1998 AEG Student Professional Paper: Graduate Division: Engineering and Geochemical Evaluation of the Middle Devonian Millboro Shale, Highland County, Virginia

Abstract

Shales have long since been used as engineering fill material due to their innate ability for easy physical break down into soil-sized material. However, also due to their potential for rapid weathering, road cuts within shale units risk failures. This paper presents results of a study aimed at evaluating the engineering properties, rock-slope stability hazards and potential for ground-water contamination of the Lower to Upper Middle Devonian shales on U. S. Route 250 W at Bullpasture Mountain in Highland County, Virginia. ASTM standard techniques were utilized for classification of the shale with regard to its engineering properties. Classifications were based on parameters including moisture-density relations, liquid limit, plastic limit and plasticity index. Determination of strength of the material in its rock mass 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) within the rock and the frictional characteristics of such surfaces. In this study, orientation data were obtained for the dominant discontinuities, and kinematic stereonet analyses were performed for identification of combinations of discontinuities that could lead to rockslides. Both fresh and altered discontinuity surfaces were tested using direct shear pull-testing techniques to ascertain cohesion (c) and friction angle (ϕ) shear strength parameters. Factor of safety calculations were made in accordance with limit equilibrium theory and compared for the discontinuity surfaces in progressive stages of weathering and alteration. Decreasing highway road-cut safety factors over time caused by chemical alteration of Devonian shale raise questions concerning the economics of short-term slope design versus long-term slope design.