Huisjongste

Ze zit eerst twee maanden in onderhuur in een ruime kamer, daarna kan ze haar intrek nemen in het HJ-hok. De HJ-taken krijgt ze deze week nog te horen, op de Huisavond. Ook wordt dan haar ‘spil’ aangewezen, een soort mentor in het huis. Navraag bij de huisoudste leert dat de taken vrij standaard zijn: vuilnis buitenzetten, telefoon opnemen, post sorteren en koffie zetten na het eten.

Over the centuries buildings in Amsterdam have been sinking steadily in the soft, swampy grounds underneath the city. Over the past few years however some buildings have started rising. According to Johan Bosch, a professor at the faculty of Civil Engineering and Geosciences (CEG), and until recently the technical manager of the North South metro line project in Amsterdam, buildings situated along the Rokin, the Dam square and the Munt Tower have been lifted up half a centimeter by compensation grouting.

This measure, which involves pumping huge quantities of a mixture of cement, bentonite and water at dozens of spots in the ground underneath a building, must compensate for settlements that occur during the drilling of the tunnels. The drilling will start in a couple of months, Bosch stated last week during the ‘Frontiers in Shallow Subsurface Technology’ conference, held at TU Delft.
Until now contractors have only partially dug out the metro stations, yet they’ve already had to deal with numerous setbacks, most importantly the subsiding of houses along the Vijzelgracht, which sank up to 25 centimeters due to leakage in the casing of the station.

So what might we expect from the most crucial part, the actual drilling of the tunnels? That will work out all right, Bosch says reassuringly (and not surprisingly). But PhD student and grout expert, Adam Bezuijen (CEG, and also employed by Deltares), warns that the lifting of the buildings prior to the tunneling, the so-called pregrouting stage, is the easiest part of the settlement compensation.

During the drilling, more grout may have to be added underneath the buildings. And, Bezuijen says, this compensation must be done simultaneously with the tunneling, in order to keep the settlements of the buildings within certain limits. “If the settlement caused by the tunneling cannot be compensated for fast enough, the drilling has to be slowed down or even stopped temporarily. They should be able to drill and compensate simultaneously”, Bezuijen concludes. “But I do wonder how easy it will be.”
Bezuijen focused his research on finding the physical mechanisms that play a role during compensation grouting and how these can be influenced by the grout mixtures for different soils. The problem with grouting is that it is not known in what geometrical form the mixtures end up in the ground; therefore, the reaction of the soil and the buildings is continuously monitored.
In his laboratory Bezuijen tried to figure out how the composition of the grout influences its shape in the ground. “In the laboratory we experimented by injecting grout in layers of sand kept under high pressure”, he explains. “We then washed away the sand to see the shape of the grout. Ideally you’d like the grout to split up the sand layer and form a pancake that has a diameter of a couple of meters and that lifts up the top sand layer. But the grout often ended up looking like a potato.”

By adding more water to the mixture, the grout tends to form into a shape more like a pancake. Add too much water however and the grout particles get mixed up between the grains of sand. That’s not good either. The ideal mixture depends on the size of the grains of sand, among other factors. All in all it’s no easy feat to obtain the best mixture.
Jan Bijkerk, who is now in charge of the construction of the metro line, isn’t worried that houses will subside. “We can’t give 100 percent guarantees, but we’re convinced the technique works. It has been used successfully around the world. Preventing houses from subsiding is our priority.”