Wirth TBM Breaks Through CTRL Tunnel SectionOn 3rd December, 2003 the first Wirth TBM of the Channel Tunnel Rail Link project broke through after record time. It took only 299 days to drive 4.7 km, an outstanding performance. The downline section of lot 240, from Stratford East Station to Barrington Road, was finished exactly one week before the upline section of the two parallel tunnels. The Wirth breakthroughs are the CTRL first ones to be achieved by EPB machines. The average daily advance of the machines was 30 metres in two shifts. The best 24-hour performance was 37 rings or 56 metres, with best four-week period of 620 rings or 930 metres.The Wirth crews expected to encounter layers of sand and clay with occasional inclusions of flint. Thus, the 8.16 m-diameter EPB shield type TB 816 H/GS was equipped with carbide teeth and prepared to be fitted with disc cutters for a harder ground configuration. Although the machine had to pass through chalk strata, up to one metre in thickness with flint inclusions, the installed teeth proved sufficient to do the job. The very hard and brittle flint produced cuttings that appeared to have an average diameter of 75 mm, with many large pieces that were up to 180 mm in diameter. A problem arose from the dewatered ground right from the start. Dewatering had been necessary for the construction of two ventilation shafts in between which the tunnels were to be bored. In addition, the alignment of the tunnels was dewatered to assist construction of the eight cross-passages between the two parallel drives, avoiding ground freezing as an alternative. Within 30 metres from the launch, the machines were required to pass underneath the London Underground Central Line with as little as 4 m of clearance. As a result of the dewatering, this zone was predominantly dry sand which was very hard to pressurise with an EPB machine. The ground just did not flow easily. In the beginning, the crew added some 15 tons of water per ring to stabilise the ground. Nonetheless, the machines advanced remarkably well. In particular, the steering followed precisely the alignment of the rather curvaceous tunnel and was very easy to handle. For breakthrough, an auxiliary reception shaft was sunk and a steel support frame and cradle installed to a tight tolerance. The TBM came in just where it should - quite a precision landing!The degree of settlement control was another prerequisite. In particular, when passing four metres underneath the Central Line, London Underground would tolerate little or no settlement. A face volume loss of 0.5% was predicted but the actual loss was no more than 0.3%. To achieve this minimal settlement, the TBM was equipped with an active front articulation to allow for small steerage corrections and rear passive articulation.The machines are fitted with a vacuum segment erector operated from a remote unit. The erector rotates through 220 degrees to place the precast concrete segments. Segment pick up is aided by a laser guidance system. The cutter head rotates at a maximum speed of 3 rpm developing a maximum torque of 15,500 kN. The head advances by means of 29 hydraulic cylinders, achieving a thrust of 63,000 kN. Installed power totals 2800 kW. The machines have an overall length of 100 m each and weigh 1,050 tonnes.The cutter head can be dismantled for transport by flat bed trailer. It is equipped with carbide teeth but is designed for rear-fitting disc cutters as well. Teeth are fitted to the arms sideways and can also be changed from the rear. Cuttings fall through openings in the cutter head into the extraction chamber and are conveyed by means of an auger and subsequently by conveyor belt.The awarding authority for section 240 is Union Railways (North) Ltd. The contractors are Costain, Skanska and Soletanche Bachy in joint venture. Each machine was manufactured at Wirth's plant in Erkelenz, Germany and its French sister company NFM Technologies in Le Creusot, France.The CTRL is UK's first new railway for over a century, running for 109 km between Central London and the Channel Tunnel. Section 1 between the Channel Tunnel and North Kent is complete and operational, reducing Eurostar journey times by 20 minutes. The twin 8,165 mm-diameter tunnels bored by the Wirth machines are part of Section 2 which, when operational in 2007, will reduce the journey time to Paris and Brussels by 40 minutes. Click uk/21. Visit www.wirth-europe.com, www.nfm-technologies.com and www.ctrl.co.uk 06/04.