China mega immersed sea link

Direct by Design

Shenzheng-Zhongshan mega sea link The Shenzhen-Zhongshan fixed sea link highway across the Pearl River estuary in China is currently under construction and is scheduled to open to traffic in 2024. Connecting Shenzhen in the east with the cities of Zhongshan and Guangzhou Nansha in the west, the 24km long ShenZhong link passes over and under three of the busiest shipping lanes of the world on a suspension bridge with a main span of 1,666m, a cable-stayed bridge with a main span of 580m and transitions on two man-made islands into a 5km long immersed tube structure and a 485m cut-and-cover section extending from the east landfall near Shenzhen. The link is a highway of four lanes in each direction, eight lanes in total, with one special element of six lanes in each direction, and designed for traffic speed of 100km/hr and a design life of 100 years. After about four years in construction, the mega link, which is similar in scope to the recent Hong Kong-Zhuhai-Macao immersed tube and bridge highway link project, is about 50% completed. The authors, as members of the ShenZhong Link Administration Center owner organisation, highlight the design and construction of the widest immersed tube tunnel in the world.

Underground in Action U

Liu Jian, Chen Yue, Jin Wenliang and Song Shenyou, ShenZhong Link Administration Center

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he ShenZhong bridge and tunnel combination across the Pearl River estuary is located at about 40km upstream of the Hong Kong-Zhuhai-Macao immersed tube and bridge link that opened to highway traffic in October 2018 and 38km downstream of the Humeng Bridge. The project is built above and beneath the busiest shipping channels in the world with more than 4,500 vessels passing every day. The tunnel at the east end of the 24km long x mostly eight lane highway is required also to maintain the clearance limitation of the Shenzhen international airport flight path.

The immersed tube section of the link between the east and west man-made islands of about 370,000m2 and 130,000m2 respectively, consists of 32 elements of 10.6m high x 46m wide and 165m long with a special 55m wide x 123m long 12 lane element and a 2m long closure joint.

Dec 2021

Artificial islands Construction of the west transition island and of the east island and it 485m cut-and-cover section, completed before sinking of the immersed tube elements, employed two different methods for their diverse conditions.

The east island, close to the Shenzhen coastline, passes under the Guangzhou-Shenzhen Riverside Highway. To safeguard it continuing operation, the highway owner required that the maximum horizontal displacement of less than 5mm for the 38 of its support piers that lie within the island. Piers near the tunnel are surrounded by 1.4m diameter steel pipes filled with reinforced concrete as retaining structures with other piers protected by steel sheet piles. In this area, the water depth is about 4m and the thickness of the underlying soft soil is between 8m and 15m.

Using traditional artificial island methods, the soft soil under the seabed is improved or removed and the selected fill of suitable Fig 1. Main structures of the 24km long 16-lane highway sea link Inset: The new Shenzhen-Zhongshan sealink highway in relation to the Hong Kong-Zhuhai-Macao link

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Model of the east artificial island

engineering materials are deposited to create a more stable foundation. Slow sand fill to protect those piers will take three years to finish the man-made island.

The west island is about 6km from the ShenZhong coastline in a maximum water depth of about 10m. In order to catch up with the overall project schedule, the vibration piling techniques, similar to those developed for the Hong Kong-Zhuhai-Macao sea link, were applied to insert 57 steel cylinders with a dimension of 18mm thick x 28m diameter x 40m high. For stability and waterproofing requirements, the cylinders had to be penetrated into the completely weathered rock zone, which was hard for steel cylinder stiffness. To prepare the area special blender equipment was used to break the bedrock and pour bentonite into the zone.

Auxiliary cells were then inserted to connect the adjacent cylinders to create a temporary impermeable wall for the seawater within to be pumped out and allow sand filling and vertical drains to be installed inside the wall on the island and achieve surcharge/precompression. Island construction was shortened to five months using the cylinder wall method.

Tunnel foundation A pre-paving method has been adopted to create the foundation of the immersed tube section in a trench excavated to a maximum 36m below sea level. The foundation consists of a double-bedded foundation layer comprising a 1m thickgraded aggregate cushion and a 1.1m thick layer of rock levelling with a ground improvement process of deep cement mixing piles to support nine of the elements. A double/singlebedded foundation layer of 1m thick graded aggregate cushion and partial rock tamping and levelling supports the other 23 elements above the natural seabed.

The DCM deep cement mixing piled reinforcement of the soft ground under the nine immersed tube elements is installed from special barges and uses cement as the curing agent, pumping and pouring the cement slurry into the soil and mixing them with special machinery to form a homogeneous cement-soil. The cement-soil can remarkably improve the strength of foundation due to the hydration reaction of the cement and the chemical reaction between the two materials. According to design

Last cylinder being vibrated into the seabed for the 130,000m2 west transition man-made island

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requirements, 60 day unconfined mean compressive strength of the DCM piles is not less than 1.2Mpa.

Steel-concrete-steel (SCS) structure For fabrication of the 46m eight-lane immersed tube elements and the 55m wide 12 lane element, the traditional reinforced concrete structure is not suitable. Instead the SCS composite structure, as first applied for immersed tube projects in Japan, is chosen.

The steel plate structure is composite of inner and outer panels as the main flexural members, with transverse and longitudinal diaphragms as the main shear members, and all held together with stiffeners and welding nails (Fig 2). The panels and diaphragms form an independent chamber for concrete pouring. Pouring holes and exhaust holes are reserved in the chamber. Water sealing must be done after concrete pouring. The longitudinal stiffeners with T-shaped steel and angle steel, and the welding nails are used as the shear connectors to ensure the connection between the panel and the infill concrete. Longitudinal stiffeners and transverse flat ribs work together to enhance the stiffness of the panels. Since the concrete is placed inside the steel chambers, high-flowability and self-compacting concrete is required.

Since the SCS composite structure is being applied for the first time projects in China, there were three challenges to be confronted.

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First, the structural design needed support of the analysis theories and design codes, including the design methodology of the shear connectors, the reinforcement of corners and the relief of construction stresses.

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Secondly, the mix proportion of high-flowability concrete and its construction management method had to be studied.

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Thirdly, non-destructive methods were required for testing, detecting and confirming the quality of the concrete filling performance.

Systematic research and study was conducted using experiment and analysis on the mechanical mechanisms and design methods of SCS composite structures were performed. Based on these studies, the mechanism of the SCS structure under bending and shearing were revealed and the analysis methods were proposed. In addition, the effects of the gap between steel plates and concrete on the bearing capacity are quantitatively analysed, and the quality control standards of concrete casting were proposed.

In another study, experiments with SCS steel chambers and a full-scale model of 18m long x 46m wide X 10.6m high were performed to confirm the types and strength grades of materials, design of steel chambers and details of design requirements. The concrete quality can be met when the flow speed of concrete is limited to 30~40m³/hr to a 20cm clearance between the face of concrete and upper steel plates in the steel chamber, and limited to 15m³/hr after that. The density of concrete should be controlled in the range of 2,300~2,370kg/m³.

Closure joint The final closing joint of the tunnel is located between elements E23 and E24, where the depth of the element invert is at -15m below the sea level. The closure joint is a special prefabricated Right, from top: Fig 2. Schematic of the steel-concrete-steel composite structure; Fig 3. Schematic of the immersed tube closure element; Intelligent plate welding line; Fabrication of element E1 in the factory on Niutou Island

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Element casting factory on Niutou Island

element with the 2.2m long pushout section of the same standard cross section as the elements either side (Fig 3). The prefabricated pushout construction procedure has the advantages of: •

greater safety for the floating and sinking operation,

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lower requirements for temporary channels and equipment,

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less dependence on diving operations, and

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faster installation speed.

Element fabrication workshop The total consumption of steel plates for all 32 elements is about 320,000 tons. It is the first time to use steel plates for immersed tubes in such a large scale and huge volume, and its fabrication quality and cycle control are highly important. According to the project construction plan, 32 elements will be fabricated by two contractors and be finished in 36 months. Because of the similarity to ship building - in reference to the construction mode, construction technology and inspection standards based on the integration of industrialization and data management digitisation the overall process for element fabrication is based on intelligent manufacturing of steel structures.

As result, manufacturing of the steel plate is divided into intelligent small section workshops, digital loading in medium Smart concrete pouring equipment at work in the Unit One factory

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section sites and automatic grouping in large section docks.

In order to improve work efficiency and ensure quality in the intelligent workshop subsection, it was required to build an intelligent workshop of four lines and one system, including an intelligent plate cutting line, an intelligent plate welding line, an intelligent block welding line, an intelligent coating line and an intelligent workshop management and control system.

Element casting factory To manage the whole project efficiently, the construction process, such as concrete pouring, first and second outfitting, floating transportation and installation, is divided into two bidding units: •

Unit One for elements E1 to E23 and the closure joint, and

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Unit Two for elements E24-E32.

The Unit One element casting facility is the same factory on Niutou Island established to cast the elements of the Hong KongZhuhai-Macao immersed tube project, while Unit Two is in the dock of a shipyard is where the steel structures are prefabricated.

The concrete pouring process is controlled by a stable intelligent system with an automatic hole-finding function. After the completion of the concrete pouring in one chamber, it will automatically address the next pre-set chamber within the required precision.

The pouring equipment and trailer pump is linked by two-way wireless communication, which automatically starts and stops the pouring process. In the process, the concrete flow speed can be adjusted in time and according to the feedback, and the pump tube could also be lifted according to the liquid level feedback to ensure a reasonable drop height of the concrete while avoiding the outlet of pump hose becoming buried. The flow speed of the concrete is also automatically controlled at 30~40m³/hr to within 20cm of the upper steel plate of each steel chamber.

Two detection methods are being used to detect any voids beneath the maximum 4cm thick upper steel plates and between the transverse and longitudinal diaphragms, stiffeners and welding nails. One is the improved impact imaging method, which scans all upper plates at the top of the SCS elements and finds possible void areas, and the neutron method to redetect those areas. Accuracy for the average void height for 10cm x 10cm area is 2.5mm, and efficiency for detecting void locations is recorded at about 90%.

Element installation For E1-E23 elements and the closure joint, the distance between the tunnel site and the concrete casting factory is up to 50km, and about 21km along the Lingding shipping channel, one of the busiest channel in the world. To minimize the impact on the shipping, it will be necessary to conduct immersion and installation construction without total or partial channel closure a self-propelling barge of huge dimension and a maximum velocity of 9km/hr has been developed to integrate the floating, immersion and installation functions.

For E24-E32 elements, the distance between the dock and sinking site is about 21km, and traditional towing and immersion methods are being deployed using eight boats for the towing works; four tugboats connected directly to the element, with one

at the fore, to provide forward momentum, and one aft and two as backup. A new control system to monitor the position of the tunnel element and all the tugboats in real time is developed to coordinate the tugs and ensures the element being towed does not exceed 10m from the centreline of the dredged channels.

Once lowered into the prepared trench, the connection between adjacent SCS immersed elements is similar to the traditional reinforced concrete immersed tunnel with the Gina seal installed before the elements are floated out to position and the omega seal installed once the elements are in place. The exterior steel plates are welded to create the SCS elements only in the manufacturing plant with concrete poured to create the links between elements once immersed into their final positions.

As one of the most important and challenging infrastructure projects currently underway in China, the ShenZhong Link has been the catalyst for new innovations and programme-saving developments.

The SCS composite structure provides significant advantages for the construction of large-width, long-span immersed tube elements and a set of code and specifications for the design, construction, and quality evaluation of the construction processes have been introduced. Development of digitized construction technologies on the project has also helped to overcome technique challenges, improve engineering quality, and decrease construction risks.

Development of the innovative closure joint is also progressing and researchers and engineers will reveal the final design and details of the installation process soon.

As with the Hong Kong-Zhuhai-Macao project before it, the ShenZhong project is advancing again the planning, design and construction of mega fixed links across the seas. n

Reference •

Hong Kong-Zhuhai-Macao sea link, TunnelTalk, October 2018

Special ship installing element E1 from the west man-made island

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