A downtown Chilliwack infill on Fraser River floodplain soils required liquefaction mitigation, tight coordination at shallow excavation depths, and careful subgrade verification during footing construction. The geotechnical program addressed loose layers susceptible to liquefaction, seasonal groundwater fluctuation, and flood considerations for the below-grade parkade.
The investigation confirmed fills over loose sands and silts with groundwater influenced by nearby river levels, so the design assumed liquefiable layers unless improved. Foundations would otherwise face low punching capacity during the design earthquake, so ground improvement was required before footing construction.
Before any improvement work, we documented baseline conditions at the adjacent buildings with a formal precondition survey that included interior and exterior crack mapping where access was available. This created a clear starting point for comparison during and after construction.
During densification, we implemented vibration monitoring to manage risk to neighbouring structures and confirm that induced motions stayed within acceptable limits.
We specified densification to establish a non-liquefiable bearing zone beneath the foundations. Post-treatment verification used SPT and DCPT data, together with fines content checks and energy calibration, to confirm that the improvement met project intent. Results showed adequate densification at depth, with some near-surface variability earmarked for local follow-up.
The improvement layout and working platform were coordinated to suit utilities, clearances, and access on a compact site.
A thin silt layer was encountered sporadically at footing subgrade. We used field vane testing at each affected pad to measure undrained shear strength. If the measured strength met the bearing requirement for that footing, the silt remained in place, if not, we directed removal and replacement with compacted structural fill, then re-tested to confirm capacity before release. This keep-or-replace workflow prevented unnecessary excavation while controlling bearing and settlement risk.
Where subgrades were sandy, we checked density by probing and compaction response, and for localized deeper elements such as the sump, we reviewed subgrade conditions and confirmed allowable bearing before construction.
Target bearing pressures and footing types were coordinated with the structural foundation plan and verified during field reviews.
Densification reduced liquefaction and punching risks, footings were placed on verified bearing soils, and the neighbour protection plan provided traceable evidence from precondition through vibration monitoring and post-work checks.
