West Dike was constructed between 1908 and 1914, and is part of the Ashokan Reservoir system, which is one of New York City’s largest drinking water supply reservoirs. Exploration work in the early 2000’s indicated significant artesian pressures at the toe of the dike, which were confirmed by a subsequent exploration program. It was believed that these conditions were primarily occurring near a former streambed.
The owner, NYCDEP, sought an evaluation of these conditions, and recommendations for mitigation measures. GZA reviewed available instrumentation data and developed a seepage model calibrated to the instrumentation readings from the past decade. Seepage and stability analyses determined that the dike had acceptable stability factors of safety under normal conditions but did not meet stability requirements for flood and seismic loading, due to the artesian pressures.
A system of relief wells at the toe of the dike was recommended. As part of the design of the system, another exploration program was executed, to better define the lateral extents of the artesian conditions near the former streambed. Updated subsurface and instrumentation information was used to refine the prior seepage and stability models and finalize the relief well design. A 2D “plan view” seepage model was used to evaluate various well layouts and spacing to optimize the proposed number of wells to achieve the necessary pressure reduction.
The relief wells were installed in 2022. As part of the relief well installation, the prior models were refined to provide a tool that could evaluate instrumentation readings and flows from the relief wells during installation, in order to evaluate if changes to the well layout were necessary. Office and field staff were in constant contact to verify readings and installation progress, in order to keep models updated in “real time”. The “real time” modeling indicated that sufficient factors of safety were afforded by the installation of only two new relief wells strategically located along the former streambed, in addition to the novel approach of converting one of the existing piezometers to a relief well. At the end of the program, the model was once again updated to reflect as built conditions and was used to develop action and threshold levels for relief well maintenance.
