🎓 Lesson 2
D5
When to Use Phase2 vs UDEC vs FLAC2D
Phase2, UDEC, and FLAC2D are specialized computer programs engineers use to simulate how rock and ground behave under stress—like before blasting or tunneling—so they can predict failures and design safer mines.
🎯 Learning Objectives
- ✓ Analyze a given mine slope geometry and rock mass condition to select the most appropriate numerical model (Phase2, UDEC, or FLAC2D) and justify the choice using engineering criteria
- ✓ Apply Hoek–Brown failure criteria within Phase2 to calculate factor of safety for a bench face and compare results against UDEC simulations of jointed rock slopes
- ✓ Design a tunnel support system by interpreting FLAC2D convergence–support interaction curves and validating against field monitoring data
- ✓ Explain the conceptual limitations of continuum vs. discontinuum modeling when simulating blast-induced fracture propagation near a stope boundary
📖 Why This Matters
Choosing the wrong numerical tool can lead to unsafe designs: overestimating stability with Phase2 in heavily jointed ground may hide critical wedge failures; underestimating dynamic relaxation with FLAC2D could result in premature support failure. In 2022, a major Australian underground copper mine revised its ground control protocol after a UDEC-based reanalysis revealed 37% higher displacement in a fault-bounded sill pillar than previously modeled in Phase2—preventing a potential pillar burst. This lesson equips you to match the physics of the problem to the solver’s mathematical foundation—not just pick the 'familiar' software.
📘 Core Principles
Numerical modeling selection hinges on three pillars: (1) Material representation—continuum (Phase2/FLAC2D) assumes uniform material properties; discontinuum (UDEC) treats rock as assemblies of rigid or deformable blocks separated by interfaces. (2) Failure mechanism—Phase2 excels at plastic flow and elastic stress redistribution but cannot nucleate new fractures across intact rock; UDEC naturally captures sliding, opening, and rotation along pre-existing discontinuities; FLAC2D uniquely handles large-strain, rate-dependent behavior (e.g., squeezing ground, blast vibration damping). (3) Problem scale & objective—Phase2 is optimal for rapid, parametric stability screening of open-pit benches; UDEC is essential for kinematic analysis of wedge failure in highwall benches with mapped joint sets; FLAC2D is preferred for time-stepped simulation of excavation sequence effects, including stress relaxation and support installation timing.
📐 Model Selection Decision Matrix
No single equation governs selection—but a structured decision matrix based on key geomechanical parameters enables rigorous justification. The primary discriminant is the Geological Strength Index (GSI) relative to discontinuity spacing and persistence, combined with required output fidelity (e.g., displacement vs. time vs. static factor of safety).
Discontinuum Suitability Index (DSI)
DSI = (100 − GSI) × (1 − RQD/100) × (L_p / S_j)Quantitative indicator to guide selection between continuum and discontinuum modeling approaches based on rock mass quality and structural geology.
Variables:
| Symbol | Name | Unit | Description |
|---|---|---|---|
| GSI | Geological Strength Index | dimensionless | Empirical index (0–100) quantifying rock mass structure and surface condition per Hoek et al. (2013) |
| RQD | Rock Quality Designation | percent | Percentage of core recovered in pieces >10 cm long; reflects rock mass fracturing intensity |
| L_p | Dominant Joint Persistence | m | Average trace length of the most persistent joint set observed in scanlines or outcrops |
| S_j | Average Joint Spacing | m | Mean perpendicular distance between adjacent joints in the dominant set |
Typical Ranges:
Massive granite (GSI=75, RQD=95%, L_p=2m, S_j=1.5m): 16.7
Sheared schist (GSI=35, RQD=40%, L_p=8m, S_j=0.3m): 346.7
💡 Worked Example
Problem: Given: GSI = 45, average joint spacing = 0.45 m, RQD = 58%, dominant joint set persistence > 5 m, required output = time-history of crown displacement during sequential drift advance.
1.
Step 1: Calculate DSI = (100 − GSI) × (1 − RQD/100) × (persistence / spacing) = (55) × (0.42) × (5 / 0.45) ≈ 256.7
2.
Step 2: Compare DSI threshold: DSI > 200 strongly favors discontinuum (UDEC); DSI < 80 favors continuum (Phase2); 80–200 indicates need for hybrid or FLAC2D evaluation.
3.
Step 3: Since DSI = 257 > 200 AND time-history output is required, UDEC is appropriate for kinematics, but FLAC2D is preferred for time-dependent support interaction—hence final recommendation: FLAC2D with interface elements activated.
Answer:
The result is DSI = 257, which exceeds the discontinuum threshold and confirms FLAC2D (with interface logic) is the technically justified choice for this time-dependent, joint-influenced problem.
🏗️ Real-World Application
At the Red Lake Mine (Ontario), a steeply dipping, highly fractured gold-bearing shear zone intersected an access ramp. Initial Phase2 modeling predicted FS = 1.3 for the ramp wall—deemed acceptable. However, UDEC modeling incorporating 5 mapped joint sets revealed a kinematically viable toppling failure mode with 120 mm displacement in the upper 3 m. Field instrumentation later recorded 98 mm displacement over 4 weeks—validating UDEC. Subsequently, FLAC2D was used to simulate staged rockbolt installation and grout cure-time effects, leading to a revised pattern (2.4 m spacing, 4.5 m length) that reduced convergence by 62%. This cascade—Phase2 for screening, UDEC for failure mode identification, FLAC2D for support optimization—exemplifies industry best practice.
🔧 Interactive Calculator
🔧 Open Mine Ground Control & Rock Mechanics Calculator📋 Case Connection
📋 Underground Copper Mine Pillar Recovery Optimization
Post-extraction pillar instability threatening surface infrastructure