Construction of a 0.81km long, 6.1m diameter sewer tunnel, a component of D.C. Water’s Clean River Project required construction of three drop shafts and connecting adits. The location of these three shafts and adits was complicated by their locations in a populated suburban neighborhood. Ground freezing from the surface was selected as the method to provide temporary earth support and groundwater control during construction. The ground freezing option was not only the most technically appropriate method, it provided significant advantages to accommodating the citizens in the neighborhood. Not only did the drilling and installation process provide a minimal equipment footprint at each site and reduce cuttings and spoils, but the refrigeration plants were located at a remote location reducing noise levels during the freezing process and construction. This paper discusses the design and implementation process of this complex freezing system that included drilling refrigeration pipes at complex angles in order to minimize traffic interruption and avoid utilities. Additionally, horizontal freeze pipes were drilled under 3 bars of pressure from the EPB tunnel in order to ensure that the adit connections were sufficiently frozen. The intricate cooling system extended over half a mile on both the surface and underground tunnel with instrumentation and monitoring along all points.
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