OARS shaft-sinking challenges
Oct 2011
Paula Wallis, TunnelTalk
- Shaft construction on the deep rock CSO tunnel in Columbus, Ohio was expected to be challenging, and it is proving to be the case for the contractor Kenny/Obayashi JV.
- To comply with design, that calls for a watertight support of excavation the contractor elected to use diaphragm slurry walls through the upper soft ground layers with a minimum 5ft (1.52m) socket into the rock interface where drill+blast takes over. The contractor ran into difficulty in July 2011 with the sudden inflow of approximately 1,200gpm (4.54m3/min) of muck and water into Shaft 2, the working shaft, with a second inflow occurring in early August said Paul Smith, of Black & Veatch, Construction Manager for the construction management team on the project.
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Fig 1. OARS subsurface geological profile
- "It was determined to be a differing site condition," said Smith. "There were a couple of features in the shaft that did not show up in any of the investigation borings for the GBR (Geotechnical Baseline Report). The contractor chased a couple of high angle vertical joints with clay seams that could not be grouted."
- Excavation was down about 120ft (36.6m) in Shaft 2 when the inflow occurred just below where the slurry walls are socketed into the rock. Smith said the flow of material from the overburden through the feature into the shaft gave some indication of the trouble ahead.
- "Once the situation was realized the contractor attempted to pump cementitious grout from the top down through some of the pre-excavation grout holes. Eventually the contractor brought in a diver to put tubes directly into the feature from the shaft. The contractor pumped over 200gal (757.1 liters) of polyurethane grout, before finally getting the grout to take and shut off the inflow."
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Fig 2. The OARS CSO control tunnel underpasses the Scioto and Olentangy Rivers...
- The Olentangy-Scioto Intercepting Sewer Augmentation Relief Sewer (OARS) is designed to take untreated wastewater, currently discharged into local rivers during heavy rainfall and route it to the city's Jackson Pike and Southerly wastewater treatment plants. The project is one of the largest capital investment elements of the City's Wet Weather Management Plan, implemented as a result of two consent decree orders from the Ohio State Environmental Protection Agency.
- The City of Columbus Department of Public Utilities awarded the Phase 1 contract to Kenny/Obayashi JV in September 2010.
- Set against the Engineer's Estimate of $234.4 million the JV submitted the lowest of three bids at $264.5 million.
- Phase 1 of the project includes excavation of the 23,317ft (7km) deep rock tunnel and three large diameter shafts. Components of a Herrenknecht slurry Mixshield TBM are set to arrive at the end of January 2012 for the under river alignment of the 20ft (6m) i.d. segmentally lined tunnel. Excavation will advance from Shaft 2, the 42ft (12.8m) i.d. working shaft, a short distance from the 52ft (15.8m) i.d. Shaft 1 pump station at the Jackson Pike Wastewater Treatment Plant. It will continue north along and under the Scioto River to the 48ft (14.6m) i.d. reception/access Shaft 6 that will house a 16ft (4.8m) drop shaft from the existing OSIS system and a 20ft (6m) ventilation shaft (Figs 2 and 3).
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Fig 3. ...to relieve the existing Olentangy Scioto Interception Sewer (OSIS)
- Sinking of Shafts 1 and 2 experienced delays due to problems with the slurry diaphragm wall construction and the inflow of water into Shaft 2. "This feature or seam of clay in Shaft 2 is at a high angle, almost vertical, and appears to be going down with us", said Smith. "Shaft 2 is about 140ft (42.7m) down and the contractor has about 35ft (10.6m) to go to the bottom of the shaft." Smith said the same type of feature is also showing up in Shaft 1, which is about 112ft (34m) down, where the slurry walls end.
- "Once the contractor figured out what they were up against, they implemented preventative measures in Shaft 2 and Shaft 1, utilizing polyurethane grout," said Smith. "The contractor has been grouting the perimeter up front before blasting and that seems to be working.
- "The contactor still has to do the starter and tail tunnels and they have horizontal grouting, because this ground is very difficult and complex. It is Columbus limestone with karstic and solution features, voids, cracks, seams, and hydrostatic water with potential pressures of up to 5.2 bars."
- Elsewhere on the project the precast concrete segment plant started production in July 2011 and has completed 445 rings, about 10% of the 4,589 rings that will be required.
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Fig 4. Vertical section of the OARS alignment
- Phase 2 of the project was awarded on August 25, 2011 to Trumbull Corporation, which is just beginning to mobilize. Phase 2 includes two additional 30ft (9.1m) i.d. access shafts (Shafts 4 and 5) with a 16ft (4.8m) drop shaft and a 42in ventilation pipe in each and a 10ft (3m) i.d. drop/access shaft (Shaft 3) with a 42in vent pipe plus approximately 1,350ft (400m) of large diameter surface pipe at Shafts 4 and 5, various manholes and flow diversion structures, and installation and commissioning of an integrated submersible pump system in Shaft 1.
- The project is designed for the City by consultants DLZ Corp in association with Jenny Engineering and CH2M Hill, and construction management is with Black & Veatch in association with local consultants H. R. Gray.
- The TBM is expected to begin its drive at the end of March 2012. Substantial completion for both Phase 1 and Phase 2 is projected for December 2014.
- Columbus mobilizes the OARS project - TunnelTalk, Jan 2011
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