Twin Tunnels Research Articles

Three-dimensional numerical investigation of ground settlement caused by piggyback twin tunnels

Induced transverse ground surface settlement during the excavation of shallow piggyback twin tunnels is examined in this study. A series of three-dimensional parametric finite element analyses were carried out to investigate the settlement behavior and interactions between piggyback twin tunnels excavated in clay using mechanized excavation method. Various geometric parameters and construction sequences play important roles in the interaction between twin tunnels and on the induced ground settlement and internal forces in the linings of both tunnels. Analyses were carried out for five cover-to-diameter (C/D) ratios, three possible construction sequences and four pillar distances (P). The total settlement of the excavated twin tunnels as well as the settlement induced solely by the excavation of a new second tunnel, are both presented along with anticipated shear strains under various excavation sequence scenarios. The observed data trends from this study are generally consistent with the data available in the literature. One of the key findings, among others from this study is that construction of twin or multi-tunnels with a small number of steps induces relatively low settlement compared to tunnel construction with a high number of steps. Furthermore, the effects of C/D and P/D ratios on the magnitude of induced settlement and the characteristics of settlement trough are identified. In addition, two design charts have been proposed to calculate the settlement induced from a new second tunnel excavation.

Experimental and Numerical Analysis of Vibrations Induced by a Twin Tunnel, Underground Railway

Experimental and Numerical Analysis of Vibrations Induced by a Twin Tunnel, Underground Railway

Performance of Side-By-Side Twin Tunnel Excavation Under a Single Pile and Pile Group by the NATM

Performance of Side-By-Side Twin Tunnel Excavation Under a Single Pile and Pile Group by the NATM

Mapping Tunneling-Induced Uneven Ground Subsidence Using Sentinel-1 SAR Interferometry: A Twin-Tunnel Case Study of Downtown Los Angeles, USA

Synthetic Aperture Radar (SAR) interferometry is a formidable technique to monitor surface deformation with a millimeter detection resolution. This study applies the Persistent Scatter-Interferometric Synthetic Aperture Radar (PSInSARTM) technique to measure ground subsidence related to a twin-tunnel excavation in downtown Los Angeles, USA. The PSInSARTM technique is suitable for urban settings because urban areas have strong reflectors. The twin tunnels in downtown Los Angeles were excavated beneath a densely urbanized area with variable overburden depths. In practice, tunneling-induced ground settlement is dominantly vertical. The vertical deformation rate in this study is derived by combining Line of Sight (LOS) deformation velocities obtained from SAR images from both ascending and descending satellite orbits. Local and uneven settlements up to approximately 12 mm/year along the tunnel alignment are observed within the allowable threshold. No severe damages to aboveground structures were reported. Furthermore, ground movements mapped one year before tunnel construction indicate that no concentrated ground settlements pre-existed. A Machine Learning (ML)-based permutation feature importance method is used for a parametric study to identify dominant factors associated with the twin-tunneling induced uneven ground subsidence. Six parameters are selected to conduct the parametric study, including overburden thickness, i.e., the thickness of artificial fill and alluvium soils above the tunnel springline, the distance between the two tunnel centerlines, the depth to the tunnel springline, building height, the distance to the tunnel, and groundwater level. Results of the parametric analysis indicate that overburden thickness, i.e., the thickness of artificial fill and alluvium soils above the tunnel springline, is the dominant contributing factor, followed by the distance between tunnel centerlines, depth to the tunnel springline, and building height. Two parameters, the distance to the tunnel, and the groundwater level, play lesser essential roles than others. In addition, the geological profile provides comprehension of unevenly distributed ground settlements, which are geologically sensitive and more concentrated in areas with thick artificial fill and alluvium soils, low tunnel depth, and high groundwater levels.

Analysis of observed performance of a deep excavation straddled by shallowly buried pressurized pipelines and underneath traversed by planned tunnels

The objective of this paper is to analyze the observed performance of a deep excavation facing severe restrictions from underground facilities or structures in Suzhou, China. The first source of the restrictions comes from two shallowly buried pressurized pipelines that straddle the excavation at an intersection angle of about 81°. The pipelines need to be well protected at the site during the construction to ensure the normal life of the local resident. The second source of the restrictions arises from the twin tunnels that are planned to be constructed beneath the excavation in the future for Suzhou Metro Line 14. The planned tunnels will be constructed using a tunnel boring machine, which necessitates the reservation of favorable conditions by the present excavation project. Because of these restrictions, the form of the excavation support system becomes complicated, and the performance of the excavation will inevitably differ from that of the common excavations. Based on a thorough analysis of the extensive field performance data collected by means of settlement points and inclinometers located within and adjacent to the excavation, the characteristics of the ground surface settlements, lateral ground movements, ground water table variations, vertical and lateral displacements atop diaphragm walls, vertical displacements atop lattice column, axial forces in concrete strut, and vertical and lateral pipeline deformations are captured. Despite the discontinuity of the peripheral retaining wall and the shortened wall height on the discontinuous sides, the ground and structural deformations are well controlled by the specially designed excavation support system. The pipelines are slightly uplifted by the suspension system, with the maximum vertical pipeline displacement being lower by an order of magnitude than the commonly observed maximum pipeline settlements in the literature. The existing empirical envelops are found to be site-specific in applicability, and are recommended to incorporate the field observations of this unusual deep braced excavation case history.

Numerical Investigation and Prediction of Side-By-Side Tunneling Effects on Buried Pipelines

With the fast development of underground space engineering, it is inevitable for buried pipelines to be crossed by twin tunnels. Previous studies mainly focused on the single-tunneling effects on pipelines. To emphasize the twin-tunneling effects on buried pipelines, we first examined the effectiveness of the ground settlement prediction method under twin-tunneling conditions. Then, the estimated ground settlement boundary condition was applied to the beam-on-spring finite element model. The numerical results show that with the decrease in tunnel depth and twin tunnel space, the values and positions of the maximum ground settlement and longitudinal pipe bending behavior both changed significantly. The biased distance of the maximum settlement position and the distance to the inflection point of the final ground settlement curve can be obtained by curve fitting. Based on that, a semi-empirical prediction method for the longitudinal pipe bending strain was proposed. The predicted values matched quite well with the numerical results, which can thus provide a quick and effective structural safety and integrity assessment approach for buried pipelines subjected to twin-tunneling conditions.

Passenger ride comfort in subway due to new subway excavation below

New tunnel excavation below existing tunnel deforms the rails inside the existing tunnel. The settlement-induced rail irregularity intensifies the wheel-rail interaction, which reduces passenger ride comfort of the existing tunnel. This research investigates the passenger ride comfort of the existing tunnel due to new tunnel excavation. A project of new twin tunnels excavation below existing triple tunnels is introduced. According to the field monitoring data, the settlement of each of the existing tunnels, although supported by different types of lining, can be fitted by the Gaussian function. Then numerical simulation is performed to study the deformation characteristics of the existing rails, which is difficult to monitor in the field. The simulation results show that the tunnel deformation can well represent the rail deformation. After that, train ride comfort is adopted to evaluate passenger comfort indirectly. The vehicle dynamic performance due to settlement-induced track irregularity is studied by a general multibody system simulation software. The results show that the vertical acceleration of the car body increases linearly with the increase of the settlement amplitude and decreases exponentially with the increase of the trough width i. When the trough width i is larger than 15 m, the vertical accelerations produced by new tunnel excavation are negligible compared to those produced by track irregularity. The settlement amplitude limits considering the passenger ride comfort in subway due to new subway excavation below are obtained.

Influence of Pipeline Leakage on the Ground Settlement around the Tunnel during Shield Tunneling

Shield tunneling is widely used in urban subway tunnel construction. Old urban underground pipelines generally have small leakages that are difficult to find. The water leakage significantly reduces the stability of the stratum, posing a threat to the safety of tunnel shield construction. Therefore, this study established 2D and 3D calculation models for analyzing the law of the leakage diffusion in the ground under water pressure, and the influences of the pipeline leakage range and leakage length on the changes in ground settlement during shield tunneling. The 2D model calculation results show that seepage water mainly diffuses vertically under gravity. As the pipeline leakage gradually reaches a predetermined depth, the simulation results tend to be consistent with the test results. The 3D model is more accurate than the theoretical solution in predicting the ground settlement because it can consider the influences of repeated disturbances in twin tunnel shield construction. The maximum ground surface settlement increases with the extent of the leakage length and leakage range, and the range is the main factor determining the settlement. At the interior of the ground, the seepage water has a greater impact on areas with strong disturbances and large soil losses.

Experimental Study on Deformation Failure Mechanism of Surrounding Rocks for the Deep-Buried Twin Tunnels in Inclined Layered Strata During the Excavation

Experimental Study on Deformation Failure Mechanism of Surrounding Rocks for the Deep-Buried Twin Tunnels in Inclined Layered Strata During the Excavation

Rockburst and microseismic activity in a lagging tunnel as the spacing between twin TBM excavated tunnels changes: A case from the Neelum-Jhelum hydropower project

A reasonable spacing between tunnels is very important in deeply buried tunnels, as it may mitigate the risk of rockburst occurrence. This spacing is particularly important for tunnels excavated using a tunnel boring machine (TBM), as the methods available for controlling rockburst occurrence are inflexible and limited using these machines (compared to tunnels excavated using the drilling and blasting (D&B) method). In this paper, a rare example was studied in situ in which the spacing between twin tunnels was changed to mitigate the rockburst risk during TBM excavation. First, the characteristics of the rockburst and microseismic activity were investigated in three sections of the lagging tunnel before, during, and after the spacing between the tunnels was changed (we concentrated on the lagging TBM696 tunnel, which is part of the Neelum-Jhelum hydropower project). We observed a significant occurrence of rockburst and microseismic activity in the lagging tunnel at the original spacing of the twin tunnels in both the sandstone and siltstone lithologies, and thus a consideration of enlarging the spacing between the twin tunnels is reasonable. A more detailed investigation was then conducted to explore the effects of tunnel spacing on rockburst occurrence and microseismicity. The rockburst frequency and microseismic radiated energy due to excavation unloading generally decrease as the spacing between the twin tunnels increases (substantially at first and then more slowly as separation increases). In particular, the microseismic radiated energy exhibits a power-decreasing type in both the sandstone and siltstone. Increasing the spacing beyond a certain point, therefore, exerts a very limited effect on the microseismic radiated energy and occurrence of rockbursts (increments of 16–21 m and 15–20 m in sandstone and siltstone, respectively). Consequently, in practice, a very reasonable approach would be to enlarge the spacing of twin tunnels by 22.48 m. Thus, microseismic monitoring is a useful technique for assessing the rockburst risk and deriving a reasonable value for tunnel spacing. This study can be used as a guideline to derive reasonable tunnel spacings when using not only TBMs but also D&B.

Design And Construction Of Roma Street Station Cavern, Cross River Rail, Brisbane

The new Roma Street underground railway station in Brisbane is being constructed as part of Cross River Rail’s Tunnel, Stations and Development (TSD) package. The joint venture of CPB Contractors, BAM International Australia, Ghella and UGL (CBGU JV) is building the 5.9km long twin tunnels from the Southern Tunnel Portal near Dutton Park station, beneath the Brisbane River and CBD to the Northern Tunnel Portal in Spring Hill. The Cross River Rail project includes excavation and construction of four new underground stations. Roma Street station comprises a 280m long cavern, five smaller connecting tunnels (adits) and three shafts. The station cavern has an excavated span of up to 24.4m with approximately 15m rock cover. It has been excavated within the Neranleigh-Fernvale Group (NFG) rock mass, which comprises weakly metamorphosed sandstone (meta-greywacke and arenite), phyllite and subordinate quartzite and meta-basalt. The station lies within the regional Normanby Fault Zone, characterised by a major fault up to 20m wide comprising a combination of intact rock, rock breccia and clay gouge. The fault zone encountered during the station cavern excavation required heavier primary support and localised foundation treatment. The initial primary (temporary) support of the cavern and adits comprised rock bolts, cable bolts and a thin synthetic fibre-reinforced shotcrete lining. In some areas a passive shotcrete arch lining was required. Overlying piled footings from an existing busway overpass structure were within a metre of the adits’ excavated profile which necessitated a complex load transfer structure at the surface and verification of pile toe levels during tunnel construction. The cavern permanent lining typically comprises steel fibre-reinforced concrete in the crown, bar reinforced concrete for the sidewalls, and bar and steel fibre-reinforced concrete invert slabs. Bar reinforcement is used in the cavern crown where it intersects the adits. Ground loads for the permanent structure had to consider the influence of future developments. This paper presents some of the challenges of the primary support and permanent lining design of the station cavern and adits. It summarises the as-encountered ground conditions, aspects of the primary support and permanent lining design that were geotechnically challenging and the solutions developed to meet the project requirements.

Analytical approach to scattering of P, SV waves by twin tunnels in saturated half-space

Analytical approach to scattering of P, SV waves by twin tunnels in saturated half-space

Influence of Construction of the following Tunnel on the Preceding Tunnel in the Reinforced Soil Layer

In coastal cities, some paralleled twin tunnels must be constructed in soft soil layers, and stratum reinforcement is necessary to enhance the stability of the tunnel. However, the influence of construction of the following tunnel on the preceding tunnel in reinforced strata is not clear. This study focuses on four kinds of reinforcement strategies: ring reinforcement of the right tunnel (RR), ring reinforcement of the left tunnel (LR), ring reinforcement of twin tunnels (TR), and cement wall reinforcement between left and right tunnels (CR). The interaction mechanism of the twin tunnels during construction, in terms of surface settlement, tunnel deformation, convergence deformation of the tunnel lining, and internal force of the tunnel lining, is revealed by a series of three-dimensional numerical analyses. It was found that the LR offers an appropriate strategy in terms of both engineering safety and cost savings, and it can significantly reduce the disturbance of the following tunnel excavation on the preceding tunnel.

Grouting at Lonato tunnel

AbstractThe planned Brescia–Verona high‐speed/high‐capacity railway line is a key project of the Milan–Verona railway link. Between Brescia and Verona, the new line is about 48 km long, running mostly alongside the A4 highway. Near Lonato (south of Garda Lake), the line goes under A4 and other buildings and structures, through twin tunnels bored by the tunnel boring machine (TBM). The soil consolidation project is deemed necessary to protect the road from subsidence before the TBM is deployed. On site, the soil is extremely heterogeneous but always characterized by a silty sand matrix. The poor capacity of the soil to be permeated led to reshaping the borehole mesh and selecting both cementitious and silicate‐based grout mixes designed to obtain rheological features leading to better permeation capability. The operation to prevent subsidence involved injecting some 10,000 m3 of grout mixture, through about 90,000 m of sleeved pipes, in multi‐directional geometries.

A practical approach for modeling twin-tunnel excavation in Ho chi minh city

The prediction of ground settlement under tunnel excavation is still challenge. Almost engineer uses Mohr-Coulomb model in practice due to the conventional geotechnical investigation data. This paper describes the study of tunnel lining behaviors and ground surface settlement under tunneling process with a typical case study of twin tunnels excavation in Ho Chi Minh city, Vietnam. The advanced material model namely Hardening Soil model is used to investigate the proposing twin-tunnel with numerical approach. The internal forces of tunnel lining and ground settlement, which achieved from Hardening Soil model and the available results from Mohr-Coulomb model, are then made comparison between two models which yields some important differences for analysis. Since the experimental works for qualifying stiffness parameters in Hardening Soil model are missed in the Metro Line 1 project in Ho Chi Minh city, an empirical formula is proposed in the paper as a guide for estimating the required data in modelling process

Analytical study on seepage field of subsea twin tunnels constructed by NATM

When two tunnels are close to each other, there will be serious mutual interference in the seepage fields. It may result in errors of more than 30% to use an independent single-hole tunnel model for seepage field analysis. An analytical model for the seepage field of subsea twin tunnels is proposed, and an analytical solution of the seepage field is obtained based on the superposition principle. The analytical solution is verified by degradation analysis, a numerical test and an engineering case. It is shown that: (1) The optimal net distance of twin tunnels is 1 times the tunnel diameter. At this time, the water inflow into one of the twin tunnels is the lowest, which is about 70% of the water inflow into the independent single-hole tunnel. (2) When the diameter of the drainage tunnel is reduced by 75% (the excavation area is reduced by 94%, and the project cost is greatly reduced), the water inflow into the main tunnel increases by only 18%. Therefore, small diameter drainage tunnel is the priority strategy. (3) The drainage tunnel should preferably be arranged at 45° obliquely below the main tunnel.

Critical support pressure of shield tunnel face in soft-hard mixed strata

Support pressure is a key factor to the stability of excavation face during shield tunneling, which further imposes a safety issue on surrounding buildings and facilities. However, several studies have been reported on critical support pressure in mixed strata regarding twin-tunnel excavation. This paper aims to obtain the critical support pressure based on minimizing the influence on the surrounding environment induced by shield tunneling in soft-hard mixed strata. A series of three-dimensional finite element models are established to analyze soil and lining responding with different support pressure caused by twin tunnels excavation. The results reveal that the ground surface settlement and lateral displacement of soil are sensitive to the change of support pressure, especially under clay-dominated conditions. Increasing support pressure slightly changes the lining movement and location of maximum stresses of the existing tunnel due to adjacent tunnel excavating. According to this study, the upper and the lower critical support pressures are 1.43 and 1.06 times of lateral earth stress at tunnel crown, respectively, which are also compared with the limit support pressure and measured results from a shield tunnel in the literature. The critical support pressure has great significance for the stability of the excavation face and minimizing the influence on the surrounding environment caused by shield tunnel excavation.

Twin tunnelling caused distortions and its effect on a hospital building on mixed ground conditions

ABSTRACT An extensive geotechnical assessment of a hospital building under-crossed by twin tunnels is presented in this paper. The case study considered in the study is Phase III of Delhi Metro Gray line, which has been built recently using Tunnel Boring Machine. Finite element analysis was carried out to simulate the construction of two tunnels underneath the building, and the soil is simulated using an advanced constitutive model. The numerical model was validated against the field monitoring data, and the effect of tunnelling on the building deformation is also studied. Also, the comparison of building settlements with greenfield conditions is carried out. The study showed that the building influences the pattern of the settlement trough. Quantitative study of the settlement showed that there was no undue distress to the building due to the construction of twin tunnels, suggesting the safe crossing of twin tunnel below hospital building.

Twin earth pressure balance tunnelling – monitoring and numerical study of an urban case

In recent decades, numerical calculations have often been used to optimise, from a technical and economical perspective, the design of underground works. Using these tools allows efficient and affordable designs to be obtained. Simulation of the tunnelling procedure when using a tunnel-boring machine and in urban areas is a complex soil–structure interaction problem. Here, a three-dimensional (3D) code using finite differences is used. The reference case of this study is Lyon metro line D project in France, where a non-symmetric load is present at the surface due to the presence of a retaining wall. An accurate on-site monitoring system was set up to assess ground movements during the mechanised excavation of twin tunnels. The collected data are used as a reference to show the validity and accuracy of the developed 3D model. Two constitutive models were used to simulate the soil behaviour: plastic hardening (PH) and linear elasticity with perfect plasticity (Mohr–Coulomb shear failure criterion). The Mohr–Coulomb constitutive model shows some drawbacks, which can be eliminated by using PH. The existence of a retaining wall induces a non-symmetric load condition; however, the settlement trough predicted by advanced numerical models shows good agreement with that obtained by monitoring data.

Centrifuge modelling of tunnelling below existing twin tunnels with different types of support

Tunnel excavation below existing tunnels produces ultimate and serviceability problems to the existing tunnels. The behaviours of induced stresses on the existing tunnels haven't yet been fully recognized. In this study, a centrifuge model test was adopted to investigate the effects of new tunnelling on two existing overlying tunnels. One existing tunnel model simulated a prototype composite lining tunnel and the other simulated a prototype segmental lining tunnel. The volume loss produced by new tunnel excavation was modelled by an in-flight actuator system. The surface settlements, the existing tunnels settlements, the soil pressures on existing tunnels, the bending movements of existing tunnels, and the joint behaviours of existing tunnels were monitored. The volume of surface settlement trough was much smaller than the soil volume moving into the tunnel, due to the heave of the tunnel bottom and the dilation of sand during shearing. The maximum settlement of the segmental lining model was larger than that of the composite lining model as the equivalent bending stiffness of the composite lining model was larger than that of the former. Due to new tunnel excavation, the soil pressures on different positions of the existing tunnel behaved differently, and the bending movements of the existing tunnels decreased. Moreover, the joint deformation of existing tunnel caused by new tunnel excavation could be classified into three types: (1) translation, (2) rotation, and (3) combination of both.