Saturday, 27 February 2010

Lost for words once more

Written 25th February    

Who would have thought, speechless twice in one trip!  We have found the man-made equivalent of Dune de Pilat.  A sight so unexpected and awesome it has quite taken my breath away.

The Millau viaduct was always on the route, right from when this trip was just pins in a map on our living room wall.  But having come down to earth with a bump on the price of petrol here, we almost, almost, decided that the 220km round trip detour from our south coast route from Bezier to Montpellier was not worth it.  Fortunately for us, my brother Tom has been and told us that he thought it was worth the petrol and even better, his girlfriend Katy, gave us a list of places they had enjoyed on their visit which we could take in en route.

I have seen many pictures of the Viaduct at Millau but you always have that slight feeling in the back of your mind that there must be clever camera angles or lenses or retouching going on and nothing prepares you for the colossal reality of the thing, when it appears in front of you.  

We didn't really know where we were going so stayed on the A75 until the very last exit before the toll and ended up going though St Rome de Cornon and back towards Millau on a route which seemed destined to take us under the bridge.  

Worth noting here that we were just 7 km from the village of Roquefort at this point but sadly I was not allowed to visit as I still have cheese to eat in the van :(  it is a large lump of not especially nice cheese bought in a Mercadona in spain and I am not particularly enjoying it but I am not allowed more until it is finished or gone mouldy... So hard being me :(  the only consolation is that Will has two unopened packets of pate which he is similarly ambivalent about and at least my cheese will go off first!

Other people making this trip might like a cheese diversion at this point...

Anyway, we still hadn't seen the bridge - and were just wondering how you hide a bridge that big!?! - and contemplating the gently sloping valley and wondering whether it really warranted a bridge on the scale we were anticipating, when we rounded a corner and there it was, stretching up and away as far as the eye could see in both directions - simply staggering.

We found the south side visitors centre on the road we were on, 

before continuing on down the valley to the foot of the bridge and the 4m wide river Tarn which its 2,460m span(!!) crosses 270m up above.  



Heading on through Millau town and out the other side we found the Aire du Viaduct, another exhibition centre, and spectacular views down onto the bridge.

The bridge came about as the result of a government commitment in 1975(!) to provide a free motorway alternative to the main north-south toll road from Paris to Montpellier which goes by way of Lyon.   By 1991, they had got as far as planning a route from Clermont Ferrand through the Massif Central, when the question of how to cross the Tarn Valley at Millau came to the fore.  Having decided that it was more environmentally sound to go straight over rather than down and up again - a decision which we are sure was in no way influenced by the realisation that going across would result in a world record breaking bridge... ;) - Norman Foster's iconic design, with its 7 pillars and arrow head shaped supports was chosen from the five proposals submitted.

It is the world's highest multi-stayed viaduct and the numbers involved are incomprehensibly immense.  The bridge is 2,460m long, the road 270m above the river below, there are seven pillars spaced 342m apart, the tallest of which is 343m and has a ground footprint the area of a tennis court.  The steel deck is 65,000m2 and weighs 36,000 metric tons, with a further 10,000 tons of bitumen road surface, and the pillars and supports equate to 205,000 metric tons of concrete.  It took three years to the day to complete and cost €400M, €80M of which was the tollbooths -  they must be some fancy tollbooths!

The most staggering thing however is the way it was constructed.  The bottom sections of the pillars are concrete.  They were constructed with self raising, internal and external forms, and continuous pour concrete, raising 4m every three days.  Each pillar had its own crane which built itself upwards at the same rate as the pillars and lifted the concrete.  All fairly obviously so far. 

What I didn't know - and to be fair, had never had cause to think about before - was how they built the road.  I had assumed that the pillars were solid to to bottom and the road some how lifted into place between them, section by section, but no.  The metal deck of the road was constructed in sections on the granite plateaux on either side of the valley and was then pushed slowly into place until the two sides met in the middle!  With the metal pylons which top the middle two pillars already erected on the deck as it slid across!!

Special jacks were constructed to to this which raised the deck 2cm then pushed it forwards 60cm each go and the whole thing progressed at an astonishing rate of 9m/h.

It took 30h of continuous pushing to get one section in place, supported mid-way between each pillar by specially erected temporary supports which looked like giant mechano structures.   They then constructed the next deck section on the hill top and started pushing once more, proceeding in this manner with a section every 4-6 weeks. Simply awesome.  The remaining 87m tall pylons were then transported across the bridge and erected in situ, and the eleven pairs of steel tensioning cables on each one connected up and tightened to support the deck.

The bridge was load tested by driving 28 fully laden trucks (a combined weight of 968 metric tons) on to each span in turn and measuring the deflection - all within expected tolerance of course!

We learnt all this from the video in the southside visitors centre - magnificent construction footage (including lots of of happy engineers drinking celebratory champagne at any and all significant events - well this is France!) as well as stunning bridge shots in all weather conditions, including the valley filled with cloud up to the level of the road with just the pillars peeking out the top -  and in the northside exhibition we found more info about the road plans, models of all five original proposals and one of the pushing jacks, which you can actually see in operation.

The jack is noteworthy for its simplicity alone.  It has two parallel fixed beams on which the deck sits.  There are then two wedge-shaped sections sitting one on top of the other in the centre.  The lower wedge section is pushed forward with a massive ram which has the result of raising the upper wedge section so the deck is now resting solely on that, 2cm above the fixed beams. 

A second massive ram pushes the top wedge section forwards, sliding the balanced deck forwards and downwards until it is resting once more on the fixed beams, 60cms further forward than it previously was.    The wedge sections are pulled back, then the whole cycle starts again.  

The jacks were conceived and purpose built for the job by the engineering and construction company Eiffage who built the bridge and 64 were used in total in positioning the deck.  Simple awesome stuff.

The video is also available on the northside exhibition centre, which does have more information than the one on the southside if you only have time for one, and which is also accessible from the A75 so you can see it all en route without leaving the motorway (€6.10 to cross the bridge) but the detour to the valley bottom is definitely worth it - and means you can freeload the whole experience if you wish ;)

The visitors centre also has eat free maps and booklets of the whole A75 from Clermont Ferrand to Montpellier which shows places of interest and little circular routes at each junction so you can take picturesque detours on you journey if you wish - all very nicely done and showing all the places Tom and Katy have recommended and interesting roads by which to get to them - brilliant.

We considered crossing the bridge just for the experience, but from above, it didn't look like you could see very much - I'm sure I read elsewhere that the safety barriers were originally designed to be clear so you got the view but it made drivers disorientated so they filled them in - so we set off southwards through the russet-hued Dourbie gorge where the icy clear river alternated emerald green and sapphire blue between bucolic mills and hilltop hamlets, all stone-coloured and barely visible against the rocky banks and cliffs.

A simply perfect day.  

We're now parked for the night in a small picnic area just outside La Couvertoirade and excitingly, Will's fishing rod has finally had its first use... Although not, sadly, for fishing, merely for holding aloft the piece of wire by which we can extend our car ariel and get intermittent radio 4 - depending on the weather between here and London.  Night all.

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