Shani Wallis, Editor
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.
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.
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.
Shani Wallis, Editor
Courtesy Der Spiegel
Site of the devastating collapse
- Failure or ‘boiling’ of the invert in a deep shaft excavation is the probable root cause of a catastrophic and fatal disaster on the new underground tram line project in Cologne, Germany. Failure of the excavation caused complete collapse of one building last week on Tuesday 3 March and claimed the lives of two residents in the partially collapsed apartment buildings either side. The disaster comes more than one and half years after excavation of the running tunnels and during construction of a crossover link between the two.
- Workers, presumably excavating the core of the large straight-sided shaft in which the crossover was to be built, evacuated the works when they sensed something as wrong and warned 10 to 15 employees and visitors in the city’s historical archive building adjacent to the shaft and others to run for their lives just minutes before the heavy concrete building and its heavy weight of historical books and papers collapsed partly into the shaft and into the gapping hole in the sidewalk and beneath the building.
Damaged apartment building
- The body of one victim was found on Sunday morning (8 March) and the other is still missing and presumed dead beneath the rubble. Dangerous conditions all around are preventing any major clearing. The damaged buildings either side are severely compromised and disturbed ground rules out the use of heavy lifting equipment until the area is secured. One of the damaged buildings was demolished yesterday (11 March) and volumes of concrete have been poured into the voids but more stabilization is required before investigations and recovery can begin.
- While the owner, Kölner Verkehrs-Betriebe AG, and the contractor are releasing little information into the cause of the collapse, others familiar with tunneling and deep foundation excavations in Cologne suspect an under performance of systems installed to control the high water content in the permeable soils beneath the city.
New underground tram line
- “Managing the area’s high ground water content together with perhaps some irregular geological conditions appears likely to be the root of this disaster,” said Dr.-Ing. Roland Leucker, Managing Director of STUVA, the Reseach Association for Underground Transportation Facilities based in Cologne. The geology of Cologne on the banks of the Rhine is notorious for its high ground water table in the sand and gravel deposits. The water table lies within meters of the surface and is influenced by fluctuations in the flow of the river. High spring melts that swell the river often cause construction of open pit foundations and basements for new buildings to be backfilled with water to counterbalance rising hydrostatic pressures outside the diaphragm walls used commonly to construct such excavations in the city.
- The collapse occurred at about 2pm on Tuesday afternoon at the height of the working day. It occurred on the southern section of the new North-South metro tram line. The 4km of twin tube running tunnels and six underground stations is divided into two contracts with the Bilfinger Berger, Züblin, Wayss & Freytag JV working on the 3km southern section with five underground stations, and a consortium lead by Hochtief and including Strabag and ground engineering companies Keller, Brueckner and Bauer, completing the 1km northern section with the sixth underground station. A spokesperson for the owner, KVB, confirmed that the crossover was the last section of excavation on the project to finish. All other excavation was complete.
- The new metro line runs between 100-500m parallel with the river and the collapse site is about 500m from the river bank, but according to Leucker, influences of the river are unlikely to have been a causing affect. “It is also unlikely that a broken water main or sewer could have caused the amount of water that appears to have run into the shaft so quickly.” The possibility of a failure in the diaphragm wall is said to be unlikely to have caused such a rapid sequence of events. Suggestion that the interfaces between the segmental lining of the four running tunnel junctions with the preinstalled diaphragm walls might have caused the problem is also deemed unlikely although sealing of the break-in and break-out eyes of the running tunnels in the diaphragm walls is believed to have been the annular grouting of the segmental lining alone. A jet-grouted block of treated ground outside the diaphragm walls at the eyes is believed not to have been installed.
Courtesy Der Spiegel
The Rhine River lies some 500m from the collapse site
- “Although still yet to be confirmed, it is almost certain that the tunnels themselves did not fail,” said Leucker.
- Instead, and also yet to be thoroughly investigated, it is suspected that a breakthrough or ‘boiling’ of the shaft’s invert caused rapid high pressure inflow of ground water, bringing with it volumes of material, and creating the void outside the diaphragm wall into which the archive building collapsed. “The line runs up to 28m below the surface, and with the water table at about 8-10m below the surface, there is about 2 bar water pressure acting on the excavation at invert level,” said Leucker. “It is unknown how deep the diaphragm walls are in this case but for most diaphragm wall constructions, they are about 1m thick and extend into the more watertight tertiary sands and gravels beneath the upper, more permeable quaternary deposits of the Cologne region.” It is believed there had been some problems with dewatering the core ahead of starting shaft excavation.
Crossover superimposed on the collapse
- General construction of the new line began in early 2004 with installation of the heavy diaphragm walls around open-cut access shafts and station excavations. To excavate the 2,700m of 7.3m i.d. segmentally lined running tunnels of the southern lot, the JV selected two Herrenknecht slurry Mixshields. The machines were launched on 6 June and 12 July 2006 and completed their respective drives on 12 July and 4 August 2007. En-route the TBMs passed through the single crossover zone - the diaphragm walls of which had been installed but the core had yet to be excavated. They also mined through, or were pulled through, the open-cut station excavations and drove either side of the biggest open-cut station excavation on the line, also supported with diaphragm walls.
Running tunnel filled with inflow material
- Core excavation of the crossover had waited until use of the tunnels for access purposes could be discontinued and allow for breakout of their segmental linings. Photographic evidence indicates that excavation of the core in the more than 30m long x 25-30m wide x more than 25m deep crossover excavation running diagonally across the alignment of the two running tunnels, had uncovered the tunnels and that dismantling of the segment lining waslikely finished. Gravel and sand filling about one-third of the running tunnels indicates that breakout of the segments was finished and that excavation was at or near the deepest level. It is reported that the excavation had reached the invert level in parts and that the first blinding layer of concrete was being poured ahead of installing the rebar reinforcement and casting the excavation’s base slab, which would be likely 1m or more in design thickness.
Buckled steel decking over the shaft excavation
- As Leucker explained, the diaphragm wall method of construction for deep excavations in Cologne is standard. The diaphragm walls are installed and the core usually dewatered ahead of dry excavation. Sometimes the core is excavated in wet conditions and water pumped out after the base slab is tremmied in. It is believed the crossover excavation had been dewatered ahead of excavation and that the base slab was in the process of being constructed. The alternative to dewatering is ground-freezing which is being used for construction of connections between the large open box excavation of the line’s big underground station and the running tunnels that pass outside the walls. Ground freezing however is expensive and alternatives are adopted where possible.
Broken dewatering pipes
- Dewatering of the large open-cut station boxes and shafts ahead of core excavation was a major operation on the contract and large diameter blue-painted pipes on the surface convey extracted ground water to the river for discharge. Excavation of the new line in the neighbourhood of the city’s main station and its famous cathedral, runs beneath the streets where possible but comes close to property lines in several areas. At the site of the collapse, the wall of the crossover excavation in the street is within 5m from the front of the buildings. Heavy steel girders and steel plate decking, with a small access opening incorporated, covered the large box to keep traffic moving during excavation of the crossover.
- Media reports state that cracks noticed in the walls and floor of the basement of the archive building in early 2007, were investigated then and again in 2008 and one both occasions were considered non-urgent and no cause for concern in such a heavy building and its particular foundation construction and were not monitored for any further movement.
- As well as the loss of two innocent victims, the loss of historic papers from the archive building is said to be tragic. Archive reaches back through the medieval period to the year 922 and is valued as a whole at an estimated €400 million. The long, complex and costly road of recovery, investigation and possible prosecution is only just beginning. Meanwhile the new underground tram line is scheduled to open in 2010.
- STUVA’s offical journal TUNNEL researched and published a comprehensive supplement on the planning, design and construction to date of the new North-South tram line for the occasion of the STUVA conference in Köln in Nov 2007.