Hong Kong-Zhuhai-Macao sea link 25 Oct 2018

The Hong Kong-Zhuhai-Macao fixed link is a bridge and tunnel combination that will transform economic development in the Pearl River region of China. The project has been built within a Chinese white dolphin conservation region and beneath shipping channels that accommodate up to 4,000 vessel passages per day. The immersed tube beneath the shipping channels consists of 33 11.4m high x 37.95m wide and approximately 180m long elements that connect to the bridges on either side of two 100,000m² man-made islands.

Vital to the Hong Kong-Zhuhai-Macao sea link highway is the immersed tube tunnel between the two man-made islands

The two transition islands had to be completed before sinking of the immersed tube elements could begin. Using traditional man-made island methods, the soft soil of the seabed must be improved or removed and engineering materials deposited to create a more stable foundation. With an underlying soft soil of up to 30m thick on the bed of the estuary, traditional methods for the island construction would take an estimated three years, leaving inadequate time for tunnel construction within the overall project schedule. A different approach had to be adopted.

With soft soil of the seabed of a consistency to be easily penetrated, the method adopted for both islands was to use vibration piling techniques to insert 120 steel cylinders of 16mm thick steel x 22m diameter x 50m high about 30m deep into the impermeable layer of the soil.

Auxiliary cells were then inserted to connect the adjacent cylinders creating a temporary impermeable wall from within which the seawater could be pumped out. This allowed for sand filling and vertical drains to be installed inside the wall on the island to achieve surcharge pre-compression by a large over consolidated ratio. Foundation consolidation and improvement took about 100 days.

Video record of the immersed tube and artificial island construction

With ground improvement meeting the requirements for building structures on the island, the need for separate pile foundations was eliminated. The cylinders were used also as a wall for construction of the 18m deep trench needed for the cut-and-cover transition ramps to the immersed tube tunnel.

Island formation was thus shortened to seven months and greatly reduced the impact on the habitat of the white dolphins and minimised the extra construction traffic in the sea channels. Island costs did not outstrip the budget mainly because the marine work was significantly reduced in time and scale.

Tunnel foundation

The foundation for the immersed tube elements consisted of a double-bedded foundation layer, an underwater surcharge and ground improvement by sand compaction piling.

The tunnel trench was excavated to a maximum of 45m below sea level, so the soil stress generated by all loads will not exceed the historical stress. The main settlements will be at, or close to, the surface, where settlement was a matter of great uncertainty due to the disturbance of underwater excavation and sedimentation.

The interlayer formed by disturbed soil or sediment is very weak compared to intact soil. To overcome this, an additional 2m thick layer of cobbles, compacted by vibration using a hydraulic hammer, was installed below the gravel bed to provide a hard base for the landing of the elements. Cobbles of 300mm to 500mm diameter enabled the multi-beam to easily determine the sediment from the cobble layer.

The bridge/tunnel transition on the artificial island

Sand compaction piles were applied to improve the soft soil layer and to increase the bearing capacity of the ground. Depending on the load level and soft layer thickness, sand compaction piling replacement ratios of 70%, 55% and 42% were used.

Settlement of the elements is controlled to between 50mm and 60mm with settlement recorded as much higher at the immersion joint between elements E32 and E31, possibly due to sedimentation interference. This was far less than the predicted settlement value of 150mm to 200mm, and monitored settlement, considered longitudinally, shows better uniformity than that of the predictions.

Special-purpose equipment was developed for preparing the trench for the elements. A grab dredger was modified to have a plane dredging function to reduce disturbance and increase accuracy to control tolerances to within ±500mm for the tunnel trench bottom. Another dredger cleaned the sediment at the bottom of the tunnel foundation trench before the gravel bed was placed. A stone compacting and levelling vessel placed, vibrated, and leveled the underwater cobble mass layer using two falling pipes and a hydraulic hammer. A gravel-leveling platform placed the gravel bed and a suction head was added to remove local sediments from the gravel bed almost without disturbing it.

Semi-rigid tunnel element

The original design envisaged a segmented immersed tube structure with vertical concrete shear keys arranged at the joints with a spacing of 22.5m. The vertical shear keys however could be damaged by differential settlement of the element. The shear keys would result in insufficient bearing capacity for the 21m thick cover of sediment over the tunnel. Two solutions for reducing the load were proposed: constant dredging for the 120-year lifetime of the tunnel or placing light material on top of the tunnel.

Steel cylinders vibrated into the soft seabed formed the islands

Changing the structure was preferable to changing the external surroundings and a semi-rigid element structure solved the problem without the need for load reduction. The main change was keeping the temporary prestressing tendon in place permanently instead of releasing it after immersion, which is normal practice for segmented tunnels.

The semi-rigid element is a segmented structure in which the frictional force at the vertical face of the segmented joint is mobilised to resist part of the shear force to strengthen the shear capacity of the segmented joint. Adequate frictional force is ensured by arranging a rational amount of longitudinal prestressing tendons to gain adequate compression at the joints.

A certain amount of rotation is allowed between segments to enable the element structure to adapt to foundation settlement through longitudinal deformation. The semi-rigid elements also feature better watertightness at the joints, particularly with an injectable waterstop, and decreased the likelihood of reflection cracking on the road surface.

Memory bearings

Another feature of the immersed tube is vertical locking by steel shear keys mounted after element immersion. Although delaying locking time, to allow some relative vertical displacement at the immersion joint, can reduce the force on vertical shear keys, sediment will accumulate and increase the load on the tunnel roof for the years, subjecting the steel shear keys and to potential damage to the concrete walls adjacent to the connections.

33 tunnel elements of 11.4m high x 37.95m x 180m long create the six-lane immersed tube highway

To protect the shear keys and avoid cracking of the concrete, especially on the external walls that have no external membrane and where cracking may lead to corrosion, memory bearings were set between the vertical steel shear keys.

A memory bearing protects the structure from shear damage by releasing the differential settlement between elements and continually sustaining a constant force during its compression. This force is set to be no greater than the capacity of the shear key or of the adjacent structure.

The memory bearing remembers the design capacity of the concerned structure and when that value is about to be exceeded by the load the bearing will divert the excess part of the load to the foundation right down the structure, rather than to the foundation of the adjacent structure via the shear keys.

Closure joint

To complete the string of 33 elements, the final closure join was inserted between elements E29 and E30, where the depth of the deepest level of the element invert is at -27.9m below sea level and exposed to waves and currents. No existing closure joint construction methods could reliably connect to the adjacent tunnel elements and stop the water, so a prefabricated deployable element of 9.6m long at the bottom and 12m long at the top was developed and inserted into position using a 12,000 tonne floating crane.

The closing element is equipped with two retractable joints that expand or retract longitudinally in relation to movements of the main structure. Once inserted, the 54 hydraulic jacks in the element extended the join ends to make contact with the adjacent tunnel elements. The joints are arranged at the outer edge of the main structure to provide space for works inside the tunnel. A waterstop system at the ends of the joint element ensured dry conditions in the connection chamber.

Final closing joint lifted into place with a 12,000 tonne floating crane

Installation of the closure joint, not counting the re-connection, took less than one day. By comparison, the panel method of completing an immersed tube tunnel would take approximately eight months of marine operations. As an advance on this closure method, the retractable joints could be built into the ends of the last two elements to be installed thus achieving closure without leaving a gap for an insertion end joint.

Element factory

The purpose-built element casting factory took 14 months to establish. Two elements were produced every two months on two production lines, with total element production taking 58 months. Production and concrete casting was progressed alongside the launch of the segments and finally the entire element, each of typically 76,000 tonne, was launched forward over 200m to the shallow dock.

Four sliding tracks were arranged below the four walls of each element and a multiple points launch was undertaken. Jacking speed reached up to 0.13m per min and transverse accuracy was controlled to within 5mm. For the first time ever, five curved elements with 5,500m plane curvature radii were launched as part of this operation.

Fitting out was undertaken in the shallow dock. The deep dock could accommodate four elements to prevent delays in element installation. A sliding gate divided the shallow dock and the production line. The sliding operation took 30 to 40 hours. The floating dock gate in the deep dock was a 60m wide 13,000 tonne gravity structure.

Element installation

Floating out and sinking of the 33 elements and insertion of the final closure joint took 48 months. All 33 elements were placed during 35 installation attempts with no major incidents.

The immersed tube and artificial islands of the project took seven years to complete

As the natural water depth was insufficient for transporting the tunnel elements, they were towed within the dredged channel. Twelve boats were deployed for the towing works. Four tugboats were connected directly to the element, two at the fore to provide forward momentum and two aft. Eight tugboats sailed alongside to control the position of the element by pushing the immersion rigs riding above the element whenever large crosscurrents were encountered.

The position of the tunnel element and all tugboats were monitored in real time. It was extremely challenging to coordinate the 12 tugboats and simultaneously keep the element within the dredged channel. Practice exercises were performed by towing a barge of similar mass while calibrating the newly developed navigation piloting system.

The current velocity at the trench bottom was sometimes greater than that at the surface. This led to an unanticipated transverse installation offset of approximately 100mm when installing element E10. The forecasting and alerting system was updated to add a window for the period of element connection with the current velocity at the trench bottom monitored and forecast.

As sedimentation was rapid and could cover the gravel bed within one day, a sedimentation forecasting system was also developed to aid installations and a sedimentation cleaning head was developed to assist with efficient removal of the sedimentation.

Construction of the Hong Kong-Zhuhai-Macao sea link highway is an achievement of modern-day engineering and of inventing solutions to engineering and logistical challenges. Construction of the immersed tube and its transition islands to the landfall bridges on either side was completed in December 2017 after overcoming all challenges, including hazardous weather conditions and withstanding the frontal assault of a super strong typhoon.