DTD has recently completed a pair of horizontal dual-phase extraction (DPE) wells beneath a busy wholesale store. The wells were necessary to address a contaminant plume under the building. The plume is a difficult challenge for several reasons: 1- it’s under a busy retail store, 2- the plume potentially introduces fugitive emissions to the building interior, and 3- the over 2 acre building is on the property boundary and the underlying plume is escaping the property. In short, the plume was creating big issues for the building occupant and the owner, and so many vertical wells would have been required inside the building that their installation would have been impossible without major cost and shutting down the business.
We mobilized our dual rod drill rig (Vermeer 36x50DR) to tackle the difficult drilling conditions expected at the project location. Provided boring logs showed difficult (and often impossible) to drill glacial till. Large cobbles and gravel were noted, with many vertical borings hitting refusal above and within the target screen zone. The dual rod rig has the ability to drill intact bedrock and soils equally well.
The risk inherent in drilling in this geologic setting is drilling past gravel, cobbles or boulders and being deflected from the planned bore path, or having the completed bore collapse completely before the well can be installed. We anticipated slow difficult drilling, and therefore mobilized our our newest mud recycling system, a Kemtron Tango 200. The rig and mud system, coupled with significant care during operation proved invaluable for the successful installation of these remediation wells. The two wells were completed as double-ended configurations with the deep well (~25′) totaling 410 feet and 390′ as the finished length of the shallow well (~12′).
The wells trended from the a traffic island between truck loading docks and a back alley, underneath the building footer, and exited in a parking lot on the opposite side of the building. Both the deep well and shallow well required sharp right-hand turns to reach the targeted position for the well screens at 12.5 feet and 23.5 feet deep, respectively.
The glacial till affected the amount of steering we were able to obtain while advancing each drill rod, and even required that one of the boring attempts be abandoned. The Vermeer 36x50DR is an excellent choice when drilling through mixed ground conditions, but in these geologic conditions (glacial till), no rig is perfect, and there is no substitute for experience. Both the drill rig’s capabilities and our crew’s experience were tested during this installation.
During October, these DPE wells were pilot tested, and the company mobilizing and operating the vacuum system said they could not believe the zone of influence expressed by a single well. The amount of screen a horizontal well puts in communication with the contaminate zone is significant, and in this case, included approximately 400′ of screen between the two wells. Considering the amount of screen, the large zone of influence is not that surprising.