Twin Tunnels Research Articles

Construction Mechanical Mechanism of Shallow Subway Tunnel Group with Large-Span Variable Cross Section

The number of tunnel crossings or line switchings in the vicinity of subway transfer stations is increasing with the construction of subway tunnels. The construction interaction of the subway tunnel group with variable cross section and the time and space effects of strata deformation has become a major problem that limits the development of subway tunnel spaces. Thus, this study analyzed the deformation law of the upper strata and surrounding rocks in a subway tunnel group with large-span variable cross section caused by different construction processes using ABAQUS numerical simulation analysis. The research focused on the construction interaction mechanism and stratum deformation law of such a subway tunnel group and considered Shenzhen Metro Line 7 as the study subject. The stratum deformation law caused by twin tunnels excavation was referred. Results showed that (1) the construction process significantly influenced the construction safety of tunnel groups with large-span variable cross section; (2) constructing small cross-sectional tunnels first increases the lateral deformation and settlement of the vault in large cross-sectional tunnels, which are far from small cross-sectional tunnels, but reduces the settlement of the vault in the small cross-sectional tunnels and the lateral deformation of the surrounding rock between the large and small cross-sectional tunnels, which benefits tunnel groups with large-span variable cross section; and (3) as shown by the comparison of the final numerical simulation data that in which the large cross-sectional tunnel was constructed after the completion of the small cross-sectional tunnel construction had the lowest project risk.

Three-dimensional numerical simulation of the Shiraz subway second line – influence of the segmental joints geometry and of the lagging distance between twin tunnels’ faces

Due to the widespread need for the urban transportation network, construction of underground railway lines by mechanised shield-tunnelling increases in metropolises. The case of the Shiraz (Iran) subway twin tunnel construction is an example of transport network extension. In the present paper, a three-dimensionalnumerical model was developed to investigate the tunnel’s lining behaviour and the ground displacement surrounding the second line of the Shiraz subway tunnels. All the excavation phases of EPB-TBM (Earth pressure balance, Tunnel boring machine) were simulated using the finite difference method. In most numerical simulations, the segmental joints were assumed parallel to the tunnel axis, despite the fact that the longitudinal joints make an oblique angle with the tunnel. Three kinds of lining pattern were then considered: continuous, straight and oblique joints. The numerical results indicated the necessity of using segmental lining with oblique joints pattern to obtain an accurate evaluation of the structural forces applied in segmental lining. The excavation of a new tunnel near to an existing tunnel with high lagging distance between twin tunnels leads to increase in the normal and the longitudinal displacements, in the internal lining forces and to a decrease in the ground surface settlement above the tunnels.

Behavior of Twin Tunnels under Different Physical Conditions

Behavior of Twin Tunnels under Different Physical Conditions

The Investigation of Twin Tunnel Stability: Effect of Spacing and Diameter

ABSTRACT The underground excavation became the backbone of developing as well as developed country due to unavailability of surface area and increase demand of space for transportation, water channeling, waste disposal and storage. Except for transportation, the crystalline rocks (high strength, low porosity and less permeability) are considered most suitable and safe space for the storage of radioactive waste as well as petroleum. The role of spacing between twin tunnel and tunnel’s diameter in its stability was carried out using finite element method. Not much research has been done on twin tunnel spacing in hard rock. In the present study, a total of ninety simulations were performed to investigate critical spacing of the twin tunnel in granitic rock mass. Twin tunnel diameter is varied from the 2 m to 10 m and their spacing varied from 0.2 to 2 times of diameter. It was concluded that minimum spacing should be 0.8 times of the tunnel diameter. It was also found that small diameter tunnel effects only near field but large diameter tunnel effect near field as well as far field. The vertical hole, approximately 3m diameter, containing radioactive waste canister should be drilled at the interval of more than 2.4m.

Influences of Dynamic Loads of Earthquake on Pore Water Pressure (Tabriz Metro First Line)

Summary Tabriz metro twin tunnels in some stations are under ground water level. In addition to the loads from the weight of overburden, the load caused by pore water pressure is applied to them. To enter the current load into model certain boundary conditions must be considered that in this study, the probability of liquefaction and the impact of the earthquake on pore pressure loading structure have been investigated. Introduction Geo-technical design of structures is encountered with a variety of different natural conditions (earthquake, liquefaction, etc.). This will be due to the complexity of behaviors and conditions. These structures during his lifetime, because dealing with different situations such as earthquake will not have fixed characteristics and behavior. Complex Mechanical behavior of the soil under statistical loads can be different and more complicated under dynamic loads. In this respect, the modeling of alternative behavior of saturated soil, especially in the interaction zone of soil – structure, has gained increasing research interest over past decades. Because the estimation of structure response in the soil liquefaction depends on pore water pressure, softening strain and decreasing of soil resistance. For correct investigating, linear dynamic analyze in effective stress is necessary. For this type of analyze couple or semi couple methods could be applied. In this regard Finn & Martin and Byrne could be regarded as semi couple models. Methodology and Approaches From aforementioned semi couple models, Byrne model was chosen in this research. After calculation of related indexes, the model implemented in Flac2D software. The possibility of liquefaction take place in structure foundation was investigated by local tests results in effective stress method. For numerical modeling, the Bam earthquake acceleration was selected in DBE level among the existing acceleration and impact of earthquake dynamic load of on project structure pore pressure was investigated. Results and Conclusions Using available data at the project structure the probability of liquefaction in the project region was investigated by cyclic stress method. It was estimated that with safety factor of 1.28 for Bam earthquake magnitude earthquake 6.5M, liquefaction will not occur. Nevertheless, due to exist of pore pressure at structure needed parameters was calculated in Byrne model to study simultaneous impact of dynamic loads and pore pressure to be used in numerical modeling. The results showed an increase in pore pressure due to dynamic loads caused by earthquakes so higher load values to the desired location (subway tunnel) is inserted and finally more effective maintenance system for the tunnel is estimated.

Lateral behaviour of a pile group due to side-by-side twin tunnelling in dry sand: 3D centrifuge tests and numerical modelling

This paper presents lateral responses of an existing 2 × 2 pile group to advancement of side-by-side twin tunnels (with 1% volume loss) at various depths in dry sand using 3D centrifuge and numerical modelling. The tunnels were located near mid-depth of the pile shaft (test SS), next to (test TT) and below the toe (test BB) of the pile group. The largest and the smallest lateral movement of the pile cap was induced after first tunnelling in test TT and test SS, respectively. The shearing force was induced in piles as a reaction of pile deflection in tests TT and BB.

Rehabilitation of Portal Subsidence of Heybat Sultan Twin Tunnels: Selection of Shotcrete or Geogrid Alternatives

The occurrence of karst phenomenon is one of the common problems in carbonate rocks in the presence of water. The rock masses constituting the ground surface are mostly of sedimentary types among which carbonate rocks are widely observed. It is therefore necessary to accept such a geological hazard in many projects. Some of the rehabilitation methods for karstic subsidence include grout injection, filling with concrete or shotcrete and making use of geosynthetic products. Few studies have been carried out on the application of the geosynthetic products. Limy rocks form the main lithology of a large part of the 72 km long access road tunnel designed for the Iraqi-Kurdistan. The occurrence of a karstic subsidence with a volume of about 2250 m3 in the portal of Heybat Sultan tunnels revealed the necessity of examining and selecting one of these rehabilitation methods. Concerns about the repetition of such collapses in other tunnels, especially in the 6740 m long Korek twin tunnels that is the longest tunnel of the Middle East located in the vicinity of the project, has doubled the importance of the issue. This paper aims to render an account of the use of shotcrete and geogrid to rehabilitate the subsidence of the portal of Heybat Sultan tunnels. The modeling results with the FLAC numerical finite difference code showed that the displacement amount of the host rock mass after implementing the second method would be 2.9 times less than the first method. Geogrid also reduces the axial forces exerted to the tunnel supporting system up to 43 tons.

Numerical Modeling of Ground Movement due to Twin Tunnel Structure of Esfahan Subway, Iran

Tunneling operations in urban areas are always associated with surface and subsurface ground movement, which may affect the stability of nearby structures and utilities. One of the main issues in the analysis and design of tunnels, is ground movement after the support systems and settlement systems caused by excavation. This paper takes the discrete element calculation software of Universal Distinct Element Code (UDEC) to set three dimensional calculations models to simulate the construction of Esfahan subway tunnel, analyzing the displacement and stress dynamic response during the construction process. The paper also carries out the comparison analysis of the field measurement of the ground movement. The result indicates that the simulation objectively reflected the rules of movement during the construction process. The results show that the tunnels are stable before support system installation and the tunnel induced movement is allowable.

Catastrophe stability analysis for shallow tunnels considering settlement

Limit analysis of the stability of geomechanical projects is one of the most difficult problems. This work investigates the influences of different parameters in NL failure strength on possible collapsing block shapes of single and twin shallow tunnels with considering the effects of surface settlement. Upper bound solutions derived by functional catastrophe theory are used for describing the distinct characteristics of falling blocks of different parts in twin tunnels. Furthermore the analytical solutions of minimum supporting pressures in shallow tunnels are obtained by the help of the variational principle. Lastly, the comparisons are made both in collapsed mechanism and stability factor with different methods. According to the numerical results in this work, the influences of different parameters on the size of collapsing block are presented in the tables and the limit supporting loads are illustrated in the form graphs that account for the surface settlement.

A FE-IBE method for linearized nonlinear soil-tunnel interaction in water-saturated, poroelastic half-space: I. Methodology and numerical examples

Currently, finite element method is the most widely used method for seismic analysis of underground tunnels. However, the finite element method has an apparent difficulty in modelling unbounded soils, therefore, artificial boundaries are often used to solve this problem, but it becomes much more complicated when the unbounded soils are water-saturated, poroelastic media. This paper provides an alternative technique for seismic analysis of tunnels in unbounded water-saturated, poroelastic soils, with excellent accuracy and efficiency. Based on the Biot's theory of wave propagation in a poroelastic medium, this paper proposes a 2-D finite element-indirect boundary element (FE-IBE) coupling method for seismic analysis of tunnels in water-saturated, poroelastic half-space. The soil nonlinearity is dealt with equivalently linear approach, while the tunnel lining remains linear. One particular advantage of the proposed coupling method is that it allows separate computation for the FE subdomain and the IBE subdomain while avoids iterative process, which is well suitable for parallel computation. Another desirable feature of the proposed coupling method is that it can account for nonlinear soil effect for both the FE subdomain (near field) and the IBE subdomain (far field). In the first part of this paper, the FE-IBE coupling method is presented in detail and the accuracy of this coupling method is verified extensively through comparisons with published results. Besides, the capability of the proposed coupling method is further demonstrated by applying it to analyze seismic responses of tunnels in water-saturated, poroelastic half-space, with emphasis on the effects of soil nonlinearity and interaction between twin tunnels. In the second part of this paper, using results obtained by the FE-IBE coupling method as benchmark, the applicability of two widely used analytical solutions to seismic design of tunnels with soil nonlinearity is investigated. Of particular interest is the influence of dynamic soil-tunnel interaction and interaction between solid frame and pore water.

Analytical stress and displacement due to twin tunneling in an elastic semi‐infinite ground subjected to surcharge loads

SummaryThe construction of twin tunnels at shallow depth has become increasingly common in urban areas. In general, twin tunnels are usually near each other, in which the interaction between tunnels is too significant to be ignored on their stability. The equivalent arbitrarily distributed loads imposed on ground surface were considered in this study, and a new analytical approach was provided to efficiently predict the elastic stresses and displacements around the twin tunnels. The interaction between 2 tunnels of different radii with various arrangements was taken into account in the analysis. We used the Schwartz alternating method in this study to reduce the twin‐tunnel problem to a series of problems where only 1 tunnel was contained in half‐plane. The convergent and highly accurate analytical solutions were achieved by superposing the solutions of the reduced single‐tunnel problems. The analytical solutions were then verified by the good agreement between analytical and numerical results. Furthermore, by the comparison on initial plastic zone and surface settlement between analytical solution and numerical/measured results of elastoplastic cases, it was proven that the analytical solution can accurately predict the initial plastic zone and its propagation direction and can qualitatively provide the reliable ground settlements. A parametric study was finally performed to investigate the influence of locations of surcharge load and the tunnel arrangement on the ground stresses and displacements. The new solution proposed in this study provides an insight into the interaction of shallow twin tunnels under surcharge loads, and it can be used as an alternative approach for the preliminary design of future shallow tunnels excavated in rock or medium/stiff clay.

Monitoring Cityringen Metro project in Copenhagen, Denmark

Due to open in 2019, Cityringen Metro in Copenhagen, Denmark, consists of 17 underground stations, five shafts and 15·57 km of twin tunnels. Construction is in dense urban areas and much of the tunnelling was in soft ground or mixed-face conditions. Continuous monitoring of the ground, structures, groundwater, noise and vibration has been critical to successful completion of the 8 year project. Monitoring data were also used to justify design changes to avoid intrusive mitigation measures. Illustrated with case studies, this paper describes the scope of the monitoring and how the extensive amount of data was managed.

2D numerical investigation of twin tunnels-Influence of excavation phase shift

The excavation of twin tunnels is a process that destabilizes the ground. The stability of the tunnel lining, the control of ground displacements around the tunnel resulting from each excavation and the interaction between them must be controlled. This paper provides a new approach for replacing the costly 3D analyses with the equivalent 2D analyses that closely reflects the in-situ measurements when excavating twin tunnels. The modeling was performed in two dimensions using the FLAC2D finite difference code. The three-dimensional effect of excavation is taken into account through the deconfinement rate l of the soil surrounding the excavation by applying the convergence-confinement method. A comparison between settlements derived by the proposed 2D analysis and the settlements measured in a real project in Algeria shows an acceptable agreement. Also, this paper reports the investigation into the changes in deformations on tunnel linings and surface settlements which may be expected if the twin tunnels of T4 El-Harouche Skikda were constructed with a tunneling machine. Special attention was paid to the influence of the excavation phase shift distance between the two mechanized tunnel faces. It is revealed that the ground movements and the lining deformations during tunnel excavation depend on the distance between the tunnels\' axis and the excavation phase shift.

Field Observation and Theoretical Study on an Existing Tunnel Underpassed by New Twin Tunnels

The methodology of the existing displacement control is illustrated taking the shield of twin tunnels of Line 4 underpassing the upline tunnel of existing metro Line 1, for example. Vertical, horizontal, and convergence displacement of the existing tunnel is monitored and analyzed in detail in this paper. Shield parameters are predefined and adjusted based on the feedback of the displacement of Line 1. Short‐term displacement of the existing tunnel is greatly influenced by the relative distance between the shield face and the existing tunnel and shield parameters. The shapes of horizontal and convergence displacement curves are similar. Line 1 is reinforced, and a new analysis method is firstly proposed for the design of reinforcement of the existing tunnel which is verified by the analytical methods derived from prior studies. The results show that the change of reinforcement stiffness has a greater effect on the normalized bending moment and the normalized shear force of the existing tunnel, and reinforcement of 25 rings on either side of the intersection point is the best choice in this case. The proposed model can be widely applicable for reinforcement design and safety check of the existing tunnel.

Monitoring and Analysis of Ground Settlement Induced by Tunnelling with Slurry Pressure‐Balanced Tunnel Boring Machine

A case study of monitoring and analysis of ground settlement caused by tunnelling of stacked twin tunnels for underground metro line construction through the densely populated area using the slurry pressure‐balanced TBM is presented. Detailed ground settlement monitoring was carried out for the initial stage of down‐track tunnelling in order to estimate trough width factor and volume losses including face, shield, and tail losses. In addition, using the gap model, prediction of volume loss and ground settlement was carried out with consideration of the ground condition, TBM configurations, and actual operation data. The predictions of the gap model were compared with the observed results, and adjustment factors were determined for volume loss estimation. The adjusted factors were applied to predict ground settlement of the up‐track tunnel, and its results were compared with the field measurements.

A new method for predicting ground settlement caused by twin-tunneling under-crossing an existing tunnel

Tunneling in densely populated urban areas frequently brings about under-crossing the existing tunnels, which may induce serviceability problems to the existing tunnels. In order to obtain accurate prediction of ground settlement, the current study synthetically analyzes the coupling effects of new tunnels, in-between soil and the existing tunnel, and proposes a new method for predicting the ground settlement caused by construction of a new subway twin tunnel under-passing an existing tunnel. This paper classifies the influence zone caused by construction of the twin tunnels under-passing existing tunnel into strong, weak and no influence zones based on synthetically considering the coupling effects of front-tunnel, in-between soil, rear-tunnel, and the existing tunnel. Further, by extending the proposed method and applying it to the case of twin-tunneling under-crossing an existing tunnel, detail processes for predicting the ground settlement are presented for the cases of strong and weak influence zones. Two case studies were carried out to verify the proposed method, and the results show that the predicting settlements well agree with the measured data.

3D Centrifuge Modeling of the Effect of Twin Tunneling to an Existing Pile Group

In densely built urban areas, it is inevitable that tunnels will be constructed near existing pile groups. The bearing capacity of a pile group depends on shear stress along the soil-pile interface and normal stress underneath the pile toe while the two would be adversely affected by the unloading process of tunneling. Although extensive studies have been conducted to investigate the effects of tunnel construction on existing single piles, the influence of twin tunnel advancement on an existing pile group is merely reported in the literature. In this study, a series of three-dimensional centrifuge tests were carried out to investigate the response of an existing pile group under working load subjected to twin tunneling at various locations in dry Toyoura sand. In each twin tunneling test, the first tunnel is constructed near the mid-depth of the pile shaft, while the second tunnel is subsequently constructed either next to, below or right underneath the pile toe (Tests G_ST, G_SB and G_SU, respectively). Among the three tests, the 2nd tunnel excavated near the pile toe (Test G_ST) results in the smallest settlement but the largest transverse tilting (0.2%) of pile group. Significant bending moment was induced at the pile head (1.4 times of its bending moment capacity) due to the 2nd tunnel T. On the contrary, tunneling right underneath the toe of pile (i.e., Test G_SU) results in the smallest tilting but largest settlement of the pile group (4.6% of pile diameter) and incremental mobilisation of shaft resistance (13%). Due to stress release by the twin tunneling, the axial force taken by the front piles close to tunnels was reduced and partially transferred to the rear piles. This load transfer can increase the axial force in rear piles by 24%.

3D Centrifuge Modeling of the Effect of Twin Tunneling to an Existing Pile Group

3D Centrifuge Modeling of the Effect of Twin Tunneling to an Existing Pile Group

Response of full-scale piles to EPBM tunnelling in London Clay

The installation and working test performance of four full-scale instrumented driven piles and their subsequent response to twin tunnels constructed below the pile bases are described. One pair was designed to be largely friction piles and the other pair end-bearing. Their locations relative to the new tunnels were carefully chosen to optimise understanding of pile responses at varying offsets from the centre-lines. The site conditions and the greenfield response to earth pressure balance machine tunnelling at the site were described in a companion paper that reported an expanding displacement field around the tunnels rather than the contracting fields usually observed. The field monitoring results indicated that, during construction, zones of influence existed around tunnels, where the ground and piles were subjected to different degrees and senses of relative vertical displacement. Redistributions of load along the pile lengths occurred as the tunnel boring machines approached, passed beneath and continued beyond the pile bases; lateral pile deflections and bending moments were also induced. Based on the results from this field study, implications for the capacity of existing piles (and design of new piles) subjected to tunnelling-induced movements are assessed for cases of expanding and contracting displacement fields.

An in-tunnel grouting protection method for excavating twin tunnels beneath an existing tunnel

A section of closely spaced twin tunnels at Shangmeilin station on Shenzhen metro line 9 was excavated by the shield tunneling method. Part of this section lies closely beneath the tunnel servicing Metro line 4. Due to restrictions on ground surface to control the extra settlement of the existing tunnel in the area, an in-tunnel grouting protection method was adopted in combination with the shield method. This method greatly reduced the effect on the existing tunnels when building the twin tunnels underneath. The settlement of and stress on the existing tunnels associated with the construction of the new tunnels were systematically monitored. Because of the twice under-crossing process, the excavation of twin tunnels had a superposed influence on the existing tunnel and surrounding soil. The behavior of the overlying tunnel was analyzed based on instrumentation records obtained from the project. The settlement profile of the existing structure displayed a “V” shape after the first under-crossing but a “W” shape after the second. The hoop stress of the existing tunnel induced by shield tunneling below had beneficial effects on the stress state of the tunnel structure. By contrast, the longitudinal stress exerts a bending moment on the existing tunnel, with a large tensile stress that had adverse effects on its structure. Based on the monitored data, three deformation modes are proposed to describe the behavior of the existing tunnel. In addition, the differences between the two under-crossing processes are discussed; both the ground loss ratio and width parameter of the settlement trough in the second profile were larger than those of the first profile due to the decrease in soil stability.