Sinkhole report: Drainage project was major factor
The investigation into what caused the 2011 sinkholes in the City of Georgetown is complete and the Highway 17 Drainage Project work has been confirmed as major contributors.
The seven sinkholes that formed in areas between Highmarket Street and Prince Street between July and November 2011 caused the collapse of Parrish Place — which housed several businesses — and damaged many other buildings and homes.
Some of the buildings still are considered to be unsafe to occupy.
The drainage project — which is still incomplete — is being conducted under the auspices of the South Carolina Department of Transportation.
The investigation was requested by the law offices of Anderson, Reynolds & Stephens of Charleston — representing the SCDOT— and was conducted by F&ME Consultants of Columbia.
A 452 page report detailing the findings of the investigation was released last week by F&ME and has been obtained by the Georgetown Times.
While the report does not cast blame on any specific person or agency, it does indicate the de-watering process that was taking place before and after the sinkholes formed — as well as the installation of king piles — were primary and accelerating factors.
The investigation revealed the areas where the sinkholes formed contain “karst” conditions, defined as a landscape that exists over fossiliferous marine limestone that contains solution cavities and is subject to sinkholes or depressions.
The de-watering process — which at one time was removing as much as 60,000 gallons of water from underground each hour — combined with the placement of pilings — caused the sinkholes in the karst areas, according to the findings.
The report lists the “primary cause” of the sinkholes as the installation of king piles around the stormwater runoff tank that is still under construction next to Georgetown City Hall.
“The king pile installation penetrated the aquitard, thereby providing a conduit for flow of groundwater upward from the lower confined aquifer,” the report states.
An aquifer is an aquifer is an underground layer of water-bearing permeable rock or unconsolidated materials (gravel, sand, or silt) from which groundwater can be extracted.
An aquitard is a zone within the earth that restricts the flow of groundwater from one aquifer to another.
The subcontractor for the king pile grouting program was Uretek.
The investigation determined the de-watering process accelerated the sinkhole formation.
The pumping activities for the de-watering...associated with the wet well construction resulted in a substantial volume of water flowing from the lower confined aquifer. This flow reduced the upward hydraulic pressure within solution cavities in the lower confined aquifer. The reduced upward pressure resulted in an increase in stress in the aquitard and localized collapses of the aquitard into the solution cavities. The collapse or breach of the aquitard allowed the sandy soil deposits above the aquitard to flow down and into the exposed cavities resulting in the observed sinkholes and depressions,” the report states.
The report continues by stating none of the city’s underground utilities in place before the drainage project work began caused the sinkholes.
“We have found no evidence that existing utilities contributed to the formation of the identified surface sinkholes and/or depressions,” the report states.
It has also been determined the drainage project work taking place above ground or near the surface did not contribute to the sinkhole formation.
It will ultimately be up to juries to determine if any particular agency is responsible for the sinkholes.
The first lawsuit was filed last week by Parrish Place owners Tony and Debbie Jordan naming Davis & Floyd Engineering, Republic Contracting, S&ME Inc. and the South Carolina Department of Transportation as defendants.
By Scott Harper
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