The diameter conversion occurred 2.8km into the excavation of the Mill Creek Drainage Relief Tunnel, Dallas, US. Having started out with a cutterhead diameter of 11.6m, the unique machine was subsequently converted to bore a more compact 9.9m-diameter (concentric to the larger bore). The TBM is boring through a relatively soft and non-abrasive Austin Chalk (12-30MPa UCS) at depths of 31-46m below the city.

To undertake the diameter conversion, the crew accessed the cutterhead to remove the ‘skins’ which had been applied around the machine. A small lateral tunnel had been excavated previously to give full access to the front of the cutterhead. Modified pieces were then reinstalled on the cutterhead to facilitate the conversion to the smaller bore. The operation was complex given the size of the pieces involved and the limited space and equipment inside the tunnel.

Two tunnel diameters were required as the two stretches of tunnel have different cross-sections and peak flow rates (PFR): upstream will have a circular cross-section and PFR of 424m3/sec; downstream will have a horseshoe cross-section and 565m3/sec PFR. The tunnel will have seven shafts and a total volume of 727m litres (160m gallons).

Currently the largest water tunnel project under construction in North America, the US$300m Mill Creek Drainage Tunnel is an 8km flood protection tunnel involving the intersection and diversion of surface stormwater drainage from various watersheds in Dallas. It is designed to provide 100-year flood protection to east and southeast areas of the city and is expected to be completed in 2023. The contractor is Southland Holdings / Mole JV; consulting engineer is Black & Veatch; and the owner is the City of Dallas.