The National Building Code of Canada (NBCC) and CSA A23.3 impose strict requirements for underground construction in sensitive clay deposits. In Terrebonne, a city of over 119,000 residents located on the north shore of the Rivière des Mille Îles, these regulations are not just paperwork. The city sits on the Champlain Sea clay plain, a post-glacial deposit notorious for its high sensitivity and potential for large deformations during tunneling. A standard desk study simply cannot capture the erratic lensing of silt and sand within this marine clay. Our team approaches every linear meter of a proposed tunnel alignment with a rigorous, phased investigation plan, combining high-quality sampling with advanced laboratory testing. This is not about ticking a box for a permit; it is about defining a safe excavation and support strategy for a lifespan measured in decades.
The Champlain Sea clay in Terrebonne can lose up to 90% of its strength when remolded, making undisturbed sampling the single most critical field operation for tunnel design.
Methodology applied in Terrebonne
Local geotechnical conditions in Terrebonne
A recent investigation for a micro-tunnel drive beneath Boulevard des Seigneurs encountered a buried esker deposit completely hidden by the uniform clay surface. The tunnel face lost stability within minutes when the auger hit this saturated, cohesionless sand lens. The groundwater pressure, fed by the esker, was 40% higher than the hydrostatic pressure assumed in the baseline report. This required an immediate switch to a closed-face earth pressure balance mode and an emergency grouting program to stabilize the overlying roadway. In Terrebonne, the legacy of the post-glacial environment means these hidden features—eskers, boulder pavements, and abrupt rock pinnacles—can appear without warning. Ignoring a detailed, phased site investigation that includes pore pressure monitoring and lateral variability mapping exposes a project to catastrophic face loss and surface settlement damage to the historic masonry buildings along Rue Saint-Pierre.
Our services
A tunnel project in the Mascouche formation requires more than just boreholes. We integrate field exploration, advanced lab testing, and numerical modeling into a single workflow. The two core service packages are designed to address both the global alignment risk and the local face stability challenge.
Tunnel Alignment Geotechnical Baseline Report
We establish the geotechnical baseline for your tunnel alignment along the Rivière des Mille Îles corridor. This includes a factual report with borehole logs drilled to a minimum of 1.5 tunnel diameters below the invert, piezometer monitoring, and the interpretive report defining the design parameters for the tunnel boring machine (TBM) or sequential excavation method (SEM). We define the limits of the weathered crust and the intact clay to ensure the design load matches the actual ground conditions.
Face Stability and Settlement Analysis
Using PLAXIS 3D and FLAC, we model the coupled stress-pore pressure response at the tunnel face. This package addresses the specific risk of chimney collapse in the sensitive clay by calibrating the soil model against site-specific triaxial and oedometer results. We provide the required face support pressure envelope and predict the surface settlement trough to protect the dense heritage infrastructure of Vieux-Terrebonne.
Questions and answers
What is the typical cost for a geotechnical investigation for a soft soil tunnel in Terrebonne?
The investigation scope varies with tunnel length and depth, but for a detailed alignment study involving deep boreholes, triaxial testing, and instrumentation, the budget typically falls between CA$6,400 and CA$22,790. A shallow utility tunnel with access shafts costs less than a deep sanitary sewer crossing under the Mille Îles River, which requires extensive hydrogeological profiling.
How do you handle the risk of quick clay during tunneling in the Mascouche region?
We specifically target the sensitivity of the Champlain Sea clay using a combination of field vane testing and fall cone tests on undisturbed samples. If the remolded strength drops below 0.5 kPa, we classify it as quick clay. The mitigation strategy involves a strict limit on the allowable pore pressure reduction at the face and the use of a pressurized closed-face TBM to prevent the progressive failure that triggers a retrogressive landslide.
Can you provide a single borehole to cover the entire tunnel alignment?
No. Given the lateral variability of the eskers and rock pinnacles in Terrebonne, a single borehole is insufficient and would violate the standard of care under the NBCC. We space the investigations at a maximum of 50 to 75 meters for soft ground, with a targeted drilling angle to intersect vertical geological features that could compromise the face.
What parameters do you test in the lab for the tunnel lining design?
We run a full suite, but the critical tests for the lining design in the plastic clay are the oedometer tests to define the coefficient of earth pressure at rest (K0) and the creep rate, and the CU triaxial tests to define the effective stress parameters. We also measure the undrained shear strength anisotropy using direct simple shear (DSS) tests to ensure the lining resists both the vertical and lateral loading. More info.