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talking point

Doubts have surrounded NATM in the wake of the Heathrow Express collapse but the method is still the pinnacle of economic engineering, says Helen Nattrass.

My old mate Derek, a structures foreman, once said to me in a rather raised voice after I'd made some silly setting out mistake: 'Now then, lass, you're allowed three mistakes on a job. We've only been here a week and you've already made two. So that leaves one more for the next two years - and by the way, it's not allowed to be the same as the previous two. Never make the same mistake twice!'

As I sloped off with that distasteful feeling of justified castigation lingering over me, he called me back.I was expecting a second volley, but he said, 'Don't be so down in the mouth, lass.It's part ofmy job to tell you when you've made a mistake and try and keep you right.Them as never made a mistake has never done any real work and never learned owt.Learn from what went wrong.'Most of us have been the 'engineer'in that scenario.

Derek's words came back to me as I listened to the recent presentation about the Heathrow Express tunnel collapse.

The events of autumn 1994 have had a profound impact on the design and construction of tunnels throughout the world.At the presentation, Sir Alan Muir Wood said it was an expensive way to relearn what we knew already.I found myself agreeing.

More than 12 years ago, I and my team at the Channel Tunnel were involved with the first big sprayed concrete works in the UK for the underground development work at Shakespeare Cliff.This led on to the undersea crossover - a giant cavern constructed in sprayed concrete using NATM principles.We were the geotechnical measurements team and had to fight the miners to get the instruments in as close to the face as possible (the early readings are so important).

We then had to battle with them to do the daily readings.The readings were plotted religiously by hand every day and thus were subject to the inevitable daily review.We had a couple of Austrian engineers working with us to keep us right.One of them once said to me, 'Helen, you never get a catastrophic failure in NATM without warning.You can always see from the geotechnical measurements that something is wrong.As long as you act quickly and correctly, you can usually save the situation'- words which again I found creeping into my head last month.

From very early in my career I had always been a passionate advocate of NATM design and construction.The ease of forming underground intersections with complex geometry seemed so fast and efficient.The semiempirical/observational approach to refining the design and responding to anomalies was to my mind the pinnacle of balanced economy.I write 'was', but believe, with the greatest conviction, 'is still'.

I accept that the cautious actions and analysis in the aftermath of the Heathrow collapse had to occur for many reasons.We heard earlier in the year at the British Tunnelling Society how extensive changes to the design and construction of the Jubilee Line Extension tunnels and stations were occasioned by the collapse.Not least among the effects has been the increase in conservatism among clients and insurers.The words NATM or sprayed concrete seem to invoke negativity on an unprecedented scale.The risks are perceived as unacceptable.The potential for catastrophe is viewed as huge.

I would like to see a campaign for the 'political rehabilitation'of sprayed concrete/observational method work.There is a huge amount of data about the Heathrow collapse which has not been presented to the engineering community.I am not in a position to know what story this would tell when presented in full, but I am sure it could be of importance in the greater understanding of structure/soil interaction and the behaviour of sprayed concrete and London Clay (and by extrapolation, other types of soft ground).Designs are often so conservative that no significant movement occurs.Here we have a set of data which shows movement up to the point of failure - invaluable.We could surely salvage some good from it for the future.In addition, all the geotechnical monitoring done on the Jubilee Line Extension sprayed concrete work could be compiled and collated to give engineering support to designs for future projects.

A concerted effort not to 'talk NATM down'could plant seeds of sense and proportion back into the minds of the non-specialists and the public.Armed with this backanalysis and knowledge, I would like to see the success of future sprayed concrete/ observational method tunnelling in the UK.I hope that such designs, possibly proposed for the Channel Tunnel Rail Link and other large projects, could be accepted without undue 'paralysis by analysis'.

I know, by the questioning I receive from my church choir, that sprayed concrete work currently evokes an almost hysterically negative reaction from those who are informed merely by the popular media.It is time to knock down the myth and get back to work.We have stood the castigation and see the lessons for the future.To quote from a recent BTS meeting: 'Let us not throw away the advantages of NATM on the back of one unfortunate incident, but let us learn from the mistakes of the past and blend the old with the new to the best advantage.'

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