Case History: Dewatering for DC Metrorail Terminal

Case HistoryDTD recently completed a series of six dewa-tering wells to support construction of the DC to Dulles Metrorail in Washington, D.C. The project was completed during record blizzards (including more than 50 inches of snow in 12 days) ex-perienced in early 2010. The harsh winter conditions, abundance of buried 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 be-low grade rail line, in order to reduce groundwater encountered during the tunneling process. Control of excessive seepage is often a high-cost effort in many tunneling projects. Horizontal wells can reduce this problem by controlling the groundwater before the tunneling equipment reaches the dewatered zone. DTD mobilized their CMS 6015 drill rig for this project. The rig is compact enough to fit on tight con-struction 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 mod-ern HDD technologies, when crossing bores were typically within a few feet of the pipeline. For this project, the hori-zontal 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 inter-ceptions with vertically drilled inspection wells. Starting at a distance 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 con-firmation was made, each bore was continued along its design profile through the targeted screened inter-val. Due to the unavailability 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 con-struction of the Metrorail will soon be initiated. Client Name and Address TBD Construction Dewatering Drains Metrorail Tunnel – Washington, D.C. Client Technical Point of Contact TBD Contract Value TBD Directed Technologies Drilling, Inc. 8700 State Highway 3 SW Port Orchard, WA 98637 800-239-5950 info@horizontaldrill.com © 2010 Directed Technologies Drilling Inc. All rights reserved Case History—Escambia Superfund Map adapted from Google Earth