IT WAS always going to be a major construction challenge, and so it proved. On the A23 Coulsdon Relief Road project, Hochtief was tasked with jacking a 37 m long concrete tunnel box less than 2 m below a live railway station.
Thankfully, all those at the main contractor can breathe a collective sigh of relief. The one-month jacking operation, for client Transport for London, was completed just a day late at the end of April.
But the period before when the team was trying to stabilise the ground between the tunnel and railway station may give them nightmares for some time to come.
The problems started when Hochtief tried to install a 'steel table top' of forty-six 610 mm diameter tubes to stabilise the ground beneath Smitham railway station and prevent settlement of the track.
As the tubes were pushed through the earth by a hydraulic ram they hit large pieces of concrete foundation with flint boulders beneath. Dealing with the obstruction is one of the main reasons why the complet ion date has been delayed by some six months.
'We hit what we think is a reinforced concrete substructure to the railway platform and underneath it huge lumps of flint, ' says Julian Spiller, project manager at Hochtief. 'From a tunnelling point of view this is the worst possible thing to find and extremely hard to remove.' The job started with a normal drilling auger at the front of the tube. This was changed to a tungsten cutting head that would cut through the f lint as the tube was forced through, but that was quickly ground down to nothing.
Hochtief and designer Tony Gee & Partners, which was responsible for all the temporary works, including the steel tubing system, also experimented with roller cone cutting heads that were effectively mini tunnel boring machines but they kept going out of alignment.
Eventually a cutting head was adapted from an Italian product that did the job, although it had to be replaced many times.
'The challenge was to come up with a cutting head that could cut through the material and open out a wider diameter, ' says Mr Spiller. 'We ended up working through six types of cutting head. None of those items were off the shelf so we were designing a different cutting head each time, which was very time consuming and quite morale sapping.' Hochtief has put in a claim for recovery of time and cost to client Transport for London and negotiations towards a settlement are 'ongoing'.
The contractor is claiming the concrete and flint did not show up in any of the tender drawings provided by TfL. Despite the fact that the steel tubing system was designed by the cont ractors, Hocht ief is mak ing a claim under a clause in the ICE 7th contract that covers incorrect information in the tender drawings.
A second problem added considerably to Hochtief's costs. It designed a steel tubing system that would allow settlement above of up to 25 mm, but Network Rail then insisted on no more than 10 mm set tlement during the tunnelling operation.
'The contract allowed for 25 mm settlement but that was shortened to 10 mm just as the contract was awarded, ' says Mr Spiller. 'With the trains pulling into the station at such a slow speed, the line could have taken movement of 50 mm.' Hochtief thickened the walls of the steel tubes to 19 mm and the tubes were filled with concrete to add stiffness. Sensors on all the sleepers gave a readout of their position every half an hour and the 10 mm settlement mark was breached on at least 15 occasions.
This required night-t ime possessions to jack up the railway sleepers.
The third problem was that the tubes would pass through a cavity of air, the passenger subway underneath the railway. In the event the tubes were fed either side of the subway. Plate girders were fitted to stabilise the walls of the subway area and the cavity was filled with foam concrete.
With the steel table top in place, 22 massive horizontal jacks with a pushing force of 15,000 tonnes could start to move the tunnel shield and the framework of ribs behind it which prevent the tunnel from collapsing.
The large and small jacks were operated by hydraulic pumps programmed to work to a particular pressure then stop.
The face of the tunnel was divided into 12 bays, each of which had a mini excavator with a clay spade and pneumatic breaker attached.
The ground was largely gravelly clay with chalk underneath but there were large bits of flint at the top that required hand work. The bottom of the concrete subway underneath the railway platform also required some hand work to dismantle but th is was kept to a minimum at the insistence of the Health & Safety Executive because of concerns about vibration white finger.
The tunnel jacking was divided into three pushes, each roughly 12 m in length and taking around eight days. During this time the speed averaged around 200 mm in 30 m inutes.
The operation was particularly painstaking because the ground dipped slightly and there was the added danger that the jacking could go out of alignment.
After the jacking, the space behind was infilled with pre-cast concrete units and left for two days before the jacking started again.
Apart from two breaks at eight-day intervals to reset the jacks, the 29-day operation carried on 24 hours a day with staff rotating in 12-hour shifts.
It was one of the largest tunnel boxes of its type in Europe to be jacked all in one go and the team proceeded cautiously.
The first push was stopped at 10.6 m 'because the guys needed a break after working non-stop'. The second jacking was 11.9 m and the third and final jack was 12.4 m.
This created a problem for the final jack because the packers in front of the hydraulic jack only move 12 m.
The team had to find concrete infill blocks to cover the final 40 mm.
'The jacking operation was completed one day late because the team ran out of packers for the hydraulic jack to push against for the final few millimetres, ' says Mr Spiller.
With the jacking completed, it is hoped that the road tunnel will open in October, six months later than scheduled. 'It's the culmination of two years of design, development and construction which has tested the project team to the limit, ' says Mr Spiller.
Main contractor: Hochtief Client: Transport for London Engineering support and supervision: Atkins Railway box design: Scott Wilson Tubing system: Tony Gee + Partners Jacking operation: Mammoet/de Boer Mining team: McBurney We lde r s : McBu r ney