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  Mountain Tunnelling Method of Excavation Work between Two Shield Tunnels in Permeable Sandy Soil

  Yokohama Circular Northwestern Route tunnelling project consists of two shield tunnels of about 4km in length and 12m in diameter, and two perpendicular tunnels which connect them. This paper deals with one of the connecting tunnels to show design and construction technique employed to overcome permeable ground condition. The connecting tunnel was constructed using curved pipe roofs at the crown, straight pipe roofs at the spring lines, and no support at the invert for soil retention other than chemical grouting for waterproofing. They were implemented to reduce construction time without obstructing material and equipment flows for other construction works, to safely carry out the construction in a permeable sandy soil at a depth of 40m. The series of construction operations were carried out in the space not in use in the circular tunnel, and a number of ground monitoring methods were used to avoid scouring the sandy soil caused by water leakage. The excavation was completed without accidents, and the employed technique contributed to reducing months of construction time.

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