Cologne - The KVB stonewall when asked for the cause of the accident.
The participating engineers who work hand in hand with the Baugeologen, meanwhile, assume that a "hydraulic base failure" is the cause of the collapse of disaster.
On Friday, details were known to have been aware that in one of the three wells in the 28-meter deep pit KVB the ground water for weeks as a problem. For these wells, the groundwater is pumped strongly to keep dry the building site.
The well, which lies closest to the cave-in, that was not initially successful, until finally a much stronger pump was running. Due to the huge pumping gravel and soil material could have been pumped out, and thus have originated with voids, was plunged into the ground under the buildings until its collapse.
The second theory: The ground water was coming from outside the pit against the towering up to 37 meters deep, pressed a foot-thick concrete diaphragm wall. "It could have been undermined," says Matthias Scholz engineer from Munich.
Episode: water shoots from below uncontrolled into the pit and attracts large amounts of gravel and soil from the outside with - the hydraulic base failure. "Construction workers have reported excavation in the area of the diaphragm wall," said project manager Rolf fits KVB Pope on Thursday.
The immense water pressure but also may cause that there had bent the concrete slurry wall - due to faulty construction - or was even broken. And although below the pit bottom, ie from 28 to 37 feet below the surface. Same result: Groundwater shoot from the outside with tremendous pressure in the excavation and carries everything with - like a champagne bottle, which is shaken until the cork flying.
"With a hydraulic base failure there is no stopping it," said Andreas Roeder, whose engineering firm, worked for the KVB. At all points the KVB struggling with the groundwater pressure - the North-South underground station is located below the water table.
Before every Subway construction must generally be made test borings to see where the water lies. "But we can make a hole and find nothing, and five yards is a Wasserkaverne" said Andreas Roeder.
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