PRELIMINARY RESULTS FROM A FOLLOW-UP STUDY ON RATES OF SHALE UNDERCUTTING

Pennsylvanian-age cyclothemic deposits of shale-sandstone and shale-limestone are common throughout the Appalachian Plateau areas of Ohio, Pennsylvania, Kentucky and West Virginia. The shales can be subdivided into massive mudstones, laminated claystones, argillaceous limestones and silty shale. Natural slopes and roadcuts that expose these materials are subject to differential weathering of shales that then undercut the more resistant sandstones and limestones. Tensile stresses that develop in unsupported ledges of sandstone or limestone can lead to potentially troublesome and hazardous rock falls and slides.

Determining a reliable undercutting rate at a specific roadcut requires knowledge of the date of the roadcut and the existence of pre-split blast holes along the face of the limestone or sandstone layer above the receding shale. The rate of undercutting is determined from the amount of shale recession relative to the unweathered limestone/sandstone surface, averaged over the age of the roadcut (e.g., inches per year).

Shakoor and Rogers (1992) examined 14 roadcuts from the Appalachian Plateau and determined average rates of shale undercutting between 0.5 to 3.75 inches per year (median rate of 1.4) based on weathering periods of 12 and 25 years. The current study consisted of re-measuring eight of the same roadcuts and recalculating the rates of undercutting based on weathering periods of 27 and 40 years. Results indicate average rates of undercutting between 0.1 and 2.0 inches per year (median rate of 0.8). Shakoor and Rogers (1992) were able to correlate the rate of undercutting with the slake durability index (SDI) of the shale units.

The general decline in the rate of undercutting might be expected as roadcuts reach equilibrium with weathering and slope conditions over time. However, the reduced rate of undercutting may not correlate to a reduced rate of slope failure, as undercutting has continued to destabilize a number of the sites studied. Evidence for continued deterioration includes recent falls or slides at most of the eight sites studied; the reduced length of measurable roadcut relative to the previous study at six of the eight sites; and the regrading of two roadcuts and partial repair of a third since the previous study.