A warehouse expansion near the Transcona rail yards hit refusal at 2.4 meters—loose sand over Lake Agassiz clay. The structural engineer needed 150 kPa bearing capacity but the SPT N‑values barely reached 8. We proposed a vibrocompaction design that increased relative density to 78% without over‑excavating the entire footprint. In a city built on 12,000‑year‑old lacustrine deposits, Winnipeg’s stratigraphy alternates between soft clay, silt, and pockets of outwash sand. Standard fill compaction often fails below 3 meters, especially where the Red River has reworked the upper layers. Our approach ties the test pit logs directly to the compaction grid, and when the sand fraction is too fine we cross‑check with CPT soundings to confirm the probe‑to‑grid spacing before mobilizing the vibroflot.
A well‑designed vibrocompaction grid can raise relative density from 45% to over 75% in two passes, eliminating deep foundations on marginal sand sites.
Frequently asked questions
What soil conditions in Winnipeg make vibrocompaction suitable versus unsuitable?
Vibrocompaction works best on clean to slightly silty sands with less than 15% passing the #200 sieve and a plasticity index below 10. The loose outwash sands found in parts of southwest Winnipeg and along the floodway are ideal candidates. It is not effective on the thick Lake Agassiz clays that dominate much of the city—those require stone columns, rigid inclusions, or surcharge with wick drains.
How long does a typical vibrocompaction design and field program take for a one‑acre commercial lot?
Desktop design and modeling take about 8 to 12 working days after receiving the geotechnical report. On site, a one‑acre treatment with a single vibroflot and a two‑pass pattern typically completes in 5 to 7 working days, including the trial panel and post‑treatment CPT verification.
What is the cost range for vibrocompaction design and QA‑QC on a medium‑size project in Winnipeg?
For a site between 0.5 and 2 acres, the engineering design and full‑time field QA‑QC monitoring generally falls between CA$1,880 and CA$7,820, depending on the number of CPT verification points and the complexity of the pre‑treatment data review.
Can vibrocompaction be used directly beneath shallow footings or only under floor slabs?
It can be used under both. We frequently design the treatment to extend 2 meters beyond the footing perimeter so the stress bulb stays within densified soil. For isolated pad footings we tighten the grid to 1.8‑meter spacing within the column footprint to achieve the 150‑kPa bearing target required by the NBCC.
What monitoring data does the engineer need during the production passes?
The vibrator’s onboard recorder must log depth, amperage, lift rate, and hold time for each probe point at a minimum 0.5‑second interval. We also install two standpipe piezometers at the edge of the treatment zone to track excess pore pressure dissipation, which guides the wait time between passes in silty ground.
