Mysterious collapse of Cologne's archive building

The underground excavations for the new section of Cologne's light railway have come under the spotlight following the collapse of the city's archive building, which killed two people earlier this month.

It is thought the building had not been underpinned or compensation grouted, despite its proximity to work being carried out for the 3,9 km north-south light railway tunnel.

Local news reports about what caused the collapse vary but there are concerns over the diaphragm walls for the underground structure, which had been in place for two years before excavation of the ground between them began late last year.

It is thought the contactor for this section of the project - a joint venture between Bilfinger Berger, Weiss & Freytag and Züblin- was working on the base slab between the diaphragm walls at the time of the collapse. The JV has so far declined to comment.

The diaphragm walls were supported by ground anchors in predominantly river gravels. Local news reports state that failure of the anchors could have led to the collapse of one of the diaphragm walls supporting the area in front of the archive building.

Residential properties also collapsed on either side of the 1971-built six-storey archive structure.

According to reports staff first noticed cracks in the archive's cellar early last year but the building was deemed safe.

Cologne police stated the collapse was caused by the ground under the building slipping into an underground excavation. However, authorities are still investigating the cause of the collapse.

Modernisation of Cologne's tram network has been ongoing, having first begun in 1956. City tram routes and interurban routes were amalgamated in 1968 and the 15-route tram system upgraded. Most subways in the city centre were constructed up until 1974 and the tunnel networks gradually expanded in the following decades.

The north-south tunnel, which will run between Breslauer Platz/Hauptbahnhof to Bonntor via Heumarkt, Severinstraße and Chlodwigplatz, is set for completion in 2011.

Some of Germany's most valuable documents may have been destroyed in the collapse of the archive building including manuscripts and essays written by Karl Marx, letters written by philosopher Hegel and lyrics and notes written by composer Jacques Offenbach. The earliest document stored in the building dated back to AD.922.