Immersed tube tunnelling is a firmly established engineering method, yet a tunnel now under construction in the heart of Europort is giving Dutch engineers plenty of fresh challenges.
With 27 immersed tube tunnels completed in the Netherlands, constructing a 1,500m long, six lane, twin-bore road tunnel 20m beneath the surface of the heavily trafficked Caland Canal is far from a fearsome task for experienced Dutch engineers.
However, the route of this tunnel cuts through some major obstacles. Located on the canal is a giant windbreak to prevent ship drift and the site is crisscrossed by large diameter pipes feeding the port’s huge chemical complexes and refineries, with often highly explosive chemicals. Tight site boundaries and very soft ground conditions added to the problems.
It all demanded an innovative design solution, the result of which is a considerably widened canal, excavated 225m into each bank with the help of deep retaining walls, permanently supported by 1m diameter steel props. This will allow the immersed tube sections to be extended far into the canal banks, dramatically cutting costs by reducing use of cut and cover, a more expensive option in these conditions.
But why build a tunnel in such complex surroundings? Because the Caland Canal crossing is a major bottleneck on the A15 motorway, the main road access to Europort, one of the world’s largest port complexes. And this road will face even heavier traffic in 2010 when Europort is expanded still further. More than 1,000ha of land will be reclaimed from the sea and added to the huge port with the construction of Maasvlakte II harbour.
The A15 crosses the canal on a vertical lift bridge. This must open for long periods up to eight times a day to let seagoing vessels pass through.
Primarily for economic reasons, an immersed tube tunnel was always the favourite solution for client Bouwdienst Rijkswaterstaat (RWS), the civil engineering division of the Dutch Ministry of Transport, Public Works & Water Management. At 250m wide and 14m deep the ship canal is narrow and deep. A bored tunnel would have to be 2.5km long to meet requirements for cover and road alignment. A bridge was out of the question, given the 50m headroom demanded by Europort authorities to ensure free passage for shipping.
However, the immersed tube brought other problems. The minimum draft of 12m in the canal demanded by the port authorities could not be reduced.
Since the tunnel box is 8.5m high and the maximum water depth is 16m, the tunnel boxes had to be laid in an excavated trench.
This means that the tunnel road base level is 24m below ground. A maximum road gradient of 4.5% means the total enclosed length of tunnel is 1,100m. On the eastern bank it meant businesses had to be relocated.
And there is another problem: founding the cut and cover section. ‘The whole area is a delta from the Rhine and Maas, ’ explains RWS assistant project manager Albert Gerrits. ‘So it is all silt, sand and clay. The piles have to be very deep’. Deep means expensive, with every 3540m pile costing close to 1,000.
Minimising the length of the cut and cover section was therefore vital. Consultant Tunnel Engineering (TEC), carried out a thorough cost optimisation of where immersion should end and in situ cut and cover begin.
The verdict was exactly 225m into the canal bank.
Immersing tubes into solid ground, no matter how soft and unsound, is clearly impossible, so TEC’s design called for for a new channel to be excavated into the canal banks, cutting across the main canal.
Yet even with cut and cover kept to the minimum, the cost of piling is immense. Over 2,600, 28m deep, pre-tensioned concrete piles were needed, equating to almost 2M in the cost of materials alone.
‘But the immersed solution is still cheaper, ’ maintains Gerrits.
‘We may have to make an artificial channel, but the elements themselves are quite cheap and you need no piled foundations.’
However, it is still not straightforward. Site constraints left insufficient room to form the new channels with sloped banks. Instead, permanent steel sheet pile walls have been driven, supported by permanent 1m diameter props at 5m centres.
These will create their own problems when the tunnel units are floated in, as the locating mast and access shafts can only be attached to the units once they have passed beneath the props.
And the difficulties do not end there. What may be the world’s largest windbreak is right in line with the new channel. The break was constructed in 1985 to prevent the gales which blow freely across Holland’s flat landscape from driving heavy shipping off course into the bridge piers. It consists of a series of 4m diameter and 20m high half cylindrical shells. Ten of these had to be removed while the channel was excavated and then replaced, seated on a new 35m long 6m tall concrete box girder bridging the new channel and immersed tubes.
Further work was also needed to divert several pipelines, up to 1m in diameter and carrying highly flammable chemicals including benzene and diesel.
These had to go into a new pipe duct and absolutely minimal disruption was allowed.
‘These firms cannot be disrupted at any stage, ’ explains Gerrits. ‘They are all heavily dependent on logistics. Cut them off for just one day and the claims are incredible.’
Work is now well advanced.
Excavation of the new channel into the river banks is complete, and delivery of the tunnel elements is now awaited.
Four out of six of the 114m long tube elements are floating in a secure dock near to the tunnel site, having completed a 20km float from Scheepswerf Verolme, a dry dock accessed from the River Maas and commandeered by RWS for the 18 month construction period.
Each tube element consists of five segments, 22.8m long and weighing 30,000t each. The segments are cast in three continuous pours: one of 1,000m 3to1200m 3for the floor, one for the inner walls, and another 1200m 3to 1400m 3for the outer walls and roof. At a rate of 80m 3/hr, each pour takes up to 16 hours.
The final two units are under construction and should be finished in mid October. They will then make the 12 hour voyage around the Europort complex, ready to be positioned and sunk in early December.
Before this the river bed will have been excavated and concrete plates laid in the trench.
The six sections of the tunnel will be lowered into position by built in jack props on the plates. The trench will then be back filled with sand to a maximum of 1m above the top of the tunnel.
All six units will be in place by the end of January, with a total of 16,000m 3of concrete ballast to ensure they stay grounded. Road laying and fit out can then begin, with the tunnel due to open in September 2003.
Solutions to technical challenges are not the only innovations on display at the Caland tunnel. A courageous approach to project management is also adding to the tension on the 130M project.
At first glance, the project’s organisational chart is no different to any traditional bill of quantities contract. Client RWS runs the project from the top, with technical support provided by consultant TEC and with the role of the contractor - a joint venture between Ballast Nedam, Strukton and Van Oord ACZ - largely limited to construction itself.
But beneath the surface there is a real culture shift, to one of trust and confidence throughout the project team, permitting the contractor to carry out his work largely unchecked. It translates into a vastly streamlined project process.
‘We have cut back on-site testing, and are focussed on reviewing the contractor’s process, ’ explains RWS assistant project manager Albert Gerrits.
‘The contractor has a certified quality assurance system for process control. We have a quality assurance system on top for process review.
‘We don’t even check quantities, ’ expands Gerrits.
Instead, client and contractor are bound into ‘predefined quantities’, specified at contract award. ‘If there is no basis for changing the predefined quantities then the contractor gets paid the same regardless of actual quantities used, ’ Gerrits explains.
‘It’s a culture difference, both for the contractor and our own organisation, ’ he says.
‘Traditionally the client is heavily involved in construction on a day to day basis, which makes it difficult (for him) to take a step back. It needs courage and trust.’
With the project on time and on budget, the approach seems to be working.