Floating the tunnel idea

Architects and civil engineers of a certain calibre are becoming known for their drive to build things better and more sustainably. A seminar organised last month by the Malta Chamber of Architects and Civil Engineers featured seismic structural engineer Federico Mazzolani key speaker on protection of historical heritage.

Former co-ordinator of an international project on protecting historical buildings from earthquakes using mixed reversible technologies, Mazzolani has long been connected to the development of an alternative type of undersea transport link.

While in Malta he discussed the idea with his colleague, senior lecturer Ruben Borg, a structural engineering specialist at the University’s Department of Built Environment.

The concept of the Archimedes or floating bridge has been growing over the past 40 years. The idea of a submerged bridge in a tube that would be virtually invisible won strong support from environmentalists when it was first proposed to span the Straits of Messina in “the most massive engineering feat in the Mediterranean region since the Suez Canal”.

Developments in offshore engineering technology have steered us towards a point beyond bridges or tunnels and closer to a partially buoyant structure. Anchored at 20 to 30 metres below the surface, the submerged floating tunnel would be secured to the seabed by steel cables.

This type of suspended tunnel or tube is different from the 100 or more immersed tunnels constructed in the past century in that it is intended to interact with the water and harness its weight-bearing capacity.

A research project in Norway has shown how the tunnel can withstand sea currents and waves by lateral flexing, although the movement would not be noticeable to drivers. Conventional suspension bridges move around much more, as they are designed to do, up to a certain point.

A suspended floating tunnel (SFT) would compare favourably with a suspension bridge, giving a higher level of performance in seismic events. On the other hand, an excavated tunnel under the seabed would be more vulnerable in an earthquake zone.

The SFT – a large, buoyant concrete tube – would be anchored underwater at a position deep enough to avoid rough weather and marine vessels without the need for high water pressure resistance required of a structure in deeper water.

The submarine tunnel bridge could hold the environmental edge over a conventional tunnel dug into rock under the seabed which would need to descend to a greater depth.

A tunnel dug into the rock would have to be longer than a submerged floating tunnel, and therefore drivers would burn more fuel as the distance is longer and inclines are steeper. The result from an environmental viewpoint would mean increased emissions from vehicles using the tunnel.

Building a tunnel which rests on the seabed would have a significant impact on the marine environment. Tunnelling underneath the seabed would produce massive amounts of excavation waste, causing a similar environmental dilemma.

EU funding for development of a prototype SFT brought Italian, Norwegian and Danish experts together at an international forum for European highways research laboratories, made up of 22 public roads adminstrations.

Italy has been very active in this direction with provincial administrations in Como and Lecco showing interest in a submerged floating tunnel solution for getting traffic across Lake Lario. A feasibility analysis has been verified by the Italian naval register for a Calabria-Messina connection in the Strait of Messina where the idea was originally born.

The design patent for the floating underwater tunnel bridge, first proposed for the Straits of Messina in 1969 by British engineer Alan Grant, was acquired by an Italian company, Ponte di Archimede nello Stretto di Messina SpA.

The submerged floating tunnel concept narrowly missed coming into being in Indonesia’s Sunda Strait. A European consortium had shown interest in investing in a suspended tunnel that was to be completed by 2018 as part of the Asian highway connecting Bali to Thailand. Since the choice veered back toward a conventional bridge the project has met with delays.

Structural engineers at Norway’s University of Science and Technology have been looking at creating a floating tunnel in a 155-metre deep fjord. The project was evaluated as feasible by national authorities but put on hold ‘for political reasons’. The design of the 1,400-metre floating tunnel includes a pedestrian path and bicycle track.

In 2007, a Chinese-Italian consortium started to plan the construction of a STF prototype in Lake Qiandao, a popular tourist spot.

The consortium, which started out with the building of a 100-metre demonstration tunnel in the lake as proof of concept, is engaged in assessments on the prototype model for larger connections in the straits and archipelagos of eastern China.

A railway development project in the area was halted as it was considered damaging to the natural sights of Lake Qiandao. Construction of a planned highway link to the lake, increasing its prominence as a tourist destination, was to begin at the end of the year.

In 2010, civil engineers from Zhejiang University gave an assessment on preliminary SFT design at environmentally sensitive Lake Qiandao, showing how public safety risks can be improved by design measures.

Cost-effectiveness studies revolve around whether SFT solutions will prove cheaper than traditional bridges and tunnels.

How relevant the discussion might be to widening economic options for a Gozo Channel highway link would have to be seen in the context of social and environmental studies, since the island’s remoteness is often considered to be its salvation.

Another possible factor is the amount of funding that has already gone into the construction of modern ferry terminals on either side of the channel, bearing in mind that a tunnel could permanently change the way people travel between the islands.

Some fear that such a connection would turn the sister island into something of a clone, as they pertinently ask: “Do we really want Gozo to become just another Malta?”

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