Where in Zuidas
At the end of August, the Nieuw-Zuid Construction Consortium (Mobilis, Boskalis, Van Gelder) got off to a rapid start in removing a total of 50,000 cubic metres of sand from underneath the A10, metro and railway tracks. The construction workers have already made significant progress – but they are doing much more than just digging underground. In order to enable excavation of the new Brittenpassage, large numbers of sheet piles and tubular piles have been inserted into the ground at both ends of the passageway. This is necessary in order to prevent the sand collapsing inwards during digging. Installing them was far from easy: the tubular piles have to be inserted around 25 m into the ground and the sheet piles around 6 m, yet the passage itself is currently just around 3 m in height. Despite this, it proved successful. ‘During the last weekend of November, we used vibration to successfully insert the final sheet piles’, says Alexander Bor, planning engineer at Nieuw-Zuid Construction Consortium (BCNZ) proudly. ‘That means we’re now entering a new phase in the digging process. By the start of 2024, the southern side of the Brittenpassage will have reached the right depth.’
Preventing settlement
At both sides of the passageway, a so-called combination wall has been installed from the south side to metro track M1 (see map). This is a combined system of tubular steel piles and sheet piles that can withstand a heavier load than a traditional sheet pile wall structure. ‘We had to install both of them into the ground in sections of 1.8 m. The next section was then welded on top and it continued like that until everything was at the right depth.’ This was a time-consuming job, but still not the end, explains Bor. ‘The combination wall cannot be allowed to collapse forward. This is because there is a layer of soil behind it and behind that there are abutments that support the roof sections of the Brittenpassage. This soil must not be allowed to settle. In order to prevent it from settling, we need to thoroughly reinforce the combination wall, which is what we’re currently working on.’
Drilling in tieback anchors
The combination walls are reinforced by means of so-called tieback anchors. These are lateral foundation piles that are drilled into the tubular pile before a mixture of water and cement (grout) is injected into the ground around them. When this grout has hardened, the anchors can be tensioned. Bor: ‘We’re installing two rows of these anchors across the length of the combination walls. One row will be at ground level (where the floor of the passageway will be) and one row will be 2 m above that.’ The row at a height of 2 m has already been installed and tensioned. This means that the walls are sufficiently robust to allow the row of anchors at ground level to be drilled into position. ‘We aim to have them all in position before Christmas. They will then be able to harden during the Christmas holiday, enabling us to dig the final section of the south side after that.’
Tieback anchors
In the week of 11 December 2023, we will be working for four evenings and nights drilling the row of tieback anchors into the tubular piles at ground level. Any noise disruption will be limited.
Several metres more
How much more digging will be needed after the final row of tieback anchors has been tensioned? ‘Around 1.5 m below ground level’, says Bor. ‘When we reach that far, we can start drilling the foundation piles for the passageway.’ In some places, the construction workers will go deeper: on the Eduard van Beinumstraat side, for example, there will be an underground car park and that will involve digging around 3 m below ground level. More sand will also be removed in other parts of the passageway, for example to enable the construction of large foundation blocks that will support the columns of the passageway.
Ground hardening
Underneath railway platform 1 and track T2-3 (see map), there is an area of several metres when no sheet piles or tieback anchors have been inserted into the ground. This is because behind this there are remnants of what used to be the so-called Amstelveenboog, the tunnel used by metro and tram line 51 until early 2019. If you were to drill tieback anchors into the ground here, they would hit parts of the old Amstelveenboog. In order to be able to excavate here after all, a clever idea was devised: ground hardening. We inject a hardening gel into the ground that binds the sand grains to each other. Bor: ‘You can’t see anything with the naked eye, but when you dig into it, you see the sand stays upright.’
Can’t just dig down recklessly
Excavation work on the northern side is not progressing so fast. ‘We can’t just dig down recklessly there because the most northerly metro track is still supported by a coffer dam structure’, says Bor. The coffer dam is made up of sheet piles on both sides of the track that are connected to each other by means of large anchors underneath the track. This ensures the track remains in place during the excavation process. Bor: ‘The coffer dam structure itself owes its stability to a supporting verge, so it will need to remain in place for now. We will remove the coffer dam again in the summer of 2024, after which we will be able to dig here as well.’ As soon as the first sheet pile has been removed from the coffer dam, something quite special will happen. ‘Direct access from north to south will be possible for the first time. I definitely want to be there on that date. I’d love to be the first person to walk through the new Brittenpassage.’
Give your opinion