Dual-diameter Robbins TBM makes steady headway
20 August 2020An innovative 11.6m-diameter Robbins main beam TBM continues to make good progress on the excavation of the 8km Mill Creek Drainage Relief Tunnel in Dallas, Texas. The machine, which is the largest hard-rock TBM ever to bore in the US, began excavating in April 2020.
Apart from its record size, the TBM also has the notable ability at any point along its drive to reduce its diameter to a more compact 9.9m.
The tunnel is designed to provide 100-year flood protection to east and southeast Dallas, areas with drainage systems that are up to 70 years old.
Contractor Southland/Mole Joint Venture is preparing to make the diameter conversion at the 2.8km mark along the drive. It is required as two lengths of tunnel have to accommodate significantly different peak flow rates of 42m3/sec and 565m3/sec.
Named ‘Big Tex’, the Robbins TBM has a specialised cutterhead including removable spacers and adjustable bucket lips to convert to the smaller diameter. Once the machine has completed its 11.6m-diameter boring, it will be backed up by about 21m to a transition zone, where it will undergo the conversion over a period of six to eight weeks.
Boring at depths below the city of between 31 to 46m, the machine has so far excavated 580m and has encountered mainly Austin Chalk (12-30MPa). On average, around 12m of advance per shift has been achieved with the best shift so far achieving 19.2m. Probe drilling will be required as the route is expected to encounter gas at various stages.
Ground support measures include installing eight rockbolts at 1.5m spacing with wire mesh and channel straps as required. Insitu-poured concrete 380mm-thick will form the tunnel’s finished internal lining. The project is expected to be completed in 2023.
