Another Atlanta Sinkhole – 30 Years After the Fatal 14th Street Sinkhole.

Sinkholessinkhole swallowed a large SUV on Ponce De Leon Ave. - a 4 lane undivided highway providing an east-west artery in Midtown Atlanta.  Thankfully, the driver and passenger of the SUV were able to climb to safety, without suffering any significant injuries.  The Ponce sinkhole was caused by the rupture of a lateral combined sewer overflow or CSO line, servicing an adjacent business. Approximately 8’ deep and 16’ across, the Ponce Sinkhole opened at 2 p.m. on a Tuesday afternoon causing the closure of 2 lanes of the City street for nearly one week. Ironically, the Ponce sinkhole appeared just two weeks after the 30^th anniversary of a much larger fatal sinkhole in West Atlanta.

have become commonplace in Atlanta as well as many other older cities with aging infrastructure.  Last month, on June 27, 2023, a

On June 15, 1993, a sinkhole swallowed 3 people and 3 cars in the parking lot of a Marriott Courtyard hotel, at the corner of 14^th Street and I-75/85.  The 14th Street sinkhole was caused by the rupture of a 70-year-old CSO trunk line and was approximately 50’ deep and 100’ across.  It appeared at approximately 6 a.m. on a Monday morning during a shift change at the hotel. Two women and one man were swallowed up the 14th Street sinkhole and drowned in a mixture rainwater from a severe thunderstorm and wastewater flowing through the ruptured CSO trunkline. 

The first woman, was driving to an early morning shift at the Marriott, when her car was swallowed by the sinkhole trapping her inside. The man was leaving his night shift and standing in the parking lot, awaiting a service call to jump start his car, when he was swallowed by the sinkhole. Their bodies were discovered at the bottom of the 14th Street sinkhole amongst debris and recovered that night.  A second woman, never publicly identified, was trapped in a car swallowed by the 14th Street Sinkhole and washed into the 12’ CSO trunk line at the bottom of the sinkhole.  The woman's car and body were later found inside the CSO trunk line, 3 and 4 miles downstream, from where the rupture occurred.   

The causes of sinkholes in Atlanta materially differ from those in other States. Unlike sinkholes in Florida and Kentucky, typically caused by the dissolution of limestone formations forming underground cavities, Atlanta sinkholes are typically caused by the rupture of aging underground infrastructure, most often aging CSO pipes, and to a lesser extent broken water mains,  Whereas limestone dissolves in a chemical reaction with water forming limestone sinkholes, ruptures in CSO pipes allow soils to fall or “ravel” into the CSO that are then carried away by wastewater flows forming the cavities giving rise to CSO sinkholes.

Cavities creating limestone sinkholes are typically irregular in shape, size, and locale, whereas cavities creating CSO sinkholes are typically conical in shape, with size dependent upon the depth of the CSO rupture, and are located under public roadways or within other utility easements. The formation of limestone sinkholes typically take years, if not decades to materialize, whereas CSO sinkholes often materialize within weeks or months following the initial CSO rupture.

The failure mechanism for the formation of CSO sinkholes in Atlanta is predictable.  A rupture occurs in an aging CSO pipe or trunk line often made out of brick or an aged concrete culvert.  [See  "How Tunnel Construction Solved Sewer Overflows in Many Older US Cities", posted 7/5/23].  The soils above the rupture gradually begin to fall or “ravel” into the ruptured CSO below.  Heavy rains or wastewater flows erode away the ruptured CSO pipe and carry away the soils “raveling” into the CSO through the rupture.  

The cavity created by soils “raveling” into the CSO from above the rupture assumes a conical shape, with the cone tip at the ruptured area and the cone circumference expanding upwards towards the earth surface above.  The slope of the cone face is dependent upon the the "angle of repose" or slope at which the soil would naturally assume under its own weight without flowing downward under the forces of gravity. The diameter at the top of the cone is dependent upon two factors: the "angle of repose" and the depth of the CSO rupture at the tip of the cone.  "Angles of repose" for soils typically range between 35 and 45 degrees, meaning the deeper the ruptured CSO, the greater the diameter of the CSO sinkhole at the earths surface.  The diameter of an average CSO sinkhole is twice its depth. 

When CSO sinkholes form under roads or parking lots they become particularly dangerous, because the asphalt or concrete pavement often "bridges" the developing cone cavity creating a trap, until collapsing under the weight of a load.  Often, the only symptoms of a CSO sinkhole are telltale signs of cracking and/or subsidence of the road or parking lot.  Back in 1993, the City of Atlanta was monitoring cracking and subsidence of the pavement in the Marriott parking lot at least one week before the 14^th Street sinkhole appeared.

As infrastructure within the City of Atlanta continues to age, sinkholes are becoming far more common.  The need to inspect and rehabilitate CSO trunk lines has been and remains a critical component of maintaining the infrastructure of Atlanta, along with every other major US city.  Advances in technology allow the inspection and investigation of CSO lines using robots, camera's and GPS tracking making identification of the need for repairs and rehabilitation of CSO lines highly efficient. However, once identified, the repair and rehabilitation of CSO lines remains an expensive proposition, especially in light of the aging infrastructure in many older US cities. Unfortunately, it appears recent federal legislation (i.e. "Infrastructure Investment and Jobs Act") is aimed more at new technology as opposed to maintaining and rehabilitating old technologies upon which all major cities continue to depend.