CONTRACTORS ON the M1 widening are this week preparing for the project's most ambitious challenge to date - demolishing five bridges in 72 hours.
A series of Saturday night motorway closures planned over the next seven weeks will give the Balfour Beatty/Skanska joint venture and demolition subcontractor Armac time to divert traffic onto new bridges before demolition starts.
Hertfordshire police will begin a rolling block from about 8pm this Saturday between junction 8 at Hemel Hempstead and junction 9 at Redding.
Police will first close the northbound carriageway and then the south, clearing the motorway for the 12-hour demolition of the 1950s reinforced concrete single lane road bridge at Flamsteadbury Lane.
Joint venture project manager John Hollaway explained that the method used will be the same as that used for the Slip End Lane Bridge demolition late last year (NCE 23 November 2006). A dozen hydraulic excavators filled with pinching shears will peck away at the solid reinforced concrete deck, using 500t of pressure delivered through their shears. As the structure weakens, the deck will eventually collapse onto timber mats protecting the carriageway below.
The replacement bridges have already been built and erected to ease the traffic management but this restricts access to the existing structure - at one point there is just 5m clearance between the two decks.
To overcome this four breakers will work on either side of the main deck span with two sat on each of the side spans.
All three supports will then be demolished, beginning with the central pier. But if work is running late the team will first collapse the central deck and pier section. The side spans can then be collapsed into the banks and the road re-opened.
As a final contingency the team will leave the southbound side span of the bridge erected, as the two lanes nearest the verge will remain closed as part of the widening work.
Hollaway said: 'The real push is for the whole bridge to be taken out, including the foundations, which need to be removed so you don't get a hard spot on the carriageway.' The same method will be used to collapse Nicholls Farm Bridge and Redding Lane Bridge on the same stretch of the M1 on the weekends of 16 and 23 June.
The team will then tackle the demolition of the two junction 10 bridges on 30 June and 14 July, each of which will need a closure of 18 hours because of their size.