Interstate Technical Group on Abandoned Underground Mines
Fourth Biennial Abandoned Underground Mine Workshop
Abstract: Abandoned Underground Coal Mines and the Pennsylvania Turnpike
Donald V. Gaffney
For over 60 years, the Pennsylvania Turnpike has taken a positive approach to dealing with abandoned underground coal mines. During its original construction completed in 1941, geologists on leave from the PA State Topographic and Geologic Survey assisted in assessing mining conditions and overseeing remediation in the Bituminous coal field east of Pittsburgh. Extension of the Turnpike in 1957 from Philadelphia to Scranton brought the Turnpike through steeply dipping Anthracite coal beds, with their own special problems and solutions. Rehabilitation and improvement projects along the existing Turnpike since the 1970's and expansion of the Turnpike system near Pittsburgh since the 1980's have continued to highlight the importance of addressing abandoned underground coal mines during design.
Early remediation techniques included slushing and simple over-excavation and backfill. Rehabilitation and improvement projects along the main East-West Turnpike and the Northeast Extension took advantage of more elaborate investigation, design and construction techniques. These included the use of camera surveys of mine workings, structure foundations set below mine floors, and concrete grout for Stabilization. Expansion projects including the James E. Ross (Beaver Valley) and Amos K. Hutchinson (Greensburg) Expressways (completed in the 1990's), the Mon-Fayette Expressway (currently under construction), and the Southern Beltway (in final design) all followed general guidelines for dealing with abandoned underground coal mines. The guidelines covered investigation and remediation alternatives, and included details and specifications for performance of related construction activities. The guidelines are revised with each expansion project on the basis of previous experience.
Early underground coal mine strategies primarily addressed mineral value and positive prevention of subsidence. Over the years, however, secondary impacts of acid mine drainage and mine fires have also become prominent concerns. Risk assessments and cost-benefit analyses during design, and value engineering during construction, are beginning to play a role in determining the extent and type of remediation. As the complexities of specific project sites increase, it is natural to expect the complexities of investigations to increase. But the simple solutions are still the best.