Tunnelling triumph Down Under for McConnell Dowell

Pringle Hill Pipejack believed to be the longest ever completed in the Southern Hemisphere.

McConnell Dowell Corporation has completed the Pringle Hill Pipejack, which forms part of the construction work on the Northern Pipeline Interconnector project being undertaken by the Northern Network Alliance (NNA) in South East Queensland, Australia. The NNA comprises LinkWater Projects, McConnell Dowell, Abigroup and KBR. The Pringle Hill installation is recognised as a major achievement given that, at about 1,032m in length, the 2,100mm i.d. pipejack is believed to be the longest yet completed in the Southern Hemisphere. Whilst there are longer completed pipejacks in Europe the Pringle Hill installation is different in that it includes a significant vertical curve contributing to a 60m climb from the tunnel launch position to the final target destination. After careful consideration of the options available for the Pringle Hill installation, the pipejack technique was chosen with a state-of-the-art guidance system to control the navigation around the curves and the necessary elevation change over the course of the drive. The tunnelling machine chosen was a Herrenknecht AVND-2000AB microtunnelling system. The unit would be jacked through the drive using the 3m long, 2,100mm internal diameter reinforced concrete pipes that would form the final installed tunnel. During the course of the installation a total of ten interjack stations were deployed throughout the pipe string alignment to keep jacking forces within the necessary limits of the jacking pipe through the curves and alignment changes and the designed capacity of the launch shaft. Ground conditions over the course of the drive comprised silt stone and partial sandstone. The installation was completed with an average production rate of 7.5m per shift, with two 12-hour shifts per day, working seven days a week. The micro-tunnelling system worked well throughout the course of the installation which started in August 2010. One of the most important aspects of the construction was the required alignment and navigational control. The construction plan called for the drive to cover some 1,030m in a straight line in the horizontal plane. In the vertical plane, the first 50m were straight followed by 780m curve on a radius of 8,000m. The drive was to be completed with the final 215m being straight but with a +10% grade. This meant that the navigation utilised for the drive would be a key element in the success of the tunnel. All services and products used for the navigational control system were provided by tunnel navigation systems specialist, VMT, with the unit used on the Pringle Hill installation being supplied through the company’s Australian subsidiary, VMT Tunnel Guidance, which is based in Brisbane. Given the level of directional control required for the installation at Pringle Hill, the correct choice of navigational equipment and technique was vital. After considering the options available, it was decided to operate the microtunnelling machine with a VMT SLS-Microtunnelling LT – Laser Total Station system. The SLS-Microtunnelling LT guidance system was chosen because, according to VMT, it offers significant advantages for longer or curved tunnelling projects. The ability to determine, continually update and display the tunnel boring machine’s (TBM) current position, independent of drift or refraction, guarantees optimum control in long pipeline excavations and in complex curves. The guidance system software employs a self-referencing algorithm that uses the measured system points to create a whole, uninterrupted representation of the actual pipe string position. The motorised laser total station is mounted in a fixed position in the pipe. The system measurements are carried out during the machine’s ongoing advance, meaning that no interruption to production is required. Control measurements for system adjustment are taken in intervals of between 60m to 120m, depending on the route. A surveyor requires only minimal time and effort for supervision during the project. Reduced machine down time therefore makes this high performance system very cost-effective, especially for quick excavations. Daily remote monitoring services offered by VMT allowed for smoothing of the reference track of the guidance system. The navigation technique allows the TBM measurement to determine precisely the laser target position and offsets. The PLC Communication Link also offers data logging of TBM data. All of this means that the DTA can be calculated and verified fully. The system also allows for real time internet visualisation of the relevant TBM data including both navigation and machine performance data. All survey surface network measurements on the project were completed by Surex. During the course of the drive, two gyro-theodolite survey campaigns were implemented to check the position and route and, once completed, there was almost no orientation corrections required to the ongoing tunnel route/alignment. The first campaign was completed at a chainage of 400m with the second being done at a chainage of 780m. Ultimately the pipejacking section of the tunnelling works was completed on November 1 last year, some five weeks ahead of programme. Breakthrough was achieved to a high accuracy with the drive being less than 50mm off the target point in both horizontal and vertical planes. www.vmt-gmbh.de

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