The 2020 National Building Code of Canada (NBCC) and CSA A23.3 provide the structural and geotechnical framework for any underground work in Alberta, but what makes Medicine Hat genuinely distinctive is its layered Cretaceous bedrock mantled by highly variable surficial deposits. The city sits at roughly 690 metres elevation along the South Saskatchewan River, a valley that has been carved through the Bearpaw Formation, leaving a complex stratigraphy of stiff clays, soft shale, and unconsolidated alluvium that demands a careful geotechnical analysis for soft soil tunnels. In our experience, the transition zones between the upper glacial till and the underlying shale are where we most often identify critical ground loss risks, a pattern we have documented across multiple infrastructure projects in the region. Whenever a tunnel alignment crosses beneath the river or its tributary coulees, we integrate pore pressure data from in-situ permeability testing to model the hydraulic connectivity between the riverbed and the excavation face.
Understanding the swelling potential of the Bearpaw shale is the single most consequential factor in designing a durable tunnel lining in Medicine Hat.
Our approach and scope
Local considerations
The risk profile for a tunnel alignment near the Flats neighbourhood, with its thick alluvial sands directly overlying shale, differs markedly from a drive through the higher benches of Crescent Heights where the glacial till is denser and less water-charged. In the lower terraces, we have measured standing groundwater within three metres of the surface during spring melt, a condition that produces a highly unstable mixed-face at the tunnel crown and drives the need for pre-excavation dewatering combined with continuous face support. The greatest geotechnical hazard we track in Medicine Hat is the rapid slaking of the Bearpaw shale when it is exposed to humidity cycles in the tunnel invert; if left unprotected, it can generate heave pressures that distort the temporary lining within 48 hours. We address this by specifying shotcrete application within the same shift as excavation and by verifying the undrained shear strength through field vane tests at regular intervals along the drive.
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Reference standards
NBCC 2020 – National Building Code of Canada, CSA A23.3 – Design of Concrete Structures, CAN/CSA-S6 – Canadian Highway Bridge Design Code (geotechnical provisions), ASTM D2487 – Unified Soil Classification System
Complementary services
Pre-construction ground characterization
Rotary sonic drilling through the glacial till and into the Bearpaw shale, with downhole geophysics and comprehensive laboratory testing to define the engineering properties of each unit along the tunnel horizon.
TBM face stability analysis
Numerical modelling of EPB and slurry shield modes using site-specific pore pressure and strength parameters, calibrated against CPTu profiles and permeability tests performed in Medicine Hat's layered deposits.
Settlement and building damage assessment
Transverse settlement trough prediction based on empirical and finite element methods, correlated with the condition surveys of existing structures along the alignment, in accordance with the NBCC deflection criteria.
Typical parameters
Common questions
What is the typical cost range for a soft soil tunnel geotechnical study in Medicine Hat?
A site characterization program for a tunnel alignment in the Medicine Hat area typically falls between CA$5,440 and CA$25,150, depending on the number of boreholes, the depth of investigation, and the laboratory testing suite required to satisfy the NBCC and CSA A23.3 design requirements.
How do you handle the swelling behaviour of the Bearpaw shale in your analysis?
We conduct both free-swell and swell-pressure tests on undisturbed shale samples at varying moisture contents, then incorporate the results into a finite element model that simulates the long-term interaction between the swelling ground and the permanent lining, ensuring the design accommodates the predicted strain without serviceability loss.
What makes the river valley geology in Medicine Hat different from other Alberta cities?
Unlike the more uniform lacustrine clays of Edmonton or the coarse gravels of Calgary, Medicine Hat's subsurface is dominated by the Bearpaw Formation, a marine shale that is unusually sensitive to moisture and prone to rapid slaking, combined with a thin, discontinuous till cover that creates unpredictable mixed-face conditions during tunnelling.
Do you provide real-time monitoring during the tunnel drive?
Yes, we deploy automated inclinometers, piezometers, and surface settlement points along the alignment, with data streamed to a central dashboard that allows the project team to compare measured ground movements against the predicted envelope and adjust the TBM operating parameters on the fly.