NUMERICAL DETERMINATION OF BEDROCK OUTCROP STABILITY OF SELECTED PENNSYLVANIAN AGE FORMATIONS IN WESTERN ARKANSAS

Failures of bedrock due to oversteepening of slopes by construction of roadways and buildings cost Arkansas state agencies and private organizations millions of dollars each year. Traditional rock strength evaluations upon which slope stabilities are determined are mainly based on rock-sample stress/strain laboratory analyses which do not include outcrop features such as fracturing, layering thickness, and inclination of strata that can significantly reduce rock strength of large-scale exposures. A rock strength classification developed by New Zealander, Michael J. Selby, that assigns numerical values to stress resistance and outcrop features for crystalline igneous rocks has been adapted to evaluate the strength and predict the stability of selected sedimentary formations in Western Arkansas. Nearly 1,750 outcrops were screened and approximately 175 outcrops of the Hartshorne Sandstone and Atoka and McAlester formations have been analyzed for adaptation of the Selby's Numerical Model. Stability was assessed by visual inspection of bedrock exposures. Features that have proved to have the most significant impact on stability of sedimentary strata of Western Arkansas include dip of bedding, bedding plane thickness, inclination of fracture systems, fracture width, fracture spacing, rock hardness, and degree of weathering. By creating a numerical model incorporating aspects of outcrop features into a strength/failure prediction model, it is estimated that greater than 95% of potential bedrock failures in Western Arkansas can be predicted, and an inexpensive screening tool can be made available to state and private agencies.