Hong Kong's Record Tunnel Borer Built an Underwater Highway, Then Changed Size Below the Seabed

A 17.63 Meter Machine Beneath Hong Kong's Waters

Hong Kong's Tuen Mun-Chek Lap Kok Link Northern Connection opened on December 27, 2020, after a construction effort that included an extraordinary operation beneath the seabed: engineers reduced the size of a giant tunnel boring machine underground, then sent it onward to continue digging.

The machine was the Herrenknecht Mixshield S-880, named Qin Liangyu. Its shield measured 17.63 meters in diameter, a dimension that still holds the Guinness World Records title for the largest tunnel boring machine by shield diameter as of July 17, 2026. Longer than a football field and heavier than most ships, it was built to excavate under pressures that would crush ordinary equipment.

The highway it helped create connects the northwestern New Territories with Hong Kong International Airport and the Hong Kong-Zhuhai-Macau Bridge. The Northern Connection forms a route about 9 kilometers long, costing approximately HK$18.2 billion, or US$2.3 billion. Its underwater road tunnel is Hong Kong's deepest, longest and widest.

At the center of the project were two tunnel tubes extending about 5 kilometers beneath the water. Their route passed below shipping lanes and through soft seabed deposits, close to sensitive coastal ecology. Engineers needed to create room for traffic while keeping seawater and unstable ground outside the excavation.

The S-880 combined digging, ground support and installation of the tunnel lining in one moving system. Its work also depended on another large boring machine, smaller equipment for connecting passages, specialist workers and a supply network that kept concrete segments, slurry and replacement parts moving underground.

Why the Tunnel Needed Such a Large Shield

The seabed between Tuen Mun and Chek Lap Kok contains up to 30 meters of marine deposits and alluvial clays. These soft soils are saturated with water, making an unsupported opening vulnerable to collapse and flooding. In some places, bedrock lies about 90 meters below sea level.

A conventional tunnel boring machine would struggle with those conditions. The project used a Mixshield design, which balances pressure at the excavation face through a controlled combination of slurry and compressed air. The excavation face is the area of soil or rock directly ahead of the machine.

Traffic requirements also shaped the dimensions. The planned tunnel needed an internal diameter of 15.6 meters to accommodate two highway lanes in each direction, together with safety space, ventilation ducts, electrical systems and maintenance access.

The machine had to make an opening larger than the finished interior. Prefabricated concrete lining segments would occupy part of that space, while the machine's outer casing, known as its tail skin, and its cutter head also required room. With those elements included, the required shield diameter reached 17.63 meters.

That diameter was roughly equivalent to the height of a building with six stories. Qin Liangyu's size followed the geometry of the structure it had to build, as well as the demands of the ground surrounding it.

How Slurry Held Back Mud and Seawater

The S-880 operated less like a simple drill than a moving factory. A rotating cutter head at the front scraped away soil and rock. Immediately behind it, an excavation chamber contained bentonite slurry, a thick fluid made with clay, together with a cushion of compressed air.

The slurry pushed against the exposed ground. By maintaining the right pressure, operators could keep mud and seawater from rushing into the machine. The compressed air above the slurry allowed fine adjustments as excavation progressed through changing conditions.

Excavated material mixed with the slurry and traveled through pipelines to a separation plant at the surface. There, equipment removed soil particles so the cleaned bentonite could be sent underground again. This circulation system allowed excavation to continue while carrying the spoil away from the tunnel face.

Behind the excavation chamber, a robotic segment erector lifted curved pieces of concrete into position. The pieces joined to form rings, supporting the surrounding ground and creating the cylindrical tunnel walls. Roads, lighting and ventilation would later be installed within that structure.

Every part of this production line had to keep pace. Pressure control, pumping, slurry chemistry, hydraulic thrust, segment deliveries and surveying all worked together. A delay in one system could stop the entire operation, making coordination as necessary as the machine's cutting power.

Maintenance Under Nearly Six Times Surface Pressure

Water pressure at the excavation face reached up to 5 bar, described as five times atmospheric pressure at sea level and comparable to the pressure a diver encounters at roughly 50 meters deep. Maintaining that pressure helped keep the excavation stable, but it made work around the cutter head much more demanding.

The Hong Kong Government News Service described the depth and the conditions encountered by specialist hyperbaric workers, whose jobs took them into environments with pressure above normal atmospheric levels:

The deepest section of the Tuen Mun-Chek Lap Kok Tunnel sits about 60 meters below sea level, and specialist hyperbaric workers faced pressure nearly six times greater than at the surface.

The S-880 used airlock chambers to move personnel between ordinary atmospheric pressure and the pressurized excavation chamber. These sealed compartments allowed pressure to change in stages while the cutter face remained supported.

Cutting tools still needed maintenance, inspections and repairs. Trained workers entering the pressurized area followed strict hyperbaric medicine procedures, with limited exposure times and medical supervision. Their work resembled a diving operation conducted inside a machine beneath the seabed.

Decompression was part of the job. Workers had to return gradually to normal pressure to avoid decompression sickness, commonly called the bends. The maintenance schedule therefore had to account for the time needed to enter, work in and safely leave the pressurized zone.

The Machine Changed Diameter Underground

The northern approach structure required a tunnel diameter of 17.6 meters, so Qin Liangyu began excavation at its full size. After the first 650 meters, the remaining underwater route could be built with a diameter of 14 meters.

Engineers converted the S-880 inside the tunnel instead of bringing it back to the surface and launching another machine. They removed external parts of the 17.6 meter shield, reconfigured internal systems and installed a new shield measuring 14 meters.

The conversion took place below the seabed, under sea pressure, with restricted working space and no direct access to daylight. Changing a machine's shield size within a tunnel is extremely rare. In this case, it allowed the same machine to continue beyond the larger approach section.

The S-880 excavated the section measuring 17.63 meters between March 25 and November 3, 2015, before its conversion. It then continued on the underwater route alongside a second Herrenknecht machine measuring 14 meters. The two machines broke through on February 27, 2019.

The process was documented as an advancement that could be applied to future projects needing different tunnel diameters within a single drive. Its practical achievement was straightforward: equipment built for one size of opening was rebuilt underground to make another.

Connecting the Two Tubes Under Pressure

The main road tunnels needed 57 cross passages. These smaller corridors connect the two tubes, providing routes for evacuation and maintenance. Constructing them meant opening paths through the ground between tunnels already beneath the sea.

Pressure at the cross passage locations reached 5.8 bar. Herrenknecht recommends professional divers for pressurized chamber work above 3.6 bar, so the work demanded specialist precautions even beyond those associated with the main excavation.

One conventional option for underwater tunnel work is ground freezing. This method freezes water within the soil to make the ground more stable during construction. In the soft, saturated seabed clay along this route, that approach would have been slow and risky.

The team instead used a mini slurry tunnel boring machine measuring 3.6 meters. It bored through the walls between the main tubes while a slurry system maintained pressure around the excavation. The method was a global first for cross passage construction under such high pressure.

Using the smaller machine avoided the need for freezing, shortened construction time and reduced risks where seawater, soft clay and high pressure combined. The passages were much smaller than the highway bores, but they required their own specialized engineering solution.

Power, Speed and the Supply Chain Behind Them

The S-880 reached a maximum daily advance of 30 meters and a weekly record of 167.2 meters. The American Society of Mechanical Engineers recorded its power output as 5,600 kilowatts and its ability to advance about 30 meters per day through saturated ground and rock.

Those figures depended on keeping the entire operation supplied. Concrete segments had to arrive as the lining was assembled. Bentonite had to circulate through the separation plant, and replacement parts had to be available when tools or machinery required attention.

The machine's speed therefore reflected both mechanical performance and construction logistics. Cutting faster could produce useful progress only while spoil removal, pressure control and installation of the lining continued at the same pace.

The project received the International Tunnelling and Underground Space Association's ITA Tunnelling Award in 2019, recognizing the technical innovations used during construction. The work combined the unusually large shield, its underground conversion and methods for building connections between the main tubes.

Another solution to receive an award was the caterpillar cofferdam system used in the northern approach structures. It provided an alternative to conventional diaphragm wall construction in very soft soils, addressing a different part of the same difficult ground conditions.

Why Boring Reduced Disruption Above

Building beneath busy waters affected more than the construction site. Urmston Road carried heavy marine traffic, submarine power cables served the airport, and Chinese white dolphins and other marine life occupied the surrounding waters.

An older construction approach, the immersed tube method, would have required extensive dredging of the seabed. The Hong Kong Highways Department described the environmental benefit of using tunnel boring machines in an engineering feature:

Using tunnel boring machines instead of the traditional immersed tube method reduced dredging and disposal of about 11 million cubic meters of marine sediment, roughly the capacity of 4,900 standard swimming pools, and helped minimize impact on Chinese white dolphins and other marine life.

The department also said the chosen method reduced disruption to shipping along Urmston Road and avoided the need to divert existing submarine power cables supplying the airport.

These were practical reasons for excavating beneath the seabed. Traditional construction would have involved large amounts of sediment removal, interference with marine traffic and years of environmental conflict. Boring allowed the tunnel to be formed underground while limiting disturbance above it.

The equipment remained expensive, energy intensive and technically demanding. Its value lay in creating transport capacity beneath an already busy coastal area while reducing the need for dredging and additional construction sites along the coastline.

From Excavation to an Airport Connection

The Northern Connection became part of a wider transport network linking the northwestern New Territories, the airport and the Hong Kong-Zhuhai-Macau Bridge. The southern connection had already begun opening in 2018, before the northern route entered service in December 2020.

The main construction and opening dates show how the project moved from its largest excavation to a working highway:

• March 25, 2015: The S-880 began excavating the section with a diameter of 17.63 meters.
• November 3, 2015: Excavation of that section ended, before the machine was converted to a diameter of 14 meters.
• 2018: The southern connection began opening, linking into the wider Hong Kong-Zhuhai-Macau Bridge network.
• February 27, 2019: The two main tunnel boring machines broke through.
• 2019: The project received the ITA Tunnelling Award.
• December 27, 2020: The Northern Connection opened to traffic.

The finished route turned a complicated construction operation into an everyday road connection. The two underwater tubes provided highway capacity, while the cross passages, ventilation equipment, electrical systems and maintenance spaces supported operation beneath the sea.

The approximately HK$18.2 billion project was conceived around the movement of people and vehicles between land, sea and air. Its engineering choices followed that purpose, from the size of the main bores to the decision to keep excavation below shipping lanes and existing cables.

A Record With a Precise Meaning

Qin Liangyu retains the Guinness title for shield diameter. Other Chinese and international tunnel boring machines introduced in recent years have brought greater thrust, more automation or different cutter head designs. The S-880's record concerns its 17.63 meter shield.

The Hong Kong project combined several unusual demands in one machine's journey: a very large starting diameter, deep underwater excavation, high working pressure and a shield conversion performed within the tunnel. Together, they made the S-880's work distinctive beyond the diameter record itself.

Coastal cities seeking more transport capacity face a similar physical problem: ports, airports and urban districts compete for limited space. Machines such as the S-880 allow new routes to pass underground and underwater, with less surface disruption, dredging and ecological disturbance than older methods.

The machine's name also carried a historical connection. Qin Liangyu was a Chinese general of the Ming Dynasty, born in 1574 and known for courage and loyalty. Naming the machine after her followed an underground engineering tradition of giving large tunnel boring machines female names, a custom stretching back to the nineteenth century.

For Hong Kong, the lasting result is the road rather than the record. A machine designed to hold back wet ground and seawater helped create a permanent connection beneath them, while its underground transformation demonstrated how one excavation system could serve sections requiring different sizes.

Key Points

• The Herrenknecht Mixshield S-880 Qin Liangyu retains the Guinness World Records title for largest tunnel boring machine by shield diameter, at 17.63 meters.
• The Northern Connection opened on December 27, 2020, linking the northwestern New Territories with the airport and the Hong Kong-Zhuhai-Macau Bridge.
• The route is about 9 kilometers long, includes roughly 5 kilometers of underwater tunnel and cost approximately HK$18.2 billion.
• Bentonite slurry and compressed air supported excavation through soft, saturated seabed ground.
• Engineers reduced the machine's diameter from 17.6 meters to 14 meters inside the tunnel after its first 650 meters.
• A mini slurry machine measuring 3.6 meters constructed 57 cross passages at pressures reaching 5.8 bar.
• The S-880 achieved a maximum advance of 30 meters in a day and 167.2 meters in a week.
• The Highways Department said boring reduced dredging and sediment disposal by about 11 million cubic meters while limiting disruption to marine life, shipping and airport power cables.