ABSTRACT
While much is known about the seismic behavior of tunnels, limited literature is available which sheds light on the seismic behavior of circular shafts, which tend to undergo ovalisation on the cross-sectional plane and racking in the vertical plane. This paper summarizes the design methodology adopted for the seismic design of multiple shafts on Watercare’s Northern Interceptor Phase II, New Zealand. This includes the risk-based approach in the overall design philosophy, where an initial racking assessment was carried out to identify shafts that are impacted by liquefaction or cyclic softening, and subsequently bring them forward for ground structure interaction analysis. The shafts in question adopted a secant pile wall permanent lining, which is composed of alternate high-strength, reinforced “hard” piles and low strength, unreinforced “soft” piles. As a result, the axial stiffness of the shaft structure varies along the hoop direction. Structural analysis was therefore carried out to study the three-dimensional seismic behavior of the secant pile wall shaft structure. The overall seismic design approach, as well as the design methodology and findings of the geotechnical and structural analysis are presented in this paper. The validation measures carried out to reconcile the findings of geotechnical ground structure interaction analysis and structural analysis are also discussed. These resulted in a robust design of shafts with a non-typical structural form in ground prone to liquefaction and cyclic softening.
