One of the most costly mistakes we see on Terrebonne construction sites is treating all temporary retaining systems the same—assuming a driven pile or a generic tieback will hold in the Champlain Sea clay that underlies most of the city. It doesn't. Between the Rivière des Mille Îles and the newer developments north of Autoroute 640, the marine clay can lose up to 70% of its undrained shear strength when remolded. An active anchor design that pre-loads the tendon against a competent bearing stratum is often the only reliable path to keep an excavation open without excessive deformation. We've had to redesign more than one project where the original passive system crept until the soldier pile wall deflected beyond serviceability. In those cases, complementing the ground investigation with a precise Sondaje SPT helps us map the clay-to-till transition depth before any anchor bond length is calculated.
In Terrebonne's Champlain clay, a passive anchor designed without pre-load verification often fails the creep test long before reaching its design load.
Methodology applied in Terrebonne
Local geotechnical conditions in Terrebonne
Part 4 of the NBCC and CSA A23.3 Annex D set clear requirements for anchor testing, but in Terrebonne the real risk isn't the code—it's the sensitivity of the marine clay. When the Liquide de Champlain, as our francophone crews call it, gets disturbed by auger drilling or high-pressure grouting, it can liquefy locally and lose all bond. We've seen permanent anchors in sensitive zones creep at loads well below 60% of the ultimate capacity calculated from site investigation data. That's why our team insists on sacrificial load tests on the first production anchors, not just on a separate test anchor away from the wall line. The 2018 landslide near the Rivière des Prairies in a neighboring municipality was a stark reminder that sensitive clay terrain doesn't forgive shortcuts in anchorage design; a passive system installed without a locked-in pre-stress simply cannot mobilize resistance fast enough to prevent progressive failure.
Our services
Our anchor design methodology in Terrebonne covers the full lifecycle of the system, from geotechnical investigation through construction-phase testing. We work directly with the excavation contractor to ensure the installation method matches the ground conditions.
Active Pre-Loaded Anchor Systems
Complete design package for active tiebacks including bond zone calculation in competent till or bedrock, free length verification, lock-off load specification, and step-by-step testing procedures per CSA A23.3. We handle the tendon selection, corrosion protection class, and head detail to match your waler or pile cap geometry.
Passive Soil Nail and Dowel Design
Passive anchor design for cut slopes and temporary excavations where deformation can be tolerated. We use limit equilibrium methods calibrated with site-specific CPTu data to determine nail spacing and length, ensuring the mobilized pullout resistance exceeds the demand with a factor of safety not less than 1.5.
Questions and answers
What is the typical cost range for anchor design on a Terrebonne project?
For a standalone active/passive anchor design package covering a single retaining wall or slope stabilization, our fees typically range from CA$1,600 to CA$5,550. The final figure depends on the number of anchor rows, whether we're designing active post-tensioned anchors or passive soil nails, and how many distinct soil profiles the wall crosses. Projects requiring multiple load test specifications or complex staged excavation analysis fall at the upper end.
Do you need to run a proof test on every anchor in Terrebonne's clay?
Per CSA A23.3, 100% of the anchors on a project must be subjected to a performance test or a proof test depending on their classification. In Terrebonne's sensitive clay, we typically recommend proof testing a higher percentage—often every anchor on the critical wall sections—because the variability in the Champlain Sea deposits can cause adjacent anchors to behave very differently. Creep tests extended beyond the code minimum are also prudent for permanent anchors in this geology.
How do you decide between an active and a passive anchor system?
The decision hinges on allowable deformation and the depth to a competent bearing layer. If the structure is sensitive to movement—like an adjacent building on shallow footings—we design an active system that pre-loads the anchor to lock off a portion of the earth pressure before any excavation-induced movement accumulates. If the ground has a reliable dense layer at shallow depth and the facing can tolerate 25 to 50 mm of movement, a passive grouted anchor or soil nail system can be the more economical choice without compromising safety.