231.7 Backslope Grade

231.7.1 Introduction

Consult the district geologist to obtain the preliminary geotechnical report sufficiently early in the project development process so the slope ratios can be established in the preliminary plan phase. The slope ratio given in the preliminary geotechnical report is a maximum and is not to be exceeded. Also, refer to the guidance found in Roadside Ditches since it also impacts the backslope.

231.7.1.1 Rock Cuts

The maximum backslope allowed is included in the preliminary geotechnical report. Rock bench details are included in the preliminary geotechnical report and are typically set at 30 ft. (9.0 m) vertical with 15 ft. (4.5 m) horizontal benches. When good quality rock is present, it is recommended a 1:1 backslope be used from the back of the ditch to establish the theoretical slope limit. This slope limit can then be utilized to determine the location of the R/W line. If there is a significant amount of soil above the rock, then the soil cut slope recommended in the preliminary geotechnical report must be used to establish the theoretical slope limit.

231.7.1.2 Special Conditions

Often the preliminary geotechnical report will reveal the presence of different materials or conditions such as shale, sandstone, pinnacle rock, etc. This may necessitate site-specific requirements for the backslope.