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Angle of Repose: A Structural Perspective on Ontario Footing Design

  • Negin Amani
  • 1 day ago
  • 2 min read

The Angle of Repose (AoR) is the natural slope at which loose soil or granular material stays stable without sliding. Though it isn’t a formal design parameter, it provides engineers with a quick visual indicator of how soil behaves on-site—especially useful before lab or geotechnical data is available.


In structural engineering across Ontario, foundation stability relies not only on concrete design but also on how the supporting soil transfers loads. Knowing the AoR helps engineers judge excavation slopes, backfill behavior, and footing conditions in line with safe construction practice and the Ontario Building Code (OBC).


The Angle of Repose bridges the gap between theory and field judgment.

Structural Importance of the Angle of Repose


From a structural engineering standpoint, the AoR provides valuable insight into how effectively the soil can support and stabilize footings.


  • Footings near slopes: If the ground slope is steeper than the soil’s repose angle, the bearing zone may extend into unstable soil, increasing the risk of shear movement.


  • Backfill stability: Retaining and basement walls rely on stable fill for lateral support. Keeping fill slopes below the AoR prevents excessive lateral pressure and local collapse.


  • Soil strength indicator: A high AoR (>35°) suggests dense, frictional soil with good interlocking, while a low AoR (<30°) points to loose or fine-grained material requiring verification or improvement.


AoR vs. Angle of Internal Friction (Φ)



These two concepts are often confused. The AoR represents the visible surface stability of loose soil, while the Angle of Internal Friction (Φ) defines the internal resistance of confined soil under load.In simple terms:


  • AoR: What you see — the slope of a loose pile.

  • Φ: What you calculate — the hidden strength within compacted soil.


While the Ontario Building Code (OBC) does not reference AoR directly, it complements on-site evaluation under O. Reg 213/91 (Construction Projects) and OBC 9.12.3.2 (Excavation Stability) for safe excavation and slope assessment.


Typical Angles for Ontario Soils

Soil Type

Typical AoR (°)

Engineering Note

Granular A (Crushed Stone)

40–45

Excellent interlock; stable footing base

Granular B (Sand & Gravel)

35–40

Good strength with proper compaction

Clean Sand

30–35

Moderate friction; monitor settlement

Silty or Moist Soil

<30

Unstable; needs shoring or slope reduction


Field Application: The 1:1 Excavation Rule


For safe excavation, engineers often apply the 1:1 rule—for every 1 ft of depth, cut back the slope 1 ft. This 45-degree “layback” aligns with the typical AoR of granular soils and prevents wall collapse. Where space is limited, engineered shoring systems are required to maintain safety.


Conclusion


The Angle of Repose bridges the gap between theory and field judgment. It helps engineers recognize how soil actually behaves under real conditions—beyond what drawings or reports show.


At Parsways, we integrate code-based design with field observation to ensure each footing rests on stable, well-compacted, and compliant ground.The AoR may not appear on a calculation sheet, but it remains an essential concept in every structural engineer’s toolkit for safe and reliable foundation design.

 
 
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