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Diaphragm walls probed after Cologne collapse

Investigations into the fatal collapse of the six storey city archive building in Cologne are focusing on neighbouring underground excavations for a new light railway.

It has emerged that diaphragm walls supporting the investigation had been in place for two years before excavation of the ground between them began late last year. The base slab for the structure had not been cast at the time of the collapse.

The archive building collapsed on 3 March along with residential properties on either side (News last week). One person was killed and another was still missing as NCE went to press on Tuesday.

Cologne police said the collapse was caused by the ground under the building slipping into the underground excavation. “Earth under the archive building slid into the tunnel of the subway, causing the building to collapse,” said a city of Cologne spokesman.

“The whole building has collapsed, everything has gone including the two buildings to the left and right of it.” The north to south light rail line – known as Nord-Sud Stadtbahn Köln – run at 28m depths (to the base of the bored tunnels).

These give way to a cut and cover crossover section in front of the archive building. The crossover section is built between two 37m deep diaphragm walls using a bottom up technique.

The diaphragm walls were constructed first before the ground between them was excavated. Ground anchors were inserted to support the diaphragm walls.

The reinforced concrete base slab was then to be cast immediately below the base of the tunnels. The diaphragm walls for the underground structure had been constructed in 2005, but excavation work between them did not start until the end of 2008.

“In some areas, they were still excavating [at the time of the collapse] and in some areas they had reached bottom and were laying reinforcement,” said a city transport authority Kölner Verkehrs-Betriebe (KBV) spokesman.

The 1m thick diaphragm walls for the cut and cover crossover structure were supported by ground anchors in predominantly river gravels.

The groundwater level in Cologne is relatively high at about 12m below the surface and the site of the collapse was around 300m from the River Rhine.

Work on this section of the project was being carried out by a joint venture comprising Bilfinger Berger, Weiss & Freytag and Züblin.

It refused to comment. Cracks had been noticed in the archive building since 2007 but the structure had been declared safe by structural engineers.

“[The cracks] were supervised by specialists who said that there was no problem for stability. There was a second examination in 2008 by another specialist who also said there was no danger,” said the city spokesman.

The archive building collapsed almost without warning. Archive staff working inside were unaware of problems until they heard noises in the structure, about five minutes before it collapsed.

KBV is sending engineers to assess stability of other surrounding buildings, and is reviewing its procedures for the remainder of the construction work on the project which is currently about 90% complete.

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