DTD recently completed a series of six dewatering wells to support construction of the DC to Dulles Metrorail in Washington, D.C. The project was completed during record blizzards (including >50″ of snow in 12 days) experienced in early 2010. The harsh winter conditions, abundance of utilities, and site space restrictions required the use of innovative construction techniques to complete the project.
The dewatering wells were drilled in advance of the below grade rail line, in order to reduce groundwater encountered during the tunneling process.
DTD mobilized their CMS 6015 drill rig for this project. The rig is compact enough to fit on tight construction areas, but has ample power to drill the 300 foot long bores required for the dewatering wells. The rig was supplied with drilling fluid from an MTI mud system that included shakers and cyclones to clean the mud for recycling.
A unique aspect of the project was the requirement to visually observe the horizontal downhole assembly as it passed beneath a 12-inch high pressure gas transmission line that traverses the construction zone. This regulation was written many years prior to the development of modern HDD technologies, when crossing bores were typically within a few feet of the pipeline. For this project, the horizontal drains were installed approximately 50 feet beneath the gas pipeline, making it unfeasible to simply trench or “pothole” to meet the visual inspection requirement.
To fulfill this requirement, DTD used a TruTracker wireline locating system to provide precision interceptions with vertically drilled inspection wells. Starting variably from 120-180 feet from the vertical inspection wells, the horizontal drill string was guided to intersect with the inspection wells. A downhole camera was used to confirm that the horizontal string had passed through the vertical well, meeting the requirements of visual spotting of the bit without the need for massive excavation. After the visual confirmation was made, each bore was continued along its design profile through the targeted screened interval. Due to the unvailabilty of an exit for the bores (the area is a concentrated commercial and retail area), the wells were completed as single-ended or blind installations with 4-inch diameter EnviroFlex well screens.
Since completion, the wells have been in continuous operation, dewatering the ground where the construction of the Metrorail will soon be initiated.
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