Can you build a tunnel first, then lower it into the sea?
Yes: the immersed-tube method builds hollow elements ashore, seals their ends, floats them to the site and lowers them into a prepared trench. Ballast water enters dedicated tanks. The traffic corridor stays dry.
A bored alternative tunnels beneath the seabed, supporting the excavation face with suitable earth or slurry pressure and installing lining behind it. Soft ground can be bored too. Geology, depth, cover, shipping, approaches, environmental impacts and cost all influence the choice.
Follow one element through construction
- Cast and seal. Reinforcement and concrete form a hollow element. Ballast tanks are separate from the corridor. After watertight bulkheads have been checked, flooding the dock floats the element.
- Tow, ballast and lower. Tugs bring it into position. Ballast creates a small downward force; placement cables control the descent. This film uses a prepared gravel bed. Other foundation treatments also exist.
- Join and drain. Pulling against the installed element makes an initial GINA seal around the end face. Water remains between two temporary bulkheads. Draining that pocket lets external water pressure compress the joint further.
- Double-seal, inspect and remove the mating bulkheads. Install the internal Omega seal and complete inspections and testing before removing those bulkheads. The sea-facing construction front stays sealed.
- Backfill, fit out and commission. Side fill locks the element in position; protective cover goes above it. Roadway, drainage, ventilation, fire safety and monitoring require completion and commissioning before traffic opens.
Pause, seek or jump chapters to inspect the joint while it is still wet. Exploration lets you vary ballast, prescribed depth and alignment offset.
A layer deeper
Why can such a heavy element float?
Archimedes’ principle compares weight with displaced water. Our illustrative 180 × 37.95 × 11.4 metre envelope displaces about 79,820 tonnes of seawater. A teaching self-weight of 74,800 tonnes leaves about 5,020 tonnes of fully submerged reserve. Add a 600-tonne downward allowance, and the illustrated ballast target is about 5,620 tonnes.
At a free-floating waterline, displacement adjusts until static forces balance. The screen’s fully submerged buoyancy reserve is buoyancy minus weight when completely underwater. It is not the actual upward force on a freely floating element. Real ballast control also accounts for trim, roll, currents and cable forces.
Why doesn’t the joint water flood the corridor?
The wet pocket lies between two temporary bulkheads, separate from both sealed corridors. GINA forms a continuous perimeter; the longitudinal section cuts its top and bottom. Draining creates a pressure difference that helps compress GINA. The internal Omega seal adds a second barrier. Alignment alone does not establish watertightness or replace pressure testing.
What has been simplified?
This is a construction-principle model, not an engineering design or safety calculation. Dimensions, weights and the 44-metre roof depth are teaching inputs, rather than one tunnel’s complete design. Horizontal and vertical lengths use different scales. A real element contains multiple cells, walls, reinforcement and detailed foundation work; this section emphasizes one dry corridor and its joint.
Hydrostatic pressure uses (p=\rho gh); the atmosphere readout includes surface atmospheric pressure. The model omits wave and current dynamics, earth pressure, structural stress and cable dynamics. Exploration prescribes depth rather than computing descent speed from ballast; wave attenuation is illustrative. The 10-centimetre alignment threshold is a teaching limit, not a universal construction tolerance. The waterplane stability formula applies only while freely afloat, not after full submergence.
The film presents drainage, bulkhead removal and backfill sequentially for inspection. Real projects can overlap work in different zones. The final traffic view cuts to a representative completed section, rather than sending a car towards the sealed construction front.
See ITA-AITES’ immersed-tunnel construction sequence, Trelleborg’s GINA installation guide and DAUB’s soft-ground face-support recommendations. Continue with the tunnel boring machine or the sea bridge.