THE NATURE AND SIGNIFICANCE OF ALTERED MATERIAL ALONG JOINTS IN SHALE BEDROCK IN OHIO, PENNSYLVANIA, AND WEST VIRGINIA

The presence of a zone of altered rock material along joint surfaces in shale bedrock is a characteristic feature of a number of road-cut sites in Ohio, Pennsylvania, and West Virginia. Three such sites, designated as Belle Valley, Ohio, Hopewell, Pennsylvania, and East Steubenville, West Virginia, were selected to investigate the nature of the altered material and its effect on rock block failures occurring along the joints. A detailed-line survey was conducted at each site to determine the orientation, spacing, and continuity of joints. The principal joint sets were analyzed to determine the various modes of failure. The shale bedrock and the altered material were tested for slake durability, unconfined compressive strength, and shear strength parameters. The altered rock material was also subjected to petrographic and x-ray diffraction analyses.

The results of the study indicate that the altered material at all three sites consists of original shale bedrock that has been modified by the precipitation of hematite, geothite, and pyrolusite from ground water flowing through the joints. However, the thickness of altered material and the frequency of fractures within it vary among the three sites. The altered material at all three sites is stronger and more durable than the adjacent shale bedrock. The friction angle is higher for the altered material compared to the unaltered shale bedrock for the Belle Valley site but lower for the other two sites. Because of this, the altered rock material increases the factor of safety against plane and wedge failures at the Belle Valley site but reduces it at the hopewell and East Steubenville sites.