Comparison of Pile Foundations and Ground Improvement for Reduction of Seismic Displacements for a Wastewater Treatment Plant-Paper

Paper

Pdf Summary

The study compared ground improvement and pile foundations for reducing earthquake-induced movements of two proposed trickling filters at the Annacis Island Wastewater Treatment Plant in British Columbia. The site has liquefiable Fraser River sands over deep, compressible silts and clays. Nonlinear FLAC analyses using the UBCSAND constitutive model evaluated performance during the 2,475-year design earthquake and subsequent consolidation.<br /><br />Without mitigation, the filters were predicted to move more than 1 m horizontally, with short-term differential settlement of about 400 mm over 50 m. Three options were assessed: vibro-replacement stone columns; stone columns supplemented by cutter soil mixing (CSM) walls; and steel pipe piles without ground improvement.<br /><br />Stone columns extending 20 m beyond the structure footprints substantially reduced liquefaction-related movement. Predicted horizontal displacement fell to about 130 mm, and short-term differential settlement to 140 mm over 50 m. Adding CSM walls provided little additional seismic performance in the modeled configurations, though walls could help limit installation-related settlement of nearby structures.<br /><br />Piles installed beneath the filters also offered comparable performance. Piles connected to the raft reduced horizontal displacement to approximately 230 mm, with differential settlement gradients broadly similar to those achieved by stone columns extending 20 m beyond the structures. Unconnected piles were less effective at controlling differential settlement. Pile-supported cases had greater long-term settlements than the extensive ground-improvement case, and connected rafts may need to span between piles if surrounding soil settles away.<br /><br />Overall, all mitigation options could provide similar seismic and post-seismic performance. However, ground improvement must extend well beyond the structures, whereas piles can remain within their footprints, potentially increasing separation from existing facilities and reducing installation impacts. For conceptual budgeting, the study considered stone columns with CSM and piles unconnected to the raft to have comparable cost and construction duration.

Keywords

earthquake-induced movements

liquefiable Fraser River sands

ground improvement

pile foundations

stone columns

cutter soil mixing

UBCSAND

FLAC analysis

differential settlement

Annacis Island Wastewater Treatment Plant