Few things demand immediate attention more than an erosion or piping condition at a dam or levee. No matter how small initially, the condition has the potential to be catastrophic and impact a wide downstream area. Addressing the problem quickly and effectively is critical. When a turbid water discharge developed from the downstream rockfill embankment at the toe of the NYC DEP Cannonsville Reservoir Dam, in the Catskill Mountains, Moretrench was ready and able to respond.
The source of the problem was quickly identified as resulting from the interception of artesian water in the underlying glacial till during drilling of exploratory boreholes for construction of a new hydroelectric plant. Work was halted, 24-hour monitoring was initiated, and as a risk reduction measure NYCDEP began to draw down the reservoir’s total storage at a rate of a billion gallons a day. They then turned to qualified contractors within the geotechnical community for a solution to control the ground loss and piping. Moretrench was given a verbal commitment for the work based on its familiarity with this type of problem, and its ability to not only develop and implement a pressure relief system to lower the hydrostatic head within the glacial till, but also to permanently seal off the artesian aquifer.
Moretrench immediately began coordinating its work force and gathering materials, and worked with the owners’ geotechnical consultant to develop the pressure relief system. With the preparation well in hand, Moretrench was then able to mobilize to the site and begin installation of the first of a series of pressure relief wells within three days of the notice to proceed.
The 75-ft deep wells were installed using sonic drilling, which allowed a cased hole to be created with little or no water– a critical consideration under the circumstances– and screened in the glacial till. Each well was activated as it was completed. One week after mobilization, the artesian pressure was under control. The following day, the NYCDEP was able to discontinue the reservoir drawdown.
Moretrench just completed grouting of the borehole locations to seal off the aquifer.