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| 3 minutes read

How Tunnel Construction Solved Sewer Overflows in Many Older US Cities?

Many older cities, especially those East of the Mississippi, rely upon combined sewer overflow systems or “CSO’s” that collect and deliver rainwater runoff, domestic sewage, and industrial sewage into one pipe.  Back 100 years ago, this was a good idea, for a variety of reasons.  First, rainwater flushed animal waste from horses, cattle and humans off the streets into the CSO through drainage systems and required treatment, more commonly known as wastewater or sewage.  Second, before hydraulic excavators existed, and excavation was performed through human labor, it was far more efficient and economical to combine sewer and drain into a single trench.  Finally, before large diameter pipe could be mass-manufactured, large diameter pipes and culverts were slowly constructed from brick at great cost and low productivity.  Consequently, back 100 years ago, it made perfect sense to combine the collection and transport of raw sewage and rainwater into a single pipe in a single trench.

Population growth in our older cities and urban areas, coupled with advances in excavation and manufacturing rendered CSO’s obsolete.  By the year 2000, over 75% of the US population was served by 16,000 centralized wastewater treatment plants while the remaining 25% were served by septic or other on-site treatment systems.  However, the impacts resulting from unbundling the drainage systems from the sewage systems would be prohibitive with respect to cost, time, and disruption to the population.  CSO's required the treatment of both rainwater or stormwater and sewage or wastewater equally.

Consequently, when heavy rain events coincided with increased  domestic and industrial sewage flows, the wastewater treatment facilities could not handle the influx of untreated stormwater and wastewater flows.  This created a dilemma – the treatment facilities had to either allow the CSO to back up effluent from sewer and drain into streets, homes. and businesses, or release the untreated rainwater and wastewater into surrounding rivers and waterways.  Given these choices, all major cities chose the latter and released untreated mixture of stormwater and wastewater polluting our rivers and waterways and killing both vegetation and wildlife.

The growing pollution of rivers and waterways contributed to the creation of the Environmental Protection Agency or “EPA” under President Richard Nixon in 1970.  In 1972, the EPA adopted the National Pollutant Discharge Elimination System or “NPDES” permitting and restricting the volume of pollutants that could be discharged into rivers and waterways by wastewater treatment plants.  While rainwater was not a pollutant, it became such when combined with wastewater within a CSO.  To avoid the huge fines imposed by the EPA for the failure to comply with the discharge of pollutants and untreated sewage into rivers and waterways under NPDES, major cities faced another different dilemma.  Older cities had to either construct enormous wastewater treatment facilities to treat all rainwater and sewage during the worst possible peak flows, or develop a place to store untreated sewage during peak flows that could be treated in the middle of the night when wastewater flows are negligible..

Given the costs of constructing enormous plants that would not be used on a regular basis, coupled with public resistance towards the storage of sewage above ground, older Cities turned to tunneling contractors.  Many of the largest Cities in the World, including New York, Los Angles, Chicago, London, Paris, Berlin, and Singapore, opted to construct large bore, tunnels hundreds of feet below the surface in which to store untreated wastewater. this untreated wastewater is then stored in tunnels until nighttime when flows are negligible and then pumped to a wastewater treatment facility above ground.  Constructing tunnels deep in the earth through bed rock was logical because it is self supporting, and therefore faster and more economical to tunnel through bedrock as opposed to soft ground and it greatly reduces the possibility of polluting groundwater.  Most of these tunnels were constructed 100’ ton 300’ beneath the surface of the earth and work in conjunction with pumping stations to pump the wastewater back to the surface when flows allowed for treatment.

The City of Atlanta is another older City who solved its CSO problems through the construction of two deep hard rock tunnels.  The first tunnel was completed in 2005, known as the Nancy Creek Sanitary Sewer Tunnel, lies up to 300 feet below the ground surface, is 8.0 miles long, and has a finished diameter of 18’.  A second hard rock tunnel known as the West Area CSO Storage Tunnel was completed in 2008, lies up to 170 feet below the surface, is 8.5 miles long, and has a finished diameter of 24’. Together the 2 tunnels are designed to store 77M gallons of sewage and wastewater pending treatment.  When combined the 2 tunnels reduce the discharge of untreated wastewater into the Chattahoochee River from 300+ events per year to less than 10 events per year, just 6 over the NPDES permit requirements. 

Many of these same cities, including Atlanta, now also use tunnels to transport drinking water from its point of origin or source to water treatment plants  for treatment, often through chlorination, and then distribution to citizens.  Tunnel construction, while seldom seen. has a major impact upon the infrastructure of our country!  [See, "Does your Building Project Require an Easement for Construction" posted 3/1/23].   


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