Transducers are installed on Mount Rushmore to measure the relative movement of four critical rock blocks located within the sculpture. Measurements are performed using high resolution displacement gauges mounted astride fractures that form the rock blocks. The purpose of the monitoring program is to assist in determining the long-term stability of the sculpture. The monitoring system has been continuously recording data for over 5 years. Inspection of the displacement data indicates that the rock blocks are responding to both daily and seasonal temperature changes. However, any long-term permanent offset of the rock blocks is difficult to discern because of the temperature-dependent movement. In an attempt to identify any permanent offset, a regression analysis was performed whereby displacement data were fit to rock temperature measurements taken at various locations in the sculpture. The regression fits between block offset and rock temperature resulted in generally good correlations as measured by the coefficient of determination, R2. Analyses of the residuals of the least-squares fits indicated no permanent, measurable rock block offset of statistical significance.
The sculpture, which the general public identifies as Mount Rushmore, is carved in pegmatitic granite that forms this peak in the Black Hills of western South Dakota. A close inspection of the carving reveals that the sculptor, Gutzon Borglum, had a very good understanding of the dangers that discontinuities in the rock mass could cause the monument. Gutzon Borglum continually adjusted the position of the presidential carvings to avoid fractures in the rock and, at one point, abandoned the work that had been performed to locate the likeness of Thomas Jefferson to the left of George Washington. Gutzon Borglum also understood the deleterious effects that water, infiltrating the rock through fractures, could have on the sculpture. Borglum developed a patching compound composed of linseed oil, white lead, and granite dust. The National Park Service (NPS), which has been entrusted with the preservation of the monument, currently uses a silicone-based compound to seal surface cracks located on the sculpture.
After carving ended in 1941, no work was performed to assess the overall stability of the sculpture until the 1980s. Beginning in 1989, RESPEC undertook a three-phase process to assess the global stability of the monument. The stability analysis began (the first phase) with three-dimensional mapping of the sculpture using both aerial and ground-based photogrammetry [1]. The second phase required the identification of discontinuities on the rock surface and mapping these discontinuities onto the surface of the threedimensional model [2]. A total of 144 discontinues were identified. Some of the discontinuities were oriented such that they intersected to form a continuous trace on the surface of the sculpture. In this manner, 22 distinct rock blocks were identified.
The third and final phase of the stability study was the key block analysis [3]. The key block analysis individually studied the 22 distinct rock blocks and determined whether any of these blocks have the potential to move out of the sculpture. Of these 22 rock blocks, 2 were identified as having the potential to slide out of the sculpture under gravitational loading (i.e., key blocks). Further analyses indicated that, even under conservative assumptions, frictional forces are great enough to prevent these blocks from moving.
