PRACTICAL ASPECTS OF ROCK CUT SLOPE DESIGN

The geology and the orientation and condition of the discontinuities present in a rock mass are the most significant factors affecting rock slope stability; therefore, they are the primary controlling elements of rock cut slope design. In practice, the design process is a balance between stability and other items such as constructibility, economics, potential environmental impacts, and the accepted level of risk. Steeper cut slope angles result in lower construction costs, impact less right-of-way, require a smaller volume of rock excavation, and because the cut height is generally less and the excavation footprint smaller, may be perceived as a more environmentally sensitive design.

Rock cuts made steeper than the controlling geologic structure allows may require expensive measures to maintain overall slope stability and wider fallout areas to contain increased rockfall. In addition, superimposing a steep cut slope on an existing slope configuration may result in thin (sliver) cuts. Often, sliver cuts pose impractical construction limitations such as difficult access to begin the cut and may be deemed unsafe to build because the narrow work area provides inadequate room for construction equipment.

When the geologic conditions affecting slope stability are favorable and constructibility is not a controlling factor, a steep cut (0.25H:1V or vertical) is generally preferred. It is important to note that changing the strike or dip of a cut slope by a few degrees may decrease slope performance by increasing the rockfall potential and the number of discontinuities adversely impacting the cut. This could require rockfall mitigation and protection measures, and increase design-life maintenance costs. In such cases, the economic advantages afforded by a steeper slope can be lost and a flatter, more stable slope (0.5H:1V or less) may be warranted.

Since there are no simple, universal rules that can be used to guarantee the performance of a rock cut slope, a thorough understanding of the controlling design factors is essential. It is important to have experienced rock slope design professionals investigate and evaluate the safest and most economical design, which best accommodates the site geology and project-specific constraints.