United States Department of Transportation
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Abstract:Based on the stratigraphy and the type of slope stability problems, the flat lying, Paleozoic age, sedimentary
rocks of Ohio were divided into three design units: 1) competent rock design unit consisting of sandstones, limestones,
and siltstones that may exhibit discontinuity-related failures; 2) incompetent rock design unit consisting of shales,
claystones, and mudstones that may exhibit raveling and gully erosion; and 3) inter-layered design unit consisting of
both competent and incompetent rocks where differential weathering may result in undercutting-induced failures. Data
regarding geological parameters (stratigraphy, joint orientation, joint spacing, bedding thickness, total thickness of rock
unit), geotechnical parameters (point load strength index, slake durability index, plasticity index, geologic strength
index, rock quality designation), and geometrical parameters (slope height, slope angle, catchment ditch width,
catchment ditch depth) were collected for 26 cut slopes containing the three design units. Twenty three additional sites
were later added to the study for a more detailed investigation of undercutting-induced failures within inter-layered rock
sequences and the instability caused by raveling of incompetent rock. The data were used to perform slope stability
analyses including kinematic analysis using discontinuity data, global stability analysis using the geological strength
index (GSI) and the Franklin shale rating system, and an analysis for determining the stable slope angles using the
approach described in the Ohio Department of Transportation Geotechnical Bulletin 3 (GB 3).
Results show that slopes cut at 0.5H:1V and 0.25H:1V are adequate in minimizing the potential for discontinuity related
failures in competent rock design units and second-cycle slake durability index (Id2) values can be used to select stable
slope angles for incompetent rock and inter-layered rock design units. Based on Id2 values, these angles range from <
2H:1V to 0.5H:1V. RocFall analysis indicates that either a 13 ft (3.9 m) wide by 1 ft (0.3 m) deep ditch with a 10 ft (3 m)
wide flat bottom and a 3H:1V foreslope or 16 ft (4.8 m) wide by 1 ft (0.3 m) deep ditch with a 10 ft (3 m) wide flat
bottom and a 6H:1V foreslope would adequately contain at least 95 % of the rockfalls, as long as the slope height does
not exceed a certain limit. For higher slopes, either rockfall barriers or wider and deeper catchment ditches will be
required. The choice between a rockfall barrier and a catchment ditch will depend on economic considerations and/or
space limitations. Based on these results, detailed cut slope designs, including slope angle, catchment ditch and bench
design, and stabilization techniques, are recommended for each of the three design units.
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