The excavation pits were then drained in stages, and the construction of the subterranean structures could begin.

The first rough sketches for the new Berlin Hauptbahnhof – Lehrter Bahnhof were prepared at the beginning of the 1990s. On 12 September 1995, the statutory project approval procedure was finalised for the North-South Link passing through the new Berlin Hauptbahnhof – Lehrter Bahnhof, which meant that the preparation of the site could now begin. By 1998, the first excavation pits had been constructed and, with the laying of the foundation stone on 9 September 1998, the actual construction of the station began. The close proximity to the River Spree, the Tiergarten park and the adjacent buildings meant that only cautious, fully monitored changes to the groundwater regime were permitted. As was the case with the construction of the subterranean facilities of the new north-south long-distance link, for the construction of the excavation pits for the new Berlin Hauptbahnhof – Lehrter Bahnhof, a decision was made in favour of a method that eliminated the need for lowering the groundwater table.

1.5 million cubic metres of soil excavated

One after another, a total of nine excavation pits were constructed over an area of 90,000 square metres, which is equivalent in size to eleven football fields. The work involved excavating 1.5 million cubic metres of soil, which was subsequently removed by barge from the Humboldthafen. If this quantity had been transported by road, there would have been a convoy of dump trucks some 1,300 kilometres long, stretching from the Brandenburg Gate to the French Atlantic coast. Out of environmental considerations, the excavation sites were constructed with diaphragm walls and an underwater concrete base slab.

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Uplift pressure of the groundwater: 20 tons per square metre

Using this method, a cut-off wall is erected around the excavation pit, to produce a completely watertight area in which the underground structural elements can be erected. The graphics show the steps necessary to make such an excavation pit. Initially, the some 25-metre deep and 1.5-metre thick cut-off wall was erected in the form of a diaphram wall and secured with inclined anchors. Soil was then excavated from the pit, which filled with groundwater. When the bottom of the pit was reached at a depth of some 20 metres, the 27-metre long uplift anchors were driven into the subsoil and the 1.5-metre thick concrete base slab was cast under water. The anchors secure the base slab against the uplift pressure of the groundwater of 20 tons per square metre. As soon as the excavation pit was watertight, the groundwater was pumped out. The actual construction of the structures then began in the dry excavation pit.