🎓 Lesson 1
D1
Getting Started with Retaining Wall Engineering
A retaining wall is a structure built to hold back soil or rock so it doesn’t slide or collapse onto roads, buildings, or mine infrastructure.
🎯 Learning Objectives
- ✓ Explain the fundamental forces acting on a retaining wall (active, passive, and at-rest earth pressure)
- ✓ Calculate lateral earth pressure using Rankine’s theory for cohesionless and cohesive soils
- ✓ Analyze wall stability against overturning, sliding, and bearing failure using factor-of-safety criteria
- ✓ Select appropriate wall type (gravity, cantilever, anchored, or mechanically stabilized) based on site constraints and loading conditions
- ✓ Apply minimum safety factors per industry standards (e.g., ≥1.5 for sliding, ≥2.0 for overturning)
📖 Why This Matters
In open-pit mines, steep excavated slopes often require engineered support to prevent catastrophic failures that endanger personnel, halt production, and damage equipment. Retaining walls protect access roads along pit rims, stabilize waste dumps, confine leach pads, and safeguard processing facilities — making them indispensable for safe, compliant, and economical mine operations.
📘 Core Principles
Retaining wall behavior hinges on soil-structure interaction. Earth pressure develops due to soil weight and surcharge loads; Rankine and Coulomb theories model this pressure distribution. Walls must satisfy three primary stability criteria: (1) resistance to overturning about the toe, (2) resistance to sliding along the base, and (3) adequate bearing capacity to avoid foundation settlement or rupture. Wall geometry, backfill properties (φ, c, γ), drainage, and construction sequencing all critically influence performance — especially in dynamic or water-saturated mining environments.
📐 Rankine Active Earth Pressure
Rankine’s theory calculates the horizontal pressure exerted by dry, cohesionless backfill on a smooth, vertical wall with no wall-soil friction. It assumes elastic, homogeneous, isotropic soil and is widely used for preliminary design in mining applications where rapid assessment is needed.
💡 Worked Example
Problem: Given: dry sand backfill with unit weight γ = 18 kN/m³, internal friction angle φ = 32°, wall height H = 6 m. Calculate active pressure at the base and total active force per meter width.
1.
Step 1: Compute coefficient of active earth pressure: Kₐ = tan²(45° − φ/2) = tan²(45° − 16°) = tan²(29°) ≈ 0.307
2.
Step 2: Calculate pressure at base: σₐ = Kₐ·γ·H = 0.307 × 18 × 6 = 33.16 kPa
3.
Step 3: Compute total active force per meter: Pₐ = ½·Kₐ·γ·H² = 0.5 × 0.307 × 18 × 6² = 99.5 kN/m
Answer:
The active pressure at the base is 33.2 kPa, and the resultant force is 99.5 kN/m acting at H/3 = 2 m above the base — consistent with triangular pressure distribution and within typical design bounds for temporary mine walls.
🏗️ Real-World Application
At Newmont’s Boddington Mine (Western Australia), a 12-m-high reinforced concrete cantilever wall was constructed to retain weathered granite and overburden along a critical haul road adjacent to the main pit. Design incorporated groundwater pressure from seasonal infiltration, seismic loading (Mw 5.0), and 15 kPa live surcharge from heavy vehicles. Stability analyses confirmed FS_sliding = 1.82 and FS_overturning = 2.41 — exceeding AS 4678–2022 requirements — validated via inclinometer monitoring over 5 years showing <2 mm lateral movement.