GEOTECHNICAL ENGINEERING
QUEBEC CITY
HomeSlopes & WallsActive/passive anchor design

Active and Passive Anchor Design for Quebec City Soils

Evidence-based design. Reliable delivery.

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Quebec City sits on a geological stage built from the St. Lawrence Platform, where deep deposits of sensitive Champlain Sea clay meet fractured shale of the Appalachian foothills. With a frost depth reaching 1.5 meters and a seismic hazard model that demands ductile detailing per the 2020 NBCC, anchoring into these formations is never a routine task. A single misjudged bond length in the soft silts of the Limoilou district or an overlooked ice lens in the Cap-Rouge terrace can set a project back by months. We design deep excavation support systems and permanent retaining structures by correlating grout-to-ground bond values with the specific mineralogy of the site rather than applying textbook defaults.
The design process integrates CSA A23.3 provisions for tendon corrosion protection and ASTM A615 material specs, but the real value comes from understanding how the anchor will behave during the January freeze-thaw cycle versus the August construction window. Whether the project involves stabilizing a historic masonry wall in Old Quebec or securing a cut-and-cover metro station, the anchor type—active or passive—must be selected after evaluating the allowable deformation of the adjacent structures and the long-term creep potential of the native clay.

An anchor isn't just a bar in the ground—it's a pre-assembled load path that must survive Quebec City's freeze-thaw cycles without losing a single kilonewton of design force.

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Our approach and scope

The most common mistake we see on Quebec City projects is specifying a single anchor type across an entire mixed-profile cut. A contractor will mobilize a crew and install passive bar anchors through the upper till, only to hit a pocket of sensitive clay at 9 meters where the grout column cracks under even modest pre-load. That scenario ends with re-drilling, change orders, and a nervous structural engineer. A proper design differentiates between active prestressed anchors, which control movement in critical zones like the footings of a neighboring building, and passive anchors that mobilize resistance only after some displacement occurs. We model the load-transfer mechanism using finite element software calibrated to the consolidation characteristics of Champlain Sea sediments.
Key design elements we control include: unbonded length calculation to avoid blowout behind the critical failure surface; tendon-to-grout adherence testing using ASTM A1080 methods; staged stressing protocols that compensate for seating losses in the wedge plates; and long-term monitoring plans for anchored retaining walls where tendon relaxation could compromise performance over the 50-year service life. For projects adjacent to the St. Lawrence River, we also factor in cyclic loading from ice scour and debris impact during the spring break-up.
Active and Passive Anchor Design for Quebec City Soils
Technical reference — Quebec City

Site-specific factors

In Quebec City, we often see contractors who have worked mainly on the Canadian Shield assume that a 15-meter anchor bond length will behave identically in the Champlain Sea clay of the St. Lawrence Lowlands. It does not. The sensitivity of these marine sediments means that remolding during augering can reduce the undrained shear strength by 40% or more if the drilling parameters aren't tightly controlled. A progressive collapse of an anchored wall usually starts with a single anchor failing at the grout-soil interface, then cascading to overload its neighbors.
Seismic performance is another layer of risk that some generic designs overlook: under the NBCC ground motion spectra for Quebec City, an anchor system must be detailed to either fuse in a ductile manner or remain elastic under the design earthquake. We address these risks by requiring on-site sacrificial anchor testing to failure before production drilling begins, and by designing the unbonded length to extend well beyond any potential slip surface aggravated by seismically-induced pore pressure spikes.

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Applicable standards

CSA A23.3 - Design of Concrete Structures (Anchorage to Concrete), NBCC 2020 - National Building Code of Canada (Seismic Hazard), ASTM A615 - Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement, ASTM A1080 - Standard Practice for Hot Isostatic Pressing of Steel, Stainless Steel, and Related Alloy Castings (Anchor Testing), Canadian Foundation Engineering Manual (CFEM) - 4th Edition

Reference parameters

ParameterTypical value
Design StandardCSA A23.3, NBCC 2020, ASTM A615
Anchor TypesActive (prestressed) and passive (non-prestressed)
Grout-to-Ground BondSite-specific, verified by field pull-out tests
Corrosion ProtectionClass I (double-corrugated sheathing) or Class II
Seismic DesignDuctile yielding mechanism, reduced bond length per NBCC
Freezing Depth1.2 m to 1.5 m (accounted in unbonded length design)
Proof TestingASTM A1080 or CSA A23.3 performance test regimens

Quick answers

How do active and passive anchors differ in a Quebec City excavation project?

Active anchors are prestressed immediately after grouting to apply a controlled force against the wall, minimizing soil movement before excavation proceeds—essential when adjacent to sensitive Old Quebec foundations. Passive anchors only develop resistance once the wall deforms, making them suitable for temporary cuts in competent till where some displacement is acceptable. The choice turns on the allowable lateral movement of nearby structures and the stiffness of the ground at the anchor bond zone.

What is the typical cost range for designing an anchored system in the Quebec City area?

For a complete design package including anchor capacity calculations, corrosion protection detailing, and construction-phase testing specifications, project budgets typically fall between CA$1,300 and CA$5,550 depending on the number of anchors, the complexity of the ground profile, and the level of seismic analysis required per the NBCC.

How do you account for the Champlain Sea clay when designing anchors here?

We characterize the clay's sensitivity and creep potential through laboratory triaxial and consolidation tests, then apply a reduction factor to the grout-to-ground bond strength found in the CFEM. For active anchors, we also design the lock-off load to be lower than the creep threshold to avoid long-term relaxation; for passive anchors, we extend the bond length through the weathered crust into intact material to ensure a reliable pull-out capacity.

Location and service area

We serve projects in Quebec City and surrounding areas.

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