SEEPAGE ISSUES AT THE WARREN OHL DAM

摘要

The Warren Ohl dam is located in central Pennsylvania and was built in 1964 as arnzoned soil and rock embankment with a length of 915 feet and maximum height of 55 feet.rnThe dam is underlain by nearly flat-laying sandstone, siltstone and shale that are generallyrnthinly bedded and vertically jointed. The dam is one of two water supply dams for the City ofrnLock Haven.rnSince its construction, seepage issues have plagued the dam. During 1965, upon initialrnfilling, seepage greatly exceeded the rate of inflow to the dam, and a foundation groutingrnprogram was conducted for the west abutment to enable the first filling of the reservoir. Inrn1978, an inspection of the dam indicated the need to control seepage exiting the toe area ofrnthe dam. Shallow drainage measures were initially installed, but were not sufficient to addressrnthe issue. An exploration program found the seepage through the embankment was beingrncontrolled satisfactorily by the central core; however, filtered pipe drains and a granularrnbuttress were added to control seepage exiting the lower portion of the downstream slope ofrnthe embankment. In early 1995, a sinkhole was found along the outside of the east wall of thernemergency spillway. Subsequent evaluations concluded that concentrated seepage along thernbase of the spillway from the underlying rock and cracks in the spillway base had permittedrnerosion of the soil subgrade from below the base of the concrete spillway. A filtered underdrainrnand replacement slab were designed for the spillway. During construction of the spillwayrnrepairs, springs were encountered in the newly exposed rock requiring increased drainagerncapacity to be incorporated into the repairs. In 2002, the Pennsylvania DEP Division of DamrnSafety requested that the City perform a rock grouting program to reduce and control thernseepage below the dam. This paper presents the history of the dam, the geology causing thernexcessive seepage problems, a geotechnical exploration of the dam, the basis for the designrnof spillway repairs and a grouting program, observations during construction, and conclusionsrnon the effectiveness of the repairs.