An Examination Of The Loyalhanna Limestone’s Structural Features And Their Impact On Mining And Ground Control Practices

Field: Earth and Planetary Sciences (Physical Sciences)

A close look at the Loyalhanna Limestone of southwestern Pennsylvania reveals a complex structural environment. Most exposures of the Loyalhanna occur along Chestnut Ridge and Laurel Hill within the Allegheny Mountain Section of the Appalachian Plateaus Province. Because the Loyalhanna is rated as a superpavement aggregate by the Pennsylvania Department of Transportation and because of its proximity to the population centers of southwestern Pennsylvania, the Loyalhanna has been extensively mined along these prominent anticlinal structures. Geologic and engineering analyses were performed using gas well and core logs, outcrop examinations, underground observations, and mine maps. Strata exposures from both outcrops and quarries were used to construct geologic maps, stereonet, and rose diagram projections so that the Loyalhanna’s structural environment could be understood better. Many of these structural conditions cause ground control problems at local quarries. These problems ranged from small-to-moderate sized rock falls associated with roof jointing to pillar failures associated with dipping discontinuities. Stresses ranged from tensional on structural domes where weathering dominates, to high levels of compression within structural saddles. A greater understanding of these characteristics is a prerequisite to develop engineering controls, such as improved mine layouts, pillar sizes, etc., that lessen miner exposure to these hazards. INTRODUCTION The Loyalhanna Limestone varies in composition from a quartzose limestone to a calcareous quartz arenite (1). It underlies 17,000 square miles of West Virginia, Maryland, Pennsylvania, and Ohio and averages 60 ft in thickness with a maximum thickness of 103 ft (1). Wells (2) has since expanded the aerial extent of the Loyalhanna to Sullivan Co. in the northwestern portion of Pennsylvania. While this limestone is found under an extensive area in four states and is a major source of crushed stone for southwestern Pennsylvania, it has a limited area accessible to mining. The Loyalhanna, which is typically greater than a thousand feet below the surface in the Pittsburgh Low Plateau Section of the Appalachian Plateaus Province, crops out at numerous sites along the crest of Chestnut Ridge, Laurel Hill, and Negro Mountain Anticlines in the Allegheny Mountain Section of the Appalachian Plateaus Province (figure 1). These outcrops have historically been the sites for commercial aggregate mining, especially those closest to the population centers in and around Pittsburgh, Pennsylvania. Currently nine quarries mine the Loyalhanna through underground and surface operations in Monongalia Co., West Virginia, and Fayette and Westmoreland Counties, Pennsylvania, (table 1) along the linear trace of Chestnut Ridge. The Chestnut Ridge Anticline is a fold with limbs dipping away from each other and, with erosion, exposes older geologic beds nearest its central core. As this fold was produced in conjunction with the formation of the Appalachian Mountains, the rocks fractured and broke to form joints and faults. Understanding the characteristics, such as orientation, dip, and spacing, of these structural features is necessary for mine design and development.