Aarsleff Piling has completed the restricted headroom phase of its £4.5M contract on CTRL contract 310 in nearly half the expected time.
This section of the high speed line runs over the soft West Thames marshes at Havering and Thurrock, Essex. Ground conditions are typically 4m to 10m of alluvium over flood plain deposits, tertiary clays and sands or chalk.
Working under three bridges with headroom as low as 8m, Aarsleff has installed 230, 600mm by 600mm precast piles to 15.3m.
The company believes this is the first time this size of precast concrete pile has ever been installed in such low headroom.
The piles are made up from 3.5m segments using speciallydesigned rigid mechanically tensioned joints that are stronger than the piles.
The restricted headroom project is part of Aarsleff's subcontract for main Contract 310 contractor Morgan Vinci, to supply and install 2,500 precast concrete piles up to 21m long (GE August 2002).
The piles, typically in rows of four and six at 5m centres, will support sections of the 7km long, 10m wide reinforced concrete track slab that will carry the high-speed line over the marshes.
For the low headroom phase, Aarsleff used one of its own adapted Banut 500 self-erecting fixed leader hydraulic piling rigs with 5t drop weight.
The firm teamed up with piling rig manufacturer Banut to develop a leader system, which included a modified leader, cathead, hydraulic raking arms and drop hammer. This was specifically to handle the pile segments within the 8m restricted headroom under the two main A13 trunk road flyovers and another bridge.
'The modified rig, which can be adapted to cater for restricted headroom ranging from 8m to 11m, worked extremely well, ' says Aarsleff piling manager Philip Chippindale.
'Despite initial teething problems with the system, which was untried and untested, and a considerable amount of very hard driving, we nearly halved our contract period.
'We progressed better than expected and managed to install an average of 207 linear metres per week to exceed our target and finish in 13 weeks, 11 weeks ahead of programme.'
Installation of the main phase of the piling started in July 2002 with Aarsleff peaking at 81 piles per week. The CTRL contract for Morgan Vinci is the largest in Aarsleff Piling's history. It drew on its Danish parent group's extensive plant fleet and used one of its 105t Hitachi KH180.3 GLSK self-erecting leader rigs, modified to handle the heavy piles.
Designed by CTRL project manager and designer Rail Link Engineering, the 600mm square section continuously reinforced concrete piles were cast at the factory of Aarsleff subsidiary Centrum Pile in Newark, Nottinghamshire using Centrum's freeflowing, self-compacting concrete mix.
The mix, which does not need any vibration in the moulds, has been successfully used for over three years in the manufacture of Centrum's smaller and standard square section continuously reinforced precast concrete piles.
The CTRL piles incorporate Centrum's standard pile design features of full length chamfered corners and a domed head, which enables the firm to dispense with the normal reinforcement bands needed for driving conventional flat-topped piles.
The CTRL piles were cast in maximum 14.3m single length segments which weigh 12.5t.
Rigid mechanical joints were cast into the ends of piles longer than 14.3m. Casting was done in purpose built steel moulds that were individually formed and connected by long flexible rubber joints.
A combination of cams and hydraulic rams lock the sides of the moulds in the vertical during casting. After casting and initial curing the rams operate the cams, causing the moulds to flex for easy stripping or striking the pile.
The CTRL piles typically reached a strength of 24-28N/mm 2at 21 hours before stripping.
'The low headroom piling has gone well, ' says Morgan Vinci section agent Steve Meadowcroft.
'It was a big risk for us, but to Aarsleff's credit it developed a rig and installed the piles quickly, efficiently and to price.'