A rock strength classification system 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 the Arkansas River Valley Region and closely adjoining sections of the Ozark Plateau and Ouachita regions of western Arkansas. Over 2,000 outcrops were viewed. Of these, 200 outcrops of the Hartshorne Sandstone, and Atoka and McAlester Formations have been analyzed for adaptation of Selby’s Numerical Model.
Stability was assessed with this method by classifying and measuring several outcrop parameters. Features proven to have the most significant impact on stability of sedimentary strata of Arkansas River Valley Region include dip of bedding, dip (inclination) of fracture plane systems, rock hardness, and degree of weathering. Probably the most important factors in stability of sedimentary strata are dip of bedding and/or dip of persistent structural fracture planes (joints). One of the primary findings of the study is that sedimentary strata and persistent structural fractures behave in essentially the same manner in terms of rock failure. Almost all sandstones studied failed where bedding and/or joint planes dipped "out-of-the outcrop" at 26o and more. In contrast, shale formations failed where bedding and/or joint planes dipped “out-of-the-outcrop” at 7o and more.
Statistical analysis using Number Cruncher Statistical Software™ (NCSS) indicates an approximately 60% or greater reliability in failure prediction based solely upon rock hardness, degree of weathering, and dip/fracture inclination; however, other features have been shown to influence failure of rocks with borderline strength values. When aspects of bedding thickness, bedding plane width, fracture width, spacing of joints, and groundwater outflow were added to the model, a greater than 95% success rate in predicting failure was recorded for the 200 outcrops surveyed during the project.
The modified Selby strength model provides a reliable means of predicting failure and providing state and private agencies with an inexpensive screening tool for sedimentary bedrock of the Arkansas River Valley Region of western Arkansas.,