🎓 Lesson 18
D3
Overcoring & Hydraulic Fracturing Field Protocols
Overcoring and hydraulic fracturing are field methods used to measure the natural stresses locked inside rock underground—like finding out how hard the rock is being squeezed or stretched before mining begins.
🎯 Learning Objectives
- ✓ Calculate principal stress magnitudes from overcoring strain data using elastic theory
- ✓ Design a hydraulic fracturing test sequence compliant with ASTM D6532 and ISRM guidelines
- ✓ Analyze stress tensor orientation and magnitude consistency between overcoring and hydraulic fracturing results
- ✓ Explain the limitations of each method in anisotropic, fractured, or low-permeability rock masses
- ✓ Apply stress measurement uncertainty budgets to ground control risk assessments
📖 Why This Matters
In deep mines (>1000 m), unexpected rockbursts and pillar failures often stem not from poor support design—but from inaccurate assumptions about in-situ stress. Overcoring and hydraulic fracturing provide the only direct, quantitative measurements of these hidden forces. Without them, engineers rely on proxies (e.g., geological indicators or numerical modeling) that can mispredict major stress concentrations by 30–50%, risking catastrophic failure. These protocols are mandated in Australia’s Code of Practice for Mine Ground Control and required for all new deep-level developments in South Africa’s DMR regulations.
📘 Core Principles
Stress in rock is a second-order tensor defined by three orthogonal principal stresses (σ₁ ≥ σ₂ ≥ σ₃). Overcoring exploits the elastic recovery of a relieved core: as the outer annulus is removed, the inner core expands/contracts proportionally to the local stress state—captured by rosette strain gauges. Hydraulic fracturing relies on linear elastic fracture mechanics: the fracture initiates perpendicular to σ₃ and propagates parallel to σ₁; pressure-time signatures reveal breakdown (P_bd ≈ σ₃ + tensile strength), shut-in (P_si ≈ σ₃), and reopening (P_reopen ≈ σ₃ + frictional offset) pressures. Critical assumptions include isotropic elasticity (overcoring) and planar, vertical fracture propagation (hydraulic fracturing)—both violated in highly jointed or laminated strata, requiring correction factors or complementary methods like borehole breakout analysis.
📐 Overcoring Stress Calculation (Kirsch Solution)
The radial strain change (εᵣ) measured at the center of a triaxial strain gauge cell during overcoring relates linearly to the far-field stress components via elastic constants. For a circular borehole in isotropic, linear-elastic rock, the stress magnitude is derived from calibrated strain coefficients and Poisson’s ratio.
💡 Worked Example
Problem: A CSIRO HI cell installed in granite (E = 65 GPa, ν = 0.22) records ε₁ = −85 με, ε₂ = −42 με, ε₃ = −12 με after overcoring. Gauge calibration matrix yields stress coefficients C₁=0.18, C₂=0.12, C₃=0.09 MPa/με. Calculate σ₁.
1.
Step 1: Apply the linear superposition model: σ₁ = C₁·ε₁ + C₂·ε₂ + C₃·ε₃
2.
Step 2: Substitute values: σ₁ = (0.18)(−85) + (0.12)(−42) + (0.09)(−12) = −15.3 − 5.04 − 1.08 = −21.42 MPa
3.
Step 3: Interpret sign convention (compressive stress is negative in strain-gauge sign convention); thus |σ₁| = 21.4 MPa — consistent with typical deep-granite vertical stress (~25 MPa at 1000 m depth)
Answer:
The calculated major principal stress magnitude is 21.4 MPa, which falls within the expected range of 18–28 MPa for granite at this depth.
🏗️ Real-World Application
At the TauTona Mine (South Africa), overcoring at 3.6 km depth revealed a near-vertical σ₁ (27 MPa) aligned with gravity, but hydraulic fracturing in the same horizon showed σₕₘₐₓ rotated 42° clockwise due to regional tectonic compression. This discrepancy triggered re-evaluation of stoping sequence orientation: changing cut-hole azimuth by 45° reduced microseismic event rate by 63% over six months—demonstrating how integrated stress mapping directly enables safer, more productive mining.
🔧 Interactive Calculator
🔧 Open Mine Ground Control & Rock Mechanics Calculator📋 Case Connection
📋 Underground Copper Mine Pillar Recovery Optimization
Post-extraction pillar instability threatening surface infrastructure