Armand van Wijck, TunnelTalk European Correspondent

With a population of 191,000 Almere, just 10km outside Amsterdam's suburbs, is the seventh largest city in the Netherlands. By 2030 that population is set to reach 350,000, a situation that demands infrastructural improvements and in particular a metro link with the Dutch capital. Armand van Wijck examines the underground options being considered.

Immersed tube metro tunnel could link Amsterdam with Almere

The city edges of Amsterdam and Almere are no more than 10km apart, but with Lake IJmeer lying between them a direct land connection is impossible. As both cities expand at their eastern and western edges respectively, the Dutch are planning a public transportation connection through the water.

In 2009 the Dutch Government rejected the first plan for the IJmeer connection: €4.5 billion for a bridge and €5.4 billion for a tunnel was considered too expensive. To come up with a cheaper solution the Government founded the Operating Company Amsterdam Almere, which in turn appointed three consortia to identify the best public transport connection. This involved finding a solution that would incorporate a more affordable design that would also be eco-friendly.

Three different plans were presented; a bridge and two tunnel solutions. Dutch engineering company Movares designed a €2.5 billion tunnel, while M55, a consortium led by the Dutch office of Mott MacDonald (UK), designed a €2.9 billion tunnel solution. The new proposals are nearly 50% cheaper than the 2009 design because they involve extending the existing metro line instead of using a heavy rail system. Both new proposals also feature significantly shorter tunnels than the 2009 plan.

Fig 1. Route of Movares' metro line between Amsterdam (west) and Almere (east). The white dotted line marks the tunnel option. Circled in red is Fort Pampus island

On a daily basis 54,400 passengers are expected to use the IJmeer metro link according to model calculations made by the Operating Company.

The Movares plan
So why the €400,000 difference between the new tunnel plans? "Unlike M55, we were assigned by the Operating Company to design a tunnel as short as possible," explained Willem Ottevanger, Project Leader for Movares. The Movares tunnel is just 4.2km long compared to M55's is 9km design.

Movares' solution involves extending the mainland on both sides of the lake by creating dikes and small islands for recreational and nature development use, an eco-friendly solution that shortens the tunnel distance considerably. A dike is much cheaper to build than a tunnel, Ottevanger points out.

At the Amsterdam side, just before the tunnel portal, the metro line would pass underneath a planned recreational beach via a small cut-and-cover tunnel, then cross a dike before becoming IJmeer tunnel. On the Almere side the tunnel emerges on one of the recreational islands then travels via a dike towards the mainland (Fig 1).

Movares' proposal increases the mainland on the Almere side with recreational islands

Movares' tunnel alignment is designed to be as straight as possible, meaning it has to pass the UNESCO heritage site, Fort Pampus, a small island in the middle of the lake. Ottevanger said: "We calculate that an immersed tunnel is the cheapest option, however, mud will be disturbed during trenching. To prevent altering the water quality of the IJmeer, we will install floating screens at the sides of the trench to contain mud swirls."

Movares has used the island's presence as an extra tunnel safety opportunity, with emergency evacuation to the island's surface designed in. "It is a safety measure which is not obligatory by Dutch law at the moment, but the law might change within 20 years, so we want to be ahead of our time," said Ottevanger.

The M55 plan
But Martijn Donders, M55 Project Leader, questions both Movares' design and its cost estimates. He said: "They need to include all capital costs for both designs to be compared on an equal basis. Land reclamation is an important aspect of Movares' design, and the final cost depends on who will pay for reclamation. I also think they miscalculated when measuring the tunnel length, it appears to measure more than 4.2km. We are also working on a shorter tunnel for the Operating Company, but it is difficult to see how to get it any shorter than 6km."

Fig 2. M55's metro route. Fort Pampus island is circled in red, the sail-and-ride station circled in black. The blue dots are optional metro stations

M55's 9km alignment also uses an immersed tube. It has three cells, two for the rail tracks and one for safety and maintenance. There is also a small cut and cover section underneath the Amsterdam district of IJburg, near the edge of the lake. Both tunnel portals are situated within the lake, and sand dredged for the trench will be used for land reclamation.

"To finish, basalt blocks will be deposited on top of the tunnel," said Donders. "These blocks are beneficial for mussel growth, which is an important nutrient for the migratory birds living in the area."

Unlike Movares' design, the M55 tunnel passes Fort Pampus island on the south side (Fig 2). "The lake bed is only about 12-20m deep in that location, and our tunnel is at least 12m deep anyway. The depth is not an issue and it might even save some trench digging," said Donders.

Sail-and-ride station
M55's project may also include the first ever sail-and-ride station in the world. Its proposal includes an underground metro station in the newly-planned lakeside district of IJburg, which is also to have an inner urban waterway. "We want to give the people who live there the opportunity to moor their boats and travel onward by public transport", said Donders.

On December 15 (2011) the Operating Company submitted its report on all three designs to the Dutch House of Commons. The Government is expected to advise on a plan that will combine the best aspects of all three designs submitted to the Operating Company.

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