White marble dam, North Adams, Massachusetts

Ground penetrating radar (GPR) was used to determine the thickness and configuration of a 156 year-old white marble block dam, believed to be the only one of its kind in North America. The dam, currently in disrepair, is within Natural Bridge State Park and is owned and managed by the Commonwealth of Massachusetts, Department of Environmental Management (MADEM). System calibrations on exposed marble blocks determined that average .GPR signal propagation velocity is approximately 0.105 mlns (0.344 ft/ns). The dam's thickness east of the sluice is approximately 1.8 meters (6 feet) at the spillway crest and 2.7 meters (9 feet) at the toe. In contrast, the dam's crest west of the sluice has a measured thickness of only 1.2 meters (5 feet). The marble dam appears to be constructed directly on top of bedrock, and bedrock may be incorporated into the dam's structure in some areas. By identifying the dam's thickness and upstream-face geometry, this GPR survey helped MADEM's consulting engineers to stabilize the dam and design a dredging program for the silted-in upstream impoundment while preserving the structure's historical integrity.