A sinkhole under the foundation of the Pennington parking garage under construction in Leesburg is expected set the county back five months and $3.5 million.
According to a staff report, during construction of the foundation in June, a sinkhole developed under drilling on the western end of the structure, nearest the existing Pennington parking lot off Church Street.
County representatives met with Howard Shockey & Sons Inc., the firm building the parking garage, and ECS, the geotechnical engineering firm on the project, to review hundreds of drill logs on the site for any signs of similar trouble elsewhere.
In August, a second sinkhole developed nearby and was deemed to be part of the same, 60-foot diameter sinkhole.
The fixes proposed for that sinkhole and to prevent future sinkholes from developing are expected to cost more than $3.5 million. Shockey expects that work will add 150 days to the project schedule, pushing its completion to November 2018.
The project was originally awarded to Shockey for $12.7 million. According to the staff report, it has already seen more than $308,000 in cost overruns for additional security conduit, repaving existing parking lot, additional rock excavation, and installing additional micropiles, an element of the structure’s foundation.
County government spokesman Glen Barbour said the staff would hold off on answering questions about the sinkhole until after the Board of Supervisors has a chance to consider the item at its Sept. 5 meeting.
As of Thursday, the item was on the board’s consent agenda, which is a collection of uncontroversial items that are passed without discussion at each board meeting. Any single board member can pull an item off the meeting’s consent agenda.
However, Barbour said, “the standard process” was conducted of many types of geological testing on the site before construction began, and that the work ECS did on that site was “consistent practice in areas underlain by limestone conglomerate bedrock.”
“Based on these studies, our consultant concluded that there was no evidence of an existing or incipient/imminent sinkhole on site and the foundation was designed accordingly,” Barbour said. “On occasion, projects do encounter unforeseen rock and unsuitable soils among a variety of subsurface conditions once construction begins and, should that occur, further review is required. The county typically prepares for those unforeseen conditions in its construction contracts with allowances and/or contingency.”
Sinkholes are common in areas of limestone—such as Leesburg, which in 2015 saw a 30-foot-by-40-foot sinkhole develop on Currant Terrace in the Exeter neighborhood after heavy rains. Almost all of Leesburg is built over limestone, which can cause sinkholes when underground water dissolves the rock and carries it off, forming subterranean spaces and caverns.