CUT SLOPE DESIGN FOR SUB-HORIZONTAL COMPETENT ROCK UNITS IN OHIO

Although most cut slopes in Ohio consist of inter layered sub-horizontal competent and incompetent rock units (sandstone, limestone, dolomite, shale, claystone, mudstone), slopes consisting of relatively thick competent rock units are not uncommon. Competent rock units require independent design that takes into consideration rock mass strength and orientation of discontinuities with respect to slope face. Twelve sites with thick competent rock units were selected for stability analysis using rock mass strength-based analysis and kinematic analyses. Discontinuity orientation data, geologic strength index (GSI) data, and rock samples were collected from each site. Unconfined compressive strength, friction angle, and density were determined in the laboratory. Results of stability analyses showed that rotational slope failures, caused by low rock mass strength, and conventional plane and wedge failures were much less likely to occur. However, toppling of rock blocks, bounded by steeply dipping discontinuities, was observed during field investigations. This type of toppling failure is promoted not only by the steeply dipping discontinuities but also by the presence of thin friable layers within the more competent rock unit that can cause undercutting. Stereonet-based kinematic analysis is not suitable for determining slope angles that would prevent the occurrence of undercutting-induced toppling. Cartoon models were used to analyze the effect of varying slope angles on the occurrence of such toppling failures. The models showed a slope angle of 45⁰ (1H:1V) to be the most appropriate angle. However, a 63⁰ (0.5H:1V) slope angle can significantly reduce the potential for such failures and is recommended for design of sub-horizontal competent rock units in Ohio.