Breakthrough on Parmer Lane Wastewater Interceptor10 September 2019
Texas – Breakthrough on the Parmer Lane Wastewater Interceptor saw the longest rock tunnel bored by a double shield TBM under 2.5m. The machine, a 2.46m Robbins, completed the 3,475m drive with no intermediate access.
Contractor S.J. Louis Construction navigated two 150m curves and unexpected ground conditions that required modifications to the cutterhead and disc cutter arrangement. Advance rates reached 380m per month while mining single 12-hour shifts per day.
The tunnel is located in an environmentally sensitive aquifer, with ground conditions ranging from soft dolomite with clay to limestone from 13 to 68 MPa UCS. While the team tunnelled through the softer material, the best advance rate was close to 0.9m per hour. When they tunnelled through the expected limestone, advance rates were over 5.2m per hour. The best day was 25m in a single shift.
Zach West, Project Manager for S.J. Louis said, The challenges for the TBM and its crew were varied, explained West. “The pairing of this tunnel length, which is on the longer side, and the diameter, which is on the smaller side, is challenging. The surveying in a small tunnel with tight radius curves and limited surface access for over two miles is very difficult.” He added that the shallow tunnel depth, and the alignment to within a few feet of sanitary lines, high-pressure gas mains, and fuel tanks for petrol stations, made TBM guidance critical. West added, “I would say that I am most proud of our ability to guide the machine successfully through these obstacles and into our retrieval shaft within our expected tolerances.”
Through one stretch, the tunnel advanced directly between a 300mm diameter, high-pressure gas main and fuel tanks for a gas station with limited as-built information. “Navigating this section took a great deal of coordination with the local utility companies. Because the tunnel diameter was too small for an automated guidance system, we manually surveyed the front of the machine at every push to ensure the machine was on track.”
The majority of the tunnel used a simple two-rock-bolt pattern for support. In the last 10% of the tunnel, ribs and lagging were used as support. Final carrier pipe, which is now being installed, consists of 1,100mm diameter fiberglass pipe.
The Parmer Lane Wastewater Interceptor connects to two existing lift stations at Lake Creek and Rattan Creek. The tunnel allows for these lift stations to be decommissioned, and will provide additional flow capacity by gravity, reducing operating costs for the City of Austin.