A site plan from the most recent engineer’s report on sinkhole activity in south Harrisburg, showing potential fracture lines and potential voids in the soil underneath the survey area.
A second engineering report of sinkhole potential in south Harrisburg shows additional areas of concern, in a region that a city official described as “fragile, but not unstable.”
The report, prepared by Camp Hill-based engineering firm Gannett Fleming, is based on an expanded survey of the surrounding neighborhood, as opposed to the single city block that was studied in a prior report.
The prior report, released in August, showed evidence of five fractures in the limestone bedrock and several potential voids beneath the 1400-block of S. 14th Street, where a series of sinkholes opened in March.
For the latest report, engineers surveyed an expanded area bounded by S. 12th Street to the west, Scott Street to the east, Hanover Street to the north and Cloverly Terrace and S. 13th Street to the south, a neighborhood encompassing some 200 buildings, most of them single-family homes.
The latest report shows evidence that previously detected fractures extend across the larger area and also introduces evidence of possible additional fractures and voids throughout the neighborhood.
Limestone fractures can contribute to the formation of voids beneath the street surface, as soil subsides into openings in the rock. The voids can eventually migrate towards the surface, causing the street to collapse and sinkholes to open.
On Wednesday, city officials were quick to emphasize that the new report did not significantly change the status quo. “I don’t think the study changes the dynamics at all,” Mayor Eric Papenfuse said.
City Engineer Wayne Martin said that the report showed the area was “fragile, but not unstable,” adding that the city had determined it was safe to keep roads in the neighborhood open.
City Council will hold a public meeting to discuss the new report at 5:30 p.m. on Monday, Dec. 22, at the public works building at 1820 Paxton Street. Councilwoman Sandra Reid, who chairs council’s public works committee, will host the meeting, council members confirmed at their legislative session Tuesday night.
The August report concluded that the “potential for future sinkhole activity” was “high” in the affected block of S. 14th Street. The more recent study does not offer a conclusion about the likelihood of future sinkholes, although it does end with a handful of suggestions about ways to prevent them or to mitigate them if they occur.
The August report relied on site observations, drill samples and multi-channel analysis of surface waves, or MASW, a method of creating and measuring seismic waves to detect likely fractures and voids underground.
The latest report relied on site observations and an MASW survey, but did not include drilling for samples. Both reports also tapped historical records, including aerial photographs showing the development of the neighborhood over the years.
Meanwhile, the city has continued its efforts to address the more immediate problems facing residents of S. 14th Street, where sinkhole activity has continued. Since March, the city has declared 10 homes on the block unfit for human habitation, according to Dave Patton, the city’s codes administrator.
Martin said the city hopes to submit an application this week to the county for $1 million in disaster relief funds. The city has already sent in a notice of intent to apply for an additional $3 million in federal disaster relief funds, he said.
Together, those applications would seek to secure the $4 million it would take to mitigate the problem on S. 14th Street, according to an estimate provided in a third report by Gannett Fleming issued earlier this year.
The mitigation efforts will likely entail a combination of buying out certain homeowners and reconstructing the street, Martin said.
There was some question as to why Harrisburg officials did not publicize the report until Wednesday, two weeks after it was sent to the city, and only after the topic was raised during public comment at Tuesday night’s council meeting.
The mayor addressed this question Wednesday morning, saying there was “no decision to hold the report,” but that the city was simply “looking for a time to hold a public meeting.”
“It’s complicated,” the mayor said of the report, adding that the best way to inform the public was to have the document explained at a hearing by an engineer.
The choice of date for the hearing was left to the discretion of council’s public works chair, the mayor said. “We would have been happy to have met sooner,” he said.
You can download the complete Gannett Fleming report here: Expanded Sinkhole Investigation, 11/25/14. For the site plan of the neighborhood, click here: Sinkhole Report – Neighborhood Site Plan.
This story has been updated to provide a time and address for the public works committee meeting, which will take place Dec. 22 at 5:30 p.m. at 1820 Paxton St.
