Originally built around 1925, Watres Dam is a 135-foot high earth fill structure, with a partial concrete core, that extends for a crest length of more than 1400 feet and retains a 167-acre reservoir. A rehabilitation program undertaken by owner Pennsylvania American Water included the installation of a permeable toe drain and a series of permanent passive collection wells at the base of the dam. The permanent well and drainage system would intercept approximately 40 gpm of water seeping from several places at the toe of the dam as a result of piping and lower the phreatic surface within the dam, improving overall stability in addition to the permanent well system, the project specifications, written by engineering firm Gannett Fleming of Camp Hill, PA, also required temporary dewatering to facilitate excavation for toe drain installation. Moretrench drew on its dewatering expertise to design a closely-spaced temporary well system that could then be reused for the permanent system once toe drain installation was complete. This innovative approach reduced the overall project schedule and costs for general contractor Popple Construction of Laflin, PA.
WELL DESIGN & INSTALLATION
Geology varied widely across the site, but generally consisted of silty/clayey sand till with occasional gravel pockets, overlying weathered sandstone. Depth to the water table varied from approximately 35 feet below ground at the dam crest to just below ground at the toe. Key to the success of the dewatering program was a system specifically designed to address these challenging variables. For the primary dewatering system along the base of the dam, 36 wells were spaced at 25-foot intervals. A supplementary system, consisting of five wells on the crest of the dam and four at mid-slope, provided additional drainage capacity during excavation and toe drain installation, and were then abandoned.
At each well location, 8-inch nominal diameter boreholes were advanced into the underlying rock using mud rotary drilling techniques to ensure sidewall stability through the overburden soils. Well construction elements included 4-inch Vee-Wire PVC screen installed through the water-bearing soils, a 2-inch thick annular sand filter pack, and a submersible pump designed to run efficiently with the anticipated flow, calculated to be >10 gpm per well. The efficiency of the operating system in meeting the design drawdown requirements was verified by means of an array of instrumented monitoring wells, including vibrating wire piezometers, installed on the dam crest, slope and toe. Pumped water was piped to designated nearby surface water for discharge. The temporary dewatering system lowered the groundwater enough to allow safe excavation of the toe drain and allowed Popple Construction to complete the toe drain well ahead of schedule.
Overall dam remediation included expansion of the existing bare rock spillway to provide improved flood relief. Moretrench injected a balanced stable cement-based grout through a single line of grout holes to reduce the permeability of the weathered rock upstream of the spillway and eliminate leakage potential. Computerized monitoring by Gannett Fleming, together with detailed manual record keeping, provided verification of the grouting program.
Ensuring safe working conditions was an important consideration on the primarily steeply sloping site, requiring earth work to cut temporary benches in some areas to provide adequate access and to support equipment and personnel during well installation. With active dewatering discontinued, gravity flow of groundwater into the passive well system and toe drain will maintain the phreatic head at a level that will ensure continued dam stability.