The Hangzhou Bay Cross Sea Bridge of the Nantong Ningbo high-speed railway has a total length of 29.2km. After completion, it will be the longest and highest construction standard high-speed railway cross sea bridge in the world. It is the first high-speed railway cross sea bridge with strong tidal bay, ultra-low water resistance, and fully embedded pile foundation, involving many key technologies and difficult issues. There is little engineering experience to learn from at home and abroad, and engineering construction faces enormous difficulties and challenges.
2 Construction conditions
The strata in the engineering site mainly include muddy soil, cohesive soil, silt, and sandy soil, and artificial fill is distributed near the seawall. The unfavorable geology in the bridge site area is mainly caused by shallow gas and sand liquefaction caused by seismic effects.
The average depth of Hangzhou Bay is 8-10m, and the seabed terrain is flat. The bay top is located near Ganpu, with a width of about 20km, and the bay mouth is located from Shanghai Nanhuizui to Ningbo Zhenhai, with a width of about 100km. The Hangzhou Bay estuary is a sedimentary estuary, with the engineering river section showing "flood siltation and tidal erosion" and "north siltation and south siltation". It accumulates during the rainy season and washes out during the dry and high tide periods. The water dynamics are complex and there are numerous geomorphic units.
2. Hydrological characteristics and tidal bore of rivers
The sediment exchange between Hangzhou Bay and the Qiantang River estuary is frequent, and most of the water in the bay belongs to the irregular semi diurnal tidal zone. The phenomenon of diurnal and nighttime tides is obvious, and the tidal range is large and the flow velocity is fast.
The surface water (seawater) in the site has sulfate attack and carbon dioxide attack on concrete, and the level of chemical attack on the environment is H2. The environmental action level of chloride salt is L3; The level of environmental damage caused by salt crystallization is Y1. The groundwater in the site has sulfate attack on concrete, and the action level of chemical attack environment is H1; the action level of chloride salt environment is L3; The level of environmental damage caused by salt crystallization is Y1.
Main control factors
The Nantong Ningbo high-speed railway spans Hangzhou Bay, with a width of 20-50km. It has the characteristics and difficulties of wide sea area, high winds and waves, strong tides and currents, poor geology, severe seabed erosion and sedimentation, poor water depth, shallow gas influence, busy shipping, multiple and high-grade waterways, strict nuclear and maritime management, and multiple sensitive ecological environment areas.
There are numerous major control factors distributed in the Hangzhou Bay area and its two sides. The sea area of Hangzhou Bay mainly includes the entrance channel of Haiyan Port Area, the Zhapu Hangzhou channel, the Hangzhou Outer Sea channel, as well as the Baitashan anchorage and planned Haiyan anchorage. The north bank of Hangzhou Bay mainly includes Haiyan Port Area, Qinshan Nuclear Power Plant, Qiantang River Tide Rising Point, Haiyan County Urban Area and Shanshui Six Banner Project, Nanbei Lake Scenic Area, Qianmu Dang Water Source Protection Zone, Haiyan County Biodiversity Ecological Protection Zone, Haiyan Zhongran Energy Co., Ltd., and docks. The south bank of Hangzhou Bay mainly includes Hangzhou Bay National Wetland Park, Yuyao City Planning Industrial Zone, Fangte Park, etc.
3 Overall design
3.1 Bridge site selection
Through analysis and comparison from the aspects of engineering technical difficulty, planning conditions, passenger flow attraction, construction period, and project investment, and through multiple thematic studies, it is recommended that the Haiyan West Passage pass through Haiyan Cixi, which is in line with road network planning and urban development, meets waterway requirements, and has controllable engineering risks.
3.2 Cross sea approach
There are currently Hangzhou Bay Cross Sea Highway Bridge and Jiashao Bridge in the Hangzhou Bay area, and the bridge construction technology is mature, so it is advisable to prioritize the selection of bridge schemes. Due to the significant impact of factors such as port, navigation, Qiantang River tidal bore, seabed evolution, and meteorological conditions on the bridge scheme, the tunnel scheme is also being studied.
Overall Bridge Plan
According to the conditions for bridge construction in the sea area, the requirements for ship navigation, and the research results of sea related topics, the span arrangement of cross sea bridges is as follows:
Firstly, the North Channel Bridge adopts a cable-stayed bridge with a main span steel box steel truss composite beam.
Secondly, the middle channel bridge adopts a main span steel truss cable-stayed bridge with a length of 1430.8m.
Thirdly, the Nanhang Road Bridge adopts a main span steel truss cable-stayed bridge with a length of 814.5m.
Fourthly, the mid sea approach bridge adopts span prestressed concrete continuous beams.
Fifth, the approach bridge in the shoal area adopts span prestressed concrete simply supported beams.
3.2.2 Overall Tunnel Plan
Taking into account factors such as geological conditions in the tunnel area, seabed erosion, shallow gas, waterproofing and drainage, artificial islands, and surrounding structures, the longitudinal section of the underwater tunnel is determined,
The design, construction, and operation management technology of cross sea bridges is relatively mature, and the risks are generally controllable; Low construction cost, short construction cycle, and guaranteed quality; Convenient operation and maintenance, low cost; The bridge is located in an open space, making disaster prevention and rescue convenient and fast, with a low degree of accident impact. Cross sea tunnels are greatly affected by geological conditions, requiring long-distance crossing of the bottom of the seabed and the construction of islands in the sea, resulting in poor geological conditions and unpredictable high risks; High construction cost and long construction period; The operation and maintenance costs are relatively high.
3.3 Construction scale
The conditions in the Hangzhou Bay area are complex, with numerous constraints on both sides, and scarce bridge resources. It is necessary to study the plan of sharing cross sea channels between railways and other projects.
Requirements for Highway Railway Combined Bridges
Combining the cross bay demand of highways, the joint construction of highways and railways is an important possible option. Based on the planning and layout of the highway network, carry out traffic demand prediction, allocation, and analysis in different scenarios, compare the plans for building railway bridges separately with those for jointly building bridges on highways and railways, and study and demonstrate the necessity of jointly building bridges on highways and railways.
The demand for road traffic within the Hangzhou Bay Cross Sea Railway Channel is small, and the demand for co construction of cross sea bridges by road and railway is not strong; The social and economic benefits of the joint construction of highways and railways are poor; The Hangzhou Bay Jinshan Highway Channel has a more significant diversion effect on the Hangzhou Bay Highway Cross Sea Bridge. It is recommended to build a separate cross sea railway bridge, and it is recommended to start the preliminary research work on the Jinshan Highway passage in Hangzhou Bay as soon as possible.
3.4 Main Technical Standards
The design speed of the Nantong Ningbo high-speed railway is 350km/h, and CRTS III slab ballastless track is laid. To improve transportation capacity and ensure consistency of standards, and reduce the maintenance and repair workload of cross sea bridges, it is recommended to lay ballastless tracks on the Hangzhou Bay Cross Sea Bridge, with a design speed consistent with the entire line.
4 Key Technologies in Engineering Design
The Hangzhou Bay Cross Sea Railway Bridge involves numerous key technologies and key and difficult issues. There are few engineering experiences that can be used for reference at home and abroad, and engineering construction faces many difficulties and challenges. This article only discusses a few typical key technical issues in engineering design.
Laying Ballastless Track for 450m Class Long Span Cable-Stayed Bridge
The Hangzhou Bay Cross Sea Bridge is equipped with three channel bridges, and laying ballastless tracks on the 450m long span cable-stayed bridge is the key to controlling the design speed.
4.1.1 Control standards for track irregularities
The standard for long wave geometric irregularity of high-speed railway tracks based on absolute measurement and vector distance difference method is no longer applicable to the analysis of track smoothness of large-span bridges. Based on the dynamic analysis of the "vehicle rail bridge" system and referring to similar engineering experience, the relationship between the vertical acceleration of the vehicle body at a speed of 350km/h and track irregularity is proposed, and the midpoint chord measurement method is used instead as the control standard for long wave static irregularity.
4.2 80m span ultra long continuous concrete beam seamless track
Long span simply supported beams require high longitudinal stiffness and displacement at the pier top, with a large foundation scale and heavy beam weight, requiring the use of large floating cranes for operation, resulting in high engineering costs. The movable pier of the continuous beam does not consider the requirements for longitudinal stiffness and displacement limits at the pier top. The foundation is small in scale and lightweight, and has low requirements for the draft depth of large floating crane operations, but requires the installation of steel rail expansion regulators. For cross sea railway bridges, in order to reduce the number of brake piers and expansion regulators, the length of the connection should be increased as much as possible.
4.2.1 Comparison and Selection of Simply Supported Beam and Continuous Beam Schemes
4.2.2 Length of 80m span long continuous beam
The sea approach bridge adopts a long continuous beam structure, and the length of the continuous beam connection should be reasonably determined to minimize the number of track expansion regulators. The distance between the channel bridges is about 6km, and three continuous concrete beams are arranged between the channel bridges, with a maximum temperature span of 1840m. The maximum specification of the beam end expansion device is ± 500mm.
4.3 Intensive layout of four electrical equipment for ultra long distance cross sea railway bridges
The Hangzhou Bay Cross Sea Railway Bridge has a long distance, narrow deck, and high humidity, strong wind, and high corrosion environmental characteristics. It is an extremely complex task to arrange a large number of four electrical equipment and facilities in a narrow space.
Therefore, it is necessary to conduct in-depth research on the selection and layout of the four electrical equipment on the bridge, while meeting the functions of the four electrical equipment. This will enhance the intensification of the four electrical equipment and enhance the feasibility and maintainability of the project.
4.3.1 Offshore Four Electric Integration Platform
Based on the requirements of the four electrical equipment, research intensive and decentralized layout plans, with the biggest influencing factors being whether to set up an offshore platform, whether to adopt a direct power supply plan or an AT power supply plan for the power supply plan. The AT power supply scheme can ensure the power supply capacity, reliability, and flexibility of ultra long cross sea bridges in complex marine environments to the greatest extent. One AT station is set up in the cross sea section.
4.3.2 Power and communication pier top integrated platform
The Hangzhou Bay Cross Sea Railway Bridge is equipped with 10kV power transmission lines, 10/0.4kV box substations, maintenance socket boxes, bridge work lighting, bridge aviation obstacle lighting, bridge culvert channel lighting, bridge evacuation lighting, and bridge landscape lighting.
4.4 Emergency rescue of ultra long distance cross sea railway bridge
The Hangzhou Bay Cross Sea Bridge spans the entire area of the sea, with piers in the deep water area located between 30 and 60 meters high from the sea surface. The bridge deck is a relatively narrow and elongated space. Once a parking accident occurs due to extreme weather, natural disasters, fires, and other emergencies, it will cause a large number of people to gather on the narrow bridge deck. If evacuation and rescue are not timely, it can easily lead to secondary accidents, seriously threatening the safety of passengers' lives and property, Therefore, it is necessary to develop passenger evacuation and emergency rescue plans.
In order to ensure the safety of rescue personnel, facilities and equipment, high-speed rail facilities and passengers, it is difficult to use air and sea rescue methods to evacuate a large number of passengers on the cross sea bridge.
Referring to the research results of tunnel emergency rescue, one emergency rescue station is arranged within the range of 6 80m box girders on the approach bridge. The width of the emergency rescue station platform is 1.85m, with a distance of 0.3m from the track surface and 0.95m from the train carriage floor,
4.5 Operation and Maintenance of Ultra Long Distance Cross Sea Railway Bridge
The operation and maintenance environment of ultra long distance sea crossing bridges is harsh, and they have been exposed to strong corrosive natural environments such as high temperature, high humidity, salt damage, and sea fog for a long time. The structural performance and health condition deteriorate earlier, faster, and more severely.
The bridge operation and maintenance channel can only be set up based on the bridge's own structure. However, this bridge has a long travel distance, a long time, poor conditions, short effective on-site operation time, low efficiency, and cannot adopt conventional maintenance plans. It is difficult to operate and maintain the bridge. Therefore, research on the bridge operation and maintenance channel should be carried out to reduce the amount of maintenance and repair work online, and strive to build an all-weather open and all-weather maintenance channel, achieving maintainability, accessibility, and repairability Replaceable design concept.
The Hangzhou Bay Cross Sea Railway Bridge, as a world-class project, faces numerous key and difficult issues, involving multiple key technologies. Through the overall design and key technical research of the Hangzhou Bay Cross Sea Railway Bridge, the following conclusions have been drawn:
(1) It is appropriate to choose the Haiyan West Passage as a bridge to cross Hangzhou Bay and build a double track railway bridge.
(2) The 450m, 448m+448m, and 364m span waterway bridges of Hangzhou Bay Cross Sea Bridge should be laid with ballastless tracks, with no speed limit for the entire bridge.
(3) The ultra long distance approach bridge should adopt an 80m span ultra long concrete continuous beam seamless track, equipped with a steel rail expansion adjuster.
(4) For ultra long distance cross sea railway bridges, it is advisable to intensively arrange four electrical equipment, and set up offshore four electrical platforms and pier top integrated platforms.
(5) Emergency rescue stations should be set up for ultra long distance cross sea railway bridges, combining centralized and random rescue.
(6) Long distance cross sea railway bridges should be reasonably arranged for operation and maintenance according to the needs of engineering, electrical, and power supply operations