🎓 Lesson 23
D5
Diagnosing Common Failure Modes: Sliding, Overturning, Scour
Shallow foundations can fail by sliding sideways, tipping over, or having soil washed away from under them — like a book slipping off a tilted desk, a chair toppling backward, or sand eroding from under a stepping stone.
🎯 Learning Objectives
- ✓ Calculate the factor of safety against sliding using effective soil parameters and interface friction
- ✓ Analyze overturning stability by computing restoring and destabilizing moments about the foundation toe
- ✓ Design scour protection measures by estimating critical velocity and maximum scour depth using empirical equations
- ✓ Explain how groundwater table elevation influences both sliding resistance and bearing capacity reduction
- ✓ Apply Eurocode 7 (EN 1997-1) partial factors to verify combined sliding–overturning limit states
📖 Why This Matters
In mining infrastructure — such as crusher pads, conveyor tower footings, or blast monitoring station bases — shallow foundations often rest on weathered rock or alluvial soils near drainage channels. A single undetected sliding or scour event can collapse critical equipment, halt production for days, and trigger forensic investigations. Real-world failures like the 2018 conveyor tower tilt at the Pilbara iron ore site were traced to underestimated scour during monsoon runoff — not structural design error. Diagnosing these modes early prevents costly retrofits and ensures regulatory compliance with mine safety codes.
📘 Core Principles
Stability against sliding depends on the ratio of resisting shear force (friction + passive earth pressure) to driving horizontal force (e.g., wind, seismic, conveyor thrust). Overturning is assessed by comparing the sum of clockwise (destabilizing) and counter-clockwise (restoring) moments about the foundation’s toe — where rotation would initiate. Scour involves two phases: initiation (when flow velocity exceeds critical threshold for sediment entrainment) and development (where local vortexes excavate a hole whose depth depends on flow hydraulics, sediment gradation, and foundation geometry). All three modes interact: rising groundwater reduces effective stress → lowers friction angle → decreases sliding resistance AND increases buoyancy → reduces restoring moment → exacerbates overturning risk.
📐 Factor of Safety Against Sliding
The most fundamental check compares resisting horizontal force to applied horizontal load. Interface friction dominates for competent soil–foundation contact; passive resistance is included conservatively only if actively retained backfill is present and verified in construction.
💡 Worked Example
Problem: A 3 m × 3 m square concrete footing (γ_conc = 24 kN/m³) carries a vertical load V = 1200 kN and horizontal load H = 180 kN. Soil beneath has c' = 0 kPa, φ' = 32°, γ_soil = 18.5 kN/m³. Foundation embedment D_f = 1.2 m. Interface friction δ = 0.8φ'. Calculate FS against sliding.
1.
Step 1: Compute effective normal force N' = V − u, where pore pressure u = γ_w × (D_f − d_w); assume groundwater at surface → u = 9.81 × 1.2 = 11.77 kN/m² × 9 m² = 106 kN → N' = 1200 − 106 = 1094 kN
2.
Step 2: Compute sliding resistance R = N' × tan(δ) = 1094 × tan(25.6°) = 1094 × 0.479 = 524 kN
3.
Step 3: FS = R / H = 524 / 180 = 2.91 → exceeds minimum required FS = 1.5 (EN 1997-1 DA1)
Answer:
The factor of safety against sliding is 2.91, which exceeds the Eurocode 7 Design Approach 1 minimum of 1.5 for persistent situations.
🏗️ Real-World Application
At the Red Lake Gold Mine (Ontario), a 2.5 m × 2.5 m shallow footing for a high-frequency blast vibration sensor failed after spring snowmelt. Forensic analysis revealed 0.4 m of scour beneath the upstream edge due to concentrated sheet flow across a 5% grade access road shoulder. The resulting eccentricity increased base pressure beyond allowable bearing capacity, causing differential settlement and sensor misalignment. Remediation involved installing a 1.2 m deep riprap apron with graded filter fabric — reducing local velocity below critical threshold (V_c = 0.65 m/s for D_50 = 2 mm sand) and extending foundation effective width by 0.8 m.