A Durability-Based Approach for Designing Cut Slopes in Weak Rock Units in Ohio

Abstract

Seven cut slope sites along Ohio highways containing thick (>10 ft/3.3 m) units of weak rocks (shales, claystones, and mudstones) were selected for investigating slope stability problems and developing design recommendations. Stability problems affecting weak rock slopes in Ohio include raveling, gully erosion, mudflows, and, occasionally, deep-seated rotational failures. Field data regarding slope height, slope angle, slope stratigraphy, catchment ditch dimensions, natural slope angle, and talus slope angle were collected for the seven sites. Laboratory data pertaining to the sites included point load strength index, second-cycle slake durability index (Id 2 ), plasticity index (I P ), and geologic strength index. The Franklin shale rating system, global stability analysis using SLIDE software, natural slope angle, and talus slope angle were used to determine safe cut slope angles for weak rock units. Based on the correlation between Id 2 and stable slope angles, as indicated by the shale rating system, we propose cut slope angles for weak rock units as follows: Id 2 2 = 20–60 percent—2H:1V (27°); Id 2 = 60–85 percent—1.5H:1V (34°); Id 2 = 85–95%—1H:1V (45°); and Id 2 >95 percent—0.5H:1V (63°). These angles are corroborated by talus slope angles, the natural angles attained by talus material covering cut slopes in weak rocks. Redbeds, consisting of very weak claystones and mudstones, should be analyzed on a case-by-case basis. Additionally, surface drainage, including backslope, midslope, and downslope drains; jute matting to promote vegetation growth, and adequate catchment ditches should be provided for all weak rock slopes.