A 14-story residential tower on Chemin de Chambly hit groundwater at 4.2 meters during excavation—three weeks ahead of schedule, with no dewatering plan in place. The geotechnical report had relied on lab permeameter values from Shelby tube samples, which underestimated the true hydraulic conductivity of the fissured till by nearly two orders of magnitude. In Longueuil, where the stratigraphy transitions from stiff Champlain Sea clay to glacial till and occasionally into fractured shale bedrock, laboratory permeability is rarely representative of field conditions. The Lefranc test (constant or falling head in boreholes) and the Lugeon test (pressurized water injection in rock) provide direct, in-situ measurements that capture the effect of fissures, sand lenses, and macro-porosity. Our team runs these tests across Vieux-Longueuil, Saint-Hubert, and Greenfield Park, always following ASTM D4630 for overburden and ASTM D4631 for rock mass. Before finalizing a foundation drainage design, it’s common to cross-check results with a CPT test to identify thin permeable layers that a standard borehole log may miss.
Lab permeameter tests on Champlain Sea clay can underestimate field hydraulic conductivity by 10 to 100 times—Lefranc and Lugeon tests capture the fractures that control real groundwater flow.
Methodology and scope
Longueuil’s population of roughly 250,000 and its position on the Saint-Lawrence South Shore mean that urban infill projects are the norm—often on lots where previous buildings had buried fuel tanks or old foundations. These disturbed soils have unpredictable permeability, sometimes varying from 1×10⁻⁷ to 1×10⁻⁴ cm/s within the same block. A single boring can intercept a sand pocket that creates a preferential flow path, yet the adjacent borehole remains in tight clay. The Lefranc test isolates a specific interval, typically 50 to 100 cm, using a pneumatically inflated packer, and measures the water level recovery or injection rate directly at that depth. When bedrock is encountered, the Lugeon protocol is applied—five pressure stages up to 10 bars in competent rock, recording water take in Lugeon units (liters per meter per minute at 1 MPa). For deep excavations near the Saint-Lawrence River, where hydraulic uplift can compromise a mat foundation, we often recommend combining these field permeability measurements with an
excavation monitoring program that tracks piezometric levels throughout construction. In the till-derived soils along Taschereau Boulevard, we’ve documented Lugeon values below 3 in sound rock, but exceeding 25 in fractured zones—a critical distinction for grouting decisions. The ASTM D4630 standard specifies minimum waiting times for equalization in low-permeability clays, sometimes up to 24 hours, which we rigidly observe. A
grouting campaign informed by accurate Lugeon values can reduce inflow by over 80% compared to a blanket treatment approach.
Local considerations
The most common mistake we see on the South Shore is designing a permanent dewatering system based on a single falling-head test in a monitoring well that screens multiple geological units. A well screen crossing a thin sand seam within the Champlain clay will show the composite permeability of the sand, not the clay—leading to an undersized sump pump system that fails during spring thaw, when the Saint-Lawrence River is high and the water table rises by over 2 meters. In Longueuil, the groundwater regime is heavily influenced by river stage and winter freeze-thaw cycles; a Lefranc test run in August can yield a permeability half of what the same soil exhibits in April. For rock sockets beneath bridge piers or high-rise foundations, a Lugeon value above 15 Lu signals the need for curtain grouting before excavation proceeds. Ignoring this step has resulted in flooded shafts on two highway projects in the Montérégie region in the past decade. We document every test with a field log including GPS coordinates, packer depth, pressure readings, and temperature corrections—because CSA A23.3-19 and the Ontario Building Code (referenced in NBCC for geotechnical scope) both demand defensible hydrogeological data when groundwater control is part of the permanent works.
Frequently asked questions
What is the difference between a Lefranc test and a Lugeon test?
The Lefranc test measures hydraulic conductivity in soil or highly weathered rock using a constant or falling head in a borehole, typically over a 50–100 cm interval. It follows ASTM D4630 and is suited for sands, silts, clays, and tills. The Lugeon test is a pressurized water injection test in competent rock, using five pressure stages to evaluate fracture permeability. It is standardized by ASTM D4631 and the ISRM Suggested Method. In Longueuil, we use Lefranc in Champlain clay and till, and Lugeon when we reach the underlying Utica or Lorraine shale.
How much does a field permeability test cost in Longueuil?
A single Lefranc test in overburden or a Lugeon test in bedrock generally ranges from CA$950 to CA$1,370 per interval, depending on depth, access conditions, and whether a drill rig is already on site. Mobilization, traffic control on boulevards like Roland-Therrien, and reporting are quoted separately. We provide a fixed-price proposal after reviewing the borehole program.
How long does a Lugeon test take in the field?
A complete 5-stage Lugeon test on one 3–5 m interval typically requires 90 to 120 minutes, including packer inflation, pressure stabilization at each stage, and depressurization. In tight rock with very low water take, waiting times between stages may extend the test to 2.5 hours. We always allow extra time in Longueuil’s Utica shale, which can exhibit pressure-dependent flow behavior requiring careful interpretation.
Can you run a Lefranc test in the same borehole as an SPT?
Yes, and that is standard practice on our sites. After completing the SPT at the desired depth, the borehole is cleaned, the casing is advanced to the test interval, and the packer is set. We typically run the Lefranc test immediately after the SPT to minimize borehole instability. The soil classification from the split-spoon sample directly informs the permeability interpretation, which is essential in Longueuil’s interbedded till and clay sequences.
