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Cologne latest: site had flooded in September

Contractors and checking engineers had recorded excessive groundwater ingress into an excavation next to Cologne’s collapsed City Archive building last September, the German city’s council claimed this week.

Daily logbook records show that the contractor and client were aware of problems with controlling groundwater from Septemberlast year onwards.

Contracting joint venture Bilfinger Berger/ Weiss & Freytag/Züblin and Cologne Transit Authority (KVB) were responsible for monitoring work on site. The joint venture was building a cut and cover crossover tunnel in front of the former archive building to allow light rail trains to switch tracks on the new Nord-Sud Stadtbahn Köln.

The 28m deep top down underground excavation was supported by 1m thick diaphragm walls restrained by ground anchors driven into river gravels.

Ground water had to be removed from the excavation to facilitate construction work.

KVB confirmed the existence of the contractors’ records which mentioned the excessive groundwater ingress. They show that on 8 September 2008, the contractor noted that work was impeded as a result of water ingress in the area of an extraction well within the excavation.

One day later, KVB was informed of a “small hydraulic groundbreak.”

Cologne city council was unaware of this until 12 March, nine days after the collapse. Then, Cologne city building department representative Bernd Streitberger informed accident investigators and the KVB.

“[Streitberger] told the KVB and told the legal department,” said a Cologne City council spokesman. “Therecords had been written by the contractor and the KVB. He said that the KVB hadn’t known about it when he told them. He had to tell the KVB about its own records.”

The records show that 15 extraction wells had been dug in September when the contractor only had permission to build four.

KVB said that more wells had been needed than originally anticipated because the ground turned out to be more water permeable than expected.

By mid to late October, all pumps were working and on 26 January this year, the contractor announced that the ground water levels in the excavation had been was successfully lowered.

The archive building collapsed on 3 March. Two people died and Cologne police said the collapse was caused by the ground under the building slipping into an excavation in front of the building (NCE 5 March).

A Cologne Transit Authority (KVB) spokesman said work on the excavation was at full depth or nearing full depth when the building collapsed.

The water table is relatively high in Cologne at just 12m below the surface and the site is just 300m from the river Rhine.

The contracting joint venture was unavailable for comment as NCE went to press.

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