🎓 Lesson 7
D4
Overhead Pool Construction Exercise
Overhead pool construction is the process of grouping shared costs—like supervision, equipment maintenance, and safety programs—into a single budget bucket so they can be fairly assigned to individual blasting or mining operations.
🎯 Learning Objectives
- ✓ Design an overhead cost pool by classifying indirect costs using activity-based criteria
- ✓ Calculate overhead allocation rates using appropriate cost drivers (e.g., meters drilled per blast)
- ✓ Analyze the impact of pool granularity on cost accuracy and decision-making sensitivity
- ✓ Explain why combining dissimilar activities (e.g., geotechnical monitoring and fuel logistics) into one pool distorts cost signals
- ✓ Apply SAMIC cost classification standards to validate pool composition
📖 Why This Matters
In open-pit blasting operations, up to 35% of total production cost stems from indirect activities—supervision, blast design engineering, regulatory compliance, and fleet maintenance—that don’t occur at the drill-hole level but are essential to safe, efficient fragmentation. Without a well-constructed overhead pool, engineers risk underpricing blasts (eroding margins) or over-allocating costs (misguiding optimization efforts). Real-world consequence: A major copper mine recently revised its blasting cost model after discovering that lumping ‘drill maintenance’ and ‘seismic monitoring’ into one pool inflated blast unit costs by 18%, delaying ROI approval for a new electronic detonation system.
📘 Core Principles
Overhead pool construction rests on three pillars: homogeneity, causality, and scalability. Homogeneity requires all costs in a pool to respond similarly to changes in a common cost driver (e.g., ‘blasting engineering labor’ and ‘blast simulation software licenses’ both scale with number of blasts). Causality ensures the chosen driver reflects operational reality—not convenience (e.g., using ‘tonnage blasted’ for explosives procurement overhead violates causality; ‘kg of ANFO used’ is superior). Scalability demands pools remain stable across production volumes without constant reconfiguration. Advanced practice distinguishes between facility-level (e.g., site security), operational-level (e.g., blast design), and campaign-level (e.g., pre-split sequence QA) pools—each requiring distinct drivers and review frequencies.
📐 Overhead Allocation Rate
The allocation rate converts pooled overhead into per-unit cost using a validated cost driver. Accuracy depends on driver relevance—not just mathematical fit—and requires periodic validation against actual cost behavior.
Overhead Allocation Rate (OAR)
OAR = \frac{\text{Total Overhead Pool Cost}}{\text{Total Volume of Cost Driver}}Computes the per-unit overhead cost assigned to a cost object (e.g., per blast, per meter drilled, or per tonne fragmented).
Variables:
| Symbol | Name | Unit | Description |
|---|---|---|---|
| OAR | Overhead Allocation Rate | $/unit | Monetary overhead cost assigned per unit of driver activity |
| C_p | Total Overhead Pool Cost | $ | Sum of all indirect costs aggregated into the pool for the period |
| D_v | Driver Volume | units | Total quantity of validated cost driver activity (e.g., blasts, drill meters, tonnes) |
Typical Ranges:
Blast design & engineering pool: $120 – $195 per blast
Drill maintenance pool: $8.50 – $14.20 per drill meter
Regulatory compliance pool: $420 – $980 per permit issued
💡 Worked Example
Problem: A mine’s ‘Blast Design & Engineering’ overhead pool totals $247,500 annually. Historical data shows 1,650 production blasts were conducted last year. Calculate the OAR per blast and verify against typical industry range.
1.
Step 1: Identify total pool cost = $247,500 and driver volume = 1,650 blasts
2.
Step 2: Apply OAR = Total Pool Cost / Driver Volume = 247500 / 1650
3.
Step 3: Compute result = $150.00/blast; compare to SAMIC 2023 benchmark range of $120–$195/blast for mid-tier open-pit operations
Answer:
The result is $150.00 per blast, which falls within the safe range of $120–$195 per blast.
🏗️ Real-World Application
At Newmont’s Boddington Mine (Western Australia), engineers constructed four distinct overhead pools for blast-related costs: (1) Blast Modeling & Simulation (driver: blast designs finalized/month), (2) Regulatory Compliance & Reporting (driver: blast permits issued/quarter), (3) Electronic Detonator Logistics (driver: detonator units handled), and (4) Post-Blast Geotechnical Review (driver: m³ of high-wall scanned via LiDAR). This structure enabled precise attribution of $3.2M annual overhead—revealing that 62% of compliance costs were tied to permit complexity (not blast count), prompting automation of form generation and reducing per-blast compliance cost by 29%.