On May 1, 2024, Alberta implemented the National Building Code - 2023 Alberta Edition (NBC(AE) 2023), setting a detailed technical standard for the construction of residential footings, especially where disturbed soil is present. Across Calgary and the province, the safe support of a building’s vertical and lateral loads hinges on the correct use of granular fill under footings, incorporating both practical experience and code compliance.

Footing Support: Mandate for Undisturbed Soil or Compacted Granular Fill

The foundation of every multifamily project in Alberta starts beneath the ground-where footings must bear on “undisturbed soil, rock or compacted granular fill” (NBC(AE) 2023, Article 9.15.3.2.). While undisturbed native soils provide predictable load capacity, projects involving cut, fill, utility trenches, or reworked sites often encounter disturbed subgrades. Here, compacted granular fill becomes not just an option, but a code-mandated necessity for a stable bearing stratum.

Failure to provide a properly prepared base invites differential settlement, undermines structural capacity, and can trigger both warranty and insurance issues from premature cracking, misalignment, or loss of support. Numerous Alberta multifamily projects have demonstrated that neglecting appropriate fill thickness or compaction results in costly remedial work and, in severe cases, reconstruction of portions of foundation systems.

Granular Fill Material Specifications: Code-Driven Properties

Material selection for granular fill directly influences performance under footings. The NBC(AE) 2023 specifies several objective criteria:

  • Maximum Fines: No more than 10% (by mass) passing a 4 mm sieve, ensuring robust drainage and minimizing settlement-prone fines content.
  • Cleanliness: All material must be free from debris, organics, wood, or deleterious substances.
  • Pyrite Exclusion: The fill must avoid pyritic materials in concentrations that could compromise building stability due to swelling or heaving.

In Alberta practice, most commercial granular fills for foundation support are chosen from crushed stone, 20-40 mm clear gravel, or engineered blends specifically produced to these gradations. The choice of crushed, angular material over rounded gravels is often preferred for improved interlock, friction angle, and reduced long-term settlement under cyclic groundwater conditions common in clay-rich subgrades south of Calgary or on glacial tills near Edmonton.

The presence of excessive fines, especially silts or clays, increases the risk of capillarity, frost heave, and post-construction settlement. In several multifamily case studies from Southern Alberta, post-warranty differential movement has been directly linked to the use of “builder’s pit run” containing uncontrolled fines beneath strip footings. Projects built with well-graded, code-compliant aggregates have shown minimal long-term movement even where aggressive ground moisture cycles are present.

Minimum Required Granular Fill Thickness: Code and Practice

The NBC(AE) 2023 mandates the following dimensional minimums for granular fill beneath residential footings on disturbed soil:

  • Minimum Depth Under Footing: 125 mm (5") of compacted granular material directly beneath the full width of the footing.
  • Lateral Extension: The fill layer must extend at least 300 mm (12") in all horizontal directions beyond the outer edge of the footing.

These dimensions serve a dual purpose: dispersing the building load over a broader area, reducing stress concentration on localized pockets of loose or variable soil, and providing a drainage path to prevent super-saturation beneath critical load paths. In the context of deep wall footings or pad footings beneath columns, Kingsway Builders’ multifamily projects routinely extend this fill laterally up to 450 mm, especially where poor backfill conditions are suspected or when supporting heavy elevator cores.

From a constructability perspective, the uniform thickness prescribed by code offers the baseline; however, advanced projects often increase granular fill thickness locally where:

  • Subgrade bearing capacity is marginal following re-excavation or deep utility crossings.
  • Localized perched groundwater conditions are encountered.
  • Frost-susceptible silts or highly plastic clays are present at the fill/soil interface.

Field experience across Calgary has shown that increasing granular fill thickness to 200 mm (8") beneath strip footings and even 300 mm (12") beneath concentrated point loads yields negligible additional cost but substantially reduces risk of settlement, especially in wet years or thaw cycles. However, the legal minimum remains set at 125 mm, with any reduction below this requiring site-specific geotechnical review and signoff by a professional engineer.

Granular Fill Compaction: Practical Standards and Testing

Granular fill’s effectiveness depends not just on thickness, but on its compaction. NBC(AE) 2023 provides a qualitative mandate (“compacted granular fill”), but Alberta practice has established quantitative targets:

  • Compaction Standard: Minimum 98% Standard Proctor Maximum Dry Density (SPMDD) for all fill beneath footings.
  • Layering: Mechanical compaction in loose lifts of 150 mm (6") is recommended to achieve even density throughout the fill layer.
  • Testing: Field density tests (nuclear or sand cone) at the base of footing excavations, typically every 20 m along strip footings or at each pad location in multifamily configurations.

Compaction deficiencies are among the leading root causes of post-construction settlement in Alberta residential construction. In a comprehensive 2022 review by major Alberta insurers, more than 40% of warranty claims for slab and wall movement were traced back to inadequate granular fill compaction or poor moisture control during placement.

Field experience highlights several pitfalls:

  • Insufficient effort to compact fill at the base of narrow or deep trenches due to access limitations.
  • Failure to rework wet or frozen fill, resulting in future thaw settlement.
  • Overcompaction leading to particle breakage in some softer granular blends, reducing drainage capacity and increasing capillarity.

Best practice on Alberta multifamily sites is to employ plate compactors or small padfoot rollers in confined footing trenches, moisture-condition the fill to within 2% of optimum, and demand independent verification of compaction results prior to concrete placement. Where granular layers exceed 250 mm, compaction and field testing should be staged by lift, ensuring no loose interfaces remain.

Footing Thickness and Width: Load Transfer Safeguards

Minimum bearing area for footings is crucial for load transfer and settlement control. NBC(AE) 2023 states the footing must be:

  • Thickness: Not less than 100 mm, or the projection beyond the supported wall or column-whichever is greater.

This ensures edge breakage or punching is avoided. For instance, a 250 mm wide footing beneath a 150 mm concrete or wood-framed wall, projecting 50 mm per side, mandates a minimum 100 mm thickness. For pad footings supporting point loads, thickness may be increased to resist shear and avoid punching failure.

Footing width is keyed to building load and number of supported floors. Typical NBC(AE) 2023 minimums:

  • Single-storey multi-unit: 250 mm (typical for exterior strip footings).
  • Two-storey: 350 mm.
  • Three-storey with heavy bearing: 450 mm.

In high-density multifamily projects, engineers frequently oversize footings-sometimes up to double the minimum width-where soil bearing capacity is low (e.g., less than 100 kPa), or to minimize long-term settlement on questionable fill. Infill construction and brownfield urban sites often warrant even more robust foundation design, particularly where granular fill replaces variable or inconsistency backfill from previous structures or excavations.

Site Preparation: Sequence, Moisture, and Contaminant Control

Site preparation establishes the long-term performance of the structure:

  • Debris Removal: All organics, topsoil, demolition debris, roots, and oversized rocks must be removed well beyond the footing line. Code and industry guidance make no allowances for "spot cleaning"-full strip and replacement of the subgrade zone is essential.
  • Base Leveling: The subgrade must be trimmed to a uniform plane to ensure consistent fill thickness and eliminate bridging or voids beneath the footing.

Moisture is the hidden variable in both fill performance and long-term settlement risk. Concrete Alberta and Kingsway’s own QC teams recommend:

  • Ensuring the subgrade and granular fill are damp, but not saturated, prior to concrete placement; this minimizes suction, prevents rapid dehydration, and ensures correct early hydration of the footing concrete.
  • Avoiding placement when standing water is present. Chronic wetting zones raise settlement and frost heave risk even if granular fill is thick and well-compacted.

Contaminant control has risen in prominence due to insurance and municipal liability. Multiple Alberta jurisdictions now require certification that fill beneath residential footings is both clean and non-pyritic-a lesson learned from past heaving failures in areas underlain by pyrite-laden subgrades in both urban and rural settings.

Case Study: Urban Redevelopment and Variable Fill

Redevelopment sites, particularly infill projects within Calgary’s Beltline or Edmonton’s mature neighborhoods, often present soils extensively disturbed by previous structures or utilities. In one 2023 Kingsway multifamily project, a main building pad spanned three previous basements. Granular fill thickness reached up to 450 mm beneath portions of the new strip footings where compacted native soils could not be economically recovered. Compaction was staged in three lifts, and additional lateral extension (600 mm beyond footing) distributed loads over potentially inconsistent city fill and utility backfill zones. Monitoring indicated settlements less than 6 mm over two years-well within code and warranty thresholds.

Frost Protection: Proactive Measures Under Alberta Conditions

Alberta's climate, with historical frost penetration regularly below 1.2 m in Calgary and 1.5 m in Edmonton, demands that the placement level of all footings-including those on compacted granular fill-must be below the local frost line. Frost action in disturbed soils can create destructive heaving forces, even when granular fill is properly installed.

Several practical implications flow from this climate reality:

  • Where thick granular fill is used to raise the bottom of footing excavations to a design elevation, the total fill/soil/foundation profile must still place the bearing level below the frost depth. Raising footings above this level with fill requires additional frost protection methods, such as insulation skirts or thicker fill zones, designed and certified by a professional engineer.
  • Frost-susceptible soils (e.g., silty clays) immediately beneath or adjacent to granular fill demand either complete removal or lateral insulation to block upward migration of the frost front.

Experienced teams sequence multifamily construction to avoid extended exposure of open footing trenches through freeze-thaw cycles, and often backfill lateral edges of granular fill with free-draining, non-frost-susceptible aggregate to blunt edge frost penetration.

Drainage and Moisture Control: Foundation Life Cycle Protection

Drainage below and away from the foundation is fundamental. The NBC(AE) 2023 reinforces the necessity for:

  • Foundation drains at or below footing level to redirect groundwater.
  • Granular drainage layers beneath slab-on-grade floors (minimum 125 mm, matching footing granular fill requirements).

In Alberta’s multifamily sector, engineered weeping tile systems discharge collected water to sump pits or municipal storm, paired with non-woven geotextile envelopes to prevent fines migration (common in fill replacement jobs). The 125 mm minimum granular base beneath footings functions both as structural support and as a means of diffusing hydrostatic pressure, especially where high water tables or perched water conditions persist following spring melt or extended rain events.

Best practices emphasize providing positive surface drainage away from all excavations prior to backfilling, extending the granular “mat” supporting footings well beyond the wall, and utilizing materials with consistent drainage characteristics. Post-construction, perimeter grading and maintenance play as critical a role as initial construction, as settlement of poorly compacted or poorly drained soils outside the 300 mm zone can result in ponding and eventual migration of water beneath the structure-adding long-term risk even when initial code provisions are met.

Professional Oversight: Engineering Review and Certification

While experienced field teams routinely recognize obvious subgrade failure or poor compaction, Alberta's regulatory environment and the risk profile of multifamily projects all but demand engagement of a licensed professional engineer for:

  • Soil classification and detailed fill design, especially where the structure is supported over extensive disturbed or replaced soils.
  • Development of site-specific compaction protocols, including minimum required granular fill thicknesses above code minimum based on field and laboratory testing.
  • Verification (by independent testing) that all granular fill has met or exceeded compaction and material standards before footing concrete is placed.

Engineers play a pivotal role in cases of deviation from code minimums-such as where geotechnical investigation confirms adequate bearing capacity exists with less than 125 mm of granular fill, or where engineered fill blends (e.g. flowable fills, reversible geofoam bases) are proposed to counter extraordinary settlement risk, chemical attack, or construction schedule acceleration.

Insurers, lenders, and municipal code officials increasingly require stamped soil compaction and fill thickness reports as a condition of facility signoff, further underscoring the criticality of granular fill compliance at every phase from tender through construction and warranty closeout.

Extended Best Practices: Lessons Learned from Alberta Multifamily Projects

Several patterns have emerged from post-occupancy evaluations and site investigations across Alberta’s high-density residential sector:

  • Over-excavation for Uniformity: Where the native subgrade is variable, over-excavation of the entire footing zone (including a buffer outside projected building lines), with uniform replacement by engineered granular fill and compaction in multiple lifts, outperforms patch-fill approaches every time. Such uniformity sharply reduces local differentials in settlement, even decades later.
  • Geotextile Use: In finer or “pumping” subgrades, the installation of non-woven separation geotextile between ambient soils and the granular fill layer prevents downward migration and fouling of the fill, preserving drainage and bearing properties.
  • Continuous Inspection and Documentation: Digital compaction logs, daily photographic records, and trap rock tag tickets (certifying material gradation and source) form the documentary backbone for regulatory review and warranty defense.
  • Real-Time Monitoring: For large or phased multifamily pours, settlement monitoring points (surveyed before, during, and after primary construction) can confirm fill performance and detect early warning signs of differential movement-critical in brownfield or accelerated build schedules.

Key Compliance Risks and Mitigation

Experience on hundreds of Alberta residential and multifamily sites underlines recurrent risk areas:

  • Placing footings at or near finished grade where insufficient granular fill thickness is present.
  • Extending granular fill less than 300 mm outside footing edges, often for cost saving, leading to stress concentrations and local settlement.
  • Failing to retest fill compaction after heavy precipitation or site traffic has degraded previously prepared zones.
  • Substituting on-site excavated “gravelly” soils for engineered granular fill without strict control of fines content or compaction.

Risk mitigation strategies proven in Alberta include mandatory third-party material tickets, written compaction certification, and strict enforcement of compaction retesting after every major rain, thaw, or cut/fill operation.

Summary Table: Essential Dimensions and Performance Criteria

  • Granular Fill - Minimum Depth (Under Footing): 125 mm (5")
  • Granular Fill - Lateral Extension: 300 mm (12") beyond footing edge (minimum)
  • Footing Thickness: ≥100 mm or equal to supporting width projection, whichever is greater
  • Footing Width (one storey): ≥250 mm
    Footing Width (two storey): ≥350 mm
    Footing Width (three storey): ≥450 mm
  • Compaction Target: ≥98% SPMDD by field testing
  • Cleanliness: ≤10% passing 4 mm sieve; no pyrites, organics, or debris
  • Moisture Conditioning: Damp but not saturated prior to concrete placement
  • Frost Protection: Bottom of footing below frost line (Calgary: 1.2 m, Edmonton: 1.5 m); fill to not raise bearing above frost level
  • Drainage Layer: ≥125 mm granular material below slab/foundation; functional perimeter drains to daylight, sump, or municipal storm
  • Engineer Involvement: Required for deviations, certification, and all non-standard site or fill conditions

Conclusion: Integrating Code, Practice, and Engineering Oversight

Ensuring residential and multifamily footings on disturbed soil in Alberta achieve durable performance and code compliance requires disciplined adherence to granular fill requirements: a minimum 125 mm compacted, laterally extensive, well-graded, clean aggregate layer; robust compaction (min. 98% SPMDD); correct footing sizing; and correlated drainage protection. High-performance sites systematically exceed code minimums, guided by geotechnical review, field testing, and post-occupancy monitoring. Building upon disturbed soil always increases risk, but consistent application of Alberta’s current standards and best practices sharply reduces the incidence of differential movement, structural distress, and costly warranty claims for multifamily and residential construction.

Kingsway Builders delivers code-compliant, durable multifamily foundations across Alberta, integrating best practice granular fill and foundation solutions tailored for complex site conditions.