The ‘De Groene Waarden’ consortium, comprising BESIX Nederland, Mobilis TBI, Van Gelder Groep BV, and Mourik, is currently preparing to widen the Houten bridge on the A27. This expansion is a key part of the infrastructure contract awarded by Rijkswaterstaat, the Dutch Ministry for Infrastructure and Water Management, in April 2023.
As part of the upgrade of the busy A27 highway, this project includes the widening of a 7-km section south of Utrecht, notably expanding the Houten box girder bridge with an additional lane, which will be placed against the bridge on the west side. To ensure precision and safety, the consortium is conducting extensive tests on this bridge, originally built in 1980.
A set of four tests for a representative model of the bridge
To precisely determine the expansion approach for the Houten bridge, the consortium is conducting four tests. The data gathered from these tests will inform the experts whether adjustments are needed to the assumptions made in the current bridge's construction model. This testing process is an invaluable tool in designing the bridge's widening.
Load tests with 14 trucks
On the night of 9-10 December, the bridge underwent a significant load test. “14 trucks, totalling 420 tonnes, were positioned on the bridge by my team in various set-ups and weight configurations to assess its structural response”, Don van Hees, Project Manager BESIX Nederland, explains. By measuring deflections, and deformations on top of the bridge, engineers on the project will be able to determine the structure's stiffness and load-carrying capacity.
Previous calculations had already indicated that the bridge was strong enough for an expansion, with a total load-bearing capacity of up to 960 tonnes. The load test serves to examine this in practice, with a comfortable safety margin. To carry out this test accurately, the bridge had to be closed to other traffic.
The consortium had previously placed measuring points under the bridge as a monitoring system to measure the deformations and deflections of the bridge for one year. This allows us to observe and analyse the behaviour of the bridge under seasonal effects (temperature) and long-term effects (creep). With this monitoring system, the measurements on top of the bridge during the load test could also be verified.
Pre-tensioning tests
Prior to the load test, two specialists went into the two western box girders of the bridge to measure the current tension in the bridge's post-tensioning cables.
One of the specialists, Didier Droogné, Design Manager at BESIX Nederland, explains: “This type of inspection is an excellent opportunity to see how well the existing bridge, built in 1980, resisted the effect of time. One thing we did was to measure how high the tension still was in the post-tensioning cables, after more than 40 years. During the construction of the bridge, post-tensioning cables were installed in casing tubes in the walls, floor, and roof slab. These post-tensioning cables were tightened and the casing tubes were injected with grout, a mixture of water and cement. We have used scanning technology to determine the exact position of the cables and exposed them by hydro demolition, so as not to damage them. Then we measured the force in the cables with special equipment. After testing the tension in the cables, the casing tube has been re-cast on-site.”
State and lifetime of the bridge
By measuring several strands of cables, it is possible to determine relatively accurately what the tension in the cable bundle is. The Houten bridge was built to last about 100 years. Therefore, these measurements are not expected to show any big surprises. “These measurements are a verification of what we already think, based also on recent visual inspections. Coupled with all the other tests we carry out, they enable us to have a complete picture of the state of the bridge, which is that it can last for another 30 years at least. And of course, all this helps us set up our design plan for the expansion”, adds Didier.
Safety in a confined space
A stair tower was installed to access the tubes via crawl hatches. Didier reassures: “My colleague and I were well prepared. When you work in a confined space, you need to be trained properly. For example, we never enter a box girder like that without an oxygen metre.”
Next steps: Concrete and dynamic load tests
From 18 December on, there will be additional concrete tests. Cores of concrete will be drilled out and examined in a laboratory. Finally, mid-January, there will be a dynamic load test, during which a special truck with a vibrating load will be used to map the 'stiffness' of the bridge.
With these thorough preparations, De Groene Waarden and Rijkswaterstaat are set to gain valuable insights into the bridge's behaviour, paving the way for a successful and well-informed expansion project thanks to the best possible design plan.
