Investigations into the collapse earlier this month of the six storey archive building in Cologne this week focused on groundwater extraction.
It has emerged that contractors exceeded authorised dewatering limits while excavating a section of crossover tunnel for the city’s subway.
Cologne City Authority investigators said that groundwater pumping logbooks showed that main contractor Bilfinger Berger/Weiss & Freytag/Züblin joint venture, had built 15 extraction wells when it only had permission to build four.
The records also showed that from December last year this had nearly doubled the amount it was allowed to pump from 450m3/hour to 750m3/h.
A Bilfinger Berger spokesman said that the JV could not discuss the scheme’s construction methods.
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 the excavation.
The joint venture was building the cut and cover crossover to allow light rail trains to switch tracks on the new Nord-Sud Stadtbahn Köln. The cavern site is in front of the building which collapsed and is just 300m from the river Rhine.
A Cologne Transit Authority (KVB) spokesman said work on the 28m deep excavation was at full depth or nearing full depth when the building collapsed. The 1m thick diaphragm walls supporting the excavation were supported by ground anchors in predominantly river gravel.
Groundwater level in Cologne is about 12m below the surface. Geotechnical experts said if the ground was more porous than initially thought, the volume of groundwater that had to be removed could have been greater than expected.
“[The amount of overpumping] means that there was far more water coming into the excavation than they expected and they were probably struggling to control it,” said HPR director Scott Steedman.
“There could have been underground channels or voids that were producing the flow.”
The transportation of fine material out of the soil matrix during pumping along with the high groundwater velocities could have combined to cause soil erosion and the formation of sink holes behind the diaphragm wall.
“If they were over-pumping the water they could be pumping fines – probably silt and fine sands – with the water and basically causing internal erosion in the material behind the wall,” said Card Geotechnics director Nick Langdon.
“You could wash out the fine gravel and silt in patches to leave clays and gravels [if present] that are not as easily transported without the matrix that held them together.
“That in itself would not directly cause the collapse but if this internal erosion happened near an anchor then the anchor would lose strength and ‘unzip’, closely followed by a few more and then of course the wall follows.
“The key will be the physical interrelationship of the dewatering points and the anchors if this is indeed the mechanism.”
A KVB spokesman said there were measuring devices on the diaphragm walls which did not show anything unusual immediately before the accident. Groundwater experts said the removal of fines should have been monitored and rectified.
“The flow of water carrying fines into wells can accumulate to the point of mining but that would be noticed early on,” said WJ Groundwater chairman professor Jim White.
“You would monitor for fines and if they [excessive amounts in groundwater extractions] were found to be a problem, the well would be turned off. [Overpumping] would be no reason to cause a catastrophic collapse, it would be more likely to be a dewatering failure.”
Experts also suggested that the increase in groundwater flow and corresponding pumping could have increased the hydraulic gradient under the excavation to the point where the ground liquefied, losing the passive resistance of the toe of the retaining wall.
“The hydraulic gradient [pressure loss/distance] would be at its steepest due to the confined space. If the critical hydraulic gradient or velocity is reached, the soil fluidises.
“The diaphragm walls would have a great deal of restraint from passive resistance of the toe. “If it was taken away, the wall would fail,” said independent consultant Andrew Hawkes/
Cologne mayor Fritz Schamma has ordered KVB and the environment department to give a full explanation of all aspects of the dewatering this week.