Couplers speed up Channel Tunnel Rail Link project

"To minimise the amount of time the Thameslink line was closed, CORBER put in as many piles as possible before closure of the box."

Advance work on the project started in June 1997 with the diversion of 15km of electricity cables. Work began in earnest on Section 1 of the new high-speed line in October 1998 and that portion of the railway opened in September 2003. Work on Section 2, which is still in progress, began in mid-2001 and will conclude in 2007. The complete line will be 109km in length. Estimated cost of the project, including land, is 5.2 billion (inflation adjusted).

Work on Section 2 includes St Paneras Station and St Paneras Chambers - formerly the Midland Grand Hotel. The first train pulled into St Paneras Station on 1 October 1868. At the time, the station was the largest undivided enclosed space in the world. The Midland Grand Hotel, once a luxurious testament to Gothic architecture, opened in 1873. It closed in 1935 because it had become too expensive to refurbish and run. Now called St Paneras Chambers it currently stands empty until planning permission is granted for the renovation required to turn it into a deluxe hotel and well-appointed residential apartments fit for the 21st century.

Piling installation

Piling work on the foundation of St Paneras Station was completed towards the end of 2004 by CORBER (a joint venture of Costain, O'Rourke, Bachy and Emcor Rail). CORBER installed more than 2000 piles on what is purported to be one of Britain's most complex foundation contracts. Piles were sunk within 1m of rail, gas main and sewer tunnels, all of which had to remain live throughout the contract.

Thameslink 2000 project

Work is also being completed on the Thameslink station box. The Thameslink line is part of the underground rail network providing service throughout London and Southeast England. Previously, Thameslink trains used to run through a quaint Victorian brick tunnel. Modernisation of the tunnel and the box, as part of the Thameslink 2000 project, will provide the capacity required to increase the number of trains from eight to 24 per hour at peak times between St Paneras and Blackfriars. This underground line runs below St Paneras station. The Thameslink box must be completed before the deck extension of St Paneras station can be completed. Currently blockaded for construction, the Thameslink box is due to be completed in June 2005, with trains being put back through the box by 16 May 2005.

To minimise the amount of time the Thameslink line was closed, CORBER put in as many piles as possible before closure of the box. Large diameter cast-in-place piles with high reinforcement densities formed the sidewalls of the new Thameslink box. The walls were both secant and contiguous and piles were typically 1200mm to 1800mm in diameter and 30m deep.

Positional couplers

As a consequence of the high reinforcement densities (up to 400kg/m^sup 3^) on a number of the large-diameter tension piles, the engineer specified that couplers should replace tension laps on T40 and T50 bars. For this reason, CORBER used approximately 5000 LENTON P13 positional couplers in the piling operations. P13 couplers from ERICO splice the bars where the ongoing bar is restricted in its axial movement and adjustability of the coupler is required.

There were typically 100 couplers in each of the piles. Express Reinforcements threaded the reinforcing bars and supplied bars and couplers to William Taylor Steel (WTS) Fixing Limited for prefabrication. WTS then prefabricated the pile cages in Sunderland and transported them to the site in a maximum of 14m lengths due to craneage, transportation and site logistic restrictions. So far, Express Reinforcements has supplied about 24,000 tonnes of reinforcement and will supply another 5000 tonnes before the project is completed.

When the cage sections arrived on site, each cage element already had one end of the coupler installed on it. WTS installed the coupler during the fabrication process.

Completion of full pile cage

A full pile cage was then completed by splicing the three separate elements vertically over the pile bore, prior to concreting. The bottom cage was lowered into the bore first and suspended over the pile casing with a steel beam beneath the lifting band. The bottom of the middle element, which incorporated a thread on the end of the reinforcement, was then lowered into the couplers and the top part of the coupler was torqued up by the site team. The process was then repeated. The bottom and middle cages were suspended and the top element lowered into position.

Each element was very rigid when tack-welded. This was one of the factors that prompted the use of a tapered thread. Due to the critical nature of the relative bar spacings within the three cage elements, they were also fabricated in one length and then dismantled for transportation purposes. Before dismantling the cage elements in the workshop, a bar was arbitrarily chosen and marked in all three elements to enable the site team to easily understand which bars were supposed to be joined together.

Reducer couplers

According to CORBER, the couplers were versatile and, compared with other bolted couplers, quicker to use and easier to handle. The piling operations also incorporated a number of reducer couplers (40-50mm and 32-50mm), which were purpose-made to specifications and delivered within 14 days of order. LENTON R11 reducer couplers from ERICO splice different diameter bars together when one bar can be rotated. The unique taper-threaded design of the coupler provides continuity and structural integrity.

CORBER has used approximately 50,000-60,000 couplers on the project in pile cages, beam and column applications and roof-slab connections. In addition to the R11 and P13 couplers, CORBER is also using A12 couplers.

Concluding remarks

The CTRL has been identified by the European Union as one of the 14 most important routes of the Trans-European Railway Networks. In addition to improving travel, the CTRL will also play a key role in London's 2012 Olympic Games bid.