Quarry Wall Slope Stability Study

At Draper Aden Associates, we take pride in the work we do and in the people chosen to perform the work. Beautiful stories and interesting facts often emerge when carrying out our work. What we are able to capture is, oftentimes, too educational, inspiring and visually appealing to not share with our staff and our followers.

Recently, a DAA client purchased some land containing an abandoned quarry. Because the quarry had been cut into the side of a mountain (Figure 1), the rim of the quarry had the potential for a beautiful view of the adjacent valley (Figure 2). The owners wanted to build three cabins on the rim of the quarry, with a conceptualization to have part of the structures extending over the rim of the quarry as in the concept drawing in Figure 3. However, the building inspector would not issue a building permit for such a structure because the code required the building to be 130 feet back from the rim. Discussions with the building inspector suggested that a waiver to the code might be issued if a geotechnical study would demonstrate that building closer to the rim could be accomplished safely.


FIGURE 1


FIGURE 3

We undertook a rock wall stability study that involved having our aerial services group take oblique aerial photography with an unmanned aerial vehicle (UAV). Photogrammetric analysis from the UAV was used to create 3-D point cloud of the rock face. This point cloud was divided into three sections based on the generally characteristics of the rock face (Figure 4). The point cloud was used to derive the planar surfaces or “facets” that represent the discontinuities that would control any prospective slope failures (Figure 5). Stereonet analysis revealed that the most likely type of slope failure would be a planar failure.


FIGURE 4


FIGURE 5

We calculated the factor of safety (FOS) of the rock wall using ROCKPACK III developed by C.F. Watts & Associates. The analysis revealed that it would be unsafe to put a load on the rim of the quarry, but just a few feet back from the rim the FOS was over 9.0 for a building. This means that the resisting forces of the rock are more than 9 times the forces driving a failure. It also means that a structure could be extended over the quarry rim as long as the load was transferred back from the rim a few feet.

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