Victoria Park Tunnel – Drawdown and Settlement in the Auckland Central Business District

摘要

Construction of the Victoria Park Tunnel (VPT) project required the excavation of a 450 m long, 3 lanernroad tunnel through Auckland’s CBD in close proximity to the Waitemata Harbour. The tunnel,rnconstructed using cut and cover techniques, had a maximum excavation depth of 11 m bgl (some 9 mrnbelow the groundwater table) through contaminated fill, compressible alluvium and sedimentary rock.rnAlthough the tunnel is now sealed, short term dewatering to facilitate construction was necessary. Arncombination of low permeability diaphragm and secant pile walls was used to retain the excavationrnand limit groundwater inflows; however the base of the excavation was left open for periods of up to 4rnmonths before the floor slab was laid. In order to evaluate the potential for consolidation settlement ofrnexisting structures including nearby historic buildings, the potential for contaminant and salt waterrnmigration, and guide detailed design, numerical groundwater modelling was undertaken. Groundwaterrnlevel and ground settlement monitoring was undertaken before, during and following construction, withrnpre-construction monitoring providing valuable records of naturally occurring large seasonal variations.rnRecorded drawdown, settlement and inflows have generally remained within the consented levels.rnSome exceedances were recorded however a pragmatic approach to monitoring and management ofrnthese exceedances allowed works to continue with minimal disruption and no adverse environmentalrneffects. This paper presents a brief comparison of calculated effects against those which occurred,rnwith a focus on two isolated groundwater level trigger exceedances (their causes and management).