A 20-meter deep excavation off Lonsdale Avenue hit running sand at 12 meters. The contractor had water flowing into the pit within hours. That job taught us a hard lesson about North Vancouver’s glacial geology. You cannot guess the ground model here. The advance and outwash deposits sitting on till create abrupt transitions from dense gravel to loose sand. We design shoring and dewatering systems that hold these transitions. Our approach starts with a tight stratigraphic model. Then we run staged excavation analysis to size walers, struts, and tied-back walls. For projects near creeks or the Burrard Inlet shoreline we add seepage modeling. The water table in Lower Lonsdale sits high year-round. Combine that with seismic demands from the NBCC and you get a design problem that demands both local data and rigorous analysis. We often integrate CPT testing to map thin silt seams that conventional boreholes miss, and we pair it with slope stability checks when the excavation backcut approaches an adjacent building.
In North Vancouver’s glacial sequences, the difference between a dry excavation and a flooded pit is a single silt seam you did not model.
Frequently asked questions
What is the typical cost range for geotechnical design of a deep excavation in North Vancouver?
Design fees for a deep excavation in North Vancouver typically range from CA$2,720 to CA$11,480 depending on excavation depth, shoring complexity, and the number of retained stages. A 10-meter excavation with a single-tier tieback system sits at the lower end. A 25-meter excavation requiring a secant wall, multiple tieback levels, and groundwater cutoff falls at the upper end. We provide a fixed-fee proposal after reviewing the structural drawings and geotechnical baseline report.
How do you handle an excavation next to a 1960s building on shallow footings?
We set a lateral deflection limit of 10 mm at the adjacent footing and design the shoring system to meet it. This usually means a stiffer wall type, like a secant pile wall instead of soldier piles. We model each excavation stage in PLAXIS and specify preloaded tiebacks to control movement from the first cut. If the building has no foundation drawings, we do a partial exposure or a test pit to confirm footing depth and condition before finalizing the design.
What seismic requirements apply to deep excavations in North Vancouver?
The NBCC 2020 governs seismic design. For excavations classified as post-disaster or high importance, we run a site-specific seismic hazard analysis. North Vancouver’s glacial soils amplify short-period shaking, so we use MASW and downhole shear wave velocity data to build a calibrated site response model. The shoring wall and anchors are designed for the amplified ground motions. Post-earthquake serviceability checks are included when the excavation must remain open for months.
How do you deal with groundwater during excavation in North Vancouver?
The water table in much of North Vancouver sits within 3 to 5 meters of the surface. Once the excavation goes below that, passive sump pumping is rarely enough. We design deep well systems or eductors depending on the permeability of the soils at the site. Where drawdown outside the excavation could cause settlement of adjacent buildings, we design a grout curtain or a secant pile cutoff wall to isolate the excavation hydraulically. The design includes contingency triggers in case inflow exceeds predicted rates.
