Whenever a water body cuts through a roadway, engineers create bridges to make sure that the traffic on those roads is not obstructed. Imagine reversing these roles ,where a roadway runs across a busy waterway. In such a situation it is made sure that the waterway traffic as well as the roadway traffic is not hindered. For this task engineers come up with something called an Aqueduct. An aqueduct can be imagined as a water channel which helps in guiding the water flow ( specially at hindered paths).
It is a structure which is very frequently used around the world. But, in Netherlands the engineers and architects came up with a very interesting version of an Aqueduct, a water bridge. Let's try and get across this bridge in detail.
The Veluwemeer Aqueduct water bridge :-
This is a navigational aqueduct situated in the city of Harderwijk in eastern Netherlands.
It is a water bridge that links two sections of a river with a channel of dimension 25m length and 19m breadth. This bridge is located over the N302 highway which has a busy traffic of 28000 to 34000 vehicles per day . So basically these vehicles pass through a tunnel which has been constructed below this Aqueduct. The water bridge itself is a shallow one with a depth of 9.83 feet.
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| Veluwemeer Aqueduct Water bridge |
Architectural beauty :-
The architectural brilliance of this bridge allows it to present an optical illusion where the isometric view of the aqueduct is such that it seems as if the road vehicles enter below the bridge and disappear, somehow managing to teleport to the other end through the water. The truth is revealed when the elevation exposes the presence of an underwater tunnel through which these vehicles pass.
Engineering Challenges :-
This was a complex project considering the type of challenges it offered :-
WEIGHT OF THE WATER ABOVE THE TUNNEL,
The volume of water above the underwater tunnel is 1425 cubic meter. This applies a load of 14250 kN on the concrete sheet(25 into 19) that makes the base of the water channel.
This exerts a bending moment of about 90000 kN-m about the center of the concrete beam.
Adding on to this load, there are movable loads of ships that pass through the aqueduct as well. This much force meant that the probability of the water bridge collapsing would be very high.
This problem was solved by using 22000 cubic m of high strength concrete to create the floor of the bridge. The base was layered with steel sheets(modulus of elasticity = 200000N/mm2) to improve the Young modulus of the base. Improving the strength of the floor was crucial with respect to the fact that unlike a normal bridge there was no room for giving columns to support the compressive loads and bending moments in this aqueduct.
DESIGNING THE ROAD UNDER THE BRIDGE,
The N302 highway links the mainland Netherlands to Flevoland. Flevoland happens to be the largest artificial island in the world. Since the two pieces of land were separated by a busy water way, engineers decided to go for a combination of an aqueduct and an underwater level highway instead of a conventional bridge. But this meant, designing and constructing a stretch of roadway which would be 610 meters long and would have a small symmetrical dip of 0.01 radian. The dip would allow the road to go below the water level so that a depth of 3 meters is obtained in the middle of the stretch for the water bridge.
So, to protrude the artificial island for roadway, a piling perimeter was formed that approaches the other end of piece of land. This was done by inserting hollow steel rods of 22m diameter into impervious sea bed which was later filled with sand and silt.
Next up this perimeter was pumped off dry and containment walls were constructed using RCC concrete with water admixtures. The cement used was a quick setting cement with a small percentage of Aluminum sulphate( Al2SO4).
Now, this empty region was filled with soil(low moisture content)and gravel to create a surface upto the level of Flevoland island. This surface was used as a starting point for the road that would go across the lake with a slope of 0.01 radian. The side containment walls under the aqueduct were designed for the shear loads that would exist due to the water currents.
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| Steel pipe piling |
FUNDING THE AQUEDUCT: -
This masterpiece took 4 years of construction(1998 to 2002). The total cost was a hefty sum of $61 million. Still it was a smart choice considering that construction of a bridge or a tunnel would have cost more than $300 million dollars. This project was funded by the government of Netherlands and the purpose was to link their artificial island(Flevoland) with the mainland. This would be crucial for transportation of various resources in and out of the island. It was also an innovative way of flood control since the pilers and containment walls within the sea bed would cut the water currents to some extent.
An aqueduct would always be a better alternative than a dam with respect to flood control considering the problems associated with dam construction and problems faced by the civilization living around it.
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