Abstract
To determine the source of seepage from a Soil Conservation Service (SCS) high hazard flood prevention dam in New York, a geologic and engineering investigation was made which showed unexplained high permeability in the foundation and embankment and a possibility of voids in the trench drain system. The decision was made to immediately breach this 65 ft high earth fill structure. During the breaching a detailed study was made of the embankment, cutoff trench and drains. The embankment contained some oversize material but was in excellent condition. Unexplained high permeability rates measured in the embankment appeared to be the result of hydraulic fracturing during drilling operations. The drains were intact and were not overloaded; however, they had relatively low permeability. An ancient landslide was discovered in the lower left abutment which was not recognized during construction; therefore, the cutoff trench had not been extended through the slide material. The seepage appearing near the toe of the dam was moving through and under the slide material.
The dam was redesigned and reconstructed with higher capacity drains. The cutoff trench in the left abutment was extended well below the slide zone and the embankment was replaced to its original design.
