High-Powered Robbins TBMs jump start Jollyville
Deep below Austin, Texas, USA, the sprawling Jollyville Transmission Main is set to dramatically increase capacity of the city's main drinking water reservoir. The 10.5 km (6.5 mi) waterway is being constructed using three TBMs, including two Robbins machines, up to 107 m (350 ft) below the city. The Southland/Mole JV is building the 2.1 m (84 in) finished diameter pipeline below residential areas and the protected Balcones Canyonlands Preserve.
Three TBMs will excavate the pipeline, with one contractor-owned machine having completed its 1.4 km (0.9 mi) section in mid-2012. Robbins supplied an additional 3.25 m (10.7 ft) Main Beam TBM, and refurbished a 3.0 m (9.8 ft) Double Shield TBM in its Solon, Ohio, USA manufacturing facility. Both machines were launched in August 2012 from deep shaft sites.
“We needed the Robbins High Performance Main Beam TBM for the longest bore, which is on the critical path for the project. It’s built to beat our 230 day schedule, and for even tougher conditions than are foreseen here,” said Tim Winn, Director of Southland Contracting.
Conditions along the way are expected to consist of uniform limestone and dolomite rock. Although karst features are present throughout the formation, the depth of the tunnel should circumvent these features. Other obstacles are associated with the protected wildlife area—endangered cave-dwelling invertebrates including six species of arachnids and insects are present in and around the karsts. Because of this, no probe grouting can be performed due to the risk of seepage into the water features. “We don’t expect any features that will need significant support. Rock bolts will be the predominant form of support, and there may be some areas requiring wire mesh. Anywhere there is a water feature, we will install a liner that will be grouted in place to seal those zones,” said Winn.
Within a week of its launch in late August 2012, the Main Beam machine had advanced about 90 m (300 ft), and it is currently keeping up a strong pace, having bored ahead 760 m (2,500 ft). The fast advance is a result of extensive planning, as logistics are often a limiting factor at small tunnel diameters: “We need to plan well in advance how ventilation and muck removal will work towards the end of tunneling. Ventilation is the biggest issue. We have multiple trains in the tunnel at once, so the requirements for ventilation are significant. Our second biggest problem is getting people and materials in and out, which takes quite a bit of time,” said Winn. In the Main Beam tunnel, Southland has planned for two California switches and one shaft switch. An oversized vent duct and additional fans will help aerate the tunnel.
Once complete in 2013, the pipeline will transfer up to 190 million liters (50 million gallons) of treated water per day from Lake Travis. The tunnel, for the Austin Water Utility, will connect up with the new Water Treatment Plant 4 currently under construction—part of a larger scheme to provide increased water capacity for a projected 60% increase in population over the next two decades.