Site investigations and analyses have been performed for the large rock sculptures carved at Mount Rushmore and Stone Mountain. Because of the low stresses at the surface and the strengths of the granitic rocks, a major focus of the investigations and analyses were the discontinuities in the rock mass. For both sculptures, the discontinuities were identified, located, and measured for use as input in key block analyses. The analyses of the Mount Rushmore sculpture identified the rock blocks that comprise the sculpture. All blocks are stable, but three blocks have been identified for additional observation. At Stone Mountain, the lengths and spacings of the joints are such that no blocks of significant size were identified that are bounded by natural discontinuities. However, during the carving of Stone Mountain, artistic changes required the placement of several large blocks or patches. Analyses of the replacement blocks show that they are stable.
A concern at both sculptures is the role of water in stability and weathering. At Mount Rushmore, a study was undertaken to identify the most appropriate sealing material to replace the traditional sculptors' sealant, which has been used since the carving was finished. This sealant will inhibit the ingress of water and minimize the effects of freeze-thaw cycles. At Stone Mountain, the deleterious role of water has to do with its contribution to the growth of lichenous materials.
Geomechanical site investigations and analyses have been conducted for the large rock sculptures at Mount Rushmore and Stone Mountain. Both sculptures are carved in strong, durable rocks that have generally low stresses relative to the rock strength because of the sculptures' locations at the ground surface. As is common with other rock structures under low stresses near the ground surface, the major sources of potential instability and displacement are the movements of rock blocks bounded by the discontinuities in the rock mass. Key block analysis, developed by Richard Goodman and Gen-Hua Shi specifically for such blocks, was utilized in the investigation and analyses of these sculptures. This paper describes the site investigation programs, the key block analyses, and other geomechanical programs such as sealing and monitoring for both sculptures.