Evaluating selected factors affecting the depth of undercutting in rocks subject to differential weathering Yonathan Admassu a , Abdul Shakoor b, ⁎, Neil A. Wells b a Department of Geology, Kent State University — Ashtabula, 3300 Lake Road, Ashtabula, OH 44004, USA b Department of Geology, Kent State University, Kent, OH 44240, USA abstract article info Article history: Received 28 January 2011 Received in revised form 10 September 2011 Accepted 13 September 2011 Available online 21 September 2011 Keywords: Multiple regression Rockfalls Cut Slopes Differential weathering Undercutting One of the most important considerations in designing cut slopes in sub-horizontal, inter-layered, sedimentary rocks subject to differential weathering is predicting the total depth of undercutting. Undercutting-induced rock- falls are a major problem for many roadways in Ohio. The total depth of undercutting was measured for 59 pro- files from 18 cut slope sites in Ohio with references to cut faces of hard rock units containing pre-split blast-hole traces. The presence of blast hole traces ensured that the rock face represented the slope surface that was cut dur- ing construction. Additionally, initial design plans were used for reference. Step-wise regression was used to de- termine the geological, geotechnical, and geometrical factors that have the highest influence on the total depth of undercutting. Selected independent factors used in the regression analysis included the vertical distance of the undercut unit from the slope crest, the relative position of the undercut unit from the slope crest, the total thick- ness of the undercut unit, the spacing of orthogonal joints within the undercut unit, the slake durability index value of the undercutting unit, the initial slope angle, and the age of the road cut. Factors that showed the most significant correlation with the total depth of undercutting were found to be the vertical distance of the un- dercut unit from the slope crest, the relative position of the undercut unit from the slope crest, the total thickness of the undercut unit, joint spacing within the undercut unit, and the slake durability index of the undercutting unit. The regression analysis resulted in an R 2 value of 0.61, with the depth of undercutting correlating most strongly with a closer relative position of the undercut unit to the slope crest above it and to a closer spacing of orthogonal joints within the undercut unit. These results can be attributed to the fact that more porous and fractured rock units closer to the slope crest intercept and transport greater amounts of infiltrating groundwater, producing greater depths of undercutting. Also, closely jointed rock units are more permeable, allowing more groundwater seepage, which also leads to greater undercutting. Published by Elsevier B.V. 1. Introduction Cut slopes in sedimentary rock sequences can fail due to unfavor- able discontinuity orientations, low rock-mass strength, and rapid weathering of softer rock units, posing a major hazard to motorists and in aggregate costing millions to taxpayers. Cut slopes consisting of hard sedimentary rocks (e.g., sandstones, limestones, and dolostones) fail mainly due to unfavorable orientation of discontinuities or low rock mass strength, whereas those consisting of soft rocks (e.g., shales, claystones, and mudstones) fail mostly due to weathering and subse- quent surface sloughing. The Ohio Department of Transportation (ODOT) constructs catchment ditches and provides drainage to collect fallen materials and remove water, and attempts to design cut slopes at appropriate angles to minimize failures in the various types of bed- rock. Cut-slope angles for hard rocks with discontinuity-caused failures can be selected using Markland's criteria as summarized by Hoek and Bray (1981). For hard rocks with low rock-mass strength, criteria from Hoek and Brown (1997) can be used to choose appropriate slope angles for various factors of safety values. For slopes comprised of soft rock units, Franklin (1983) recommends selecting stable slope angles on the basis of the shale rating system, which numerically rates soft rock units using the slake durability index, the point load strength index, and the plasticity index. In cases where slopes consist of inter-layered hard and soft rock units, differential weathering of softer rocks results in generation of rockfalls from the overlying harder rocks (Shakoor and Weber, 1988) (Figure 1). Such undercutting-induced rockfalls are very common in the eastern and southern parts of Ohio, where the geology is character- ized by sub-horizontally layered sequences of hard and soft rock units. A statistically significant relationship has been shown between the rate of undercutting of hard rock units and the slake durability index values of the underlying soft rock units (Shakoor and Rogers, 1992). Ferguson and Hamel (1981) state the importance of valley-stress-relief joints in promoting undercutting-induced rockfalls. Engineering Geology 124 (2012) 1–11 ⁎ Corresponding author. Tel.: + 1 330 672 2968. E-mail address: ashakoor@kent.edu (A. Shakoor). 0013-7952/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.enggeo.2011.09.007 Contents lists available at SciVerse ScienceDirect Engineering Geology journal homepage: www.elsevier.com/locate/enggeo