🎓 Lesson 10
D5
Interpreting Inclinometer & Piezometer Data Trends
Inclinometers measure how much the ground is tilting, and piezometers measure how much water pressure is building up underground—both help predict if a slope might slide.
🎯 Learning Objectives
- ✓ Analyze inclinometer cumulative displacement vs. depth profiles to identify slip surface location
- ✓ Interpret piezometer pressure trends to assess seasonal groundwater fluctuations and their impact on effective stress
- ✓ Correlate concurrent inclinometer and piezometer data to diagnose causality between pore pressure rise and acceleration in displacement
- ✓ Calculate rate-of-change metrics (e.g., mm/day displacement velocity, kPa/day pressure gradient) to classify deformation stage per ISRM guidelines
📖 Why This Matters
A 2018 landslide at the Mount Polley tailings storage facility was preceded by >3 mm/day inclinometer displacement and sustained piezometer pressure spikes above 80% of lithostatic pressure—yet these signals were not correlated in real time. This lesson equips you to detect, interpret, and act on these dual monitoring signals before failure occurs—turning raw data into life-saving decisions.
📘 Core Principles
Slope instability evolves through stages: elastic adjustment → creep → accelerated deformation → failure. Inclinometers track cumulative horizontal displacement (Δx) versus depth (z), revealing zones of concentrated shear (e.g., inflection points indicating potential slip surfaces). Piezometers measure pore water pressure (u), which reduces effective stress (σ′ = σ − u) and thus shear strength per Mohr-Coulomb theory. Rising u lowers the factor of safety (FoS); when combined with increasing Δx rates, it signals transition from stable creep to imminent failure. Critical insight: displacement alone may reflect consolidation; but *accelerating* Δx *coincident with rising u* strongly indicates loss of shear resistance.
📐 Displacement Velocity & Pore Pressure Gradient
Displacement velocity (v_d) quantifies kinematic urgency; pore pressure gradient (G_u) reveals hydraulic forcing. Both are first-order derivatives used to classify deformation stage (e.g., 'Stage III' per ISRM 2013).
💡 Worked Example
Problem: An inclinometer at 15 m depth shows cumulative displacements: Day 0 = 4.2 mm, Day 7 = 9.8 mm, Day 14 = 22.6 mm. A nearby piezometer at same depth records: Day 0 = 85 kPa, Day 7 = 112 kPa, Day 14 = 158 kPa.
1.
Step 1: Compute average displacement velocity over Days 7–14: v_d = (22.6 − 9.8) mm / 7 days = 1.83 mm/day
2.
Step 2: Compute average pore pressure gradient over Days 7–14: G_u = (158 − 112) kPa / 7 days = 6.57 kPa/day
3.
Step 3: Compare to ISRM thresholds: v_d > 1.5 mm/day + G_u > 5 kPa/day indicates Stage III (accelerating deformation)
Answer:
The result is v_d = 1.83 mm/day and G_u = 6.57 kPa/day — both exceed ISRM Stage III thresholds, signaling urgent review and potential intervention.
🏗️ Real-World Application
At the Chuquicamata open-pit copper mine (Chile), a north wall monitoring array detected inclinometer displacement accelerating from 0.4 mm/day to 2.1 mm/day over 12 days at 80–110 m depth. Simultaneously, piezometers in the same zone showed pore pressure rising from 142 kPa to 218 kPa (exceeding 75% of overburden pressure). Engineers halted blasting, installed additional drainage, and stabilized the slope—avoiding a predicted 300,000-m³ slide. Root cause: heavy rainfall infiltration into fractured diorite without adequate surface runoff control.
🔧 Interactive Calculator
🔧 Open Slope Stability & Landslide Risk Calculator📋 Case Connection
📋 Tailings Storage Facility (TSF) Slope Reinforcement — Pilbara, Australia
Existing FoS < 1.1 under Mw 6.5 scenario; limited space for buttressing; strict environmental containment requirements