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Mudrock sequences usually occur as harder, more competent strata (siltstones, sandstones, limestones) alternating with softer, less competent strata (claystones, mudstones, shales). This type of stratigraphy is highly susceptible to differential weathering that results in undercutting of the competent layers by the incompetent layers. Undercutting promotes a variety of slope movements such as rock falls, plane failures, and wedge failures that may not occur otherwise. Examples of such failures from selected sites in Ohio, Pennsylvania, and West Virginia are presented. Because of their high speed, suddenness of occurrence, and occasionally large volume of rock involved, undercutting-induced failures can be quite hazardous.

A variety of remedial measures are employed to reduce the potential for undercutting-induced failures. For a timely implementation of these remedial measures, it is essential to estimate the anticipated rate of undercutting for a given site. In order to develop a method that could be used to predict the rate of undercutting, the amount of undercutting was measured for 14 road-cut sites in Ohio, Pennsylvania, and West Virginia. Information regarding the excavation dates of these cuts was obtained from the highway department records. The maximum amount of undercutting at each site was divided by the time since excavation to obtain the rate of undercutting. The rate of undercutting was then correlated with the slake durability index values of the undercutting units to develop prediction equations for estimating the maximum expected rate of undercutting for different types of mudrock. The paper also reviews various methods of treating slopes subject to differential weathering.

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