Cannonsville Dam Emergency Borehole Response-Paper

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In July 2015, turbid water began discharging from the rockfill toe of Cannonsville Dam in New York after three exploratory boreholes were drilled for a proposed hydroelectric plant. The open, inadequately sealed boreholes connected an artesian aquifer to the overlying dam foundation, allowing pressurized groundwater to erode fine soils and create a potential piping and dam-stability hazard.<br /><br />Drilling stopped immediately. The New York City Department of Environmental Protection began rapidly lowering the reservoir and implemented continuous monitoring of piezometers, discharge turbidity, sediment, and settlement. A system of deep relief wells was installed to reduce artesian pressure. Sonic drilling allowed the wells to be installed with minimal disturbance; three wells intercepting a buried channel of clean sand and gravel proved especially effective. Pumping from these wells stopped the turbid discharge, and reservoir operations were later permitted to return to normal.<br /><br />To permanently address the problem, the team used low-mobility compaction grouting. Contractors successfully retraced the original boreholes, flushed and grouted them, and then grouted surrounding points to densify loosened soils, fill potential voids, and seal the permeable layer. Supplemental grouting was unnecessary.<br /><br />After the grout cured, the relief wells were shut down one at a time. Piezometer responses indicated that pre-incident hydraulic conditions had been restored, with continued water-level monitoring providing long-term verification. The response’s success depended on rapid action, close coordination among the project team, effective pressure relief, and the use of monitoring to guide and assess remediation.

Keywords

Cannonsville Dam

turbid water discharge

artesian aquifer

piping hazard

dam stability

relief wells

sonic drilling

compaction grouting

piezometer monitoring

groundwater remediation