Shani Wallis, Editor

Courtesy DPA

The city's famous cathedral

Köln continues to reel in the aftermath of the fatal collapse of buildings adjacent to open cut works for the city’s new metro line on Tuesday 3 March. Speculation about the causes, anger among the citizens, and sidestepping of responsibility by public authorities sets the current mood in the city.

Lack of information from the client, the city-owned KVB (Kölner Verkehrs-Betriebe AG), and public statements by the mayor that he and his department are not responsible, motivated calls by an angry mob at the town hall to closed down and abandoned the entire project. This was resisted. City Mayor Fritz Schramma stated the project will be completed but did call into question, during a radio interview, the responsibility of building more underground metro lines in the future. At the same time, activity at some sites along the route is reported as suspended indefinitely, which in tunnelling and underground excavation carries risk in itself.

In the vacuum of official details, possible causes for the catastrophe are fueled by speculation. While the depth of the excavation inside the cut-and-cover crossover box is set by most reports at 28m deep, the depth of the diaphragm walls around the large and irregularly shaped excavation are reported as being anywhere from 37m to more than 40m deep. Suggestions that it was the number of dewatering wells installed and the rate of ground water extraction from the core ahead of its excavation within the diaphragm walls are also assuming credibility.

The body of the accident’s second victim was recovered on Tuesday evening, 12 March, more than a week after the mid-afternoon crash of the archive building on Tuesday 3 March. The body of the first victim was recovered on Sunday morning, five days after the event. Both were in the neighbouring apartment house that was partly destroyed by the complete collapse of the city’s historical archive building. Alarms raised by workers fleeing the failing crossover excavation saved the lives of many more on that Tuesday workday afternoon. Two fatalities is a tragedy but the accident could have been much worse. Just nine days earlier, a parade and festival onlookers and participants passed along the street and crossed the decked of the shaft excavation beneath.

Courtesy Der Spiegel

As the citizens of Köln struggle to recover from the catastrophe, it is known that the city administration and the KVB are searching for independent and bone fide experts to assist them in the legal and potential prosecution processes that will follow. There have been no arrests to date but reports confirm that three experts have been assigned to work with the city’s justice department. Finding independent experts in Germany to assist the process is proving difficult since many professional and knowledgeable individuals, companies and institutions have already contributed in some way to the planning, design, and development of the project and its construction. Conflict of interest and compromised impartiality rules them inappropriate as advisors.

In addition to legal proceedings, the ramifications of the situation on the different insurances applied to the project and third party property owners is another area of investigation and that will take some time to unravel and settle.

There are also reports of the diaphragm walls being tied back with ground anchors of indeterminate length, but this too is unlikely in a box of this size and shape, according to Dr.-Ing. Roland Leucker, Managing Director of the Reseach Association for Underground Transportation Facilities based in Cologne. “Anchors are usually applied if clear access is needed through the excavated space,” said the Managing Director of STUVA. “It is more likely that struts have been used in this excavation although this is not known for certain.”

Design criteria of diaphragm walls in Köln usually specify extension by 6m into the underlying, less permeable, tertiary sands and gravels. The difficulty, said Leucker, is in establishing the interface between the upper quaternary and lower tertiary deposits. “Geologists and expert ground engineers are able to identify the interface during excavation of the panels but this is a skill that is needed on the day.”

Another perceived source of cause are reports that compensation grouting was not applied for protection of buildings adjacent to the crossover excavation. An exposé about the project published by STUVA’s official journal TUNNEL in November 2007, states that 48 buildings on the alignment of the southern construction contract were “supported” by compensation grouting for a total 6,780m2 of tube-a-manchette coverage, some 14.5km of injection drilling, and up to 3,200m3 of grout injected to create the initial grout layers to which more grout is injected to compensate for settlement induced by ground lose during the process of excavation in the area. Protective measures of underpinning with bored piles and ground freezing were also used extensively along the route to stabilize the high ground water table and loose wet sands and gravels beneath the famous historic city on the Rhine.

Why the city’s historical archive and the buildings adjacent to the crossover excavation were not afforded particular protective measures is unknown. “Usually a client specifies the buildings and structures that will require special protective works,” explained Leucker. “The contractor’s design department then recommends protective treatment at additional cost, of any buildings not in the client’s schedule that their own calculations identify as having a low factor of safety. The decision-making process for the area of the collapse is not known.”

Published media reports have also made reference to movement and tilting of a church spire in autumn 2004 when tunnelling activity was progressing nearby. This was not the city’s famous Köln Cathedral and also not in association with subsequent passage of the metro’s two running tunnels, which were advanced by two 8.4m diameter Herrenknecht slurry Mixshields. It was in association with advanced works needed for diversion of services on the alignment and near the foundations of the relatively new 40-50 year old church.

Courtesy TUNNEL

Compensation grouting access shaft with special elevating grouting platform custom built for purpose

The diversion tunnel was completed as a 2.5-3m-diameter pipejacking operation. Confirmed over-excavation of soil by the tunnelling machine caused excessive ground loss and the tilting of the church spire by a reported and dramatic 77cm. It is believed that no protective measures were applied ahead of the new service tunnel construction and rather than dismantling and building a new spire, the situation was recovered by installing a comprehensive array of bored piles which isolated the ground beneath the church from the alignment of the following running tunnels and “lifted” the church to correct the spire’s lean.

Further discussion with Dr Leucker at STUVA explored different possible methods of constructing the crossover excavation, which was one of the last pieces of excavation to be completed on the new North-South metro line. “Ground freezing is an extremely expensive method of stabilization,” he said, “and is one that is only employed when no other method can be used. The crossover could have been completed as a wet excavation but how to tremmie in a base slab and break out the running tunnels mined through the core? Pre-excavation of the box and pull through of the TBMs would have been an alternative but it too would have been complicated and would have required stabilization of the ground adjacent to the tunnel eyes. Jet grouting as a method of ground water control for this purpose is problematic. Failure of a caisson shaft on a metro contract in Berlin some year ago confirmed that jet grouting cannot be relied on to be impermeable.

Courtesy TUNNEL

Cross section of installed compensation grouting system

The technique had been used to create a block of stabilized ground outside the eyes of the running tunnels into and out of the caisson but these did not perform as designed. Water and material flooded the shaft through a breach of about 10cm diameter when the concrete of the tunnel eye was being broken out to allow pull-through of the TBM. This caused severe delay and required application of ground freezing to recover the situation.”

Tunnelling professionals say that second-guessing the decision-making process in Köln prior to the collapse is unfair and prejudicial. Judgments must wait until all investigations are complete and the facts of the situation are uncovered and published. Thorough investigation and publication of the findings, however is needed by the industry, and is being demanded by the general public in Köln, for lessons to be learned and similar disasters are to be prevented in the future.

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