It is a recurring phenomenon in Zuidas: the installation of sheet piling. Contractors do it on work sites that they excavate several metres below street level in order to create an earth- and water-retaining structure. The result is a construction pit in which they can do the work: building foundations for a block of housing or offices, for example, or positioning a support pillar for a new viaduct. In the month of August and September 2023 alone, we installed some 200 steel sheet piles into the ground at Amsterdam Zuid station (see the illustration below). And next to Beatrixpark, we installed a sheet pile wall that actually requires less steel.
Steel or combination wall?
A wall of sheet piling prevents the excavated work site – which is actually not much more than a big pit – from collapsing in on itself. Groundwater must not be allowed to seep into the pit either, because otherwise the site becomes a pool of mud. The type of sheet piles most frequently used in Zuidas are type AZ 36-700 – during work on the Zuidasdok project, as many as 180 of them were used in just two weekends in September. These steel sheet piles are 23 m long in the shape of a Z (see the illustration below) that enables them to interlock. These steel piles offer a high level of strength for a relatively low weight, making them particularly good value for money: less steel means a lower weight and therefore a lower price.
At other times, a different type of wall is used, as was recently the case on works for the new Brittenpassage. Under the railway tracks there, Zuidasdok installed a combination of steel sheet piles and tubular piles, along which we will soon be digging the new passageway: a combi-pile wall. This was yet another very challenging process with sheets 6 m in length and tubular piles 23 m long: the working space was just 3 m high. A combination wall has also sometimes been chosen because, in some places, diagonally-positioned foundation piles – known as tieback anchors – were already present in the ground. That meant we had to use shorter sheets, slightly reducing the strength of the sheet pile wall. To compensate for that, we placed tubular piles between the shorter piles: these are stronger than steel sheet piles and can offset the loss in strength.
Pushing or using vibration
In accordance with the Buildings Decree (Bouwbesluit, 2012), construction projects in Zuidas, including those that are part of the Zuidasdok project, must make use of the best available low-noise technique. In Zuidas, we generally use two techniques for installing sheet pile walls: pushing or vibration. Pushing is the technique that makes the least noise, and we are using this in the construction of a support pillar for the new metro viaduct above Parnassusweg and for the A10 tunnel works in Matthijs Vermeulenpad. To do this, we start by using vibration to drive several sheet piles into the ground and then attach a silent piler to them. This ensures that the machine remains in position, enabling us to push the remaining sheet piles into the ground (You can see how we did that in Matthijs Vermeulenpad here). The silent piler then repeatedly shifts a sheet pile already in place in order to continue with the next one.
This silent method is not always possible. Sometimes this is because we have only limited time. For example, this was the case during the sheet pile works in two weekends in September of this year. In order to be able to hoist the sheet piles safely into position, we had to close the A10 Zuid in the direction of Schiphol and take one metro track out of service. Because we need to act quickly, we use vibration to insert the sheet piles. This is much faster than pushing. Sometimes the ground can also be too hard for pushing. If possible, we pre-drill the ground to make it easier to vibrate the sheets into position (also reducing the noise). Alternatively, we spray water at high pressure underneath the tip of the sheet to soften the ground, making it easier to install the sheets: a technique known as water-jetting.
Cutter soil mixing
But there are even quieter methods. For the construction of housing on plot 6/7, located alongside the A10 and next to AkzoNobel, we are using around 200 m of sheet piles to create a construction pit. In it, we will later be building an underground car park with two residential towers on top of it. However, just where the construction pit needed to be, there were still some underground pipelines from the thermal energy storage system (WKO) of AkzoNobel and Stibbe. We have now attached these to the new 19 m section of sheet pile wall on the A10 side, but outside the pit itself. Although the rest of the sheet piling is on hire until the parking area has been built (when they will be removed again), the section to which the WKO pipelines are attached will ultimately form part of the car park wall. This means that this section of sheet piling will remain permanently in the ground. This is why developer Breevast opted not to choose an expensive steel pile wall, but to use a cheaper technique that is also quieter: cutter soil mixing (CSM).
For this technique, rather than removing the soil in order to install a sheet pile wall, the ground itself is used to create the wall. First of all, a hollow tube is inserted into the ground with two inward-rotating drill heads inside it that inject water and cement into the ground simultaneously. While still wet, this concrete groove (or column) is then reinforced using two vertically-positioned steel profiles – considerably less expensive than a full steel pile wall. This process can be quite messy because of everything unearthed during drilling, but creates much less noise and vibration. A second column is then created using the same method. After some time when the concrete has hardened, we add a third column between (and partly inside) the two existing ones. The result is an effective three-columned wall that works well in keeping out water. And the process then starts again from the beginning.