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  Ground freezing and Jet -grouting. A hybrid solution for Tunnelling in urban

  The Northeast Boundary Tunnel (NEBT) is a 5 miles long tunnel and is the largest component of the Clean Rivers Project. The NEBT will connect the existing sewer system to significantly mitigate sewer overflow while improving the water quality of the Anacostia River. The NEBT includes construction of Tunnels, Shafts, Adits and Near Surface Structures and will be completed in 2023. The construction of two out of six Adits, were performed using a ground improvement technique such ground freezing and a unique hybrid approach consisting of ground freezing and jet-grouting. During design and construction of the Adits multiple challenges have been encountered. This paper describes the laboratory tests on frozen ground, as well as the field tests to assess the shotcrete performances on frozen surfaces enabling an efficient construction process. Furthermore, engineering analysis were conducted to predict freeze pressure induced to existing tunnel segments and the load increase over the tunnel, while post construction ground thawing occurred at the end of the excavation.

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