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Cologne collapse: don’t rush to blame ground anchors

  • I read with interest the possible explanations given by geotechnical engineers regarding the recent collapse in Cologne (News last week). While I have no doubt that the failure of one or more ground anchors may have unzipped the diaphragm wall, I think it is a little previous to blame the anchors themselves. The diaphragm wall would have been constructed in the undewatered state, anchoring would have been installed as the excavation deepened, this would have necessitated progressive dewatering.

Due to the very high dewatering discharge volumes, full dewatering would not have been implemented immediately, the groundwater levels would have been managed to allow anchoring and excavation to proceed. At the time of collapse the dewatering would have been affecting the greatest groundwater lowering to date, as the slab was being constructed. At this point the effective soil stress acting on the anchors would have been the same as when they had been proof loaded.

Each anchor should have been proof loaded to at least 150% of working loads. Any weak ground would have been identified at this point. The very high groundwater inflow velocities to the dewatering system may have exceeded the critical hydraulic gradient, this would have induced fluidised conditions removing passive pressure from the toe of the diaphragm wall. This could have locally over stressed the anchors. It is also possible that the mix of course and fine materials could have mobilised fines from the fine alluvial sediments juxtaposed with the courser gravels, within which groundwater velocities may have been high enough to simply wash out the fines.

The voids formed, which would have been near the base of the diaphragm wall, where velocities would have been at their maximum, would then have collapsed progressively upward removing all components of skin friction and bearing capacity from the grouted anchors.

Andrew Hawes, Hawes Associates, hawes@ hawes99.fsnet.co.uk

  • It is unfortunate that the article on the Cologne City Archive collapse headlines the case for ground anchors being the possible cause of this unfortunate failure. The comments offered clearly pre-date a substantive causation investigation. However, it should not be forgotten that ground anchors are probably the most intensively tested and proved support element in a foundation scheme with load tests routinely carried out, in current European practice, on every installation to between 1.25 (temporary anchors) and 1.5 times (permanent anchors) the specified working load.

If this is the case on this project, and if the anchors have not suffered from premature weakening due to corrosion or some other cause, then ground anchor failure is more likely to have occurred as a result of loads significantly above the specified working loads. For example, due to global failure of the ground or failure of the supported structures.

Therefore I believe that the article gives somewhat premature and unwarranted bad press for ground anchors.

John Gannon (M), Byland Engineering, 3 Concept Court, Kettlestring Lane, Clifton Moor, York YO30 4XF  

Whitby: is his call undemocratic?

Whatever the merits of coalfired powered stations Mark Whitby’s comments are tantamount to insurrection (NCE last week). I am sure he will deny advocating that individuals should take direct action and resort to violence. However, no matter how you read his words the intent is to encourage the fervent minority to intimidate and disrupt the activities of companies and their employees going about their legitimate and lawful business. In this case, activities approved by and in accordance with government policy. In the UK, the means of showing your dissatisfaction is through the ballot box and reasoned argument.

If you don’t like the outcome, it does not give you the right to take direct action or threaten dire consequences. If we were all to take matters into our own hands then anarchy would reign. NCE should be careful not to condone or promote sedition. It should distance itself from those who foster such an agenda.

Denis Stephens, Brackenhurst, Saltergate Lane, Bamford, Hope Valley S33 0BE  

Meccano… Lego… don’t forget K’nex

It saddens me to hear of so many of my fellow engineers bitterly entrenched in their ways and allied to ancient technologies. I am, of course, referring to the Lego versus Meccano debate. It is obvious that K’nex is the superior construction material. Compared to the clunky pressed metal strips of a Meccano set, the elegant plastic lengths of K’nex boast a greatly reduced embodied energy footprint and represent a greener solution for any would-be developer.

Phil Renforth, Newcastle University, philip.renforth@ newcastle.ac.uk

Editors note: The Magic of Meccano Show will be held at Kew Bridge Steam Museum from 18-19 April. www.kbsm.org No Lego or K’nex will be on display.  

… and stop ignoring the LibDems

I guessed it would not be too long before we were given yet another photograph of Theresa Villiers MP. (ICE News last week). Why? She can say almost anything and back off if she actually becomes the minister. A likely outcome for the next parliament is no overall majority so I would expect you to publish photographs and views from the LibDem transport spokesperson as they may get the portfolio.

Louis Solway (F), 85, Netherby Park, Weybridge KT13 0AG  

Is the return of the CEGB necessary for energy stability?

I found the Energy Report (NCE last week) to be a worrying summary of a bleak situation. Simon Harrison suggests that “the way generators get incentivised is to behave like each other”. That seems a fairly robust description of a cartel. Ian Burdon suggests that our future will be decided by the French, German and Spanish boards of directors of our generators. Well, no worries there then when the chips are down. And thank goodness 19% (or is it 29%) of our generating capacity will be provided by the wind. That’s very reassuring. At least in the brown-outs we’ll know that we’re green as well as blue! What do NCE readers think the odds will be on the Central Electricity Generating Board rising phoenix-like from this mess? It seems to be sorely needed by UK plc if we are to keep the lights on.

David Myles, dwmyles@ hotmail.com, Wingerworth, Derbyshire  

Environmental shame of the A46

Congratulations to the Bam Nuttall/Capita Symonds/Ceredigion County Council team responsible for the “value engineering” redesign of the Llandysul bypass (NCE 15 January). Residents of the Rushcliffe area of Nottingham are desperately Security of supply: Do we need a national energy planning authority? hoping a similar philosophy will be applied to the A46 Newark to Widmerpool scheme.

Construction is due to commence very soon with unnecessarily deep cuttings and excessive landtake at Saxondale and an unnecessary long high embankment at Stragglethorpe. The resulting earthworks will spoil the landscape and environment at high scheme costs. It is an embarrassment to any environmentally-conscious engineer.

John Greenwood (M), reader, Nottingham Trent University, 10 Nottingham Road, Cropwell Bishop, Notts NG12 3BQ  

Forget airports and focus on rail

Peter Fells (NCE 5 March) makes the point that a Thames Estuary airport would be remote from its users. I would agree. But who are these users going to be anyway? Growing oil shortages are likely to have a dramatic effect. Leaving aside climate change considerations, I would suggest that a reduction in air travel is more likely than a growth. High speed rail is likely to become by far the preferred option for European travel, leaving flying only for long haul, and even that will be much reduced by the sheer cost. No need for new airports, it’s railways we need.

Paul Barnard, paul@psaa. me.uk

Non-engineers can succeed

I was enthused by the two articles in last week’s NCE highlighting the opportunities to allow non-engineering graduates onto civils courses to transfer into the industry. I graduated with a BSc in Botany & Zoology, but towards the end of my course realised that engineering was where my interests lay after performing a number of structural analyses on biological specimens.

Through a slightly roundabout route I was taken on by Amey as a CAD technician and funded through an MSc in Civil Engineering at the University of Brighton. I’m now working as a structural engineer and assistant designer’s site representative on the Weymouth Relief Road, and am grateful for the vision and support of Amey that has provided me this opportunity.

I still think engineers have got a lot to learn about efficiency of structures and systems from the natural world though!

Duncan Codd, Hove, East Sussex, duncan.codd@amey. co.uk

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