OARS (OSIS Augmentation Relief Sewer) Tunnel
- Location: Columbus, OH
- Owner: City of Columbus, Department of Utilities
- Technique(s) Utilized: Cutter Soil Mixing, Diaphragm Walls, Jet Grouting, Permeation Grouting
- Subsurface Conditions: Overburden (lean clay, sand and gravel, glacial outwash with cobbles and boulers,) Limestone Bedrock with karstic features
- Approximate Key Quantities: Cutter Soil Mixing - 13,100 square feet, Diaphragm Wall - 58,000 square feet, Jet Grout Columns -135
The sewer systems originally built in many cities were designed to carry storm water and raw sewage away from homes and businesses in a single, combined pipe. Prior to sewage treatment plants, the contents of these combined sewer systems would then be transported to the nearest river.
In the 1950s, based on the development of sewage treatment plants, the contents of combined sewer systems were diverted to sewage plants for treatment and sterilization. Overflow pipes were installed in the existing systems in order to better manage higher quantities of sewage and storm water in the pipes.
Many cities are still using these systems, which function appropriately in dry weather. However, in times of excessive rain, these older systems are designed to overflow into streams and rivers, rather than back up into homes and businesses.
Combined sewer overflows (CSOs) can contaminate waterways, destroying wildlife and polluting drinking water, and can also create unpleasant odors in the air.
The City of Columbus and the State of Ohio came together in 2007 in a mutual effort to reduce the environmental impacts, including air and water pollution, caused by CSOs in downtown Columbus.
PROJECT BACKGROUND
The OSIS Augmentation Relief Sewer (OARS) was designed to intercept the waste water overflow being transported to the Scioto River and divert it to the nearby Jackson Pike and Southerly waste water treatment plants. The Scioto River's waters are used primarily for drinking and recreation.
The first phase of the OARS project involves the construction of a 23,300 foot tunnel that is 20 feet in diameter. The tunnel is being constructed 170 feet below ground in order to reduce the construction risks that exist when working in shallow levels. Vertical shafts must be installed in order to effectively direct the overflow from the existing, shallow sewers into the new, deeper tunnel.
THE WORK
Nicholson was contracted to create the diaphragm wall shafts required for tunnel access. The diaphragm wall technique for the installation of the temporary shafts was the method selected in order to provide a waterproof support of excavation system in the overburden soils with ground water just below the surface. Three-foot thick diaphragm walls from the precut grade were installed as temporary support for Shafts 1 and 2, located at the Jackson Pike site, at the south end of the tunnel. The internal diameters of Shafts 1 and 2 are 58 feet and 48 feet respectively. Shaft 6 will be installed at the North end of the site near Neil Avenue. The internal diameter of Shaft 6 will be approximately 54-feet. All three shafts are over 100-ft deep and embedded 5 feet into the limestone bedrock.
A temporary outer wall was created at Shaft 1. The annular space between the diaphragm wall and the cutter soil mixing wall was treated with jet grouting in order to provide the waterproof bottom plug in the excavation between the two walls. In addition, a row of anchors was provided at mid-span of the excavated height.
Up to three phases of grouting were completed on each shaft.
In the first phase, grout holes were installed at each shaft location external to the shaft perimeter prior to installing the shaft. The perimeter ring was comprised of a series of angled holes advanced through the overburden and into the rock. The intent of this ring was to form a grout curtain encompassing the excavation in rock.
In the second phase, a ring of grout holes was drilled and grouted via pipes installed through the diaphragm wall panels forming the shaft. This grout hole ring serves as a curtain within the curtain, and also, as the "proof curtain" that confirms the success of the phase 1 grouting.
In the third phase, the verification phase, a well was constructed in the center of each shaft and three observation piezometers were located at the midpoint between the center well and the inside perimeter wall of the shaft. These wells were used to perform pump tests, and to verify that each shaft was water tight.
Nicholson is currently installing the diaphragm walls and grouting at Shaft 1 and Shaft 2. The third and final vertical shaft will be installed in early 2012.
THE RESULT
Scheduled for completion in 2014, the OARS tunnel is the single largest project the city has ever attempted. When completed, the tunnel will span approximately four and a half miles.
Historically, the Scioto River has been flooded with sewage overflow multiple times a year. The OARS tunnel will dramatically reduce the risk to approximately one occurrence every ten years.
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