Design changes help secure project success Sep 2012
Robbins News Release
- Work is complete on a new sewer tunnel system in the USA for which the contractor helped cut costs and save time by eliminating three of the 11 shafts called for in the original design.
- The innovative approach by contractor Midwest Mole to the replacement of an outdated system that had been dumping raw sewage into Shayler Creek allowed significant and unexpected project savings for the client, Clermont County Water Resources Department of Ohio.
Midwest Mole workers with Robbins rockhead
- The changes also enabled the project to be delivered on time in spite of delays late on caused by the need to negotiate with landowners over access for the seventh and final drive.
- Since 1978, when the previous pipeline was installed, the creek had eroded, exposing the pipe and putting certain sections of the line at high risk of failure.
- A 1.8m diameter Robbins double shield rockhead was used to excavate all of the tunnels, totaling 2,900m. Because of the project location below the creek bed, ground conditions were highly varied, consisting of mixed layers of limestone and shale that ranged from dry to sticky and wet. To accommodate the mixed ground, the machine was outfitted with a mixed ground cutterhead that could be swapped out for a hard rock cutterhead based on the ground conditions at each crossing.
- The mixed ground cutterhead featured 6.5in single disc cutters and carbide bits, while the hard rock cutterhead contained 11.5in single cutters and abrasion-resistant muck scrapers. Both cutterheads had large openings that allowed for efficient cutter changes.
- The tunnels were excavated by SBU, while the shafts were constructed using a combination of drill+blast techniques and manual excavation. A primary liner of ring beams and lagging was installed every 1.5m at each crossing. Even with liner installation, high production rates of 12-18m per 12-hour shift were maintained for the project duration.
Breakthrough on one of the seven tunnel alignments
- Due to the gravity sewer construction, each tunnel had strict line-and-grade requirements of within 300mm of line at 1-2% grade, which was continuously monitored from an in-shield operator's console. Over the course of the project, the variance resulted in a vertical change of 53.9m causing the machine to bore through a wide range of ground conditions.
- Excavation of the initial 484.3m crossing began in May 2010 in mixed ground, and the rockhead broke through to its first shaft site that August. After a few minor modifications to the hydraulic and muck haulage systems, the machine began boring the second 575.4m crossing.
- Crossings 3 and 4, 321.9m and 304.8m, respectively, were excavated in December 2010 and January 2011 in adverse winter weather conditions. In order to keep tunneling during freezing temperatures, the contractor heated the machine's cooling water overnight. Cooling water was recycled and filtered, then stored in 7,500-litre tanks with heaters. Using this technique, production rates stayed high and both crossings were successfully excavated by January 2011.
- The rockhead began its fifth and longest crossing of 613.8m in April 2011. For the last two crossings, 402.3m and 196.9m respectively, the mixed ground cutterhead was replaced with its hard rock counterpart.
- Due to negotiations with local landowners and a change in shaft location, the final crossing began in January 2012. A month later, the drive was completed. In July 2012, the entire project reached completion right on schedule, with all manholes and ancillary sewers tied in and in service.
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