Coquitlam's geology presents a direct challenge for subsurface work. Glacial till, marine silts, and sand lenses alternate across the city, and the water table often sits within 2 meters of surface in the lower reaches near the Fraser River. Tunneling through these materials without a precise geotechnical campaign is a gamble. We focus on soft ground tunnel analysis to define the deformation envelope before the TBM or sequential excavation method even enters the alignment. The work integrates lab shear strength data from triaxial tests with in-situ pore pressure readings to model face stability under both drained and undrained conditions.
A soft ground tunnel in Coquitlam succeeds or fails on the reliability of the pore pressure profile and the undrained shear strength assigned to the face.
Local considerations
The primary monitoring instrument for soft ground tunneling in Coquitlam is the vibrating wire piezometer, grouted into the borehole at target depths before excavation begins. These sensors record pore pressure changes in real time as the face advances. A sudden drop in pressure can signal a hydraulic connection to a sand lens, while a spike may indicate undrained loading ahead of the cutterhead. We couple these readings with surface settlement points and in-tunnel convergence arrays. The data feeds back into the observational method: if the measured settlement exceeds 60% of the predicted value, the face pressure or grout injection rate is adjusted immediately. In the loose alluvial deposits found near the Coquitlam River, the risk of a blowout or uncontrolled settlement is highest, and the instrumentation layout is denser.
