Precast Concrete
Driving New Links Ahead Of Time
Aarsleff Piling successfully completed eleven weeks ahead of programme the restricted headroom phase of its prestigious £4.5M contract to supply and install huge 600mm by 600mm square section precast concrete piles on Contract 310 of the Channel Tunnel Rail Link. Aarsleff, using one of its own purposely adapted fixed leader piling rigs working in a minimum headroom of just 8m under three separate bridges, installed nearly 230 piles up to 15.3m long, which were made from segments up to 3.5m long using specially designed rigid mechanically joints that are stronger than the piles.
The restricted headroom project was part of Aarsleff’s specialist piling sub-contract, for main Contract 310 joint venture contractor Morgan Vinci, to supply and install approximately 2,500 precast concrete piles up to 17.5m long. The piles, typically in rows of four and six at 5m centres support sections of the 7km long, 10m wide reinforced concrete track slab, which carry the high speed line over the soft West Thames marshes at Havering and Thurrock, Essex, where ground conditions are typically 4m to 10m of alluvium over flood plain deposits, tertiary clays and sands or chalk.
“The low headroom piling has not only gone well but exceeded programme,” says Morgan Vinci section manager Steve Meadowcroft. “It was big risk for us, but to Aarsleff’s credit they developed a rig and installed the piles quickly, efficiently and to price.”
Aarsleff started on the low headroom using one of its own specially adapted Banut 500 self-erecting fixed leader hydraulic piling rigs with 5t hydraulic drop hammer for the unique project. Aarsleff, in conjunction with piling rig manufacturer Banut, developed a leader system, which included a modified leader, cathead, hydraulic raking arms and drop hammer, specifically to handle the pile segments within the 8m restricted headroom under the two main A13 trunk road flyovers and one other bridge. The modifications ensured that all components were reduced to there minimum dimensions whilst permitting the maximum pile segment lengths. The hammer was modified to reduce its overall length, whilst at the same time increasing its drop height capability, which was necessary to ensure enough energy was available to install the huge piles. The company believes this is the first time anywhere that this size of precast concrete pile has been installed in such low headroom.
“The modified rig, can be adapted to work in restricted headroom between 8M and 11.0M, has worked extremely well” says Aarsleff piling manager Philip Chippindale. “The rig was developed in close consultation with Morgan Vinci and RLE to ensure that key areas of concern were identified and subsequently accommodated in the design of the modifications to the rig to ensure peak performance of the rig on the contract. The main items identified were ensuring the hammer had sufficient energy to install the piles and would be reliable in hard driving conditions, maximising pile segment lengths and having the ability to work under structures of differing heights so that pile segments lengths could be optimised to suit individual structures. Despite a few minor problems with the system, which was previously untried and untested, and a overcoming a considerable amount of hard driving on the first two structures, we have nearly halved the contract period. We progressed better than expected and managed to install an average of 207 linear metres/week to exceed our target and finish in 13 weeks, 11 weeks ahead of programme.”
Aarsleff peaked installations levels at 81 piles/week. The CTRL contract for Morgan Vinci was the largest in Aarsleff Piling’s history and the company has drawn on its Danish parent group’s extensive plant fleet and is using one of its 105t Hitachi KH180.4 GLSK self-erecting leader rigs, which has been specially modified to handle the extremely large, heavy piles. Casting of the 600mm square section continuously reinforced concrete piles, believed to be the largest made in Western Europe, were produced at Aarsleff’s wholly owned subsidiary’s Centrum Pile factory at Newark, Nottinghamshire.
CTRL Project Managers and Designer, Rail Link Engineering designed the 600mm square section piles, which are being cast using Centrum’s special and proven free flowing, 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 also incorporate Centrum’s standard pile design features of full length chamfered corners and a domed head, which enables the company to dispense with the normal reinforcement bands needed for driving conventional flat-topped piles.
The special and perfectly symmetrical 600mm square section CTRL piles are being cast in maximum 14.3m long single length segments, which weigh 12.5 tonnes. Special rigid mechanical joints are cast into the ends of piles that exceed 14.3m. The CTRL piles are made using purpose built moulds, which operate on an identical principle to Centrum’s smaller standard pile section moulds. The steel moulds are individually formed and connected together 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 reach a strength of 24-28N/mm2 at 21 hours prior to stripping.
Prior to Aarsleff starting on Contract 310 Rail Link Engineering carried out extensive testing, monitoring and analysis of pile behaviour during and after driving of precast concrete test piles by Aarsleff in the soft ground at two locations along the proposed route. These trials, carried out by RLE contributed to a saving of about six months of the main driven piling contract as the data obtained from the trial sites was used by RLE to produce the final foundation design and pile configuration for Contract 310.
