The Hong Kong – Zhuhai – Macao Bridge (HZMB) link is one of the largest fixed links ever realised. The link comprises various bridges, causeways, artificial islands and tunnels and has a total length of more than 50km. The Main Bridge project covers the main offshore section of the HZMB link of some 30km, crossing the Pearl River Estuary from the border with Hong Kong to the Zhuhai/Macao Boundary Control Facility Island. The link will accommodate a dual carriageway with three traffic lanes in each direction.

To allow the passage of sea going vessels, major cable stayed bridges will be included in the design of the link. The crossing of the main shipping channels at the eastern side of the Pearl River Estuary will be realised using a 6.75km long tunnel, of which approximately 6km will be immersed. The transition from the bridges to the tunnel will be realised with artificial islands with a length of 625m each.

Geotechnical conditions
The immersed tunnel can be considered as one of the most challenging parts of this project and special in a number of ways. The structural design of the immersed tunnel is determined by various boundary conditions. The seabed level in this area varies between -8m and -15m. Holocene deposits of a thickness from 10-25m are found below the seabed. These soils consist of muck, mucky clay and mucky clay mixed with sand and can be classified as very soft, highly compressible and normally consolidated.

Under the Holocene deposits Late Pleistocenes are found, with a thickness that varies between 37m and 102m locally. The Pleistocene deposits appear to be over consolidated and mainly consist of clay with thin layers of loose to medium dense silty sand, silty clay and sand and gravel. The sand and gravel generally underlay the cohesive soils. Underneath the Pleistocene deposits rock/granite is encountered.

Structural design
The design of the immersed tube elements that will make up the tunnel when placed in line on the sea bed has to overcome a number of different challenges. For the HZMB tunnel a number of accidental loads had to be considered, such as:
• Explosion / implosion in one traffic tube
• A sunken ship load representing a general cargo ship
• A fallen anchor load, based upon a 300,000 tonne oil tanker – anchor weight 22 ton, impact velocity 9m/s
• Extreme high water and wave; high tide level with return period of 1000 years
• Seismic events the project area has light to moderate seismic activity

As the project site is located offshore, the transport and immersion phase of the tunnel elements has to accommodate offshore conditions. The tunnel elements will be built in a construction dock located some 10km from the project site and during the transport and immersion stages adverse wave conditions may be encountered.

For the structural design of the tunnel cross section a number of aspects had to be considered:
• Large spans of 14.55m due to road design with a three-lane dual carriageway
• The tunnel is placed at a deep level of 29m below the lowest design sea level, to allow the future passage of 300,000 tonne oil tankers in two navigation channels with a total width of 2.810m
• As the navigation channels will only be dredged in the future, the immersion trench is allowed to fill with sedimentation up to the existing sea bed level, which means a ground cover that could exceed 20m
• The ground conditions are relatively poor. Time dependent effects of the cohesive soils that are encountered have a significant impact on the tunnel support characteristics. To overcome/reduce these geotechnical complications, ground improvement has to be carried out over a large part of the tunnel alignment
• The fact that the tunnel element must be able to float during transport and immersion stages implies that there are limitations to the structural dimensions as they determine most of the weight of the floating tunnel

For the cross section design the conventional reinforced concrete option was compared with an option with post tensioning in transverse direction (in roof and base slab). Finally it was concluded that the conventional reinforced option was still preferred when considering costs, risks and the execution of the works.

For the longitudinal design of concrete immersed tunnels in general two options can be distinguished: the segmental tunnel and the monolithic tunnel. With a monolithic tunnel the individual tunnel elements are continuous and capable of absorbing normal and shear forces and bending moments. With a segmental tunnel a tunnel element consists of several segments of 20-25m.

In the operation phase the joints between the segments allow for deformations in longitudinal direction and rotations in both the horizontal and vertical plane. Shear keys are provided in the joints to avoid discontinuous displacements over the joints in both horizontal and vertical direction. In order to make the segment joints watertight provisions are required e.g. rubber water stops.

For HZMB tunnel both options were thoroughly studied. A straightforward selection for the longitudinal design was not obvious, since both options have advantages and disadvantages under the given particular project conditions. The following aspects were considered:
• Geotechnical conditions including variation
• Surcharge loads, magnitude and variation
• Accidental loads such as seismic events and sunken ships
• Internal forces: bending moment, shear forces etc
• Construction costs

Based upon the various studies, it was concluded that the segmental tunnel was more economical and capable of accommodating the adverse geotechnical and surcharge conditions and impact of the accidental load cases.

The higher costs for the monolithic option were the result of the heavy longitudinal reinforcement, the waterproofing membranes and the shorter feasible maximum length for a tunnel element, which implies a larger number of transport and immersion operations.

For the segmental tunnel the joints are identified as critical items. Shear keys in the outer walls are required to accommodate the impact of variation in tunnel support and surcharge and seismic loads. In the roof and base slab shear keys are applied to absorb the horizontal seismic shear forces. For water tightness purposes a double seal was selected to provide additional safety.

The immersion joint between the different tunnel elements is provided with a traditional Gina and Omega layout for water tightness and with a shear key arrangement similar to the segment joints.

Foundation design
The structure/soil interaction is one of the governing factors in immersed tunnel design. This includes the foundation bed that is installed between the tunnel structure and the dredged trench and the geotechnical characteristics of the underlying soils.

The foundation bed is required because dredging accuracies generally do not meet the structural limitations related to uneven tunnel support and differential settlements. For the HZMB immersed tunnel the gravel bed was selected as the most appropriate foundation bed and capable of absorbing moderate to heavy seismic events. A gravel bed can be installed in berms and with a high accuracy from a floating barge in advance of the tunnel element immersion.

As indicated, earlier ground treatment is required over a considerable part of the tunnel alignment. The objective of the ground treatment is to improve the foundation conditions for the tunnel. In this way the settlements and differential settlements can be limited and therefore the internal design forces in the tunnel. In addition, ground treatment is applied to promote a smooth transition from one tunnel part (i.e. piled cut and cover tunnel at the islands) to the other (i.e. immersed tunnel). Two design approaches were adopted for the preliminary design phase of this project:
1) Improvement of the ground properties in terms of strength and stiffness and to increase the uniform behaviour of the ground.
• Replacement of soft soils by mean of sandy gravels or gravel
• Settlement reduction piles in soft cohesive layers with no end bearing!
• Sand compaction piles

2) Foundation piles on bearing ground layers in case the ground is too weak or too unpredictable, such as close to the artificial islands where large reclamations are carried out.

In the next phase of design, the detailed design, an extensive soil investigation is planned that is supposed to provide a better insight of the soil profile and properties. This will make it possible to better assess the risks involved and introduce design optimizations, where relevant.

Transport and immersion
The immersed length consists of 33 tunnel elements, of which most have a length of 180m. With the cross sectional dimensions of 11.5m by 37.95m the elements will become the largest concrete tunnel elements in the world. The offshore transport and immersions stages are essential for the tunnel element design and challenging from a risk point of view. This, among others, includes the selection of the tunnel element production location and the design wave and wind climate conditions.

The location of the production facility of tunnel elements is very important since the transport distance and the possible wave and wind conditions are governing the design in terms of feasibility, risk and construction costs. From the potential locations that have been studied, Guishan Island, approximately 10km from the project location, appeared to be the most favourable.

For the transport and immersion design of the immersed tunnel elements the design forces during the various stages (bending and torsional moments, shear and normal forces) have to be determined and the stability of the floating body has to be considered.

As there is a large dependency on the shape and dimensions of the tunnel elements, the local wave and wind conditions, the water depths during transport and at the immersion location and as dynamic influences are involved, detailed studies needed to be carried out. It was decided to develop an advanced numerical model in combination with physical model test. By using the results of the physical model tests the numerical models can be validated and used for various parametric studies (e.g. variation in wave conditions) and alternative execution stages.

Developing an optimal transport and immersion design means that a balance needs to be obtained between structural capacity (quality), acceptable risks and costs. A tunnel element that is required to accommodate practically all wave and weather conditions may be structurally not feasible or extremely costly. Therefore a design philosophy will be applied in which a decision model based upon a wave forecast system is used, where numerous wave data are collected.

With these data and the weather forecast a go or no go decision can be made for every transport and immersion operation, thus limiting risks and enabling design optimisations. This approach was successfully applied at Busan Geoje in South Korea.

Artificial Islands and transition to tunnel
In the HZMB Link the transition between the bridges and the immersed tunnel will be realised by means of an artificial island. The islands are approximately 625m long and 160m wide. The technical service building for the tunnel will be located on the islands.

As for the tunnel, the geotechnical conditions for the construction of the artificial island are not very favourable. As large land reclamations and extensive back fill operations are involved, the geotechnical design is quite delicate in order to meet the settlement requirements that were defined. Therefore the following concept has been developed:
• Excavate soft top layers of mud
• Install sand compaction piles to improve underlying cohesive layers
• In fill with coarse sand to be compacted
• Form sea defence walls consisting of rock layers and revetments of doloses
• Install circular sheet piled walls as retaining structures for the cut and cover tunnels (locally serving as part of the sea defence in the final phase)
• Construct cut and cover tunnel founded on bored piles
• Finishing works

The interface area between the artificial islands and tunnel is very complex from a geotechnical point of view. Large land reclamations in combination with ground improvements (artificial islands) are carried out close to large excavations with ground improvements (trench excavation of the tunnel) but scheduled in different periods of the project program but with a considerable mutual and subject to time dependent behaviour. Complicated time dependent 3D geotechnical processes will take place in the sub ground. The prediction of these processes is difficult even with state of the art 3D geotechnical FE models. And however the immersed tunnel is capable to deal with differential settlements, it was decided to apply robust design solutions in which a smooth transition from the island cut and cover tunnel section to the immersed tunnel section is implemented. This involves:
• A cut and cover tunnel founded on bored piles in / on the deep rock.
• The first elements of the immersed sections will be founded on steel piles penetrating the bearing sand layers (above the rock)
• Adjacent immersed tunnel section will be direct founded, however, the soft soils will be treated with settlement reducing measures such as settlement reduction piles or sand replacement
• The middle part of the immersed tunnel will be direct founded on the sub ground

For the detailed design phase it has been advised to develop a state of the art 3D FE model in which the transition area is considered, taking into account the various construction stages. This model is supposed to be verified and validated by means of an extensive monitoring program during construction. This will enable the Designer to make more reliable predictions with the progress of the works and to adjust or optimize the design where relevant and possible.

Various items
Tunnel safety / tunnel ventilation
The length of the sub-sea tunnel, which stands at over 6km, poses specific challenges while considering tunnel safety. The HZMB needs an integrated safety concept for the whole of the project, and specifically so for the immersed tunnel.

Ventilation principle in operational phase and in case of a fire to the structural element, such as the tunnel, but also to all other elements within the system, such as measures to prevent accidents as much as possible (alignment, design speed, sightlines), monitoring and detection measures in case of an accident (fire detection, cameras etc), measures to control accidents (ventilation, sprinkler system, emergency posts, traffic management etc.), escape ways and strategies (safe haven, self rescue principle) and the tunnel operations, that for example include the emergency response procedures.

Tunnel ventilation is an essential item of the tunnel installations design when it comes to health and safety in the operational phase and regarding incident control. A ventilation concept has been selected with longitudinal ventilation in the operational phase (jet fans supported by natural ventilation induced by the piston effect and enhanced by the traffic).

In case of a fire a parallel smoke extraction system will be activated using a semi transverse ventilation system including a separate smoke extraction cell located above the escape cell combined with an additional system like a foam mist fire extinguishing system or a sprinkler system.

Durability
The design service life of main work of this project is 120 years, which is the first ever specified for such extended period for infrastructure in mainland China. In addition the HZMB project is located in a marine and very corrosive environment. Therefore specially dedicated durability design criterion that include crack control criterion and concrete production technology have been made for this project. Use has been made of field experience and of Codes from Europe and America.

As part of the HZMB Main Bridge project, special durability related studies have been carried out that used the European DURACRETE and FIB ‘Model code’ as their bases.

Even though the design has produced a configuration that will meet the 120 year life time, it is obvious that the actual life time expectancy will also be created during the construction stage.

Inadequate concrete compaction, reinforcement designs that hamper the flow of concrete, offspecification concrete mixes, all contribute to sub-design life time expectancy.

A good tunnel inspection and maintenance strategy is undeniable an essential part of the durability strategy of this project. Therefore the inspection and maintenance strategy of this project should embody the full service lifetime concept to guide the design. During the preliminary design phase, the designs have been verified against accessibility, inspectability, maintainability and replaceability.

For permanent works, i.e. those that are not replaceable, the design should meet 120-year design life while for others a shorter design life period could be used where the element should be replaced; i.e. depending on the optimal life cycle cost solution.


Plan view of the HZMB Link Indicative geotechnical profile Indicative stress levels at cross section Cross Section of tunnel element Transport and immersion Busan Geoje Settlement reduction Typical cross section of artificial island and cut and cover tunnel Ventilation in principle operation phase In case of fire