INVESTIGATING THE SUBSURFACE STRUCTURE OF THE CHESTNUT HILL EARTH EMBANKMENT DAM USING GRAVITY MEASUREMENTS

Information of the subsurface structure of the Chestnut Hill Reservoir earth embankment dam is needed to evaluate the long-term integrity of the dam for water storage and flood prevention. In New England, there are at least 400 high hazard potential dams constructed before the turn of the 20th century that are expected to cause loss of human life if they were to fail, including the Chestnut Hill Reservoir dam (National Inventory of Dams, Federal Guidelines for Dam Safety https://www.ferc.gov/sites/default/files/2020-04/fema-333.pdf). Finished in 1870, technical specifications of the dam are sparse and yet vitally important as the dam is located directly above a residential area. In this study, the gravity anomalies were determined along a transect profile on the top of the dam using a LaCoste-Romberg gravimeter. The raw gravity values become more negative toward the south along the dam, even though the dam becomes thinner in that direction. However, after drift and latitude corrections the gravity values increase to the south, providing insight into the lower-density materials that were used to construct the dam. Combined with other geophysical methods, such as HVSR spectra, seismic refraction measurements, and MASW analyses, we demonstrate that the physical parameters of legacy dams can be retroactively constrained using passive methods—providing information that can be useful for dam management and hazard mitigation.