The Iona Island Wastewater Treatment Plant (IIWWTP) is currently serving about 750,000 residents in the Vancouver Sewerage Area and expected to serve 950,000 residents by the year 2051. The existing facility is vulnerable to strong seismic shaking and sea level rise. The original facility was built in 1963 and it is one of the last wastewater treatment plants in west coast of North America that provides only primary wastewater treatment. This primary level treated water is currently being disposed to Salish Sea. However, the legislation requires the wastewater treatment plants to be upgraded to minimum secondary treatment no later than end of year 2030. Metro Vancouver is planning to upgrade the IIWWTP to provide tertiary treatment to protect water quality and marine environment and also to meet the post disaster objectives under both the 2,475-year ground motions consistent with the 2020 National Building Code of Canada and sea level rise. The subsurface conditions at the proposed IIWWTP generally consist of overbank sediments and potentially liquefiable Fraser River sand deposits with varying fines content followed by compressible marine deposits. There are artesian conditions present within the marine deposits resulting in upward flow and potential leaching of the marine deposits. The upper part of the marine deposits is expected to undergo significant cyclic softening with the potential leaching
effects under the design ground motions. The marine deposits are also expected to settle following placement site grading fill to raise the site grade to meet the flood proofing elevation as well as the loading from the treatment facilities. The existing plant has settled about 900 mm the time of construction and still settling at a rate of about 10 mm/year. The proximity of the proposed IIWWTP to the waterfront on both sides along with the compressible and liquefiable soils has increased the challenges in terms of designing the new plant to meet the post disaster performance objectives. Project definition phase of the new plant has been completed and the indicative design indicates that vibro replacement stone columns to improve the sand deposits within the plant footprint and a seismic barrier comprising Deep Soil Mixing (DSM) along the perimeter of the site to limit the lateral spreading due to cyclic softening of the marine deposit are required to upgrade the site to meet the post-disaster performance objectives. In addition, the treatment plant facilities may need to be supported on piles. The geotechnical requirements for this project become significant leading to the foundation costs alone to be in the order of one billion dollars. This project is ranked as number 8th most
expensive project in Canada based on the project value of 10 billion dollars with expected substantial completion date in 2035. Given the overall costs and the geotechnical complexity of the project, the project is delivered through a Stage Gate Framework (SGF) with early works
packages to reduce the overall project schedule. The ground improvements (i.e., stone columns, DSM, preloading) for the new plant are part of the early works and field trials for the stone columns and DSM with a cost of about three million dollars are planned to be carried out
to optimize the design of ground improvement program. This paper will present the challenges associated with the site characterization and numerical modelling to optimize the ground improvement program including preloading design. The paper will also present the design of field trials, implementation of field trails, and early results of field trials.