DTD has recently completed the installation of two horizontal remediation wells at an east coast US Air Force Base. The wells are being used to treat contaminants in groundwater at the former site of two 10,000 gallon USTs that were removed in 1983. Treatment will consist of sparging air into groundwater through the two, 120′ long horizontal screens sections that were placed 35′ bgs (~20′ below the water table).
The project specified the wells be installed using a blind or single-ended installation technique. Two wells were designed to have 140′ of blank riser pipe and 120′ of screen (two 260′ wells) installed below the plume. Wells of this length are commonly installed blind and are typically within the parameters for a successful installation, unless the formation contains non-cohesive sands or collapsing gravel.
Gravel is exactly what was encountered during. The first bore was drilled through a ~15′ zone of loose gravels and cobbles. The blind well installation process required pushing the well materials through this zone. Using several different techniques, the crew was able to push the well through the gravel zone and install it to the designed length.
The second bore (30′ to the south) also passed through the same gravel zone. However, this time the collapsing bore prevented the successful installation. Determined to get the well installed, DTD designed an exit for the bore that would not interfere with site activities/operations. The team extended the borehole and exited the ground on target, and then pulled the well back into the bore hole, placing the well screen below the plume as designed. The exit side of the double-ended bore was then grouted to insure air injected into well would not short circuit to the surface via the open borehole.