DTD ended the year 2015 with the installation of a 500 foot long horizontal remediation well at an east coast US Air Force Base. The well will be used to treat contaminants in groundwater at the former site of six Underground Storage Tanks (USTs) that were removed in the 1960’s. The bore was drilled using the traditional double-ended method under an active parking area within the flight line security fence.
The well was initially designed to consist of a 260 foot long screen section installed to a depth of approximately 30 feet below grade in a highly permeable sand formation. The sand formation is underlain by a less permeable finer grained unit. The objective was to place the screen approximately 3-5 feet above the finer grained formation to maximize the contact of the sparged air with the contaminated groundwater.
The bore was drilled to the design depth and continued level through the first 160 foot section of the screened interval. At this point drilling conditions changed from soft to hard, indicating the finer grained formation had been encountered at a shallower depth than expected. An attempt was made to to slightly rise in elevation to get back into the sand but hard drilling continued.
Because the area’s stratigraphy was not precisely known the well was redesigned following discussion between DTD and the consultant: the screen section was
shortened to 160’ and the exit casing length was lengthened by 100’. The bore was then drilled out and exited on target.
Pullback of the well was successful, taking approximately one hour. The well was developed and both the entry and exit sides of the bore were grouted to insure air injected into the well would not short circuit to the surface via the open borehole annulus.
All work was completed within a five day period, on time and within the budget. The drilling also offered additional data points on the variations in site specific geology by providing coordinates (X,Y,Z) of the transition from soft to hard drilling conditions.