Design Considerations and Deep Tunnel Construction on the Reedy River Basin Sewer Tunnel Project in Greenville, SC

Renewable Water Resources (ReWa) selected construction of a deep, hard rock-tunnel to improve long-term wastewater collection and achieve sustainable growth in Greenville, SC. The Reedy River Basin Sewer Tunnel Project (RRBST), better known as Dig Greenville, is designed to increase existing conveyance and storage capacity and mitigate wet-weather overflows for the next 100 years. The project commenced in March 2018 and is scheduled for completion in May 2021. The 5,950-ft hard-rock tunnel is located approximately 100-ft beneath the Greenville business district and represents the first deep tunnel constructed in upstate South Carolina for wastewater conveyance. After mining is completed, an 84-inch diameter carrier pipe will be installed in the 11-foot diameter tunnel and grouted in place. The deep tunnel option was selected after evaluating several options, including storage reservoir construction and microtunneling/open cut pipeline construction. Although the initial capital cost was higher than other alternatives, there were several advantages. Deep tunnel construction minimized impact to the public, which was especially important because the tunnel alignment passing beneath both residential and commercial properties intersects downtown Greenville. Open cut construction would have been highly disruptive to the thriving businesses in the city. The tunnel option was also deemed to involve the least risk because the design phase geotechnical investigation indicated competent rock conditions at the depth of the tunnel. Finally, long-term operational and maintenance costs were less for the deep tunnel. Ultimately, the tunnel option represented a cost-effective alternative to add both flow capacity and storage with minimal disruption to the community. The project scope also included construction of a 30-ft deep Access Shaft, 100-ft deep Drop shaft with vortex structure, upstream Diversion Structure, odor control system, downstream Junction Chamber, and near surface connections including approximately 400-ft of 60-inch gravity sewer and 1200-feet of 42-inch gravity sewer. Shaft construction and other near-surface work was primarily confined to two construction sites on the upstream and downstream ends of the tunnel. The downstream connection, including Junction Chamber construction and installation of the 60-inch sewer, required construction within sensitive greenspace near downtown Greenville. ReWa performed extensive public outreach and coordination with the City of Greenville to ensure minimal disruption during all phases of the project, including highly visible work near parks and greenways. Outreach efforts included development of a project website, use of social media, and public meetings. The project team later adapted to COVID-19 circumstances by hosting virtual meetings to keep the community informed about progress. Pre-construction surveys of near-by homes were also performed, which also allowed the project team to develop trust with community members. Construction started in March 2018 with excavation of the 100-ft deep Drop Shaft at the upstream end of the tunnel. Drop Shaft construction required drill-and-blast methods in hard rock, and ReWa worked closely with the surrounding community to manage concerns about blasting. During excavation of the Drop Shaft, the Contractor mobilized at the downstream site. Installation of an inclinometer for future monitoring of the downstream shaft excavation revealed more complex geologic conditions than anticipated. The drilling information indicated the bedrock surface was lower than expected, which precluded the launch of the tunnel boring machine (TBM) directly from the 30-ft deep downstream shaft. ReWa collaborated with the Engineer and Contractor to devise a path forward. Following additional geotechnical investigations and extensive coordination, construction of a 230-foot starter tunnel using drill-and-blast methods was selected because of anticipated mixed-face geologic conditions revealed by rock probing upstream of the hillside. Starter tunnel construction commenced in March 2019 and was completed in November 2019. The challenging mining conditions increased the total cost of the project and added 310 days to the contract schedule. However, the overall project cost remained under the estimated budget for the tunnel. ReWa and the project team also held frequent public meetings to communicate progress updates and schedule changes to near-by residents. Acoustical, dust, and vibration monitoring was also conducted throughout the project to protect the surrounding community. The TBM was launched in December 2019, and the tunnel is scheduled for completion in late September 2020. After construction of the starter tunnel in difficult ground conditions, TBM mining proceeded through competent rock with minimal weathered rock and groundwater infiltration, as indicated by the design-phase geotechnical investigation. The TBM achieved production rates of approximately 30-ft per day. Although there was a 4-week delay in mining for mechanical repairs to the TBM thrust cylinders, the mining progress remained relatively consistent despite slightly lower production rates than expected. During TBM mining, the project team also coordinated other design changes to improve project schedule and reduce costs. The project is currently scheduled to be completed in May 2021 after installation of 5950-feet of 84-inch diameter Hobas carrier pipe, flow diversion and near-surface connections, and final site restoration work. As the tunneling industry advances, the use of trenchless technology represents a cost-effective option for future collection system expansion projects. The deep tunnel option was utilized to reduce short-term public impact and long-term operating costs. Differing ground conditions are common for most underground construction projects, so planners should anticipate potential schedule and cost impacts. However, comprehensive geotechnical investigations, risk assessment, and communication can prepare stakeholders for uncertainty. ReWa successfully delivered the Dig Greenville project with a combination of pre-construction planning, community engagement, and adaptation to changing conditions.