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  Ground Freezing for Two Frozen Shafts at the Bergen Point Outfall Tunnel

  Construction of a 9700m, 180cm diameter effluent pipeline required tunnelling from an existing sewage treatment plant to a barrier island outfall structure. Construction of the tunnel required a launching shaft at the plant and a receiving shaft at the barrier island. Artificial ground freezing was specified as the method to provide temporary earth support and groundwater control at the launching shaft and selected by the contractor for the receiving shaft. Design of the ground freezing system was complicated by existence of lateral groundwater flow and the need to freeze the base of the shaft to provide bottom stability. Frozen ground laboratory tests were conducted to evaluate the strength and deformation characteristics of the frozen mass to design a temporary liner needed for tunnelling operations. This paper details the design process used, including a heat transfer and groundwater flow FEM model to address the specified lateral groundwater flow as well as the stress analysis using the frozen soil laboratory test data. Discussion on the approach to create a frozen bottom plug as well as insulation methods to not freeze the interior of the shaft above this plug is included.

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