Constructed in 1949, the original Lyman Run Dam was an earth-filled structure, 920 feet long and 50 feet high, with a top width of 14 feet. The dam, which retained a 45-acre recreational lake, had experienced heavy seeps and leakage and has been monitored almost since construction. In 2000, the Department of Environmental Protection determined that structural analyses showed the dam to be unsafe.
Subsequently, the lake was drawn down and the dam was breached to allow in-depth examination. The extent of the structural defects revealed, including the absence of an impervious core required in the design, necessitated complete replacement.
Subgrade for core trench excavation for the new, roller compacted-concrete dam varied between elevations +1577 and +1625 feet. Groundwater was encountered at elevation +1605 feet. Excavation below the groundwater table extended between stations 1+20 and 11+25. Moretrench designed and installed the dewatering system that ensured excavation and core construction in the dry.
Deep Well Design and Installation
The intent of the dewatering program was to lower the groundwater to three feet below any temporary excavation surface and the finished core trench foundation grade. To accomplish this, Moretrench’s design involved 43 deep wells installed at approximately 50 feet on center on either side of the proposed trench alignment, together with approximately 3,000 linear feet of piping and electrical distribution.
The wells were constructed using mud rotary drilling techniques to advance a nominal 8-inch diameter borehole through the overburden soils and into the underlying decomposed rock to a depth of 20 feet below the anticipated foundation grade. During drilling operations, borehole stability was maintained through the use of a biodegradable drilling fluid additive. Each well was equipped with a submersible pump, capable of pumping 10 to 24 gallons per minute, and an above-grade control panel. Actual flow was on the order of 3 to 8 gpm per operating pump. The system was run continuously during excavation below the groundwater table. Pumped water was directed to detention ponds and ultimately discharged into Lyman Run Creek.
During dewatering, drawdown was verified by means of water level indicators on the operating wells. Once placement of the new embankment fill reached an elevation higher than that of the static water level, pumping was discontinued and the wells were tremie-grouted and abandoned.