ABSTRACT
A series of large diameter and deep underground hydraulic shafts consisting of internal dividing walls and cascade shelves are being constructed for Watercare’s Central Interceptor (CI) project, a 16.2km long large diameter sewerage tunnel network to increase Auckland’s wastewater network capacity and improve resilience. Underground permanent structures in New Zealand are typically cast in-situ bottom-up with a separate temporary earth retaining structure. Two different methodologies were adopted for the shafts on CI:
Sunken cast in-situ caisson lining which functions as a retaining wall in the temporary stage with the internal hydraulic structures precast and installed at a later stage using vertical corbels cast into the interior lining, and
Precast lining rings with the cascade shelves and dividing wall cast monolithically, connected using vertical continuity bars.
This paper presents the design and construction of the latter methodology (precast) highlighting the benefits and challenges faced in terms of constructability, ensuring watertight joints in a seismic environment and durability considerations associated with microbially induced corrosion. Also included is an overview and discussion on the deformation-based pseudo-static approach adopted for seismic assessment, as applied to Auckland seismicity and ground conditions.
