Subsea Tunnel Research Articles

Offshore high-resolution seismic survey for a subsea tunnel across the Bosphorus Waterway, Istanbul, Turkey

Following the completion of the Eurasian Tunnel in 2016, the Municipal Government of Istanbul plans to construct a second subsea tunnel across the Bosphorus Waterway between the European and Asian sectors of the city. We conducted an offshore 2-D sparker seismic survey in the Bosphorus Waterway to delineate geometry of the sub-bottom sedimentary sequence and geometry of the sediment-bedrock interface. We constructed a geological model based on the interpretation of the seismic image associated with the center line along the planned tunnel traverse. This geological model was subsequently used to determine the locations and depths of the geotechnical boreholes. Formation tops determined from geotechnical borings confirm the geological cross-section constructed from the seismic image in depth along the tunnel traverse, except within a segment on the European side where the strong currents of the Bosphorus have severely distorted the 2-D line traverse. This investigation demonstrates the compelling need for a seismic survey prior to geotechnical drilling for accurate description of the soil column both in the lateral and vertical directions within the project area. The seismic survey provided a cost-effective planning of geotechnical drilling to determine the number of boreholes, and where and how deep to drill each borehole. Additionally, the geological model constructed from the seismic image was used for the preliminary tunnel design. Nevertheless, a 3-D survey will be required prior to the final tunnel design and construction in order to resolve the uncertainty of the slope boundary of the top-Trakya bedrock on the European side.

Seismic response of soft water-bottom sediments

The Municipal Government of Istanbul plans to construct a second subsea tunnel across the Bosphorus Waterway between the European and Asian sectors of the city, and a submerged tube tunnel across the Golden Horn Waterway which connects to the Bosphorus Waterway. High-resolution 2-D sparker seismic surveys were conducted to delineate geometry of the sub-bottom sedimentary sequence and geometry of the sediment-bedrock interface, and to determine the locations and depths for geotechnical boreholes. In this paper, we discuss aspects of the seismic response of water-bottom sediments. Unlike the usual behavior of water-bottom multiples with alternating polarity observed in the Bosphorus case, we find that the polarity of the water-bottom primary and first-order water-bottom multiples are the same in the Golden Horn case. The polarity behavior of the water-bottom multiples in the Golden Horn is convincing evidence that the low-velocity sub-bottom sediments can be described as a gas-charged suspension — a composition that consists of particles suspended in water with only slight grain-to-grain contact and biogenic gas. The results of the velocity analysis of the Golden Horn seismic data confirm the presence of low-velocity (550–650 m/s) soft water-bottom sediments with an average thickness of 18 m above the bedrock. This makes it imperative to design the foundation pillars for the planned submerged tube-tunnel with sufficient penetration deep into the bedrock.

An approach for predicting multi-support seismic underground motions in layered saturated soil under surface water

Simulation of multi-support (i.e. spatially variable) seismic underground motions in sea areas plays a significant role in the seismic analysis of cross-sea structures such as cross-sea bridges or subsea tunnels. However, existing approaches for predicting multi-support seismic motions mainly focus on the dry site soils without overlying surface water. This paper proposes an approach for predicting multi-support seismic underground motions in layered saturated half space under surface water, subjected to oblique incident P waves. The transfer function in saturated soil under surface water, as the theoretical basis of the subsequent numerical simulation, is first derived based on wave propagation theory and the calculated reflection coefficients of P wave–induced P1, P2, SV waves in saturated soils. The derived transfer function is further employed to deduce and obtain the underground (sub-seabed) power spectral density function and response spectrum function. The two obtained functions, combined with the additional cross-coherence function, are subsequently employed to construct the cross power spectral density matrix and thus to simulate multi-support seismic underground motions. The solutions are validated against the target power spectral density, target response spectrum and target cross-coherence functions. A parametric analysis is presented where the effects of the soil thickness, the incident angle and the overlying water depth are investigated. Results show that the soil thickness, incident angle and overlying water depth have significant influences on the amplitude of transfer functions, which further affect the ratios between seismic ground and underground motions.

An Analytical Model of Seepage Field for Symmetrical Underwater Tunnels

Based on the mirror image method and superposition principle, an analytical model of seepage field for symmetrical underwater tunnels is proposed. The condition is assumed as two-dimensional steady water inflow into symmetrical and horizontal underwater tunnels in a fully saturated, homogeneous, isotropic, and semi-infinite aquifer. Analytical solutions of pore water distribution and water inflow into tunnels are obtained. Two different boundary conditions at the perimeter of symmetrical tunnels are considered, constant total hydraulic head and constant water pressure. Taking the subsea tunnels of Xiamen Xiang’an in China as an example, comparisons between analytical solutions and numerical solutions are analyzed in the case of zero water pressure at the perimeter of symmetrical tunnels. The results show that the analytical solutions for pore pressure distribution and water inflow match well with the numerical solutions and that the relative deviations are all in an acceptable range. The solutions derived from the analytical model in this paper can analyze the steady seepage field of symmetrical underwater tunnels accurately and reasonably.

Research on Key Design Technology of Shenjiamen Harbour Subsea Tunnel Engineering in Zhoushan

The Shenjiamen harbour subsea tunnel in Zhoushan, Zhejiang is the first pedestrian submarine immersed tube tunnel in China. In view of the characteristics of the fine soft immersed tube tunnel with large slope, the section of the immersed tube structure was reasonably chosen, the length of the pipe was partitioned, various subsidiary structures were finely arranged, the technology and parameters of the grouting were strictly controlled, and the deformation of end corner was taken as the important control parameter for the design of fine soft pipe joint. The new design technology of final joints with the combination of thrust bearing platform and water sealing wall can ensure the construction quality of final joints, as well as reduce the risk and cost of project. In the field of tunnel engineering, a new bank protection structure that double row steel pipe pile was combined with mould bag sand cofferdam had been researched and designed. Through these key technologies, it has accumulated a wealth of design experience for submarine immersed tube tunnels, which possesses an important reference value for similar projects.

Dynamic analysis of an immersed tunnel in Izmir

The original design of the planned Izmir Bay Immersed Tube Tunnel, considered it to be a continuous subsea tunnel for the whole length of about 7.6 km. But this was later changed into having 2 shorter tubes of 2.7 km long each connected thru’ a 2.2 km long artificial island created in the middle of the bay from the excess dredged material, a concept which also reduces costs and increases efficiency. This island will serve as a venue for the 2025 Expo Exhibition, which Izmir city will apply to organize in the future. Reason for the research study was to provide a preliminary design, using a dynamic analysis, during the current pre-feasibility stage, of the immersed tunnel to show whether it can be built across the Izmir Bay. This paper takes into account the new alignment and presents the results of a 2-D dynamic analysis conducted of the prefabricated 100m long tunnel elements, sitting within a backfilled dredged ditch, dug after the recommended ground improvement was carried out. Analyses considered staged construction and the results showed that: Tunnel units and its surrounding soils inside the dredged ditch act together to provide a better earthquake response with a damping effect of the earthquake force; Tunnel units do not float to the sea surface, but continued to stay inside the dredged ditch and applied positive stresses to the foundations during the design earthquakes. As there was no floating, there was no need for anchoring the tunnel to ditch bottom; Tunnel units and immersion joints (made of specified strong elastomer material) continued to stay in compression longitudinally and provided a superb water-tightness level; There were no risky (un-tolerable) ground deformations during, after striking of the design earthquakes. Total vertical and differential displacements of the tunnel units and in the surrounding soils were all at acceptable levels; Concrete surface crack widths occurring in the tunnel units, during striking of the design earthquakes were also found to be allowable. Study results show that the tunnel elements can withstand Mw=7 short duration ( 10 sec) earthquakes without major damages to their structure

Subsea road tunnels in the Faroe Islands

The Faroe Islands north of Scotland has a disproportionally large number of road tunnels. In addition to 44 km of existing tunnels, two further undersea tunnels totalling 22 km will be completed by 2023. A further 30 km undersea crossing is planned, providing nearly 2 m of tunnel for each of the 50 000 inhabitants. This paper describes the country’s recent, current and proposed subsea tunnelling projects. When eventually completed they will reduce journey times between the 18 major islands from up to a day to no more than a few hours’ drive.

Seismic response of a submarine tunnel under the action of a sea wave

In this paper, taking the Jiaozhou Bay Subsea Tunnel with a curved wall in Qingdao as the research background, while assuming that the liquid cannot be compressed and considering the effect of surface gravity waves, a harmonic wave is used to simulate a sea wave. Considering coupling effect of stress and seepage fields, dynamic responses of subsea tunnels under the action of ocean waves and earthquakes are discussed. The results show that with an increase in water depth, the maximum equivalent stress, the first principal stress, the third principal stress and the displacement of the tunnel structure all increase. When the water depth reaches a certain value, the influence of sea waves on the calculation results is so small that it can be ignored, and the influence of the sea wave on the displacement is smaller than that on the stress. After considering sea waves, the increases of velocity and acceleration with different sea water depths are far less than 5% under the action of an earthquake. The influence of sea waves on dynamic responses of subsea tunnels may not be considered when the sea water depth is more than 40 m.

Back analysis of an operating subsea tunnel considering the degradation of ground and concrete lining

ABSTRACTAn evaluation of the stability of subsea tunnels during operation is very important considering the risks involved with subsea tunnels. Although a large volume of monitoring information can be obtained, back analysis has been performed based on the internal displacement. In this study, the efficiency of an estimation of the safety of an operating subsea tunnel has been improved by implementing the back analysis algorithm based on various monitoring information. The differential evolution algorithm was adopted for back analysis of an operating subsea tunnel. The differential evolution algorithm was improved to accommodate the multiple target variables for back analysis, such as the elastic modulus, cohesion, friction angle of the ground, and the time-dependent elastic modulus of concrete lining. In addition, the elastic modulus of the concrete lining and the properties of reinforced ground can be evaluated with the proposed algorithm using a range of monitoring data, such as the internal displacement and stress acting on the lining. In summary, back analysis with a differential evolution algorithm can be used to evaluate the stability of an operating subsea tunnel.

Theoretical Assessment of the Risk of Ocular Hypotony in Patients With Intravitreal Gas Bubbles Who Travel Through Subsea Tunnels.

PurposeThis study was conducted to investigate changes in intraocular pressure (IOP) in the presence of intravitreal gas bubbles in individuals who travel through subsea tunnels.MethodsUsing a mathematical model, we simulated alterations in ocular globe shape, aqueous humor flow, volume of intravitreal gas bubbles, and IOP due to elevation changes during travel through subsea tunnels. We simulated five tunnels with different features as case studies. The role of key modeling parameters was further evaluated in a parametric study.ResultsIn three out of the five simulated tunnels (i.e., Seikan Tunnel, Bomlafjord Tunnel, and the Atlantic Ocean Tunnel), the patients were potentially at risk at lower portions of the tunnels since the IOP dropped to values less than 5 mm Hg, the clinical threshold for ocular hypotony. During ascent, the IOP increased to the normal value of 15 mm Hg and in some cases to higher values (e.g., a peak value of 22 mm Hg in Seikan Tunnel).ConclusionsOur model predicted that in the presence of intravitreal gas bubbles, the IOP could drop to extremely low values when patients descend to lower elevations in some tunnels. Such low IOP values could cause bleeding and/or retinal detachment. Since many factors (e.g., tunnel specifications and/or patient-specific characteristics) could affect the IOP during subsea travel, caution (beyond avoiding airplane flights) should be taken in advising patients about travel restrictions following intravitreal gas injections.Translational RelevanceOur findings highlight the potential risk for hypotony in the presence of intravitreal gas bubbles during subsea travels.

The Deformation Characteristics of Weak Foundation with High Back Siltation in the Immersed Tunnel

The deformation characteristics of a weak foundation with high back siltation in an immersed tunnel lack empirical data; however, the calculation method and control of foundation settlements are highly important to tunnel design and construction. This paper takes a cross section of the natural foundation of an immersed tunnel in the Hong Kong‐Zhuhai‐Macao Bridge (HZMB) as the research object and conducts a centrifuge model test. The soil layer is divided, and the soil parameters are determined by the CPTU and the indoor dynamic three triaxial test. In consideration of the actual engineering scale and the ability of the centrifuge model test system, the similarity ratio of the model to the prototype is studied and determined. As for the immersed tube structure, the deformation characteristics are mainly studied. Therefore, the organic glass is selected as the model material by the similarity of the elastic modulus and the density. The characteristics of the resilience and recompression of the natural foundation of the immersed tunnel and the strain characteristics of the immersed tube structure are obtained by the analysis of the multiworking condition test data. Moreover, based on the actual engineering geological conditions of the subsea tunnel, a numerical calculation method is carried out to check the reliability of the centrifuge model test results. The results of the numerical calculation are consistent with the model test results.

Research on he Classification of Life-Cycle Safety Monitoring Levels of Subsea Tunnels

Abstract As a traffic engineering project across straits or gulfs, subsea tunnel is one of the oceaneering with great construction difficulties, and the key to the success of subsea tunnel engineering lies in timely and accurate assessment of the structure safety of subsea tunnel engineering construction and life-cycle. Xiang’an Xiamen subsea tunnel is China’s first subsea tunnel which crosses complex formation conditions, engineering accidents such as collapse, sudden inflow of water or mud might occur during tunnel construction and operation. Therefore, the concept of subsea tunnel life-cycle monitoring is proposed aiming at the particularity of subsea tunnels. The variation forms of subsea tunnel mainly include large deformation, collapse, primary support cracking, water leakage, water pressure increase, steel arch corrosion, concrete corrosion, longitudinal differential settlement, etc., and classification of the life-cycle safety monitoring levels of the subsea tunnel is conducted based on risk assessment theory and risk level management benchmark to determine the possible variation forms in the monitoring level segments. The research results will provide reference for the subsea tunnel life-cycle monitoring, disaster warning as well as risk management under construction or to be built at home and abroad.

Feasibility of tunnel boring through weakness zones in deep Norwegian subsea tunnels

Norwegian subsea tunnels have all been excavated with the drill and blast method. The prevailing rock mass quality is generally favorable for tunneling, but the encounter of weak and/or water bearing zones is normal, and sometimes leads to extreme challenges. Future Norwegian subsea tunnels might benefit from the use of tunnel boring machines (TBMs), but the less flexible nature of a TBM will require more effort with regards to investigations and evaluations in the pre-construction phase. This paper summarizes some of the extreme challenges encountered in Norwegian subsea road tunnels, and reviews experience from international TBM projects considered relevant for Norwegian tunnels. The focus is on geological hazards, their implications, and mitigation measures. The aim is to assess the feasibility of tunnel boring through subsea weakness zones. Due to uncertainties and limitations with pre-construction investigations/interpretations for subsea tunnels, there will always be a remaining risk of encountering difficult ground. It is shown that it can be hard to predict adverse rock mass behavior ahead of the face during tunneling. Based on recent state of the art large diameter (>12m) TBM technology, it is concluded that closed-mode excavation may be considered feasible for water pressures up to ca. 100m. Pressurized TBMs can reduce risk and may enable excavation through unfavorable rock mass conditions, but this will require continuous installation of a gasketed segmental concrete lining (undrained solution), which can mean a conservative lining design for the rest of the tunnel. Adverse rock mass behavior and/or sudden large water inflow at high pressure can be challenging to handle with open-face TBMs. Based on the above large diameter tunnel boring is considered to involve a high risk for water pressures above ca. 100m, and is therefore not recommended. The use of a pilot tunnel to investigate and treat the ground ahead of the main tunnel(s) can be a way to reduce risk. In order to reduce contractual risk, the inclusion of a drill and blast section to be used in the case of extreme challenges, can be wise. The potential for squeezing should be evaluated for weakness zones of substantial width, and 3D numerical analysis are encouraged for zones where squeezing challenges are expected.

해저터널 내진성능 향상을 위한 Flexible segment 효용성 연구

최근 대형화되고 있는 지하구조물은 평상시는 물론 지진시에도 안정성을 확보하여야 한다. 특히 지하구조물 중에서도 해저터널은 지진시 안정성 유지가 매우 중요하다고 할 수 있다. 본 논문의 목표는 해저터널의 지진시 안정성 유지에 획기적인 시설물 중의 하나인 Flexible segment에 대해서 진동대 시험을 이용한 효용성 검증이다. 검증된 결과를 토대로 3차원 동해석을 통해 Flexible segment의 최적위치를 제안하는 것이 또 다른 하나의 논문 목표이다. 본 연구 수행을 위해 선정한 가상해저터널 단면에 상사율(1:100)을 고려한 1g 진동대 시험(1g Shaking Table Test)을 경주지진파, 인공지진파, 종방향, 횡방향, Flexible segment유무 등을 고려해서 모두 8회 이상 수행하였다. 진동대 시험 수행결과 전 시험 모두에서 Flexible Segment가 해저터널 내진성능향상에 효과가 있음을 확인하였다. 또한 시험적으로 내진성능 향상에 효과가 검증된 Flexible segment의 최적 위치를 선정하기 위하여, 3차원 동적 수치해석을 수행하였다. 그 결과 분기구간에서 Flexible segment를 인접해 설치할 경우 지진가속도가 감쇠하는 것을 확인할 수 있었다. Underground structures that have recently become larger are required to be stable not only during normal times but also during earthquakes. Especially, it is very important to maintain the stability of the subsea tunnels during the earthquake. The objective of this paper is to verify the effectiveness of the flexible segment, which is one of the breakthrough facilities to maintain the stability of the subsea tunnel during the earthquake using the shaking table test. Another goal of this paper is to propose the optimum position of the flexible segment through 3D dynamic numerical analysis based on the verified results from shaking table tests. The 1g shaking table test considering the similarity ratio (1:100) to the cross section of the selected artificial subsea tunnel was carried out considering the Geongju and Artificial seismic waves, longitudinal and lateral wave, and with/without flexible segments eight times or more. As a result of the shaking table test, it was confirmed that the flexible segment is effective in improving the seismic performance of the undersea tunnel in all the experimental results. In addition, 3D dynamic numerical analysis was performed to select the optimum position of the flexible segment which is effective for improving seismic performance. As a result, it was confirmed that the seismic acceleration is attenuated when the flexible segment is installed adjacent to the branch section in subsea tunnel.

초장대 해저터널의 공사중 덕트 접속부의 누풍 성능 개선에 관한 연구

초장대 해저터널의 건설은 건설공간의 제약 및 건설비용 측면에서, 공사중에 사용되는 환기 시스템 설치에 유리한 조건을 갖고 있지 않다. 공사중 환기시스템을 제공하기 위해, 환기갱이 설치되는 인공섬들이 해저터널의 루트를 따라서 일부위치마다 건설됨으로써, 건설 비용이 많이 소요된다. 따라서 이러한 인공섬의 수를 최소화하는 것이 경제적으로 필요하다. 그러나 이것은 환기갱 사이의 거리가 더 길어지게 되므로, 필요한 환기풍량을 이송하기 위한 높은 팬 압력으로 인해 환기 덕트 접속부에서 누풍이 더 많이 발생하게 된다. 선행 연구에서 공기 누설이 중요한 문제임을 조명하였다. 본 연구는 개선된 덕트 접속 방식을 개발하기 위한 실험들을 소개하고, 또한 SIA 의 "S" 등급 누풍율 및 다양한 조건들을 비교하여 터널 연장에 따른 달성된 누풍율의 환기 성능을 제시한다. 본 연구의 결과로 누풍율이 <TEX>$1.46mm^2/m^2$</TEX>인 새로운 접속방식을 개발하였으며, 개선된 방식이 30 km규모의 장대터널에 적합한 것으로 분석되었다. The construction of long sub-sea tunnel does not provide the favorable condition for the installation of ventilation system to be used during construction due to the constrained construction space. For the ventilation system required during construction, the artificial island where ventilation shaft is located is constructed at some location along the sub-sea tunnel route, which requires a high construction cost. Therefore, it is intended, as much as possible technically, to minimize the construction of artificial island. However, this requires a longer distance between ventilation shafts, there-by causing increased air leakage at the ventilation duct connection points due to the higher fan pressure being required to deliver ventilation air. Previously the air leakage was studied as an important issue. In this study experiments were carried out to develop the improved duct connection method considering various conditions such as, tunnel length, etc. Additionally, its performance results with leakage rates are shown and compared to the "S" class leakage rate of SIA. As a result, the new duct coupling type of improved method is analyzed as applicable to such a 30 km long tunnel with the leakage rate of <TEX>$1.46mm^2/m^2$</TEX>, which is better performance than SIA leakage rates.

운영 중 해저 터널의 안정성 평가를 위한 역해석 프로그램 개발: 횡단방향

운영 중 해저 터널의 안정성 평가를 위한 역해석 프로그램 개발: 횡단방향

Hydrodynamic response of a submerged tunnel element suspended from a twin-barge under random waves

It is possible that the excessive dynamic responses of tunnel elements could jeopardize the safety and accuracy of installation procedures used during subsea tunnel construction. To investigate the motion characteristics of the tunnel element, experimental measurements of a moored tunnel element suspended from a twin-barge were conducted in a wave flume at a geometric scale of 1:50. A corresponding numerical model was developed to simulate the dynamic response of the tunnel-barge system in realistic sea conditions, using hydrodynamic parameters from a radiation/diffraction potential model. Multiple linear wave conditions and three immersion depths were tested. The results indicate that the motion response of the tunnel element increases with decreasing immersion depth, and the natural periods of the tunnel, barge and combined tunnel-barge system play key roles in the influence of wave conditions on the motions of the tunnel. It was found that the low-frequency motion of the tunnel element is large in small wave periods. The mooring system under such conditions needs to be considered carefully during system design in order to safely control the motions of the tunnel-barge system in energetic ocean environments.

Long-term Field Corrosion Monitoring in Supporting Structures of China Xiamen Xiangan Subsea Tunnel

Xiamen Xiangan Subsea Tunnel is the first undersea tunnel constructed in China. It has become the major undersea pathway connecting Xiamen Island and Xiangan District since its operation in 2010. The total length is 6.05 km with the undersea length of 4.2 km. In an effort to onsite-monitor reinforcement corrosion in concrete structures, the commercially available CorroWatch multiprobe sensors and ERE-20 reference electrodes were pre-embedded in the selected locations and positions of supporting structures during the tunnel constructions. The real-time data have been collected annually by onsite measurements for 6 years. In this paper, the feasibility of the onsite corrosion monitoring system and suitability of the measured parameters including corrosion current, potential and temperature are discussed based on the preliminary results. The measured typical cyclic-type variations in corrosion current and temperature with monitoring time might be related to the seasonal changes during the annual routine measurements. The widely scattered corrosion potentials that are fluctuated complicatedly reflected localized differences in the vicinity of rebar and concrete. These findings, along with the progressively ongoing research, will provide valuable information in structural durability for service life prediction of reinforced concrete structures exposed to marine environments.

Grouting effects evaluation of water-rich faults and its engineering application in Qingdao Jiaozhou Bay Subsea Tunnel, China

In order to evaluate the grouting effects of water-rich fault in tunnels systematically, a feasible and scientific method is introduced based on the extension theory. First, eight main influencing factors are chosen as evaluation indexes by analyzing the changes of permeability, mechanical properties and deformation of surrounding rocks. The model of evaluating grouting effects based on the extension theory is established following this. According to four quality grades of grouting effects, normalization of evaluation indexes is carried out, aiming to meet the requirement of extension theory on data format. The index weight is allocated by adopting the entropy method. Finally, the model is applied to the grouting effects evaluation in water-rich fault F4-4 of Qingdao Jiaozhou Bay Subsea Tunnel, China. The evaluation results are in good agreement with the test results on the site, which shows that the evaluation model is feasible in this field, providing a powerful tool for systematically evaluating the grouting effects of water-rich fault in tunnels.

A risk prediction method for water or mud inrush from water-bearing faults in subsea tunnel based on cusp catastrophe model

Constructing a subsea tunnel is exceptionally difficult because of the complex geological conditions involved in the process. Water or mud inrush is a geological hazard that occurs suddenly during the construction of a cross-harbor tunnel, thereby causing major disasters and economic losses. The weak structural planes in a fault zone are the naturally preferred planes for water inrush. It is necessary to analyze the mechanism and to develop a risk prediction method for water or mud inrush from such faults. In order to effectively predict the risk of water or mud inrush during the construction of a subsea tunnel, this paper analyzes the mechanism of water or mud inrush from extensional faults, shear faults and compressive faults based on cusp catastrophe model. The potential function for the risk of water or mud inrush is built, and then the index of water inrush (IP) is obtained based on the equations of equilibrium for the fault surface. Water or mud inrush occurs only when IP > 1. The proposed method of risk prediction is successfully used in Qingdao Kiaochow Bay subsea tunnel to prove its applicability and feasibility.