The five-foot-wide, 10-foot-deep sinkhole appeared less than a year after the PUC ordered pipeline operations to halt when sinkholes were found in the same neighborhood of single-family homes. The pipeline resumed operation last May. But the commissioners now say they want to formally join an existing legal action started by a group of seven Delaware and Chester county residents against Sunoco and its parent company, Energy Transfer Partners. The legal complaint by the residents alleges numerous safety risks associated with the Mariner East pipelines, which are part of a multibillion-dollar effort to bring natural gas liquids from Western Pennsylvania to the Sunoco refinery in Marcus Hook and elsewhere. A spokeswoman for Energy Transfer Partners could not immediately be reached for comment. In December, a judge denied an emergency request by the residents to halt operations of the pipelines because of safety concerns. That decision was unanimously upheld by the PUC last month.