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East Area CSO Tunnels FAQs

Are tunnels a proven technology for wastewater and CSO conveyance and storage?

Yes, large underground tunnels are a common and accepted technology for conveyance and storage of wastewater way and CSO. A recent inventory of other large tunnel projects for such use identified more than 47 tunnel projects in the United States and overseas. These projects ranged from 7 feet to 33 feet in diameter and from 2 miles to 33 miles in length. The Chicago Tunnel and Reservoir Plan (TARP) is one of the best examples of the most successful and large scale application of tunnels. This project has 109 miles of 33-foot diameter tunnels that have performed in an excellent manner in more than two decades of operation. Other large tunnel projects are in operation in Austin, Boston, Cleveland, Houston, Detroit, Los Angeles, New York, Minneapolis, and San Francisco.

What is the purpose of the West Area CSO Storage Tunnel Project?

The West Area Combined Sewer Overflow (CSO) Storage Tunnel project is one part of a comprehensive plan for long-term control measures to bring the City’s combined sewer system into compliance with water quality standards. This tunnel is part of a storage and treatment alternative that involves capturing and storing overflows from combined sewers that result from rainfall. The overflows are stored in large underground tunnels. When the rainfall is over, the captured CSO volume is conveyed to a separate treatment system for removal of pollutants and disinfection before discharge to receiving waters. As parts of the combined sewer system are separated, this same system can be used to treat stormwater runoff from the urban core portion of the CSO area.

Why is storage needed?

After the CSO problems are corrected, the peak wet-weather flows into the R.M. Clayton WRC would exceed the flow that the plant can treat without violating the discharge permit. Therefore, it was determined that storage would be required to provide equalization of the peak wet-weather flows.

How does this tunnel compare to existing deep tunnels in Atlanta?

The existing Atlanta tunnels provide a model for the design of the West Area CSO tunnel. The success of the deep sewer tunnels in Atlanta is well known in the tunneling industry. The rock through which the Atlanta tunnels were excavated is excellent for tunnel construction and is similar to the rock through which this tunnel will be excavated.

Why won’t the stormwater and wastewater be separated as it is in other areas of Atlanta?

Option C, partial sewer separation (27%) and tunnel storage and treatment plants, was recommended because it complies with the Consent Decree timeline; it provides the greatest pollutant removal in the shortest time; it is next to the lowest cost option; and it reflects public opinion by beginning full sewer separation.

Will there be any odors associated with the operation of the West Area CSO Storage Tunnel?

Primarily, opportunity for odor release will be limited to the entrance and exit shafts. During normal operation of the tunnel, the ventilation system at the pump station will pull outside air into the tunnel. This will result in continuous movement of all air within the tunnel to the pump station for odor treatment and exhaust. Although air may be ventilated from the tunnel at the vent shafts when the tunnel fills, odor problems are not expected. The City will provide an odor control system at all access shaft points and at the pump station similar to that constructed during the R.M. Clayton WRC upgrade.

How was the alignment of the West Area CSO Storage Tunnel selected?

The alignment of the West Area CSO Tunnel was selected so that wastewater flow could be intercepted from the existing trunk and relief sewers in the Clear Creek, Tanyard, and North Avenue basins. It was also selected based on the best possible geological conditions for tunneling, thus minimizing the cost of tunnel construction.

Will Easements be Acquired for this Project?

The City will be acquiring subsurface easements from some property owners along the alignment. These easements will be approximately 150 to 300 feet under ground. The acquisition of subsurface easements has been done in many communities throughout the United States where underground tunnels have been constructed. Additional detailed information will be provided to the affected property owners, who will be contacted by the City’s Real Estate Consultant.

Is there any chance of structural damage to homes and buildings above the alignment?

There is no past evidence that tunnel construction in rock formations will cause structural damage to homes and buildings above the alignment. The West Area CSO Tunnel will be excavated through bedrock. The soils that overlie bedrock along the alignment are residual soils, derived from the in-place weathering of the parent bedrock over millions of years. Residual soils tend to be very stable, such that settling is nearly impossible. Also, no settlement problems were reported over the alignment of the existing Atlanta tunnels as a result of tunnel construction.

Will I have to be concerned about sinkholes in my yard?

Generally, construction of tunnels such as this one does not affect groundwater levels significantly. However, depending upon the type of soil and/or rock encountered along the alignment, there could be changes in the groundwater level. Extensive testing has been done to determine what, if any, changes in groundwater levels can be expected.

Is there any chance that construction of the West Area CSO Tunnel will cause an earthquake or seismic activity?

There is no evidence that any tunnel installation has ever caused an earthquake or seismic activity,

How will safety concerns be addressed with this project?

Safety is a major concern during all construction projects. The City constructed two major tunnel projects between 1978 and 1984, on which construction methods similar to those to be used on this project were employed. There were no major accidents on either project.

The City and its design engineer for the West Area CSO Tunnel are committed to maintaining the City’s excellent safety record on all construction projects. Therefore, the City will continue to require the tunnel contractor to develop and implement a safety program.

If you have further questions, please contact the Clean Water Atlanta Information Line at 404.529.9211 or cleanwateratlanta@Atlantaga.gov.

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