1998 AEG student professional paper: Graduate division: Engineering and geochemical evaluation of the Middle Devonian Millboro Shale, Highland County, Virginia

Shales have long since been used as engineering fill material due to their innate ability for easy physical break down into soil-sized material, Howev er, also due to their potential for rapid weathering, road cuts within shal e units risk failures. This paper presents results of a study aimed at eval uating the engineering properties, rock-slope stability hazards and potenti al for ground-water contamination of the Lower to Upper Middle Devonian sh ales on U.S. Route 250 W at Bullpasture Mountain in Highland County, Virgin ia. ASTM standard techniques were utilized for classification of the shale with regard to its engineering properties. Classifications were based on parame ters including moisture-density relations, liquid limit, plastic limit and plasticity index. Determination of strength of the material in its rock mas s was done via shear tests, pull tests and slake-durability analysis. Stability analyses of rock slopes primarily involve a determination of the orientations and characteristics of weakness surfaces (discontinuities) wit hin the rock and the frictional characteristics of such surfaces. In this s tudy, orientation data were obtained for the dominant discontinuities, and kinematic stereonet analyses were performed for identification of combinati ons of discontinuities that could lead to rockslides, Both fresh and altere d discontinuity surfaces were tested using direct shear pull,testing techni ques to ascertain cohesion (c) and friction angle (phi) shear strength para meters, Factor of safety calculations were made in accordance with limit eq uilibrium theory and compared for the discontinuity surfaces in progressive stages of weathering and alteration. Decreasing highway road-cut safety fa ctors over time caused by chemical alteration of Devonian shale raise quest ions concerning the economics of short-term slope design versus longterm sl ope design.