How do you build a retaining wall for the tunnel?

‘In the places where we’re using an open construction pit, we’ll dig out the area for the tunnel up to a depth of 10 m below sea level’, begins Admiraal. ‘To do that, you need a strong earth-retaining structure: it’s important for the surrounding earth to remain stable.’ The most well-known way of doing this is to use vibration to insert sheet piles. Although this is quick and relatively cheap, Admiraal says that it cannot be applied everywhere. ‘It just causes too much noise. What’s more, the embankment where the A10 Zuid is located is made up of relatively loosely packed sand. If you use vibration to insert sheet piling here, there’s a risk that the A10 will subside. That already happened previously during works on the De Nieuwe Meer junction, and we want to avoid that.’
 

There is also the option of ‘pressing’ the sheet piles, where special machines are used to slowly press the sheet piles into the ground in a controlled way. This has the advantage of being vibration-free, keeping noise disruption to a minimum. ‘But there’s no guarantee that the sheet-pile walls will reach the required depth’, says Admiraal. ‘The resistance in the ground can become too much, especially with long sheets, making it necessary to use vibration after all. For this method, we need to investigate whether using shorter sheet-pile walls will be an option.’

So, using vibration to insert sheet piling is less expensive, but causes a lot of noise disruption. Creating diaphragm walls is vibration-free, but expensive. Is there something in-between? Yes, confirms Admiraal: the CSM wall. The what? CSM is short for Cutter Soil Mix – put simply, a technique where you soften the ground, making it possible to slide the sheet piles into it. ‘A cutter head creates trenches, mixing the existing soil with a type of cement mix. It’s a bit like adding milk and eggs to dry flour, to create a pancake batter. You then hang a sheet pile into it, and the mixture then hardens’, says Admiraal. ‘Because of the softness of the ground, the vibration and noise disruption is kept to a minimum. However, this method is significantly more expensive than using vibration alone.’

The CSM wall option would therefore seem to be the ideal solution. But, adds Admiraal, it also has its challenges. ‘In order to insert a sheet-pile wall – made up of steel sheet piles – into the ground, you first need to create enough CSM trenches next to each other. How do you ensure that the mixture – or pancake batter – stays liquid for long enough until the sheet-pile wall is in place? After hardening, the mixture also needs to have a certain consistency. It works like this: we start the excavation work and then, for safety reasons, we have to make sure that no loose sections of the wall become detached. VolkerWessels is carrying out extensive lab testing to achieve the ideal mixture. Various samples from Amsterdam soil are being mixed with other sand as well as clay and peat. ‘In the end, one or two ideal mixtures will be identified. The ground on which the A10 Zuid is located is very specific and you can’t excavate without first checking how it reacts to the mixtures. That’s why, as proof of the pudding, we’re doing a practical test early next year.’