Armand van Wijck, TunnelTalk

Holland's first ever double-deck four-tube tunnel, which will eventually see a 2km section of the busy A2 moved underground, is currently under construction in the heart of Maastricht. But as TunnelTalk European Correspondent Armand van Wijck discovered, excavating the cut-and-cover tunnels in such a confined space, while at the same time keeping the existing above-ground roadway open at all times, presented challenges both in design and engineering.

A2 tunnel alignment

In what can best be described as a classic example of underground space for the benefit of the environment and to improve the quality of life for long-suffering Maastricht residents, the busy A2 highway through the heart of the city is going underground.

"The A2 in Maastricht is like an open traffic sewer," said Project Manager Bart Grote of JV Avenue2, which consists of contractors Strukton and Ballast Nedam. "By putting as much traffic underground as we can, we will reconnect the eastern and western districts and make the environment liveable again."

A 2km stretch of the existing highway is being sunk into an eight-lane double-deck four-tube tunnel between Maastricht's northern and southern junctions, reducing the number of vehicles above ground by 80% and cutting high pollution levels dramatically.

When complete, all that will remain above ground will be 2,000 linden trees to be planted where the highway once run. Traffic flow, that is currently affected by the large number of crossings and intersections with local routes, will be drastically improved because the bottom layer of two tunnels will be dedicated to through-traffic, with the top level of two tunnels reserved for local traffic.

Such a construction is not unusual in Europe, but it will be the first of its type to allow for the transportation of hazardous goods.

The decision to stack the four tubes 2x2, rather than having them run alongside each other, was the solution to keeping the above ground A2 route open throughout the five-year construction period.

"By doing this we only need one narrower construction pit. This means there is just enough room now to divert the ongoing traffic to the west of the tunnel alignment throughout the construction period," said Grote. "If we had chosen to construct four tubes next to each other, we would have had to excavate twice along the length of the alignment and shift the temporary road at least twice as well."

Eight-lane 2x2 tunnel construction

Construction methods
"It is a very complex project," said Grote. "We have, therefore, tried to keep the tunnelling method as simple as possible. "Cut-and-cover is easy and trustworthy. We considered a bored tunnel, but that would have needed its diameter again in overburden and would have been a lot deeper. This would have meant longer ramps and exits, for which there is almost no space. Right now we have a cover of 1.5m, which is more than enough."

Construction of the tunnel takes place in repeated steps every 750m from north to south and excavating diaphragm wall trenches of 600mm wide x 26m deep. "Trench excavation is done with a grab excavator because fields of rock and hard flint make it impossible to drive or vibrate the sheet piles into the ground," explained Designer Bas van Aart. "After filling it with cement-bentonite we carefully lower the 20m long sheet piles into the ground."

"Thanks to an overall solid underground," he said, "we do not need a tunnel foundation." This saves around 10,000 anchor piles and 150,000 tonnes of underwater concrete - a particular situation since Dutch ground is normally very soft and past constructions have nearly always needed some sort of foundation.

A wellpoint system dewaters the construction pit, pumping about 1,000m³ of water per hour over a length of 500m. "The tunnel is surrounded by groundwater", said Van Aart "and to prevent the structure from floating, we place concrete toes 2.5m wide on both sides of the tunnel. The weight of the ground above the toes, together with the ground coverage of 1.5m, gives enough downward force to keep the tunnel in position."

Excavation progresses in 750m long x 600mm x 26m deep sections

Because the tunnel is situated close to the eastern bank of the River Maas, groundwater runs perpendicular to the tunnel on its way to the river. To prevent the water from rising too high at the eastern tunnel wall, the design calls for installation of tubes that will transport groundwater over the tunnel and into the Maas.

The JV will excavate the construction pit in three layers, removing 12,000m³ of ground each week. After excavating the first and second benches, props are placed to stabilize the construction pit.

After placing the props crews will cast the floor slab concrete and hoist the tunnel formwork into the construction pit. The next step is to install the reinforcement and cast the concrete base slab. "We work from bottom to top," said Van Aart, "first the lower tunnel deck, then the upper one we build 24m-long sections, backfill to ground level. The sheet piles are then removed and installed at the start of the next section of trenches and the whole construction process starts again."

Complex cross-sections
At the portal, where the 4.7m i.d. high tubes start, the upper ones do not sit directly above the lower ones. After a distance of 400m they move directly on top of each other. "This leads to very complex cross sections, but we left it simple from a design and construction point of view", explained van Aart. "All tubes are separated from each other and behave as individual sections. If we were to bind them together over the first 400m, we would have had to deal with uncontrollable forces. The upper tunnel sections would be hanging over the lower sections and would create cracks in the concrete."

Halfway along the tunnel, construction space is so narrow that the JV has to use a top-down method over a length of 170m. Otherwise traffic would not have had enough room to be diverted to a temporary road alongside. "Because this is such an important part of the project," said Grote, "we started this tunnel section first in Autumn 2011. We placed the temporary road on top of the tunnel roof and excavated underneath. From there on we used the same construction method as with the rest of the project."

Current highway will be replaced with 2,000 trees

Increased viewing distance
Another important aspect of the project is driver safety within the tunnels, where speed limits will be, in fact, up to double that of the 50km/hr of the above-ground road they are replacing. "When a car drives 100km/hr, it needs a gentle bend," said van Aart. "There are three bends in the alignment with a radius of about 600m, which is pretty sharp for a 100km/hr speed limit. Drivers need to be able to look around such a bend so they can brake on time if something happens in front of them. The tunnel design needed to take into account both the bend radius and the viewing distance of the driver."

To meet these criteria, the JV designed the bends of the lower tunnels - where the speed limit will be 20km/h greater than the upper ones - to be wider than the straight part of the tunnel alignment. "Next to the two lanes we will place an extra lane with a maximum width of 3.5m," said van Aart. "This lane is not for driving but to extend the drivers' viewing distances. This way the tunnel gets a variable width. This is one of the most complex parts of the design because we are dealing with four different road axes."

The project is currently on schedule for a December 2016 finish. "At the moment we are busy constructing the tunnel entrances," said Grote. "We have already constructed the western part of the walls and the roof of the top-down tunnel segment. Soon we will divert the traffic on top of the roof to progress the works."

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