Froehling & Robertson Inc were engaged by the Virginia Department of Game and Inland Fisheries to remediate seepage paths at the Laurel Bed dam. The dam had been leaking in excess of 500 gpm with water leaking in multiple locations. This water supply feeds a popular trout stream. If the dam failed, homes in the valley below the dam could be flooded.
Q: Problem you needed to solve?
“Laurel dam had a history of unsuccessful grouting attempts. We had an idea of where the seepage paths were but not a specific area. We needed to pinpoint the seepage paths and the specific area to grout to save money drilling and in remediation.”
Q: Alternatives you considered?
“We didn’t have any good alternatives for getting a focused map of preferential paths. Seismic follows soils more than water; unlike Willowstick it wasn’t built for mapping groundwater directly. Resistivity might have worked but we had a concrete spillway—it wasn’t uniform ground. Rebar would have gotten in the way. Resistivity induces an electric field from above the ground. The first thing it hits is the rebar. Resistivity is good if you are looking for underground tanks, buried things. It finds bedrock pinnacles and zones of deep saturated soil, but it also picks up rebar. I don’t think it is optimal for clearly mapping seepage paths. Too many things can interfere with its signal since it doesn’t light up the water directly.”
Q: Why did you select willowstick?
“Willowstick can and did give us a specific groundwater map showing both vertical and horizontal location. We already knew roughly how deep it was. Our lake was only 5-10 feet below the top of the DAM. We were most interested in a plan view of where the seepage paths were. Willowstick energizes the groundwater directly so you get a clearer view without interference. It doesn’t pick up clay etc.”
Q: Was it successful?
“Yes. willowstick gave us the specific target area to grout. The remediation work has shown that Willowstick was accurate. It matched up really well.”
Case Study: Laurel Bed Dam