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Ground anchors could hold key to Cologne metro collapse

Ground anchor failure could have triggered progressive collapse of diaphragm wall which swallowed up the Cologne city archive building say geotechnical engineers

Weak ground and a high water table could have contributed to the archive building collapse in Cologne last week.

Engineers believe it is possible that ground anchor failure could have led to the collapse of one of the diaphragm walls supporting the area in front of the archive building.

The 37m deep diaphragm wall supporting the excavation in the collapse area had been bolstered by ground anchors. Contractors were working on the base slab between the diaphragm walls at the time of the collapse.

One British geotechnical engineer said that if an anchor was to hit a pocket of weaker than expected gravels, it could have led to a systematic failure of the wall.

“I expect that they [investigators] would be looking at the homogeneity of the soil, the nature of the anchors and the support system and the sequence with which it was installed,” said Card Geotechnics director Nick Langdon.

“It might be difficult to assess the true strength of the gravels. If there’s a variation, when one anchor goes, a higher load would be put on another, leading to progressive failure.”

The fluvial ground conditions in Cologne are highly variable, making it difficult to assess accurately the strength of the soil. The high water table would have complicated matters further.

University of Cologne Institute for Geology and Mineralogy professor Reiner Kleinschrodt
said the collapsed building was on the innermost fluvial terrace
of the River Rhine. This comprises river gravels.

“The complete newly constructed subway is situated within these river sediments or even within anthropogenic
constructions of the Roman and Medieval periods,” said Kleinschrodt. “The sediments are rich in groundwater, which was a problem, and tremendous amounts of water had to be pumped out of the construction area.”

River sediments typically create highly variable ground conditions.
“Such ground is highly variable and contains pockets of coarse and fine material, old river courses and such like,” said HPR director Scott Steedman.

“It’s likely that they [construction workers] hit some void or local pocket of very loose material and this provoked a collapse, possibly a sinkhole type of feature outside the excavation.”

The variability of the soil would have meant that the construction works required close monitoring

“In those kinds of ground conditions with that kind of water, nothing is standard,” said Imperial College London emeritus professor and senior research investigator Professor John Burland.

“Fluvial material may have layers of compressible material and is likely to be highly variable. It would be a very challenging set of conditions in which to excavate. What might work brilliantly in
one area might not in another.”

The City of Cologne legal department is currently investigating what initiated last week’s collapse.
Cologne mayor Fritz Schamma confirmed last week that as well as looking at the construction work it would examine local geology.
Cologne transit authority Kölner Verkehrs-Betriebe is carrying out its own investigation.

“A big hole has opened up which means the wall has been pushed in bodily,” said Geotechnical Consulting Group director Hugh St John.

“It implies the anchors have failed. However, we don’t know the sequence [of the collapse] - whether there was a failure at the toe, which then progressively led to the anchors failing or whether the anchors failed first.”

Readers' comments (1)

  • After the removal of about half of the debris from inside the pit it is evident that the diphragm wall is undestroyed and undisplaced. Hence there is not much room left for the theory of a possible ground anchor failure. For the time being the load loss of the upper anchors is checked to ensure the removal of the remaining debris and emergency backfill.

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