Content Information
This section addresses the typical rock slope configurations routinely used by the Iowa DOT during the planning of roadway projects. Rock mass improvement techniques such as bolting, dowelling, shotcreting, etc., to produce a stable slope are not covered herein.
The Soils Design Section is generally responsible for determining precisely where rock excavation is necessary and where rock backslopes will be used, but coordination with the Design Section is necessary. Rock excavation procedures are outlined in Section 5B-1. The guidelines for rock cut slopes contained herein are similar to soil cut slopes configurations outlined in Section 200F-11.
Back to topRock Mass Information
Rock slope design relies heavily upon the quality of the rock, which is determined by surface mapping and rock coring to assess discontinuities (fracture/joint) patterns and conditions. Discontinuities strongly control rock mass quality and slope stability. If surface mapping is not feasible due to the presence of overburden soil, or for other reasons, rock coring should be performed to obtain the required rock mass quality data. In addition, the groundwater conditions, and particularly water seepage present in rock discontinuities, should be evaluated as part of the subsurface exploration. The rock quality data will be summarized in the S3 and/or S4 event submittals.
Back to topRock Slope Configurations
Design Details 4110 or 4111 should be used to begin design of rock backslope sections for rock cuts greater than 5 feet. Finalization of rock backslope design should be done jointly between the applicable Design Section and the Soils Design Section to insure the optimum section and to define the rock excavation requirements for the project. Steepened backslopes should be avoided in areas of shale, weathered or broken and weathered limestone, or similar inferior rock unless specifically reviewed and analyzed by the Soils Design Section and the Design Section.
Where the depth of rock excavation in any given area is 5 feet or less, design practice is to provide a normal ditch section; however, this is typically reviewed on a case-by-case basis. A flat bottom ditch with a minimum width of 10 feet is generally required for all rock cut slopes as shown in Design Detail 4101.
Limestone, Dolomites, Cemented Sandstones, and other Hard Rocks
Rock slopes in good-quality limestones, dolomites, and cemented sandstones may be cut nearly vertical or with a slight inclination. An inclination of 0.5H:1V is used for normal rock excavation, and an inclination of 0.25V:1H is used when pre-splitting is performed for rock excavation. Benches should be provided at a vertical spacing not to exceed 35 feet for cut slopes greater than 35 feet high. Benches should generally be a minimum of 10 feet wide. Benches may be provided at the contacts of different formations (not necessarily at 35 feet) and may vary in width. The typical rock slope configurations for excavation of rock with and without presplitting are shown in Design Details 4110 and 4111. The Soils Design Section determines when pre-splitting of the rock is needed and appropriate based on rock quality, rock hardness, etc., and prepares a pre-splitting tab, Tab 107-25.
Shales, Siltstone, and other Soft Rocks
Cut slopes in shale, siltstone, and other soft rocks should be inclined at no steeper than 2H:1V.
Detailed Rock Slope Design
If the project requires a detailed rock cut design or stabilization techniques, FHWA HI-99-007 Rock Slopes Reference Manual, (Munfakh, et al., 1998) should be used in the design. In addition, if development of blasting plans for rock cut excavation is required, FHWA-HI-92-001 Rock Blasting and Overbreak Control, (Konya and Walter, 1991) should be used.
Back to topReferences
- Konya, C. J., and Walter, E. J., 1991, Rock Blasting and Overbreak Control, Federal Highway Administration, FHWA-HI-92-001.
- Munfakh, G., Wyllie, D., and Mah, C. W., 1998, Rock Slopes Reference Manual, Federal Highway Administration, FHWA HI-99-007.