Selecting tunneling approach and developing the final lining design through highly variable and challenging ground

The City of Manchester, New Hampshire, USA, is embarking on one of its largest public works projects in the city’s history to construct an approximately 3.55 km long, 3.66 m internal diameter stormwater conveyance tunnel through downtown Manchester. The proposed stormwater conveyance tunnel traverses through highly variable subsurface conditions, transitioning between soft ground, mixed face, and hard rock multiple times representative of very challenging geology with an undulating bedrock profile overlain by fluvial deposits. The soils consist primarily of weak sandy silts, silts, and silty clay material, while rock is classified as the Massabesic Complex consisting of granite, schist, and gneiss. Presented herein is an overview of the project development, approach for selecting the tunneling method, and analyzing, and design of the segmental lining of the tunnel.