The 1988 Saguenay earthquake, a magnitude 5.9 event centered 35 km south of the city, was a wake-up call for geotechnical practice in the region. While structural damage was limited, the event triggered numerous landslides in the sensitive Champlain Sea clays and liquefaction features in the alluvial deposits of the Saguenay River valley. That history shapes every analysis we perform. The city sits within the Saguenay Graben, a seismically active rift zone where the Laurentian Shield meets deep quaternary sediments. Our team combines decades of post-1988 data with modern In-Situ to evaluate cyclic stress demands. For sites underlain by loose deltaic sands, we integrate CPT testing to obtain continuous tip resistance and friction sleeve data without disturbing the soil fabric, essential for triggering correlations under the NBCC 2020 framework. The analysis is not just about a factor of safety; it is about predicting post-liquefaction settlement and lateral spread displacement that can rupture utility lines and rack bridge abutments.
In the Saguenay Graben, triggering a liquefaction analysis at the wrong density index can mean a 300 mm differential settlement under a critical facility.
Area-specific notes
The Champlain Sea clay is notorious for its metastable structure: a sensitive, high-water-content silt and clay that loses 90% of its strength when remolded. In the lowlands near the Chicoutimi River, these deposits are interspersed with thin, discontinuous sand lenses. A magnitude 6.0 event could pressurize these lenses, causing a flow slide that retrogresses upslope at speeds exceeding 10 meters per minute. The 1971 Saint-Jean-Vianney disaster, 50 km southeast, remains a stark reminder of how quickly this material can fail. Modern analysis uses the cone penetration test with pore pressure measurement (CPTu) to map the exact depth and continuity of each sand layer. We then model the excess pore pressure dissipation time using the Teh and Houlsby (1991) solution. If a sand lens is trapped between two impermeable clay strata, water cannot escape during shaking, and the risk of undrained failure multiplies. Our reporting always includes a lateral spread hazard map for the project site, derived from the topographic gradient and nearest free face distance.
Quick answers
Is liquefaction analysis mandatory for building permits in Saguenay?
Under the Quebec Construction Code, which adopts NBCC 2020 with provincial amendments, a geotechnical investigation including liquefaction potential evaluation is required for all structures classified as post-disaster, high importance, or assigned to Site Class E or F. In Saguenay, much of the urban area falls on Site Class D or E due to deep alluvial soils. For a standard residential build on a Site Class C rock outcrop near Shipshaw, a simplified screening may suffice. However, if you are developing a commercial lot in downtown Chicoutimi or Jonquière, the city building department will expect a signed and sealed report from an engineer registered with the Ordre des ingénieurs du Québec.
What does a soil liquefaction analysis cost in the Saguenay region?
A complete analysis package, including a CPTu campaign, selective sampling, cyclic triaxial testing, and a final engineering report, typically ranges from CA$3,100 to CA$6,470. The lower end applies to a single residential lot with accessible native ground and a short drilling window in summer. The upper end reflects multi-borehole investigations on industrial sites with difficult access, deeper liquefiable layers, or the need for winter drilling with insulated water tanks and track-mounted rigs on frozen terrain.
How deep should we investigate for liquefaction potential here?
In the Saguenay valley, the liquefiable deposits are generally confined to the upper 20 meters of post-glacial stratigraphy. We extend the investigation to at least 25 meters or until auger refusal on competent till. The 1988 earthquake hypocentre was at 29 km depth, but the strong ground motion that triggered flow failures was dominated by surface wave amplification in the top 15 meters of soft sediment. Our SPT and CPT profiles always penetrate the full thickness of the Holocene column to ensure we do not miss a buried sand channel at depth.
Can you analyze liquefaction risk for an existing building without a foundation excavation?
Yes. We use a combination of surface geophysics and angled drilling from outside the footprint. A MASW line along the perimeter provides a shear wave velocity profile to 30 meters, and a CPT rig with a low-headroom mast can access tight spaces between buildings. If the structure has a shallow basement, we perform pushover analysis on the retaining walls to check for lateral spread displacement. This approach was used successfully for a retrofit project at an institutional building in the Jonquière health district.
