Saint-Jerome sits in a transitional zone where the Canadian Shield begins to rise beneath the St. Lawrence Lowlands, creating a geological patchwork that challenges even experienced tunnel engineers. The city's subsurface is dominated by Champlain Sea clays deposited roughly 10,000 years ago, interbedded with glacial till and occasional fluvial lenses that can vary dramatically across a single block. When a tunnel alignment passes through these sensitive marine clays, the margin for error shrinks considerably. Our technical team approaches each project by first establishing the presence and continuity of these soft units through targeted in-situ permeability testing, which reveals how groundwater will migrate once the tunnel face disturbs the native structure. In areas where the Rivière du Nord has reworked these deposits, the soil fabric becomes even more unpredictable, requiring a level of characterization that goes well beyond standard site investigation protocols.
Sensitive Champlain Sea clays can lose up to 90 percent of their undisturbed strength upon remolding, which transforms a stable tunnel face into a flowing mass in minutes.
Process and scope
Site-specific factors
Saint-Jerome's growth pattern since the 1970s has pushed residential and commercial development onto terrain that was historically considered too problematic for heavy infrastructure, including the soft clay plains east of the Autoroute 15. When a new tunnel is proposed beneath this built-up environment, the geotechnical risk multiplies because any unexpected ground loss at the face can propagate upward into a settlement trough that affects existing foundations, buried utilities, and roadway surfaces. The Champlain Sea clays beneath Saint-Jerome are particularly unforgiving in this regard, as their brittle, strain-softening behavior means that deformation is not gradual but rather occurs in rapid, sometimes catastrophic increments. A face collapse in these materials can evacuate several cubic meters of soil in seconds, creating a void that migrates upward and leaves surface structures unsupported. We address this through a rigorous pre-construction investigation that maps the spatial extent of soft units and assigns realistic deformation parameters to each identified layer, so that the contractor can select an appropriate tunnel boring machine mode and face support pressure before excavation begins.
Reference standards
NBCC 2020 (National Building Code of Canada), CSA A23.3-19 (Design of Concrete Structures), ASTM D2166/D2166M-16 (Unconfined Compressive Strength of Cohesive Soil), ASTM D4767-11(2020) (Consolidated Undrained Triaxial Compression Test), CSA S6:19 (Canadian Highway Bridge Design Code — geotechnical sections)
Other technical services
Tunnel Face Stability Assessment
We evaluate short-term face stability using limit equilibrium and numerical methods calibrated to site-specific undrained shear strength profiles obtained from field vane and triaxial testing. The output includes critical face support pressure curves for open-mode and EPB-mode TBM operation, accounting for the anisotropic stress history typical of Saint-Jerome's glacially overconsolidated clays.
Consolidation and Long-Term Settlement Analysis
Because the Champlain Sea clays exhibit significant secondary compression, we perform oedometer tests with extended load increments to capture the creep coefficient (Cα). These parameters feed into finite element models that predict surface settlement evolution over the tunnel's design life, ensuring that the lining system can accommodate post-construction deformations without distress.
Ground Characterization for TBM Selection
Our investigation program delivers the abrasivity indices, grain size distributions, and Atterberg limits needed to specify the correct cutting tools and conditioning agents. In Saint-Jerome's mixed-face conditions where glacial till underlies the soft clay, we quantify the percentage of cobbles and boulders that the TBM will encounter, directly influencing cutterhead torque and wear predictions.
Typical parameters
Frequently asked questions
What makes the Champlain Sea clays in Saint-Jerome so challenging for tunneling?
These clays were deposited in a post-glacial marine environment and later leached by freshwater, which stripped away the stabilizing salts and left a meta-stable, flocculated structure. Their sensitivity—the ratio of undisturbed to remolded strength—can exceed 50 or even 100 in the Saint-Jerome area, meaning that vibration or excessive face pressure fluctuation can trigger a chain reaction of strength loss. Once remolding begins, the material behaves more like a viscous fluid than a solid, making face control extremely difficult.
What laboratory tests are essential for tunnel design in soft ground?
The core program includes consolidated-undrained triaxial tests with pore pressure measurement (ASTM D4767), incremental oedometer consolidation tests to capture both primary and secondary compression, and index testing for Atterberg limits and grain size distribution. For Saint-Jerome's sensitive clays, we also perform fall cone tests to determine remolded strength and sensitivity, which directly inform the TBM face support pressure calculations.
How do you account for the presence of boulders in the glacial till below Saint-Jerome?
The till underlying the Champlain Sea clay contains erratic boulders transported by the Laurentide Ice Sheet, some exceeding 1 meter in diameter. We use a combination of seismic refraction surveys and targeted boreholes to estimate boulder frequency and size distribution. This data feeds into the TBM cutterhead design and helps the contractor plan for hyperbaric interventions if boulders must be removed manually at the face.
What is the typical cost range for a soft soil tunnel geotechnical investigation in Saint-Jerome?
A comprehensive investigation program for a soft soil tunnel in Saint-Jerome, including deep borings, field vane testing, installation of piezometers, and a full suite of triaxial and consolidation tests, typically ranges from CA$5,450 for a preliminary desktop and limited field study to CA$25,020 for a full-scale campaign with multiple boreholes, continuous sampling, and advanced laboratory testing. The final cost depends on tunnel length, depth, and the complexity of the geological conditions encountered.
Which sections of the NBCC are most relevant to tunnel geotechnical design?
While the NBCC does not contain a dedicated tunnel chapter, its geotechnical provisions in Part 4 (Structural Design) and the referenced CSA S6:19 bridge code provide the framework for load combinations and limit states. For the soil-structure interaction aspects of tunnel lining design, we apply the factored resistance approach consistent with the NBCC's limit states design philosophy, using geotechnical resistance factors appropriate for the sensitivity of the Champlain Sea clays.