The train viaduct

You are standing in front of the two remaining supporting columns of the old train viaduct. Years ago, the elevated train line was supported by about 200 of these columns. A bit further north along the Spoorsingel, the train viaduct was 3 meters high and the space below was a parking area for about 500 cars. The viaduct was demolished in 2015, once the train tunnel was completed and trains were running underground. The tunnel affects the local groundwater flow, and the City has installed drainage to control groundwater levels.

The train tunnel and groundwater

Elsewhere you can read what groundwater is, and how the groundwater level is determined by rainfall, by the water level in the Delft canals (the bosom level), by the clay and peat in the upper 20 meters of the ground, and by the groundwater extraction by DSM. Here you can also read how the groundwater that is extracted from the sand layer at a depth of 20 to 40 meters is replenished by water from the North Sea.

Between 1965 and 2015, trains passed through Delft on an elevated railway viaduct. At that time, there was no canal along the Spoorsingel and Phoenixstraat. Groundwater therefore had to flow to the Oude Delft canal, a significant distance. The groundwater level in the Hof van Delft district could therefore become high.

The railway viaduct in Delft and the groundwater level between the Oude Delft and the Hof van Delft (drawing not to scale).

The train tunnel, called the Willem of Orange tunnel, is supported by two concrete piles that were made over the full depth of the Holocene clay / peat layer, down to the Pleistocene sand layer at a depth of 20 m. Here you can read how the tunnel was made. These piles are made of solid concrete, and are therefore impermeable to water. The canal that is now above the underground parking garage (next to the train tunnel) is connected to the other canals in Delft, but has no connection to the adjacent groundwater. The groundwater flow from the west side of the train tunnel (Hof van Delft and Westerkwartier) is hindered from draining in an easterly direction because the of the impermeable train tunnel walls. The groundwater level can therefore be higher or lower than before the construction of the tunnel, especially in the Hof van Delft district. Only around the Buitenwatersloot the situation has not changed.

That is why the City of Delft has installed various drainage pipes in those districts. How these drainage pipes affect the groundwater level can be read here.

The train tunnel in Delft and the groundwater level around the tunnel (drawing not to scale).