Where in Zuidas
In the last weekend of November 2023, we installed soil-retaining sheet piles a few metres under the metro tracks, to the east of Parnassusweg, in order to excavate the Brittenpassage. Installing that combination wall, consisting of steel sheet piles and tubular piles, involves vibrations. We are using a special monitoring system to check that the ground, and the metro track above the ground, doesn’t sink too much. Construction manager Jeroen Monnik: ‘Of course, rail and metro traffic needs to continue to run safely.’
Predictions and measurements
We already have extensive experience of working on and around this dike, on top of which both the railway lines and the A10 motorway run. ‘Using the expertise that we have in-house, we can make some predictions in advance’, explains Monnik. ‘These predictions concern the expected degree to which the ground may sink as a result of the work.’ We then take measurements during the work using an ingenious digital system of cameras, lasers and measuring points. ‘We install measuring points on the rails called “mirrors”. Our measuring instrument fires a pulse of infrared light at these points. This is reflected back at a certain angle, allowing us to calculate not only the distance but also the height of the measuring point. Then we compare that data with the baseline measurement that we made previously to find out exactly how much the ground has dropped and where.’
Loosely packed sand
The rails and sleepers sit directly on top of a foundation layer that absorbs the weight and forces exerted by passing rail traffic. That is a fairly solid layer of sand. ‘It shouldn’t be affected by the work that we’re doing underground’, explains geotechnician Jan Ruigrok. ‘Below that is subsoil of the type that you often find in this area, consisting of peat and clay. But at certain locations, the foundation consists of more loosely packed sand.’ This means that there are relatively large spaces between the grains of sand, as a result of which the sand can ‘settle’ when there is work that involves vibrations.
Low-vibration working method
This settling process is not necessarily a problem and happens often. But in this case there is a metro track on top and the settling process can cause the rails to move slightly, both in height and relative to each other. ‘This is why we try to find working methods that minimize the amount of vibration’, Ruigrok continues. ‘For example, by opting for a combination wall of tubular piles and sheet piles rather than only sheet piles. That means we have to install fewer sheet piles using vibration, and that reduces the risk of subsidence. And, almost as importantly, it also reduces noise pollution for those living and working nearby.’
Experience
In both February and October this year, we had to close a metro track temporarily, after consulting closely with the Amsterdam Municipal Transport Company (GVB). Monnik: ‘We try to make our predictions as accurate as possible, but all that heavy earthwork and the forces exerted by rail lines sometimes produce more subsidence than expected.’ Making these predicting is also a matter of learning from previous experiences, including earlier this year. Monnik: ‘In that case, we had to excavate the area below the metro tracks to find out what was happening. That kind of investigation allows us to make ever more accurate predictions.’
Smart planning
In addition to applying existing expertise and learning from experience, smart planning is also essential. ‘We worked at weekends, for example, and in close consultation with the Amsterdam regional transport authority and GVB’, says Monnik. ‘There is less metro traffic at weekends, which gives us more space and time to complete the work and to monitor it properly. And there is always some leeway in case we need to intervene unexpectedly.’
Millimetres
All this makes the current work very interesting, according to Monnik: ‘Building the Brittenpassage often involves all sorts of huge numbers – roof sections that weigh millions of kilograms, thousands of cubic metres of soil that have to be moved. But whether we can achieve all of that sometimes depends on ground subsidence of just a single millimetre more or less.’
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