect of Considering Seismic Loads on Horizontal Twin Tunnels

A 3D Finite-Difference Analysis of Interaction between a Newly-Driven Large Tunnel with Twin Tunnels in Urban Areas

Evaluation of the interaction between a new and the existing underground structures is one of the important problems in urban tunneling. In this work, using FLAC3D, four numerical models of single- and twin-tube tunnels in urban areas are developed, where the horizontal distance between the single- and twin-tube tunnels are varied. The aim is to analyze the effects of the horizontal distances, considering various criteria such as the deformation of linings, the forces and moments exerted on the twin-tube tunnels and their safety factors, the subsidence that occur on the surface and the nearby buildings, the stability of the single-tube tunnel, and the stability of the pillar lying between the single- and twin-tube tunnels. Considering the above-mentioned criteria, the results obtained indicate that the interaction between the single- and twin-tube tunnels is virtually negligible in the distance more than three times the single-tube tunnel diameter. Also the stability of the pillar lying between the tunnels makes the distance to be chosen at least 1.5 times the single-tube tunnel diameter.

Geothermal potential of the NE extension Warsaw (Poland) metro tunnels

Thermoactive geostructures are considered an economically suitable and environment-friendly solution for heating and cooling buildings. Energy tunnels have gained growing interest in recent years because of the large ground volume involved in heat exchange in comparison to building foundations. Heat exchange is obtained by embedding a circuit of pipes into the precast concrete lining, resulting in a reduction of the initial costs of installation with respect to standard low-enthalpy geothermal solutions. In this paper, thermal activation of the twin tunnels of the II line of Warsaw metro, Poland, is considered with the aim of evaluating the geothermal potential of the line. Thermohydraulic finite-element analyses were performed on a number of representative cross-sections, considering site-specific hydrogeological conditions based on the Engineering Geological Database of the Capital City of Warsaw. Results demonstrate that the thermal activation of the two 1·6 km-long tunnels could exchange up to 5·3 and 5·8 GWh in the heating and cooling seasons, respectively. Furthermore, the influence of the local geological conditions on the exchangeable heat rate was investigated. The results form a basis for a preliminary assessment of the shallow geothermal energy utilisation potential for the design of further extensions of the metro line in Warsaw.

Evaluating closely spaced twin-tunnelling-induced shear stiffness change by subsurface settlement in clayey ground

Cases of closely spaced twin tunnels demonstrate that ground settlement above the 2nd tunnel, referred to as T2 constructed after the first tunnel (T1), exhibits pronounced differences from that above T1. The differences can be attributed to the disturbance induced by T1 construction. Based upon closed-form plasticity solutions for an unloading cylindrical cavity, this study employs subsurface settlement troughs to determine the shear modulus change during twin-tunnelling. Observations of 1 g model tests for twin-tunnelling in overconsolidated kaolin clay are interpreted from literature. After T1 construction, for positions with different radial distances but a constant angle with respect to the vertical line above the tunnel center, the graph of radial displacement/tunnel radius versus tunnel radius/radial distance exhibits good linearity. Further analyses demonstrate that the shear stiffness decreases as the angle increases. In comparison, during T2 construction, the shear modulus of soils close to T1 degrades, while the other side increases. In light of the observations of combined axial–torsional tests, the tendency of shear modulus change can be attributed to the rotation of principal stress axes. The findings can explain the differential settlements over closely spaced twin tunnels in clayey ground.

Three-dimensional centrifuge modeling of the influence of side-by-side twin tunneling on a piled raft

Although the effects of tunnel excavation on nearby existing pile foundations have been extensively studied, previous studies mainly focused on response of single piles and elevated pile groups. In this study, three-dimensional centrifuge model tests were carried out to investigate the effects of side by side twin tunneling on an existing two by two piled raft in dry sand. Upon completion of twin tunnel excavation, the measured cumulative settlement of piled raft due to tunneling near the pile toe is 2.7 times of that due to tunneling near the mid-depth of the pile shaft. For twin tunneling near the pile toe, differences of piled raft settlements due to each tunnel excavation are negligible. In contrast, settlement of piled raft due to the second tunneling is about 55% of that due to the first tunneling when twin tunnels are excavated near the mid-depth of the pile shaft. As an advancement of tunnel excavation near the pile toe, the axial loads carried by the front piles are decreased, and the decreased loads are transferred to the rear piles and the raft. On the contrary, twin tunneling at the mid-depth of the pile shaft causes a reduction in the front piles and raft, and the decreased loads are transferred to the rear piles. After completing tunnel excavation, the axial load carried by the raft is increased by 22.0% for tunneling near the pile toe, while it is decreased by 21.7% for tunneling near the pile shaft.

3D numerical investigation of the coupled interaction behavior between mechanized twin tunnels and groundwater – A case study: Shiraz metro line 2

Mechanized tunneling method using Tunnel Boring Machine (TBM) is being extensively employed in urban areas, especially when excavation takes place underneath the groundwater table. In this study, three-dimensional coupled Finite Element Analyses (FEA) have been performed to investigate the interaction mechanism between mechanized twin tunnels construction of Shiraz metro (line 2) and groundwater. Firstly, two soil constitutive models (Mohr-Coulomb and Modified Cam-Clay model) of Shiraz lean clay are validated against drained triaxial compression test data. Varying the permeability of the grout layer within the specified range, the base numerical model is then calibrated by the field data of ground surface settlements. Parametric studies are conducted subsequently to evaluate the effects of influencing factors involved in the coupled FEA of twin tunnels, including the drainage boundary condition of the grout layer, the position of groundwater level, and the rest interval between excavations of twin tunnels. The numerical results indicate that the ground surface settlements and internal forces of existing segmental lining are greatly influenced by the drainage condition of the grout layer. Also highlighted is the marked effect of the drainage condition of the grout layer in the numerical prediction of the interaction behavior between mechanized twin tunnels and groundwater in different cases of the groundwater level and the rest interval between excavations of twin tunnels.

Three-Dimensional Simulation of Interaction Between Surface Buildings and Twin Tunnelling Regarding the Surface Settlement

Tunneling-induced strain and stress distribution in a soil medium is completely three dimensional. However, the issue is usually modeled in two-dimensions (2D) in order to lower the computational costs. In this study, full three-dimensional (3D) finite element (FE) analysis of twin tunnelling and surface building are carried out to compare the results with 2D ones and to find out surface settlement behavior during tunneling process considering the presence of a building on the surface. Conducting 3D FE parametric studies, a large database is created and the results are used to evaluate different aspects of 3D modeling. Based on the outcomes of this study, comparison between 2D and 3D modeling is conducted, the situations in which 3D analysis could be substituted by 2D plane strain models and the cases in which 3D modeling is necessary are defined. The effects of different parameters on surface settlement trough are assessed and a modification design graph for prediction of maximum 3D building settlement from 2D green-field analysis is suggested.

Verifying discontinuous deformation analysis simulations of the jointed rock mass behavior of shallow twin mountain tunnels

This study extends discontinuous deformation analysis (DDA) to simulate the interactions of twin tunnels in a jointed rock mass. The ground subsidence and the rock stress distribution around the tunnels from the DDA simulation are compared to those from the trap-door model. Both the experimental and computational results show that the changing tunnel interactions in the jointed rock mass between the two tunnels are coincidently associated with the tunnel distance. In addition, the dip angle of the rock strata significantly affects the surface subsidence patterns and the stress distribution around the excavation zone. Therefore, the DDA method is a useful tool for describing the mechanical behavior of a jointed rock mass when a twin-tunnel is constructed in mountain terrain.

Effects of train operational parameters on ground-borne vibrations induced by twin metro tunnels

ABSTRACT Many underground railway lines consist of twin tunnels. The accurate prediction of ground-borne vibration levels is thus essential for this situation. This study investigates the train’s axle load and train's speed effect on the ground surface vibration response while two trains passed each other in the opposite direction in the twin subway tunnels. Firstly, a 3D finite element (FE) model of a twin circular tunnel in a homogeneous soil is developed and verified by the previous analytical studies. Then, sensitivity analyses are performed to assess the effect of train's speed and axle load on the ground surface response. The achieved results show that an increment of the train's speed in both tunnels causes increment PPV (Peak Particle Velocity) at the ground surface, but increasing the speed of one of the trains respect to the other one has a considerable influence on surface vibrations. The vibrations are increased about 2 dB and 3.5 dB by doubling the speed of both trains and one of the trains, respectively. Vibrations level at the ground surface is enhanced with increasing both trains axle load, but the ground surface vibrations are more sensitive to the difference between two trains axle load.

Parametric Study of Pile Response to Side-by-Side Twin Tunneling in Stiff Clay

A three dimensional coupled-consolidation numerical parametric study was carried out in order to gain new insight of single pile response to side-by-side twin tunneling in saturated stiff clay. An advanced hypo plasticity (clay) constitutive model with small-strain stiffness was adopted. The effects of relative to the pile tunnel depths were investigated by simulating the twin tunnels near the pile at various depths of tunnels, namely near the pile shaft, adjacent to the pile toe, and below the pile toe. It was found that the second tunneling in each case resulted in a larger settlement than the one due to the first tunneling with a maximum percentage difference of 175% in the case of twin tunneling near the mid-depth of the shaft. This occurred due to the degradation of clay stiffness around the pile during the first tunneling. Conversely, the first tunneling-induced bending moment was reduced substantially during the second tunneling. The most critical location of twin tunnels relative to the pile was found to be below the pile toe.

Parametric Study of Pile Response to Side-by-Side Twin Tunneling in Stiff Clay

Parametric Study of Pile Response to Side-by-Side Twin Tunneling in Stiff Clay