All done after tough drive in Atlanta 25 Oct 2018

Project teams watched as a 3.8m diameter Robbins main beam TBM completed an 8km long journey through hard rock conditions to create a water supply tunnel for the city of Atlanta, Georgia.

Hard granitic rock of between 117 and 310 MPa presented a challenge for the 19in disc cutters from the outset. “There was ground so hard that it would take eight hours to advance 1.5m,” said Larry Weslowski, Tunnel Superintendent for the PC Russell JV. The beginning of the job was tough, but he added that once through the learning curve “the TBM and its crews started breaking project records. Rates just kept increasing and towards the end they achieved a best day of 38.4m.”

19in cutters tested by hard granite

OFTA assembly in the Bellwood Quarry

The new tunnel is designed to increase the water capacity for the city of Atlanta to between 30 and 90 days, depending on daily usage. “Our schedule for the project was very aggressive but the project team stayed together to overcome issues related to the excavation of the tunnel,” said Bob Huie, Project Director for the PC Russell JV, the construction manager at risk for the project.

The structure of the project team is credited with the overall project success despite its challenges. “There was great leadership and supervision within the team and that provided the determination to overcome obstacles and succeed,” said Weslowski.

The project is the third such large construction project in the USA to use the CMAR (construction manager at risk) procurement structure. The PC Construction/HJ Russell JV was selected as the CMAR and in turn purchased the Robbins TBM for the long tunnel drive. JP2, a JV of Stantec, PRAD Group, and River 2 Tap, as designer for the construction works, including tunnel and shafts, specified a hard rock TBM for the excavation. Assembly and operation of the TBM was then subcontracted to the Atkinson/Technique JV.

Project team celebrates TBM breakthrough

The robust TBM was assembled at the Bellwood Quarry site with the help of Robbins personnel and using the Robbins onsite first time assembly (OFTA) process. “OFTA helped with the scheduling and the guys built everything as per the specs,” said Weslowski. “It was a challenge but there was no negativity during the process.” Despite summer temperatures up to 43^oC with humidity at 100%, the TBM was ready to launch by October 2016.

As well as very hard rock conditions, other challenges included groundwater inflows. “We also encountered groundwater contamination that required remediation, which was completed successfully,” said Huie.

With tunneling complete, the US$300 million project for the City of Atlanta Department of Watershed Management is on track to meet its scheduled project completion date of September 2019. The project will turn the inactive Bellwood Quarry into a 9.1 billion liter (2.4 billion gallon) raw water storage facility connecting with the Chattahoochee River and various water treatment facilities.