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Marine tunnel construction depends on the route, ground and water depth: some tunnels are assembled in a dredged trench, while others are bored beneath the seabed or built in a shallow, supported excavation. For an immersed-tube tunnel, the process runs from dredging and preparing a foundation to floating, sinking and joining prefabricated elements, then sealing and burying them. The waterproofing is a system—not a single lining product—and differs substantially between immersed, bored and mined tunnels.
Which methods are used to build a tunnel under water?
There is no single underwater-tunnel construction method. The main options are immersed tube, bored tunnel and cut-and-cover; drill-and-blast is also used where a route passes through competent rock. The immersed-tube method is the one that begins with dredging a trench along the tunnel alignment. A bored tunnel is excavated below the waterway, and cut-and-cover is generally limited to shallow crossings or landward approaches.
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|---|---|---|---|
| Immersed tube | Prefabricated elements are floated to the site, lowered into a prepared trench and joined underwater. | Shallower water and ground that can be dredged and prepared. | Element-end seals, watertight concrete or welded steel, and sometimes an enveloping membrane. |
| Bored tunnel | A tunnel boring machine (TBM) excavates from a land shaft or portal; lining segments are installed behind it. | Deeper routes or crossings where an open trench would be impractical. | Gaskets between lining segments and grout around the outside of the rings. |
| Cut-and-cover | An excavation is supported, a tunnel box is built or installed, then the structure is backfilled. | Shallow water and, commonly, approach sections near the shore. | Usually an external membrane and sealed construction joints. |
| Drill-and-blast or sequential excavation | Rock is excavated in stages and supported as work advances. | Competent rock, including deep routes where a trench is not suitable. | May be tanked with a membrane or designed to drain collected groundwater. |
The International Tunnelling and Underground Space Association (ITA-AITES) describes the immersed sequence as dredge, fabricate, seal, transport, lower, connect, backfill and build the approaches. That sequence is a useful starting point, but each stage depends on the local ground, navigation, water pressure and environmental constraints. ITA-AITES’s overview of subaquatic tunnelling explains the main method families.
How is the route chosen before dredging starts?
Engineers compare water depth, seabed material, route length, ground conditions, shipping and the impact of construction on the waterway. Soft, dredgeable ground in relatively shallow water may suit an immersed tube; deeper alignments or hard ground may favour a TBM or rock excavation. These are tendencies, not universal limits: the best method depends on the site and the design.
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Dredging creates a trench and changes the seabed, generating spoil that must be managed and potentially affecting turbidity and navigation. A bored route leaves the seabed along the alignment largely in place, but requires shafts or portals, controlled pressure at the excavation face and a way to remove spoil on land. Immersed-tube work can also be divided into parallel activities: elements can be fabricated while dredging and foundation work proceed.
Ground affects equipment and support requirements. In broad terms, coarse, permeable ground can lead designers toward a slurry-shield TBM, while cohesive soils may suit an earth-pressure-balance machine. Rock may be excavated by TBM or drill-and-blast, depending on its condition and the route. These are engineering choices, not a checklist that alone determines the final method.
How is the trench dredged and its foundation prepared?
For an immersed tunnel, the trench is excavated before the tunnel elements arrive. A clamshell dredger is a common tool, but dredging equipment and the sequence vary with depth, material and site conditions. The trench must account for side slopes and excavation accuracy; over-dredged areas may need to be filled with specified foundation material.
The Federal Highway Administration’s road-tunnel design manual describes bulk excavation followed by a finishing stage, including trimming the last 3 feet (about 1.0 m) above final dredge level. That figure comes from indexed manual text, so it should be understood as guidance in that source, not a universal specification for every project. Sealed dredging buckets may be used when excavating contaminated material or to limit turbidity. The trench bottom also needs to be kept clear of sloughed soil or material carried in by currents.
Once excavation is complete, the tunnel element needs a prepared bearing surface; it is not simply set down on raw seabed. Depending on the soil and design, the foundation may use a gravel bed, sand bedding pumped or jetted beneath the element, special grout, or piles. The trench may also require ground improvement.
The Busan-Geoje immersed tunnel illustrates how project-specific this work can be. A 2009 New Civil Engineer construction report describes a gravel bed rather than a grout bed, because grout would take time to set. Gravel was placed through a tremie in planned strips across and along the trench; a sensor helped control the quantity. Ground improvement used cement deep soil mixing in some areas and sand compaction piles in others. The point is not that this layout is standard, but that the foundation must be accurate and compatible with the ground, the tunnel element and the construction sequence.
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How are immersed-tunnel elements made, floated and lowered?
Immersed-tunnel elements are built onshore, in a casting basin, fabrication yard, ship-lift or factory, while work continues at the route. Their size and shape depend on what the tunnel must carry: a concrete element may be a wide multi-cell box for road lanes, services and escape routes, or a combined road-and-rail crossing. Trade sources commonly describe elements in the range of roughly 100 to 200 metres, but actual dimensions are project-specific.
Temporary bulkheads close the ends of an element so it can float. When the construction basin is flooded, the sealed element can be floated out and towed to the installation site. Ballast tanks help control its buoyancy and descent; mooring systems, pontoons and positioning equipment guide it into place. The element must be lowered gradually and accurately onto the prepared foundation, with alignment checked against the section already installed.
Busan-Geoje gives a sense of the operation’s scale. The 2009 report describes elements 180 m long and 26.5 m wide, with six water-ballast tanks holding around 6,000 tonnes in total. It reports that positioning used two immersion pontoons and 14 anchors. The report gives two different Gina-gasket thickness figures—360 mm and 368 mm—and says the gasket was compressed by about 178 mm to 190 mm. Those are inconsistent figures in the same account, so they should not be treated as one settled design specification.
Temporary ballast is not the same as the permanent weight that keeps the tunnel buried. After the element is installed, permanent ballast and the trench backfill help resist buoyancy. Removing water ballast before the structure is sufficiently secured could allow it to rise, so ballast, jointing and backfill must follow the planned sequence.
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How does an immersed-tunnel joint become watertight?
The principal immersed-tunnel joint is formed between the end of a newly lowered element and the end of the element already in place. A Gina gasket is fitted around one end; the opposing face has a steel contact plate. Jacks bring the faces into initial contact. Then workers pump water out of the chamber between the temporary bulkheads. The external water pressure pushes the new element toward the old one and compresses the gasket.
ITA-AITES puts the key mechanism plainly: “Water pressure on the free end of the new element compresses the rubber seal between the two elements, closing the joint.” This is why the joint is not simply a gasket glued in place and left to the water. Positioning, controlled contact, the pressure difference created by dewatering and acceptance of the joint all matter.
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After the primary Gina seal has been compressed, an Omega seal is installed from inside across the joint as a secondary barrier. The seal manufacturer’s product description explains that the Gina gasket and Omega seal prevent water ingress under external water pressure and allow the joint to accommodate movement, including settlement and temperature effects. The temporary bulkheads are removed only after the connection is accepted; the far bulkhead remains in place until the next element is brought in.
There are other joints within an element as well as the joints between elements. At Busan-Geoje, the 2009 report describes separate waterstops at segment joints inside each element, including an injectable stop intended to allow epoxy to be introduced into small flaws. These internal segment joints should not be confused with the main immersion joint between two complete elements.
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There is no single waterproofing product used in every immersed tunnel. Watertightness depends on the structural material, the quality of construction, the joints and the design pressure. For concrete elements, designers pay particular attention to crack control and joint seals; some designs also use a continuous external membrane around the concrete. Technical guidance notes that engineers differ on whether that membrane is necessary in every case. A Boston Society of Civil Engineers paper by Thomas R. Kuesel says membrane waterproofing is usually required when the concrete box is not prestressed.
In Busan-Geoje, controlling cracks was a major concern. Halcrow technical adviser Don Fraser told New Civil Engineer: “We don’t want cracks so each segment is cast as one section.” The project report also describes doubled waterstops at segment joints. These details illustrate one project’s response to water-tightness demands; they are not a rule that every concrete immersed element must use the same arrangement.
Steel-shell tunnels have a different watertightness basis. ITA-AITES guidance credits the quality of the shell welds, in-situ joints and any flexible joints. Some secondary descriptions also mention membranes on steel-shell elements, but that should not be turned into a universal claim: the design may rely principally on welded shell integrity, include additional membrane protection or use a combination suited to the project.
A continuous sheet membrane is not interchangeable with a Gina gasket. The gasket seals a joint between immersed elements; a membrane can wrap or line a larger surface. For example, the FHWA manual describes a dual lining for sequential-excavation highway tunnels: initial shotcrete, a waterproofing system and a final concrete or shotcrete lining. It gives a common PVC membrane thickness of 80 to 120 mil (2.0 to 3.0 mm), with other materials possible where groundwater is contaminated. That is a mined-tunnel detail, not the standard specification for an immersed-tube joint.
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How are bored tunnels sealed without dredging a trench?
A bored marine tunnel is excavated under the waterway by a TBM launched from a shaft or portal on land. The machine supports the excavation face against ground and water pressure. An earth-pressure-balance machine uses conditioned spoil in its chamber; a slurry machine uses pressurised slurry and sends excavated material to a separation plant. Hydraulic jacks push the TBM forward from the last installed lining ring.
Precast concrete segments are installed behind the cutterhead to create circular rings. Gaskets—commonly described as EPDM—are compressed as the rings close, sealing the joints. Grout is pumped into the gap behind the lining to fill the space between the ring and excavated ground. Some designs add hydrophilic waterstops that swell when exposed to water as a supplementary seal. The result is a different waterproofing system from an immersed tube: it relies on repeated segment joints and the grout-filled tail void, not a chain of long elements connected by Gina-and-Omega joints.
Pressure control and ground conditions are critical to a bored crossing. A slurry shield is often considered for coarse or mixed permeable ground, while an earth-pressure-balance machine is suited to cohesive ground in appropriate conditions. These methods avoid an open trench along the seabed, but introduce different demands: face-pressure management, segment-ring installation, grout placement and spoil handling.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How do cut-and-cover and rock tunnels handle water?
Cut-and-cover is most relevant to shallow water or the approaches where a deeper marine section meets land. The excavation may be held open by cofferdams, diaphragm walls or piles. Crews build a reinforced-concrete box, seal its joints, waterproof its exterior and backfill around it. Saturated ground may require dewatering or other ground-control measures, and the box must be designed against uplift when groundwater pressure exceeds the structure’s effective weight.
The connection between a cut-and-cover terminal structure and the last immersed element is a special interface, sometimes called a terminal joint. It is not automatically equivalent to a standard immersed joint: the connection joins structures built by different methods, so continuity of waterproofing needs to be designed and checked explicitly.
In hard rock, drill-and-blast or sequential excavation can be used below the seabed. Some tunnels are designed to keep water out with a tanked lining; others allow controlled leakage into drainage layers, collection pipes and sumps, then pump it away. A drained tunnel and a tanked immersed tube reflect different design philosophies. In particular, a drained hard-rock tunnel should not be described as if its lining were simply a watertight hull resisting all external water pressure.
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What happens after an immersed element is joined?
Once the connection is accepted, fill is placed beside and over the tunnel to complete the trench and protect the structure. Locking fill helps hold the tube in position; further backfill and, where conditions require it, heavier armour shield it from currents, waves, anchors and ship impact. The material and protection are designed for the site rather than selected from one universal recipe.
At Busan-Geoje, New Civil Engineer reported an initial locking fill of crushed stone smaller than 80 mm and said it protected the tunnel from waves below 3 m. A further stone-size figure in that article is printed as “less than 300m,” an implausible measurement as written, so it should not be silently converted into a different number. The report also describes concrete armour units weighing 25 to 36 tonnes in shallower water. After installation, permanent ballast is added; the report gives 8,000 tonnes of concrete ballast for that project.
The approaches are then completed, although approach works may begin before the immersed elements are installed or proceed alongside them. The final system has to work as one continuous route, with its portals, terminal joints, drainage, services and safety provisions integrated with the underwater section.
What are the main construction risks and misconceptions?
- Assuming every marine tunnel is dredged: dredging is central to immersed tubes and some cut-and-cover work, not to a TBM bore beneath the seabed.
- Treating the foundation as an afterthought: trench accuracy, cleanup, bedding and ground improvement determine whether an element has reliable support.
- Describing the gasket as the whole waterproofing system: element joints, concrete cracks or steel welds, possible membranes, terminal connections and backfill all contribute to performance.
- Ignoring buoyancy and sequence: temporary ballast controls sinking; permanent ballast and backfill help secure the completed tube.
- Conflating project figures: published lengths and dimensions can differ because sources may report different route boundaries or project stages. For example, sources give different lengths for the Busan-Geoje tunnel and for the Hong Kong-Zhuhai-Macao immersed section. State the source and definition rather than presenting unlike figures as directly comparable.
- Applying one project’s detail everywhere: gravel beds, sand bedding, piles, membranes and armour are options determined by design and site conditions, not universal requirements.
The broad engineering logic is consistent: choose the method for the ground and route, create a stable excavation or foundation, build a lining system suited to the water pressure and materials, and protect the completed structure. What changes is where the tunnel is built, how its loads are carried, and how water is kept out—or deliberately collected and drained.
Frequently Asked Questions
How long does it take to build an immersed tunnel?
There is no reliable single duration: the programme depends on route length, ground treatment, permitting, element fabrication and the number of installation windows available. Fabrication and seabed work can overlap, so the tunnel is not necessarily built as one continuous sequence from first dredging to final backfill.
How long are immersed-tunnel elements?
Published examples and engineering summaries describe many concrete elements as roughly 100 to 200 metres long, with project-specific dimensions outside that shorthand possible. The element length is a design and transport decision, not a fixed standard.
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Often, but the construction plan must coordinate dredging, towing, immersion and temporary marine works with navigation requirements. The available channel, work windows and temporary restrictions depend on the waterway and project.
Are underwater tunnels built to last a specific number of years?
The World Construction Network account of immersed-tunnel design describes a typical system design life of about 120 years, while other secondary guidance gives broader figures such as 100 years or more. Those are planning descriptions, not a guarantee for every tunnel or a prediction of maintenance-free service.
