In addition to further Feedback comments and thoughts on the TunnelTalk Feedback page, further comments and thoughts are added to this Discussion Forum article about the collapse of the Rastatt TBM rail tunnel in Germany and failure of its segmental lining from the TunnelTalk social media accounts.

Samples of the prevailing geology

Tim Hyett, MSc Ceng Cgeol FGS MCIOB, writes in the TunnelTalk LinkedIn group:

“Good article highlighting the major risks of shallow cover tunnelling using pressure balance TBMs, not helped, no doubt, by the far too frequent trend of designers to focus on transverse loading of the ring only, and ignoring minor longitudinal deformation of the tube lining behind the TBM, and the practice (originally imported from Europe) of removing/recycling segment bolts. Taking out tunnel segment bolts in anything other than the most stable of ground is something that would have been thought madness 20 years ago, yet seems common-place today. The investigation into the tunnel collapse in Hull in 1999 wasn't anywhere near conclusive, but ask any of the miners who were frantically trying to reinsert the bolts and they will tell you what would have prevented it.”

Auke Lubach writes: “Interesting article and problem to analyse. Shooting from the hip, what is the possible effect of stray current from the railroad on ground freezing?”

Wasim Ashraf writes: “I would love to see the cost comparison between mix-in place and/or grouting vs ground freeze technique used for this 200m given the soil strata shown in the image of the core samples in this article.”

Another reader queries the immediate response to backfill TBM tunnel drive failures with concrete and burying the TBM and its back-up in the process, suggesting that there must be a better method for stabilizing the failure zone - which in the Rastatt case was 40m behind the TBM shield bulkhead, although not clear of the trailing backup gantries - and avoid the destruction of the TBM in the process.

Although thankfully rare, the incidence of tunnel excavation failure is a reflection on the competence of the global industry as a whole. Tunnelling is one of those few industries that is truly global with the expertise and best practice knowing no political or geographical boundaries and for all the hundreds and thousands of unsung successes, it is the rare failures that shape the industry for the future. They are a reminder also for anyone considering an operation in underground excavation and tunneling it is not as simple or as easy as might be perceived. When things go wrong, the consequences can be catastrophic not only to the project itself and perhaps fatal to the workers, the impact on the environment and affected involved infrastructure can be devastating. Pushing the boundaries is a must for taking industry forward but managing the risks and learning from the causes and consequences of any failure is equally, if not more important.

Rastatt tunnel collapse a one-in-a-decade cause for industry-wide examination

The failure of the Rastatt TBM rail tunnel in Germany as reported in TunnelTalk last week (21 August 2017) has prompted a series of communications from different quarters of the industry and representing the various aspects of the event.

Details of the incident, as far as can be reported, are explained in the Ground freezing TBM drive collapse in Germany article and in short, the lead heading of a 4km long twin tube TBM rail tunnel built to bypass the city of Rastatt, while passing a reported 4m under the main twin track railway corridor between Germany and Switzerland and with the support of horizontal ground freezing through waterbearing sands and gravels, failed with failure of the ground freezing regime and loss of integrity of the segmental lining, allowing ground loss under the rail tracks into the new tunnel bore beneath.

This scenario raises a multitude of queries and questions for confirmation, investigation and speculation and the parties involved and most crucially Deutschebahn, the owner of both the rail operation on the surface and the new rail tunnel being constructed beneath, are now investigating.

Fig 1. Schematic of the leading TBM drive collapse as provided by Deutschebahn

In feedback to our reporting, it was noted by readers that, initially, news was slow in reaching a wider audience. Disruption to the hundreds of passenger and freight train traffic that uses the mail rail route per day were the general media headlines until the cause for the complete closure of the line was reported more specifically a week after the event.

It was a TunnelTalk reader in the insurance industry that notified us of the situation and investigating details for our report was hampered by both the language barrier and the understandable reluctance to answer any probing questions that might compromise investigations, but that has not closed down speculation and queries within the industry and particularly as the most probable cause of the collapse reported in the German media has been a failure of the ground freezing support caused by high summer time temperature at the time and accompanied by heavy local rainfall.

Immediate responses the to TunnelTalk article came from around the world and comprised private emails to the Editor and Feedback for publication.

In the published Letter to the Editor, Nick Shirlaw of Golder Associates queries the failure of the segmental lining and highlights other incidences of segmental lining failures that could be included and reviewed again in light of this current segmental lining failure at Rastatt.

Heathrow Express Rail collapse in 1994, the collapse of the decade that changed the tunnelling industry worldwide, some say not for the better

Other emails received offered various comment and queried for confirmation various aspects of the situation, including:

• that the cover under the mainline railway tracks really was 4m and above a 10.97m diameter TBM drive;
• that the BTS drive through the adopted ground freezing regime was adopted as a contractor alternative to an original proposal for open face NATM excavation under the mainline tracks;
• asking if other methods of executing shallow underpasses of mainline rail tracks had been considered, such as rectangular boxjacking through the freeze support of the waterbreaking deposits;
• fearing that it will be a disaster for the tunnelling industry in Germany, but not for the TBM manufacturer;
• and with a hope that it will open the eyes of certain people in the insurance industry who seem to have completely lost respect for risk exposure in tunnelling projects again and leading to a situation, as was experienced last after the Heathrow collapse of not being able to secure insurance for tunneling projects at any price.

Nicoll Highway collapse in 2004 the defining failure of that decade

Adding to these thoughts, queries and comments, TunneTalk compiled a list of questions as requested when contacting the Media Relations spokesperson in DB managing media enquiries and submitted the following for consideration and possible reply.

• Why was the alignment under the railway so shallow? On a TBM tunnel of 4.3km could not the underpass on its oblique alignment under the rail tracks have been deeper. The reported 4m of cover under a rail embankment structure is less than half the rule-of-thumb minimum of one tunnel diameter of cover.

• Can ground freezing and a slurry TBM excavation be compatible? Has it been done before? The operation of a slurry TBM relies on a fluid excavation and muck haulage system and managing the balance between keeping the slurry fluid and ensuring a frozen ground support environment will have been difficult to specify as well as maintain, for both the incoming fresh slurry and the out going loaded slurry, as well as for the slurry mixing operation in the excavation chamber.
• Study of reported figures state that the freeze pipes were installed horizontally and from two shafts located 100m either side of the railway and drilled for a 100m from each shaft. That being the case, the ends of the two freeze pipe installations are at the mid-point of the total 200m length, which will have been directly under the rail tracks.

Cologne crossover box collapse of 2009, the defining collapse of the 10 years to follow

If that is the case, what was the junction and overlap of the freeze installations at this critical mid-point? It would have been difficult to check this as the pipes were horizontal and so deviation of the pipes after 100m of HDD drilling could not have been guaranteed and there would have been no way of confirming the freeze from the surface at that point as that would have required working in or close to the trackway. Supposition of the details, but a fact is that the freeze evidently failed at that critical mid-point junction and under the tracks.

• Why did the segment lining open up? Where was the annular backfill? Maybe it too was adversely affected by the ground freezing environment. Was the lining relying on a secure ground freezing stabilisation for its integrity? Where there bolts securing the lining and in which configuration, longitudinal as well as circumferential?
• What part has monitoring and instrumentation played? Accurate monitoring of a freeze operation would/should have indicated a potential freeze failure.

The Joint Code of Practice for Risk Management initiated by the British Tunnelling Society and in joint collaboration with the Association of British Insurers, published in 2003, lead to an International Code of Practice for risk management in the tunnelling industry, both of which are currently under review and update
Download free pdf copies from the BTS website

Can a freeze operation so closely under a vital set of heavy weight, high trafficked train tracks be a good idea when a secure freeze is vital, but so too is not causing any serious ground heave under the tracks. What was the effect of the vibration from the heavy and regular trains on both the freeze and the integrity of the segmental lining and TBM operation under such a shallow cover?

Added to these technical thoughts and questions, there are an additional set of of questions to query the method of contract procurement, the risk allocation regime in that contract, and the approval process for accepting the extension of the TBM alternative over the original open-face operation under the tracks.

All of these, and many more queries and questions and points for clarification surround the tunnel failure at Rastatt and the following set of reference articles from the TunnelTalk Archive provide additional reading on various points at the centre of this discussion.

Use the Search window facility at the bottom of this article page to find keyword reference articles in the extensive and free-to-access archive of more than 2,000 articles published by TunnelTalk since its launch in 2008.