High-power TBMs jump start Texas water line Sep 2012
Desiree Willis, Technical Writer, The Robbins Company
- 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 10.5km waterway is being constructed using three TBMs, including two Robbins machines, up to 107m (350ft) below the city. The Southland/Mole JV is building the 2.1m i.d. pipeline below residential areas and the protected Balcones Canyonlands Preserve.
- Three TBMs will excavate the pipeline, with one contractor-owned machine having completed its 1.4km section in mid-2012. Robbins supplied an additional 3.25m main beam TBM, and refurbished a 3m 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," said Tim Winn, Director of Southland Contracting. "It is built to beat our 230 day schedule, and for even tougher conditions than are foreseen here."
Both TBMs were delivered from the Robbins workshops in Solon, Ohio
- 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, with endangered cave-dwelling invertebrates including six species of arachnids and insects being 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 do not expect any features that will need significant support," said Winn. "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."
- Within a week of its launch in late August 2012, the main beam machine had advanced about 90m and was keeping up a strong pace and completing 760m to end September. The fast advance is a result of extensive planning to address logistics that are often a limiting factor at small tunnel diameters. "We need to plan, well in advance, ventilation and muck removal as they extend 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 ventilate the tunnel.
- Once completed in 2013, the pipeline will transfer up to 190 million liters of treated water per day from Lake Travis. The tunnel, for the Austin Water Utility, will connect to the new Water Treatment Plant 4, which is currently under construction as part of a larger scheme to provide increased water capacity for a projected 60% increase in population in the Austin area over the next two decades. Tunneling for the water supply network adds to two other major tunneling projects in Austin - the Waller Creek flood relief project the Downtown Wastewater Tunnel project with three crossings of the city's Lady Bird Lake waterway.
Tunneling to ease flooding in Austin - TunnelTalk, April 2011
Austin prepares for city center redevelopment - TunnelTalk, January 2010
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