C+S September 2022 Vol. 8 Issue 9 (web)

“It cost us about 10 months of project time. The tunnel boring machine can dig out about 40-50 feet per day. We were only digging out about two feet per day. It was very time-intensive and labor-intensive just to get started.” The tunnel boring machine helped workers pick up the pace after com- pleting the starter tunnel. The 130-ton TBM, made by The Robbins Company in Canada, measures 249 feet and is one of only a handful of similar pieces of equipment in the world. Super Excavators of Wis- consin started tunnel digging in March 2018 and completed their work in September 2020. The TBM was critical to the completion of the project in that it offered a far more efficient drilling method. TBMs are used as an alternative to drilling, blasting and hand-mining to excavate tunnels with a circular cross-section through layers of soil and rock. The machines significantly minimize disturbance to the surrounding ground and area. They are frequently used in urban areas and reduce project completion times. Out of Sight For a project of this magnitude and duration, workers were surprisingly able to stay out of the public glare. Almost all the construction took place underground, out of sight of city residents. Teams constructed wooden fencing around the construction to mini - mize the aesthetic impact of the project. During a two-month winter period, one roadway was closed off to facilitate quicker construction for a sewer crossing across Richland Creek and to accommodate the city’s streambank restoration project. Project at a Glance What: Dig Greenville, also known as the Reedy River Basin Sewer Tunnel, is a 1.3-mile gravity-fed sewer line in South Carolina. Project details: The $46 million project started in 2018 and concluded in 2022. It is the largest infrastructure project in Greenville’s history. The project is expected to support Greenville’s wastewater conveyance needs for the next century. Why it’s important: The existing sewer line face pressure from increasing population. Without more capacity, the community would be at risk for overflow. Digging deep: The tunnel is located as far as 100 feet beneath the surface. Few people will even know it’s there. The only evidence visible are access points at each end. 13 floor access doors manufactured by BILCO provide workers access to install, remove and repair equipment. Did you know? Greenville is sixth in population and growth rate in South Carolina.

the city and water basin. Planners chose to install the new line under - ground, approximately 100 feet below the heart of the city. “While it is pricier to build, a deep sewer tunnel powered by gravity will be far less costly over its lifecycle for ReWa while providing the reliable additional capacity Greenville needed as it continues to grow,’’ said ReWa Chief Executive Officer Joel Jones, who took over as Ex- ecutive Director from Rich earlier this year. Hidden Tunnel The tunnel is 7 feet in diameter and virtually invisible to the public. Entry shafts at each end are the only hint of the massive pipe under the surface. The pipes are encased in granite, lined with fiberglass and grouted. The gravity fed system means no mechanical equipment is needed to convey the flow of wastewater. “The tunnel is bored from one end, resulting in surface impact only at either end of the tunnel, rather than all along the sewer route if conven - tional construction methods were used,’’ Jones said. The initial plan was to drill from the downstream access shaft through the hard rock below with a tunnel boring machine. Before the TBM could be launched, however, a geotechnical investigation found the tunnel zone was composed of soil and different types of rock in varying conditions. “The tunnel boring machine can only work through one type of mate- rial,’’ Jones said. “Right when we were getting started, we saw that the granite was not where we thought it was.” The complication resulted in hand-digging a starter tunnel. Starter tun- nel construction also included drill and blast methods that required 41 blasts over a 9-month period. Each blast was modified to fit the zone’s complex geology. Workers also fabricated and installed a customized steel shield to secure ground support for the 14-foot-round horseshoe- shaped starter tunnel. “What we found was about 240 feet of clay and rock that we had to dig out,’’ Jones said. The existing sewer line was near capacity and face pressure due to the increasing population in Greenville.


September 2022 csengineermag.com

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