Annacis Island Wastewater Treatment Plant Ground Improvements

The Annacis Island Wastewater Treatment Plant (AIWWTP) is owned and operated by Metro Vancouver in the Lower Mainland of British Columbia. To meet increasing demand requirements, the Stage 5 Phase 1 Expansion began in 2014.

To reduce seismic induced displacements and to meet post-disaster operational requirements, densification of loose to compact sand was required beneath and around new structures. Ground improvement consisted of densifying an area of approximately 43,000 square meters to a depth of 32 m through the installation of 5,500 stone columns.

Settlement of adjacent structures during stone column installation was a key concern. Settlement of the site and adjacent structures was monitored in real time using an automated survey system which monitored over 400 optical prisms every 4 hours. The system provided high quality data showing soil settlement and horizontal displacement as a result of stone column installation.

Settlement of sensitive structures in close proximity to the densification areas was successfully reduced through construction of 420 m of Cutter Soil Mixing (CSM) double-wall to 30 m depth. Prior to CSM production, a trial wall was constructed and instrumented to assess the effectiveness of the CSM wall in reducing settlement and horizontal movement.

Port, A, C. Johnson and D. Jullienne. 2016. “Annacis Island Wastewater Treatment Plant Ground Improvements,” in: GeoVancouver, History and Innovation, 69th Canadian Geotechnical Conference, Vancouver, British Columbia, October 2-5, 2016. Canadian Geotechnical Society.

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