A Robbins main beam TBM that will excavate a critical water supply tunnel in Atlanta, USA, is at full production following launch earlier this month (October).
Following an Onsite First Time Assembly (OFTA) period of two-and-a-half months, the 3.8m diameter machine was powered up on October 13, with City Mayor Kasim Reed and other city officials in attendance. The TBM is now boring the 8km Bellwood Tunnel after being walked forward 30m into a starter tunnel. The alignment runs from an inactive quarry, below a water treatment plant and reservoir before ending next to the Chattahoochee River.
The project is owned by the City of Atlanta’s Department of Watershed Management, and seeks to address a critical water supply problem. Currently the city’s emergency reserve water supply is only sufficient to last three days. The new water transfer tunnel will enable the former quarry to be used as a reservoir with a storage capacity of 2.4 billion gallons (11 billion liters)
The PC/Russell Joint Venture, the project’s construction manager at risk, is sub-contracted with the Atkinson/Technique Joint Venture to operate the TBM and oversee construction of various intake and pumping shafts as well as final lining operations. Bob Huie, Senior Project Manager for the PC/Russell JV, said: “Right now, downtown Atlanta’s emergency water supply is approximately three days. Once the tunnel is completed this will increase to between 30 and 90 days, so to be a part of the solution is huge. This tunnel will protect the city for a very long time.”
With the tunnel on the fast track, swift TBM assembly was key. The OFTA process involved coordination by multiple crews at the large quarry site. “The OFTA went very well. The overall assembly process was well organized and supervised by the Atkinson/Technique JV and Robbins. We had a good team of folks to put it all together,” said Huie. He added: “This is a unique job where there’s a lot of people with a variety of backgrounds, but everyone came together to make the OFTA happen.”
The Robbins TBM is currently excavating in granite, with at least 300m of zones in three separate areas that will require continuous probe drilling ahead of the face. In the section directly below an existing reservoir, monitoring will be critical to mitigate against water inflow. The Robbins machine will also be required to negotiate a number of curves. “We have one curve in the first 300m and the main 370m radius curve is 1,800m in. We plan to do short TBM strokes in this section – about 20cm to 30cm shorter than normal – to get through the curves,” said Larry Weslowski, Tunneling Superintendent for the PC/Russell JV.
Excavation is scheduled for completion in the first quarter of 2018. After final lining, the tunnel will be filled with water and the quarry site will become Atlanta’s largest reservoir and park, totaling hundreds of acres. While the park site is a bonus for the city’s 1.2 million residents, the water storage capacity it will provide is critical.
“If the city were to lose water its water supply the estimated economic impact would be at least US$100 million per day. If you consider that this is a US$300 million project, that seems a pretty good investment in comparison to what could happen,” said Huie.