Soil Liquefaction Analysis in Pickering — Seismic Ground Performance

Under OBC Part 4, any project classified as a building requires a site classification and geotechnical investigation. For single-family homes with a ground floor area under 600 m², a simplified screening may suffice, but if the borehole log reveals loose sand below the water table, a liquefaction trigger analysis becomes part of the engineer's duty of care. We have performed these on custom homes along the Rouge valley bluffs where soft sand pockets were encountered at shallow depth.

SPT provides a disturbed sample and blow count (N-value) at discrete 1.5 m intervals, which is the traditional data for the simplified procedure. CPT provides continuous tip resistance (qc) and sleeve friction (fs) plus pore pressure (u2), giving a much finer stratigraphic profile. In Pickering's interlayered glaciolacustrine deposits, CPT can detect thin sand seams that SPT might miss, and the Robertson (2009) method directly computes the soil behavior type and CRR without needing sample recovery.

A complete liquefaction assessment in Pickering usually ranges from CA$3,120 to CA$6,010, depending on the number of boreholes or CPT soundings, the depth of investigation, and whether cyclic triaxial testing is required. Basic SPT-based screening with two boreholes and a report will be at the lower end; adding a CPT campaign and laboratory cyclic testing moves toward the upper range.

Fieldwork — drilling or CPT sounding — typically takes one to two days depending on access and depth. Laboratory testing, if cyclic triaxial is specified, adds 10 to 14 working days due to specimen preparation and consolidation. The interpretive report with recommendations is delivered within three weeks of field completion in most cases.

Yes. Depending on the depth and thickness of the liquefiable layer, ground improvement methods such as vibrocompaction, stone columns, or deep dynamic compaction can densify the sand and increase the CRR above the demand. We have designed stone column grids for Pickering subdivisions where the alternative was an expensive piled raft, and the densification approach met the OBC performance criteria with a factor of safety above 1.5.