General Design of Shenzhen-Zhongshan River-crossing Link Project

XU Guoping, HUANG Qingfei

(CCCC Highway Consultants Co., Ltd., Beijing 100088, China)

0 Introduction

There are many bridges across China because bridge is always the first choice for river-crossing or sea-crossing projects[1-2]. Lately, with the continuous development in design and construction technology for underwater tunnel and increasingly stringent requirements on the environment, navigation and transportation, immersed tunnels have gradually exhibited their advantages. Some similar projects in China are listed as follow:

The overall design of Weisanlu River-Crossing Link in City of Nanjing was based on the principle of foresighted plan, a ultra-large diameter shield machine was adopted and dual-tunnel double deck driveways were built by overcoming many difficult issues, such as, high water pressure, ultra-large cross section and complex geotechnical strata[3-4].

Sanyanglu Yangtze River Tunnel in city of Wuhan is the world′s first shield tunnel with the combination of highway and railway transport. Researchers have conducted some researches and studied on several key techniques, i.e. tunnel cross section layout for both highway and railway simultaneously, construction plan for installation of internal structures and evacuation & rescue, etc[5].

Hong Kong-Zhuhai-Macao Bridge project is the first project in China integrating bridges, islands and immersed tunnels and has an approximate length of 29.6 km. It consists of a large-span steel box girder cable-stayed bridge and segmented reinforced concrete immersed tunnels[6-7]. Its design theory, construction technologies and the engineering experience have provided a good reference for follow-up projects.

However, with the variations on standards, design requirements, project scale, construction difficulty and site conditions, the engineering design solutions and construction technology are often different.

Shenzhen-Zhongshan River-Crossing Link (Shenzhong Link) is the world′s first super integrated project that consists of four types of totally different structures, i.e. ultra-long and wide immersed tunnels, super-large span sea-crossing bridges, deep-water artificial islands and undersea traffic interchanges. The total length of Shenzhong Link is approximately 24 km. It is the first time that steel-shell immersed tunnel elements are adopted in China. This project has been known as the most challenging project in the world so far due to its scale, size, engineering design complexity and construction difficulty, by comparing with other similar projects.

Based on the characteristics of Shenzhong Link project, the authors have summarized the design concept of standardization, industrialization, intelligence and integration, and discussed the overall design considerations of the project, and presented key technology innovations developed in design and construction.

The geographical map of the project is shown in Fig. 1.

Fig. 1 Geographical map of project

1 Site conditions

The project site is located in the inner Lingdingyang of the Pear River. The underwater terrain is a typical 3-shoal and 2-slot shape. The surface of the riverbed is composed of a very deep silt layer formed from fluvial deposit. In recent years, the riverbed is basically in a stable condition. While, the slot along the West Island has been partially affected by sand mining, and the topographic variation of riverbed in this area are relatively large. The tidal current is a type of irregular semi-diurnal tide and the maximum surface flow velocity exceeds 1.1 m/s during the ebb tide.

The underwater terrain of the project area is shown in Fig. 2.

Fig. 2 Underwater terrain of project area

The project area is located in a typical subtropical oceanic monsoon climate zone and is affected frequently by typhoons. The maximum typhoon velocity recorded in recent years exceeds 30 m/s. Such climate has a significant impact on the project design and construction. Strata at the project site are mainly composed of Holocene silt, Late Pleistocene clay and sandy gravel. The bedrock is deeply buried in general, while, there are bedrocks exposed in some areas along the base of the proposed immersed tunnels. The regional neotectonic movement is characterized by a strong vertical fluctuation. Eleven continuous inactive strata faults run through the entire project site in north-west direction. The distribution of fracture lines in the project area is shown in Fig. 3.

Fig. 3 Distribution of fracture lines in project area

2 Project characteristics and challenges

With an unprecedented scale, complex construction conditions, and comprehensive technical challenges, Shenzhong Link is another major sea-crossing transportation project and its design and construction are even more difficult than Hong Kong-Zhuhai-Macao Bridge project[8]in some ways.

There are several similar large integrated projects in the world consisting of bridges, islands and tunnels, such as, Oresund Strait Link between Denmark and Sweden, Busan-Geoje Island Link in South Korea, and Hong Kong-Zhuhai-Macao Bridge in China. These projects are listed in Table 1 for an easy comparison[9].

Shenzhong Link has the widest immersed tunnel (46-55.5 m long) in the world. There are two main bridges. Lingdingyang Bridge has a super-large main span of 1 666 m, which is the one of the largest span suspension bridges in the world.

Table 1 Integrated projects consisting of bridges, islands and tunnels

(1) Shenzhong Link adopts two-way and 8-lane highway design. The immersed tunnel has a large width of 46 to 55.5 m and the single-open drive way span reaches 18.3 to 24.0 m. Both the immersed tunnel width and span are the largest in the world. If the traditional reinforced concrete tunnel is adopted, it will be difficult to meet the stress and durability requirements. Therefore, it is very important and very difficult to select a correct structural type for the immersed tunnel and to guarantee the project quality and safety[10].

(2) Considering the large buried depth and high water pressure on immersed tunnels, large variations in the underwater terrain and tunnel segments E1 to E5 on the West Island side in the sand mining area with extreme complex and sensitive geological conditions, how to reasonably reinforce the selected immersed tunnel base is the most difficult technical challenge of this project[11].

(3) The main tunnel on the East Island side is open-cut cast-in-place tunnel with varied widths from 46.0 to 69.8 m. The airport interchange is an undersea interchange approach ramp with a minimum radius of 125 m and maximum slope of 3.78%. The main tunnel runs between existing highway bridge piers along the river. Some parts of the approach ramp run along or adjacent to the existing highway bridge piers and highway stretch. The minimum distance between the new tunnel structure and the existing bridge piers is only about 2 m. A big challenge of this project is how to optimize the design and how to choose correct construction methods to ensure the riverside highway daily operation during the project construction.

(4) The main span of Lingdingyang Bridge is 1 666 m and is located about 10 km away from the coast of Zhongshan. The navigation clearance under the main bridge is 76.5 m and the design wind velocity at the bridge deck is 53.7 m/s. The bridge wind resistance design and undersea anchorage design and construction are difficult technical issues of this project.

3 Engineering design solutions and methods

3.1 Design principle and philosophy

As a super integrated project, the complexity of Shenzhong Link project is far beyond any projects related to only individual tunnel, bridge or hydraulic structure. It requires to be designed and considered with crossing-domain and systematic concepts. The current construction technology and methods have to be considered as the prerequisite. In order to success, the design is required to integrate the best resources in different fields, such as tunnels, bridges hydraulic engineering, dredging, materials and equipment, etc. Also, the project design will need to be aligned with national overall industrial development level.

The engineering design and investigation of Shenzhong Link project is premised on meeting construction conditions, guided by functional requirements, supported by scientific researches, with innovation as the spirit, and aims to achieve safe transportation, green transportation, and intelligent transportation. Based on the characteristics of this project, a principle of "standardization, industrialization, intelligence and integration" has been set up to achieve a series of high technology innovations in engineering design, construction methods and equipment, and project management, to make this project as a pioneer of the technical development related to super integrated projects.

3.2 Plan layout and profile of Shenzhong Link

3.2.1 Plan layout

Shengzhong Link starts from the south of Shenzhen airport at the chainage of K5+695 (starting point of the tunnel), and it connects Phase Ⅱ of Guangzhou-Shenzhen Riverside Highway (connecting line on Shenzhen side). An airport terminal interchange is set up to connect to the airport. The Link continues westward via East Island and then converts from bridge to undersea tunnel. The undersea tunnel has a length of 6 845 m and it crosses over Dachan navigation channel, airport sub navigation channel and Fanshi navigation channel. Then, the undersea tunnel changes to bridge, Lingdingyang Bridge, at West Island. Lingdingyang Bridge, a suspension bridge with a main span of 1 666 m, crosses the Lingding navigation channel. Then, the Link turns to the northwest and passes through the downstream shoal area of Wanqingsha, where Wanqingsha interchange is reserved for future connection to Nansha New District. On the west side, Zhongshan Bridge, a cable-stayed bridge with a main span of 580 m, is designed to cross the east Hengmen waterway, then lands at Ma′an Island of Cuiheng New District and connects to Zhonshan-Kaiping Highway, where Hengmen terminal interchange is to be built. The Link ends at the chainage of K29+609.497.

The Shenzhong Link has a total length of 23 914 m. The maximum radius of horizontal curve is 6 000 m and the minimum radius is 2 300 m. The total length of horizontal curves is 45.317% of the total link length. The Link plan layout is shown in Fig. 4.

Fig. 4 Plan layout of Shengzhong Link

3.2.2 Profile

The maximum longitudinal slope of the Link is 2.98% and the length of the shortest slope is 640 m. The smallest radius of convex curve is 25 000 m, and the smallest radius of concave curve is 20 000 m. The vertical curves account for 36.551% of the total link length. The height difference between the lowest point at Fanshi Tunnel and the highest point at Lingdingyang Bridge is 124.45 m and the average longitudinal slope is 2.026%. The profile of Shenzhong Link is shown in Fig. 5.

3.2.3 Lingdingyang Bridge

To meet navigation clearance requirements of Lingding navigation channel and Longxue waterway, the suspension bridge spans of Lingdingyang Bridge are designed as 570 m, 1 666 m and 570 m. After the comparisons of three types of suspension bridges, i.e., single-shaft-tower monobox spatial-cable suspension bridge, A-Shape tower monobox spatial-cable suspension bridge and H-shape tower planar-cable suspension bridge, H-Shape tower planar-cable suspension bridge has been selected[12]. A 3-span full-floating system is applied to Lingdingyang Bridge. Stiffening girders are provided at the two towers with lateral wind-resistant bearings and longitudinal limited displacement dampers. Also, seismic-resistant vertical tension-compression bearings and lateral wind-resistant bearings are provided at the transition piers. The rise to span ratio is 1∶9.65 and the horizontal spacing between two main cables at tower top and anchor point are 42.1 m. Lingdingyang Bridge typical layout is shown in Fig. 6.

(a) Island and tunnel section

(b) Bridge sectionFig. 5 Profile design of Shenzhong Link project

Fig. 6 Typical layout of Lingdingyang Bridge (unit: m)

3.2.4 Zhongshan Bridge

Zhongshan Bridge is a steel monobox girder cable-stayed bridge with a main span of 580 m and a total length of 1 170 m. It is located on a vertical curve with a radius of 25 000 m. The longitudinal slope of the bridge deck on both sides is 2.0%. The width of bridge deck is 43.5 m with a 2.5% two-way transverse slope. The bridge is a 5-span continuous structure with spans of 110 m, 185 m, 580 m, 185 m, and 110 m, respectively. The edge span to main span ratio is 0.509. A semi-floating structural system is adopted for this bridge.

Zhongshan Bridge typical layout is shown in Fig. 7.

Fig. 7 Typical layout of Zhongshan Bridge (unit: m)

3.2.5 Immersed tunnel

The starting point of the tunnel (east landing point) connects to Phase Ⅱ (connecting line on Shenzhen side) of Guangzhou-Shenzhen Riverside Highway. The end point of the tunnel connects to the non-navigation bridge on the head of West Island. The end chainage is K12+540. Total length of the tunnel is 6 845 m, including 5 035 m long immersed tunnel. The maximum longitudinal slope is 2.98% (on West Island side). The tunnel on East Island side is located on a horizontal curve with a radius of 5 003.1 m. The tunnel consists of 32 segments, including 26 standard segments with a length of 165 m and 6 non-standard segments with a length of 123.8 m. The non-standard segments include widened segments in east curve and the first segment on the west curve. The end joint is located between segments E22 and E23 and has a length of 2.2 m.

The longitudinal section and tunnel segment layout is shown in Fig. 8.

Fig. 8 Section and segment layout of immersed tunnel (unit: m)

The tunnel sloped section (E1-E5) on West Island side is underlaid by a 30 m thick weak silt layer, which has been severely disturbed by the sand mining. Deep cement mixed (DCM) pile foundation is adopted for the immersed tunnel in this area after many researches and studies. Similarly, DCM foundations are used for tunnel segments E13 to E21 due to the same consideration. For the rest segments, the native subgrade can be directly used as the foundation base. A layer of 0.7 m thick rubbles and a layer of 1 m thick gravels are placed above the subgrade to provide a flat surface[13]. The evenness of leveling layer is ±0.03 m. The maximum allowable theoretical settlement of the foundation is 0.08 m and the maximum allowable theoretical differential settlement at both ends of each segment is 0.03 m.

Tunnel foundation distribution is shown in Fig. 9.

Fig. 9 Distribution of tunnel foundation (unit: m)

The outside dimensions of standard tunnel segment are 46.00 m (width) × 10.60 m (height). The clear height of carriageway openings is 7.60 m and the thickness of structural slabs is 1.50 m. The cross-section of standard tunnel segment is shown in Fig. 10.

Fig. 10 Cross-section of standard tunnel segment (unit: cm)

3.2.6 Airport interchange tunnel

To connect Shenzhong Link to Guangzhou-Shenzhen Riverside Highway, 4 airport interchange approach ramps (E, F, G, H) are designed on East Island side. The ramps have a designed speed limit of 60 km/h, except for Ramp H, which has a designed speed limit of 40 km/h. 2-lane traffic is designed for all four ramps, while only single-lane will be operated for Ramp E.

Design parameters of airport interchange approach ramp mined sections are shown in Table 2. The layout of underground interchange ramps is shown in Fig. 11.

Table 2 Mined sections of airport interchange approach ramps

Fig. 11 Layout of underground interchange ramps

3.3 Artificial islands

Two artificial islands, i.e. East Island and West Island, are to be constructed for Shenzhong Link.

The length of East Island is 930 m and the area of the island is 343 800 m2. The island has sloped rock riprap along the perimeter with sand backfill at the middle. The plan layout of East Island is shown in Fig. 12.

The length of West Island is 625 m, and the area of the island is 137 000 m2. West Island is to be built by combining a 28 m diameter steel cylinders with a sloped rock riprap along the perimeter. The underwater sand compacted pile foundation is used for West Island. Watertight sink-in type steel cylinders, steel arc plates and fore shafts between them are used to allow the island perimeter structure watertight. Furthermore, high pressure rotary grout spray waterproof curtain with a pressure-reducing tube well program is used for water tightness at bottom. The plan layout of West Island is shown in Fig. 13.

Fig. 12 Plan layout of East Island (unit: cm)

Fig. 13 Plan layout of West Island (unit: cm)

4 Key technical innovations

4.1 Key techniques for design and construction of combined steel-shell concrete structural immersed tunnel

In the worldwide, except for Shenzhong Link project, other projects involving steel-shell immersed tunnels are Naha Tunnel, Osaka Sakishima Tunnel, and Tokyo Port Tunnel in Japan. While, it can be seen in Table 3 that the construction difficulty and scale of Shenzhong Link surpasses all other similar projects.

Table 3 Immersed tunnels with combined steel-shell concrete structure

Shenzhong Link immersed tunnel has a ultra-wide tunnel section (46.0-55.5 m), a ultra-large single-opening clear span (18.3-24.0 m), and is covered by over 17 m thick siltation and has to resist over 35 m water pressure. All the factors above will induce huge structural internal forces on immersed tunnel. If the conventional reinforced concrete structure is used, a large amount of reinforcement will be required and the reinforcement will be very congested. As a consequence, the placement of concrete will become very difficult. Also, concrete cracking control and casting quality control will become extremely hard. In order to solve the structural stress problem and reduce construction risks, a combined steel-shell with concrete composite structure has been proposed creatively and chosen for the final design of this project.

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Based on the concept of combined steel-shell concrete structural immersed tunnel, a series of systematic testing and studies regarding load resistance mechanism and design methodology were conducted for the first time. The results have revealed the bending and shear resistance mechanism of the combined steel-shell concrete structure. A design calculation method has also been developed and this method is able to quantitatively analyze the impact of concrete cavitation of the steel-shell concrete structure and the load resistance capacity. The research results[9]help establish quality control criterion for concrete casting and have been applied to design of such immersed tunnel. The optimized steel-shell structure is described below:

The steel shell of the immersed tunnel is composed of the inner and outer panels, transversal and longitudinal diaphragms, transversal and longitudinal stiffener plates and welded studs, refer to Fig. 14. Transversal diaphragms have a spacing of 3 m and longitudinal diaphragms have a spacing of 3.5 m, which form a series of closed concrete casting compartments. The inner and outer panels, as major load resistance members, resist tensile stress and compressive stress. Transversal and longitudinal diaphragms, as major shear resistance members, resist shear loads. They also connect the inner and outer panels to integrate the steel shell as an entire rigid structure. Longitudinal stiffeners made of T-section steel, steel angle and welded studs are provided to ensure the effective bond between steel panels and the concrete. Longitudinal stiffeners and transverse flat ribs work together to enhance the stiffness of the panels.

Fig. 14 Steel-shell structure of tunnel

Both inner and outer panels of the main structure are made of Grade Q420C steel plates and the maximum plate thickness is 40 mm. Transversal diaphragms are made of Grade Q390C steel plates and the maximum plate thickness is 30 mm. The rest are made of Grade Q345C steel. All steel compartments are filled with Grade C50 self-compacting fluidized concrete.

4.2 Key techniques related to batching, casting and quality inspection of high-strength self-compacting fluidized concrete

Concrete will be placed inside the closed steel-shell compartment via casting holes and it cannot be vibrated. Therefore, self-compacting fluidized concrete will have to be used. In order to improve the quality of concrete pouring and mitigate the construction risk, a systemic testing and study regarding self-compacting fluidized concrete has been performed[10]. These researches have helped develop a "Guide to Preparation of Self-Compacting Concrete for Steel-Shell Elements in Shenzhong Link Project". The relevant research results are listed as follows:

(1)Performance requirements and key construction control parameters of self-compacting concrete of steel-shell elements;

(2)The property stability of a large volume self-compacting concrete used in steel-shell elements;

(3)Long-term performance prediction method for self-compacting concrete of steel-shell elements:

(4)Quality control technology for steel-shell element self-compacting concrete;

(5)Quality inspection technology for steel-shell element self-compacting concrete.

4.3 Design and construction technology of DCM pile foundation for immersed tunnel

To solve the issues of weak silt subgrade under segments E1-E5 and segments E13-E21 close to West Island, undersea deep cement mixed (DCM) pile technology has been used to reinforce the tunnel foundation for the first time in China. It has been stated in construction technical specification that an automatic DCM construction ship must be used to treat the tunnel foundtion. The control system has been re-developed so that it is capable to achieve real-time dynamic correlation and intelligent control of soil-layer parameters and cement consumption.

4.4 Equipment technology for integration of floating transport and immersed tunnel installation

The traditional riding, lifting and hauling method for floating transport and installation of immersed tunnels has a slow navigation speed, requires a large width of the temporary navigation channel and large dredging volume. The ship is not able to return to the harbor quickly in unexpected situations. It is not conducive to construction risk and project cost control and it may have a serious impact on the environment.

In response to the above problems, the concept of integrated element floating transport and installation has been proposed first time for Shenzhong Link Project, and the equipment has been developed and upgraded based on the proposed concept. It can be foreseen that the application of this innovated equipment will significantly reduce the dredging volume of floating transport channels as well as the impact on current daily navigation in the construction area. It will be more environmentally friendly and can create significant economic and social benefits. The upgraded ship for integrated floating transport and installation of elements is shown in Fig. 15.

Fig. 15 Upgraded ship for integrated floating transport and installation of elements

4.5 Key technology for design and construction of large-scale underground interchanges

East Island interchange of this project is a turbine-type interchange with directional ramps. Four approach ramps separate from main tunnel and merge the main tunnel underwater. The minimum space between the approach ramp tunnel and the existing riverside highway is only about 2 m. Construction site conditions are very complex and the construction risks are extremely high.

At present, there is a lack of relevant technical standards and design specifications for highway underground interchanges in China. Based on the characteristics of this project, specific researches and studies have been carried out to obtain technical standards and specifications related to underwater interchange selection, routing alignment techniques, enclosure structure construction of underground interchange tunnel passing or next to existing structures, design and construction of large cofferdam in the sea, and enclosure structures for ultra-deep and ultra-wide foundation pits.

4.6 Key technologies for design of flutter and wind stability of ultra-large span monobox girder suspension bridge

Lingdingyang Bridge is a monobox girder suspension bridge and the main span is 1 666 m. The rigidity of main cable and the aerodynamic configuration of the main beam have a great influence on the bridge wind stability. The navigable clearance of the bridge is 76.5 m and the bridge flutter test wind velocity at the deck level reaches 76.7 m/s. All of these factors make the bridge to face the most server wind stability issues, by comparing with other similar suspension bridges in the world. According to design requirements on the main structure and wind resistance of Lingdingyang Bridge, some full-bridge models, especially the main beam model, have been built and tested. The Special attentions were paid to the full-bridge model similarity verification. Furthermore, theoretical analysis and wind tunnel tests have been conducted to continuously optimize the structural design of wind deflector and central stabilizer. With all the efforts mentioned above, the wind stability issue of Lingdingyang Bridge has been successfully solved.

4.7 Key technology for design and construction of offshore anchorage in deep sea

The height of bridge tower is 270 m and the tower is to be fixed by super large and super heavy offshore gravity anchor blocks. The water depth in the anchorage zone is 3 to 5 m and the stratum is compose of deep layer of soft silt and silty sand. After the specific researches, it is proposed for the first time that in the sea, building an island with locked steel-pipe pile as cofferdam and filling it with sand to form a terrestrial land, then building 8-shaped diaphragm wall anchor block foundation on the built land. Compared with the riprap island construction, this solution has the advantages of high construction efficiency, small environmental impact, easy site clearing after construction. The island with steel-pipe pile cofferdam is shown in Fig. 16.

Fig. 16 Island with steel pipe pile cofferdam (unit: cm)

5 Conclusions and discussions

Shenzhong Link is the first super integrated project in the world that consists of four different types of structures, i.e. ultra-long and wide immersed tunnels, super large span bridges, deep-water artificial islands and undersea interchange. Such large scale immersed tunnel made of steel-shell and concrete composited structure is also first application in China, even in the world. Comprehensive design and construction difficulty of this project can be ranked as the highest in the world. According to project characteristics and technical difficulties, the design idea of the integrated project has been put forward from the overall design level, and the design concept of "standardization, industrialization, intelligence and integration" has been put it into practice. Moreover, the overall design of the project and technical solutions for main structures, such as, immersed tunnels and Lingdingyang suspension bridge, have been discussed. Key technologies and innovations in design and construction of the project have been summarized. Load-resistance mechanism and design methodology of combined steel-shell with concrete structure have been established.

Currently, the construction of this project has been started and all the design concepts and innovations are going to be tested and checked by engineering practice, more systematic researches and studies on theoretical innovation of the combined structure, large-scale precast tunnel segments, construction techniques and equipment development, wind stability and vibration reduction of ultra-large-span bridge, operation and safety of underground traffic at interchange, etc., will be carried out in the follow-up construction. It is expected that this project can provide references and guides for design and construction of similar large projects in the future.

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