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Horizontal SVE System Beneath Busy Mall Store

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Construction season is in full swing, and DTD project teams are busy working on multiple horizontal well installations across the country; work is ongoing in Hawaii, New Mexico, and Washington. A recently completed project in the mountain west typifies our small rig projects.

At this particular location there is an existing strip mall that is being affected by a groundwater contaminant plume related to an upgradient dry cleaner. A network of horizontal soil vapor extraction (SVE) wells was designed to protect the strip mall from fugitive emissions. Fugitive emissions, in this case, are VOCs in soil vapors that may enter the indoor air space through the building floor slab. And the occupant of the strip mall, a national book store chain, was sensitive to having their business disturbed, vertical wells were not an option.

Target depths for all wells were set at a maximum 6.5 feet below the floor elevation with the longest well extending 210 feet from the entry point.  Drilling at this depth allowed clearance of the footer which extends 6 feet into the ground.  Despite the well configuration and project design being fairly straightforward, the geologic setting made the drilling extremely difficult. The site is in town, at the foot of mountains rising to elevations over 10,000 feet. Glacial OutwashThe sediments on site are alluvial fan and glacial outwash deposits; or variable layers of clean sand, gravel, cobble with occasional clay lenses. To give an idea of the depositional environment, look at the pic to the right, showing glacial outwash being deposited, and note the variable grainsizes; silts, sands, gravel and cobble, all being deposited simultaneously.

This presented a variety of challenges including difficult steering, borehole collapse, losing mud returns, etc.  Also, the wells were designed as single ended (blind) installations, with the option to install double ended if borehole integrity was poor.  Through multiple attempts and reaming passes, we completed over 1,000 feet of drilling to install 750 feet of well materials.  2440 BozemanThe pilot bores for these wells were completed using multiple styles of drilling bits to steer through and around the cobble/gravel rich substrate.  Measures not typically required on a small job were necessary to complete these short wells. For instance, surface or top hole casing was necessary on some of the bores.

Our experience with HDD in gravel and cobble, and a crew and client with the flexibility and willingness to consider field adjustments allowed this challenging project to be completed on schedule, and under budget.