Columbus mobilizes the OARS project
Jan 2011
Paul J. Smith, Black & Veatch, Project Manager, Construction Management Team, OARS Project
- Equipment has mobilized to site to begin construction of a 23,317ft (7km) deep rock storm water overflow and relief sewer for the City of Columbus in Ohio. The project will augment the existing Olentangy Scioto Interception Sewer (OSIS) to further limit combined overflows into the city’s Olentangy and Scioto Rivers. The contract is awarded to the Kenny/Obayashi JV which has ordered a Herrenknecht slurry Mixshield for the under river alignment.
- OARS, the OSIS Augmentation and Relief Sewer project, is one of the largest capital investment elements in the Wet Weather Management Plan (WWMP) developed by Columbus City in July 2005 after receiving two consent decree orders from the Ohio State EPA (Environmental Protection Agency). The first mandated development of a Capacity, Management, Operation, and Maintenance program that included a System Evaluation and Capacity Assurance Plan. The second ordered the development of a Long Term Control Plan to meet requirements of the USEPA Combined Sewer Overflow Control Policy. To comply with the consent decrees, planned improvements are required to be fully implemented no later than July 1, 2025.
- The OARS tunnel is designed to collect and consolidate combined overflows from the OSIS and convey the combined flows to the Jackson Pike and/or Southerly wastewater treatment plants for biological treatment. Developed to provide adequate capacity for all storms in a typical year though the year 2047, the OARS tunnel is a massive undertaking for the City of Columbus. It has included several years of planning, detailed design, financial planning, and partnerships with many professional consultants.
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The new 23,317ft of 20ft i.d. (7km x 6m i.d.) OARS CSO control tunnel for the City of Columbus of Ohio follows and underpasses the alignment of the Scioto and Olentangy Rivers...
- 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 project is to be built in two phases. Phase 1 comprises three large diameter shafts and the 23,317ft of TBM driven, 20ft i.d. segmentally lined tunnel. Aligned at about 180ft deep, tunneling will advance from Shaft 2, the 42ft i.d. working shaft which lies adjacent to the 52ft i.d. Shaft 1 pump station at the Jackson Pike Wastewater Treatment Plant, and continue north along and under the Scioto River to the 48ft i.d. reception/access Shaft 6 that will house a 16ft drop shaft from the existing OSIS system and a 20ft ventilation shaft.
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...to relieve the existing Olentangy Scioto Interception Sewer (OSIS) and direct combined stormwater flows to the Jackson Pike or Southerly treatment plants
- Phase 2 consists of two additional 30ft i.d. access shafts (Shafts 4 and 5) with a 16ft drop shaft and a 42in ventilation pipe in each and a 10ft 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.
- With a program to have Phase 1 of the project substantially completed and in service by December 2014, the contract design was ready to be advertised in February of this year (2010) and awarded by September. At a bid opening in April, the Kenny/Obayashi Joint Venture emerged as the lowest of three bids at $264.5 million.
- Set against an Engineer's Estimate of $234.4 million, the two competing bids were submitted by Traylor/Jay Dee/Shea at $290.5 million and Kiewit/McNally at $307 million.
- The Phase 2 contract is scheduled to be advertised in February next year (2011) and to run concurrent with Phase 1 towards a substantial completion at the end of 2014. Access from the 20ft i.d. TBM tunnel to complete the 10ft i.d. horseshoe adits to Shafts 3, 4 and 5 will be allowed by the Phase 1 contractor via Shaft 6 once Shaft 6 and the tunnel drive is complete. Each contract includes construction of operational infrastructure including relief, diversion and by-pass structures, smaller diameter utility pipelines, screens and floodgates, and electrical installations. Final completion of the project is programmed for mid-2015.
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Slurry specification
The deep rock tunnel is designed by DLZ Corp and Jenny Engineering as a slurry TBM operation with a precast concrete segmental lining. Although located for 85-90% through good quality dolomite and limestone rock of good to fair to excellent RQD values, there are three anticipated bedded planes of Columbus Limestone Basal Conglomerate as well as three possible faults and a number of voids, karsts and solution features. In these anomalies, hydrostatic water pressures of up to 5.2 bar and more are anticipated. - After notice to proceed in September, Kenny/Obayashi placed an order for a new slurry Mixshield with Herrenknecht for the 7km long tunnel drive. To cope with the combination of hard rock with high ground water pressures, the Mixshield is designed to operate in different modes from fully open to semi-closed and fully closed. Much of the design of the machine is based on the experience of the two Mixshields designed and delivered by Herrenknecht to the Arrowhead project in California where conditions were a similar combination of higher-strength rock with a high water table pressures and more overbreak above the TBM. Excavated material, cut by disc cutters on the rock cutterhead, will either pass through a screw conveyor extending from the invert of the excavation chamber through the pressure bulkhead to pass through two gates and onto a continuous conveyor to the surface or extracted as a slurry and pumped to a surface treatment plant. When operating in the semi-closed mode through the screw conveyor, a tub under the discharge gates will catch water and fines that is transported to the surface also in a slurry circulation system.
- The specified bolted and gasketed precast segmental lining comprises six segments and will be cast by Hanson at a local precast yard. As the lining is built into the rock tunnel the annulus will be backfilled with a cement mortar injected through the tail shield or just beyond the tail shield.
- In these early stages of the contract, work is just getting started with submittals for the shaft sinking operations. Slurry walls will be installed through the upper soft ground layers to the rock interface and taken to depth by dill+blast. All the shafts will be treated with pre-excavation grouting to control groundwater inflows. The TBM is also fitted with the ability to install arrays of pre-excavation grouting ahead of the face. Production at the precast segment factory is scheduled to start in April 2011 and the TBM is programmed to be delivered to site in about a year from now and launched in about February 2012. Construction managers Black & Veatch with H. R. Gray has mobilized a team of 12 inspectors to supervise and manage construction for the City. Black & Veatch is also managing design of a deep tunnel as part of a proposed multi-billion CSO management program for the city of Cincinnati in Ohio and the City Government of Lorain, Ohio has approved funding to build a deep bored tunnel to meet its court-ordered CSO control schedule.
- In operation, the new OARS tunnel will remain dry most of the time. During heavy storm events, the OARS diversion pumps will divert storm water flow into the CSO tunnel to the Jackson Pike or the Southerly treatment plant. As storm water levels in the diversion structure rises, the pumps are de-energized in a different sequence until all pumps are off and combined sewage can flow by gravity to the treatment plants. Only on rare and exceptional occasions, when levels in the diversion structure rises beyond the capacity of the pumps and the Jackson Pike treatment plant, will flow be diverted to the Scioto River via the river overflow structure built as part of the Phase 2 contract.
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Clawing success from the extreme at Arrowhead - TunnelTalk, Dec 2007
Mixshield TBM designed for the extreme - TunnelTalk, Nov 2009
Cincinnati, Ohio, plans deep CSO program - TunnelTalk, Nov 2010
Lorain, Ohio agrees CSO control investment - TunnelTalk, World News Briefs
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