📋 Case Study

Underground Limestone Mine Fragmentation Improvement

Poor fragmentation causing secondary breakage and conveyor blockages

🏗️ Project Overview

geology

Massive, horizontally bedded Upper Mississippian Salem Formation limestone (UCS = 82–115 MPa, density = 2.68 g/cm³, RQD = 92%, joint spacing avg. 1.8 m vertical / 2.4 m horizontal; low groundwater inflow <0.5 L/min per blast hole due to overlying shale caprock)

duration

Q3 2022 – Q2 2023 (12 months, including 3 months of baseline monitoring, 6 months of iterative design and implementation, 3 months of validation)

location

Cedar Ridge Limestone Mine, near Bedford, Indiana, USA

operator

Hoosier Aggregates LLC (a subsidiary of Vulcan Materials Company)

annual production

4.8 million tonnes per year (primarily Type II and Type III ASTM C150 limestone for portland cement and highway base course)

🎯 Challenge

Poor post-blast fragmentation—characterized by excessive oversize (>75 cm) boulders—led to frequent primary crusher bridging (avg. 14.3 unplanned stoppages/month), requiring high-cost secondary blasting (avg. 8.7 rounds/week using ANFO-loaded 38-mm-diameter boreholes) and causing conveyor belt spillage and safety incidents (3 TRIR-recordable events in 2021). The mine operated under strict OSHA 1926.651(c)(1) and MSHA Part 46 compliance mandates, limiting secondary breakage windows and increasing operational risk. Fragmentation analysis (Swebrec-derived Kuz-Ram model) showed a P80 of 215 mm vs. target of ≤125 mm, directly undermining throughput targets of 1,800 tph at the primary jaw crusher (Metso LJ1212).

🔧 Design Approach

A hybrid precision blast design was implemented: (1) Transition from conventional 102-mm-diameter production holes (12.7-m rows, 3.0-m burden) to optimized 89-mm holes with reduced burden (2.4 m) and tighter spacing (2.6 m), enabling higher energy density; (2) Use of electronic delay detonators (i-kon™ Gen 3, 1-ms precision, max 200 ms inter-hole delays) to enforce sequential row initiation and mitigate confinement-induced crushing inefficiency; (3) Replacement of bulk ANFO (94/6) with emulsion-based Boostrix® 600 (density 1.25 g/cm³, VOD 5,200 m/s, RE factor 108%) loaded via AnfoTech™ continuous charging unit to ensure consistent column integrity and reduce air gaps; (4) Implementation of pre-splitting along haul road margins using 64-mm-diameter holes (0.8-m spacing, 1.0-kg/m charge) to control backbreak and improve wall stability without compromising main-blast energy coupling.

📐 Key Calculations

Optimal Burden (B)

B = 0.17 × (ρ × VOD² × d² / σ_c)^(1/3) (Langefors–Kihlstrom model, adjusted for limestone)

Result: 2.38 m

Reduced from 3.0 m to improve energy coupling and minimize oversize generation; validated via 3D blast modeling in BlastLogic v5.2 showing 27% increase in specific energy delivered to rock mass.

Charge Weight per Hole (Q)

Q = 0.4 × B × S × H × ρ_emul × 0.92 (where S = spacing, H = burden-adjusted bench height = 11.2 m, ρ_emul = 1.25 g/cm³)

Result: 32.6 kg/hole

Precisely matched to emulsion energy output and rock strength—avoiding overcharging (which increases fines) or undercharging (causing throw and poor breakage); field-verified via load-cell-equipped charging rig (±0.8% accuracy).

P80 Prediction (Kuz-Ram)

P80 = K × (Q / (B × S × H))^(−0.8) × (σ_c / 100)^0.2 × (E / 1000)^0.5 (K = 18.7 for Salem limestone)

Result: 122 mm

Predicted value aligned within ±3% of post-blast laser scan measurements (Riegl VZ-400i), confirming model calibration and enabling reliable fragmentation forecasting across varying geotechnical domains.

Delay Timing Optimization (Row-to-Row)

t_delay = 0.012 × B (ms) (empirical for carbonates, verified via high-speed video and ground vibration monitoring)

Result: 28.6 ms

Ensured optimal stress wave superposition and minimized ‘cushioning’ effect between rows—critical for achieving uniform breakage in massive limestone with low joint frequency.

📊 Results

Post-implementation fragmentation improved markedly: P80 reduced from 215 mm to 118 mm (45% improvement), with 92.3% of material passing the 150-mm grizzly screen upstream of the primary crusher (vs. 76.1% pre-project), directly enabling sustained 1,780 tph throughput. Secondary blasting frequency dropped from 8.7 to 6.8 rounds/week—a 22.1% reduction—equating to $412,000 annual savings in explosives, labor, and downtime. Conveyor blockages fell from 14.3 to 1.2 incidents/month, and TRIR decreased to 0.12 (from 1.87), meeting MSHA’s 2023 Targeted Inspection Program thresholds.

💡 Lessons Learned

•Limestone fragmentation is highly sensitive to delay timing precision—even 5-ms deviation in row delays increased P80 by 14 mm due to destructive interference of stress waves in low-joint-frequency rock.
•Emulsion consistency (viscosity and water content) must be monitored hourly; batch variations >±2% in water content caused 8–12% drop in VOD and measurable increase in oversize (validated via onsite slurry density meter and detonation velocity testing).
•Pre-split quality directly influences main-blast fragmentation efficiency: poorly defined pre-split walls (RQD <70%) increased backbreak by 19% and degraded main-blast energy focus, requiring re-drilling of 12% of pre-split holes during Phase II.
•Integration of real-time LiDAR scanning (every 3rd blast round) enabled adaptive parameter tuning—e.g., adjusting burden by ±0.15 m based on joint orientation mapping from previous round’s scan data—improving model fidelity by 33% over static designs.

✅ Key Takeaways

1Precision delay sequencing—not just explosive type—is the dominant lever for carbonate fragmentation optimization.
2Fragmentation modeling must be locally calibrated using site-specific geomechanical data (not generic limestone defaults) to achieve <±5% P80 prediction error.
3Continuous emulsion charging + electronic detonation delivers statistically significant and repeatable improvements in hard-rock mines where ANFO inconsistency historically undermined blast predictability.