Engineering

At the crossing, the river flows in a channel about 30 m1 deep. Beneath the riverbed lies a thick layer of recent muds and sands, soft and water-bearing. There are no published boreholes on this alignment, but 2 km downstream, at the 25 de Abril Bridge, the south pier foundation had to go about 80 m22 below water level. So we assume that, in mid-river, firm ground only appears at about 70 m Estimate depth.

This means that, in mid-river, the tunnel sits inside these sediments, not below them. Slurry tunnel boring machines are made for this kind of ground, but it demands more care in the seismic design. It is the project’s biggest uncertainty until boreholes are drilled.

The geological section shown is interpretive: there are no published boreholes on the alignment. The 2029–2031 campaign includes ~25 marine boreholes (~100 m spacing), CPTu, pressuremeter and permeability tests, high-resolution seismic reflection and tide-logging piezometers.

Expected tunnel face: mixed near Cais do Sodré, alluvium (sands and muds) for most of the crossing, and Miocene on the approach to Cacilhas. If boreholes show thinner alluvium, parts of the tunnel will be in firm ground, which is favourable.

Local precedent: during construction of Terreiro do Paço station, a section of the Blue Line tunnel beside the river collapsed and flooded26. Lisbon’s waterfront soils have a record of being unforgiving.

The tunnels descend from the stations, about 28 m deep, to a low point about 59 m Estimate below sea level in mid-river, then climb back up to Cacilhas.

• Maximum gradient 4 % Estimate; station invert −28 m Estimate.
• Minimum design cover 10 m Estimate; the proposed profile keeps ≈ 20–26 m.
• Maximum depth below riverbed 27 m Estimate; hydrostatic pressure at the low point ≈ 6.1 bar Estimate.
• Drainage sump at the low point (ch 1+150), accessed from the nearest cross-passage; redundant pumps discharging to the stations.
• The profile shown joins points with straight segments; vertical curves will be defined in design and do not change the main levels.

There are two usual ways to cross a river: bore a tunnel beneath the bed with tunnel boring machines, or sink large concrete elements into a trench dredged in the bottom (immersed tube), as at Øresund or Fehmarnbelt.

We propose two tunnels bored by TBMs. They need no dredging of a protected estuary, do not interrupt shipping or the Cacilhas ferries, and sit deeper, with more ground above them, which behaves better in an earthquake than a backfilled trench. An immersed tube would be cheaper and allow shallower stations, but its impact on the estuary and the port weighs more.

• Two slurry TBMs, ≈ 7.3 m bore diameter, suited to water pressures up to ~6 bar and permeable sands and gravels.
• 350 mm precast segmental lining, 1.5 m rings, EPDM gaskets designed for twice the hydrostatic pressure.
• Launch from Cacilhas, with spoil removed by barge. Average advance 10–12 m/day Estimate: each bore in 9–12 months Estimate, within the 2034–2037 window.
• Reference: the Marmaray immersed tube is 1,387 m18 long and reaches ~60 m18 below sea level.
• Cutterhead maintenance at ~6 bar: above the limit for routine compressed-air work. The design requires tools replaceable from inside the machine (at atmospheric pressure) and allows saturation-diving interventions only as a fallback. This affects cost and schedule.

Two tunnels or one?

The reference design is two single-track tunnels. The alternative is one larger tunnel, with the two tracks separated by a central wall, which needs no cross-passages. The cross-passages are the highest-risk element of this project: about eight hand-mined excavations between the bores, in frozen ground, in sands under ~6 bar of water.

The proposed profile (cover ≥ 20 m) suits both. The final choice is left until after the ground investigation: the worse the mid-river sands turn out, the stronger the case for the single bore.

Each tunnel carries one track. Every 240 m a cross-passage lets people move to the other tunnel in an emergency.

• Internal diameter 6.3 m Estimate: clearance for ~2.8 m wide rolling stock, slab track, third rail compatible with the network, evacuation walkway ≥ 0.85 m and cable trays.
• 7.0 m pillar between bores (≈ 1 diameter) to limit interaction during excavation.
• Cross-passages every 240 m Estimate: meets NFPA 130 (≤ 244 m13) and exceeds the European TSI SRT (≤ 500 m14). Built from inside the tunnel using ground freezing.

The 1755 earthquake had an estimated magnitude of 8.5–9 Mw5 and a tsunami with waves of 5–15 m5. Any structure in Lisbon must account for it.

Deep tunnels generally perform well in earthquakes, because they move with the surrounding ground. The sensitive points are the junctions with stations and changes in ground type, and that is where the design concentrates flexible joints and strengthening.

Because the tunnel passes through loose, saturated sands, the design must check whether they could lose strength in an earthquake (liquefaction) and, if so, treat the ground or adjust the alignment.

• Lisbon is in Zone A, the highest hazard, of the 1983 Portuguese code; 475-year hazard across the mainland ranges from 0.05–0.20 g6.
• Design to EN 1998-1 and EN 1998-5 with the Portuguese National Annex; lining ovalisation analysis and longitudinal strain from wave propagation.
• Liquefaction: both the tunnel and the station walls pass through alluvium. Assessment by CPTu and cyclic laboratory tests; checks on flotation and post-earthquake settlement; ring joints with rotation and extension capacity.
• The Portuguese National Annex to Eurocode 8 defines two types of seismic action (far-field and near-field), and both must be checked23.
• Faults: the Lower Tagus Valley fault zone is linked to historical earthquakes of magnitude 6.5–7.025, such as 1531. The seismic reflection survey must confirm whether any active fault crosses the alignment; if one does, the tunnel gets an articulated lining over that stretch.
• Flexible seismic joints at tunnel–station interfaces; watertight floodgates at the waterfront stations for the tsunami scenario.

In a fire or breakdown, passengers leave the train along the walkway and enter the nearest cross-passage, never more than 120 m away, into the other tunnel, which acts as the place of safety. Ventilation pushes smoke the opposite way.

• Longitudinal ventilation with reversible fans in shafts at both stations; critical velocity calculated for a design train fire.
• Pressurised cross-passages with fire doors; platform screen doors at stations.
• Segment concrete with polypropylene fibres against explosive spalling.
• No mid-river shaft: the maximum distance to a station (~1.15 km) is covered by the cross-passages.

The risks most likely to affect schedule, cost or safety, and how they are handled.