In-Place Solidification of Coal Tailings for Expressway Subgrade
Journal American Society of Mining and Reclamation
The Mon-Fayette Expressway is one of the largest highway construction projects undertaken in PA in recent years. A 400-foot long segment with ingress and egress ramps was scheduled for construction over an active 10-acre coal tailings (fine coal refuse) disposal impoundment in Washington County, PA. The coal tailings are deposited by a slurry pipeline, are not traversable, and are over 40 feet thick in the central pond portions. The initial approved Expressway plans included construction of a
... mporary dike, separating the active slurry pond from the highway right-of-way, followed by complete removal of the tailings upstream of the dike. The remaining void would be raised to highway grade with a structural earth and rock fill followed by removal of the temporary dike. A dam would be constructed on the highway embankment slope to isolate the highway and its supporting embankment from the tailings pond. Howard Concrete Pumping Company, Inc. and GAI Consultants, Inc. proposed an engineering value, cost saving, novel approach to solidify the tailings in-place providing a stable foundation for construction of the highway embankment and the dam. This approach eliminates the temporary dike and off site disposal of the tailings. Key advantages included lower cost and rapid implementation. In January 2000, following an intensive research and development program, the plan was approved by the concerned parties. Work began in June 2000. Procedures were developed for both shallow and deep mixing that result in a stable mixture of tailings and fly ash/cement grout. A large backhoe equipped with a custom-designed long-reach dipper stick and hydraulically driven mixing device performed shallow mixing. Deep mixing was conducted with a customdesigned three-auger mix panel supported by a Manitowoc crane. Approximately 320,000 cubic yards of coal tailings were stabilized. The project was successfully completed in March 2001 and Expressway construction remained on schedule.