TBM TUNNELING | TECHNICAL
IN-TUNNEL TBM DIAMETER CONVERSION At its most recent annual awards, the International Tunnelling and Underground Space Association (ITA) announced the 2022 winner of the Technical Innovation of the Year to be Robbins for its ‘Unprecedented in- tunnel diameter conversion of the largest hard rock TBM in the USA’ on the Mill Creek/Peaks Branch/State- Thomas Drainage Relief Tunnel, in Dallas, Texas. The task for the Main Beam TBM, a re-built machine
named ‘Big Tex’ for the job, was to bore an 8km-long (5 miles) large diameter sewer through chalk and shale. Some 2.8km-long (1.7 miles) into its drive, the 11.6m-diameter (38ft) TBM diameter was halted, as planned, and its diameter reduced to 9.9m (32.5ft) through a series of sequential steps, the machine having been designed for that purpose. The TBM’s supporting tunneling system (trailing gear, continuous conveyor system) was also adjusted. Robbins field service manager, Evan Brinkerhoff, said
in a statement shortly after the machine holed through in mid-2022 to complete its drive: “We started with a smaller machine, then we made a kit or second skin that was slightly larger. And then, when the time came, we just took that skin off.” It was that simple, he says, but the sequential process
in-tunnel took about four months to perform, in early 2021. The challenge arose from the drainage system design
requiring different tunnel sizes, and cross-sections envisaged, so as to handle different flows rates on the upstream and downstream side of a branch tunnel feeding in on the network. Upstream, a circular cross- section was proposed, downstream a horseshoe- shaped section over the final 2.8km (1.7 miles). TBM boring was to be used throughout but the
downstream end would be widened to the horseshoe- shaped section by roadheader, creating a flat invert. Different formworks would be needed for the downstream and upstream lengths of tunnel. But evaluations, including of time and cost, dropped the
two-stage tunneling plan in favor of using a single TBM for the entire length – but not a single diameter. The larger drainage tunnel flow rate for the
downstream section would be achieved with a larger diameter section, excavated during the initial phase of tunneling. The upstream section would be bored after the in-tunnel changes of the TBM and tunneling support system. The ground was not ideal where a local backup of
the machine was planned to facilitate the changes, and so with extra rock support to the crown only a miniscule backup could be done and was sufficient. A 52m (57.9yd) deep drop shaft close by, reached via a short adit opened up for the purpose, would be the access route for personnel to execute the works. Four large pieces of the cutterhead, the heaviest up
to 15 000kg, were removed and taken to the surface, re-fitted for the smaller diameter requirement and then taken back underground. While refurbishment was performed on the surface, the changes were made to the rest of the TBM, trailing gear and conveyor. By the time of the in-tunnel works, the geology in the
downstream section had been of low rock strength and low abrasivity that few discs on the cutterhead needed changing. But the opportunity of the planned stoppage was taken to swap them all out – there was full dress change of the cutters. Afterward, on the narrower upstream drive, only 14 cutter changes were needed. Breakthrough was achieved in the middle of 2022.
Final lining is being constructed. The project is due for completion by early 2025. The drainage tunnel is designed to handle flood events of 1-in-100-year return period in east Dallas.
MCNALLY SLATS SUPPORT DELIVERY Tunnel boring on the Yin Han Ji Wei water diversion tunnel was a tough challenge, as expected, and the 8m-diameter Robbins Main Beam TBM on the project came fitted with the McNally support system. Well proven of many projects with difficult rock conditions.
Spring 2023 | 31
Above left:
TBM skin removal as part of diameter change
Centre:
Workers gather as TBM breaks through into a shaft at Mill Creek
Above right: Cutterhead installation
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