Sunken and semi-sunken constructions
The HSL route has a number of (semi-) sunken constructions: for instance at the Galder junction and the junction Breda-Prinsenbeek in North Brabant, and at Bergschenhoek and Mookhoek in South Holland. The construction method for a sunken construction is in effect the cut & cover method. At the location where a tunnel or tunnel trough is planned, the earth is excavated and the tunnel is constructed. After this is done, the roof is installed, which is covered with earth if the tunnel is to be closed. In the event of an open sunken construction, the tunnel roof is simply left out. This building method has a relative great impact on the environment.
As the sunken constructions remain open, they need a facility to collect and convey rainwater: a reservoir tank. This tank is built at the lowest point of the construction. Rain water flowing into the sunken construction is led to this water reservoir tank; automatic pumps then pump the water into the sewer system. The reservoir tank is covered with concrete slabs, which support the track floor.
Pouring the concrete floor
First, sheetpile walls are driven down at both sides of the pit. A sheetpile wall consists of steel U-profiles, which a machine drills into the ground to a specified depth. Once the sheetpile walls were in place, the earth between the walls was excavated. Draining kept the construction pit free from rainwater and/or groundwater. Where necessary, piles of around 15 metres long were driven or bored into the soil. On top of these piles, the reinforcement was put in place, running as wide as the inside of the walls. The reinforcement was attached to the top ends of the piles. After this the concrete floor could be poured and leave to harden.
Pouring the walls
When the floor was ready, the walls of the sunken construction could be made. A wooden formwork was put together, in which horizontal pipes were fitted. As soon as the concrete was poured, cold water was led through the pipes to absorb the heat released during hardening of the concrete (This was not necessary in the concrete floor, because it was cooled sufficiently by the soil). Once the walls had hardened, the pipes were also filled with concrete.
Compacting, hardening and removing the formwork
It took three days for the concrete to harden. During this time, the concrete was compacted: needle vibrators vibrated extremely rapidly in the concrete to dissipate the air from the concrete and make the concrete flow into all nooks and crannies of the formwork. After this, the formwork could be removed.
Filling the gap and removing sheetpile walls
Once the concrete had hardened completely, the gap between the sheetpile wall and the concrete wall of the sunken construction was filled with earth up to surface level. After this the sheetpile walls could be removed.