Saguenay sits on a deep basin of post-glacial marine clays—the Laflamme Sea sediments—that can reach thicknesses of over 60 meters in the Jonquière sector. These sensitive clays lose significant strength when disturbed, which makes any tunnel advance through them a matter of precise face support and continuous deformation monitoring. In our experience, the Saguenay graben’s seismicity adds a second layer of complexity: a design that looks stable under static conditions may still undergo softening during even a moderate earthquake. Before selecting a tunnel alignment we typically run a seismic refraction survey to map bedrock depth variability, because the interface between the clay and the underlying crystalline shield is rarely horizontal. When the tunnel crown must stay within the soft clay, we complement the geophysical data with an in-situ permeability campaign to understand the drainage conditions that control excavation-induced pore pressures.
In Saguenay’s sensitive clay, a 2 kPa suction change at the tunnel face can trigger a progressive failure that propagates 40 meters back from the heading.
Quick answers
What makes Saguenay clay so risky for tunneling?
The clay was deposited in the Laflamme Sea roughly 10,000 years ago and has a flocculated structure that is highly sensitive. When remolded, its strength can drop to less than 1 kPa, turning the soil into a heavy liquid. A small loss of face pressure or uncontrolled vibration can trigger a run-out that travels tens of meters, which is why our design always includes a minimum face support pressure based on undrained strength and a buffer for dynamic effects during cutterhead rotation.
Do you perform seismic analysis for Saguenay tunnels?
Yes, we run site-specific seismic response analyses using deep borehole shear-wave velocity profiles. Because Saguenay lies within a graben structure, the impedance contrast between the soft clay and the crystalline bedrock amplifies ground motion at the tunnel elevation. We evaluate both ovaling (rack) deformation under S-waves and axial strain under P-waves per the FHWA-NHI guidelines, and we often recommend ductile segment connections where the soil profile predicts a site period above 0.8 seconds.
What is the typical cost range for a geotechnical investigation of a soft-ground tunnel alignment in Saguenay?
A complete investigation program—including deep sonic borings, CPT soundings, a full laboratory testing suite, and the production of a 3D ground model—generally falls between CA$5,040 and CA$23,680, depending on the length of the tunnel, the depth to bedrock, and the density of the instrumentation array required for an urban crossing.
How do you determine the undrained shear strength profile in sensitive clay?
We combine field vane tests corrected for plasticity index with CPT net cone resistance using a site-specific Nkt factor calibrated against consolidated-undrained triaxial tests on block samples. Block sampling is essential in Saguenay because conventional Shelby tubes can remold the clay and underestimate in-situ strength by up to 25%. The corrected Su profile then feeds directly into the face stability calculations and the settlement trough model.
