🎓 Lesson 7
D4
Allocating Overhead Across BOM Hierarchies
Allocating overhead across BOM hierarchies means spreading shared costs (like supervision, equipment maintenance, or software licensing) fairly among the different levels of a mining project’s Bill of Materials — from the entire blast design down to individual drill holes and explosives.
🎯 Learning Objectives
- ✓ Calculate overhead allocation rates using activity-based drivers (e.g., $/drill meter, $/ton blasted)
- ✓ Design a hierarchical BOM structure that maps overhead costs from corporate to blast-round level
- ✓ Analyze cost distortion risks when using volume-based vs. activity-based allocation methods
- ✓ Apply traceability rules to reconcile overhead allocations across SAP S/4HANA or MinePlan BOM modules
📖 Why This Matters
In open-pit blasting operations, 30–45% of total blast-related costs are indirect—supervision, geotechnical modeling, blast monitoring systems, and regulatory compliance—not tied directly to explosives or drill bits. Without structured overhead allocation across BOM hierarchies, engineers misattribute profitability per bench, over-allocate risk to low-volume zones, and fail audit trails required by ISO 50001 or IFRS 9. This lesson bridges cost accounting rigor with blasting execution reality.
📘 Core Principles
Overhead allocation in mining BOMs follows three foundational principles: (1) Causality—costs must be assigned based on measurable cause-and-effect relationships (e.g., blast design hours drive CAD licensing cost); (2) Hierarchy Consistency—the BOM must reflect physical and functional dependencies (e.g., 'Blast Round' → 'Bench' → 'Pit Section'); (3) Traceability—every allocated dollar must be auditable to source transactional data (ERP logs, time sheets, sensor telemetry). Modern practice moves beyond simple tonnage-based apportionment toward driver-weighted multi-tier allocation, where each BOM level inherits overhead from its parent while contributing drivers to its children.
📐 Activity-Based Overhead Allocation Rate
This formula computes the rate used to assign indirect costs to a BOM node based on its consumption of a measurable activity driver. It ensures proportional, defensible allocation aligned with actual resource usage—not arbitrary percentages.
Overhead Allocation Rate (OAR)
OAR = C_total / D_totalComputes the per-unit rate used to allocate indirect costs to BOM nodes based on a selected activity driver.
Variables:
| Symbol | Name | Unit | Description |
|---|---|---|---|
| OAR | Overhead Allocation Rate | $/unit of driver | Cost assigned per unit of activity driver (e.g., $/drill meter, $/blast design) |
| C_total | Total Indirect Cost in Pool | $ | Sum of traceable overhead costs assigned to the cost pool (e.g., blast engineering salaries, software subscriptions) |
| D_total | Total Activity Driver Volume | units (e.g., m, designs, hours) | Aggregate consumption of the chosen driver across all BOM nodes served by the pool |
Typical Ranges:
Drill-meter-based allocation (hard rock): $0.60 – $1.20/m
Blast-design-based allocation (complex geology): $1,800 – $4,500/design
💡 Worked Example
Problem: A mine’s blast engineering department incurs $1,248,000/year in indirect costs (software licenses, senior engineer salaries, blast vibration monitoring). Total annual drill meters across all benches = 1,560,000 m. Allocate overhead to Bench B-7, which accounts for 42,800 drill meters.
1.
Step 1: Compute OAR = Total Indirect Cost / Total Driver Volume = $1,248,000 / 1,560,000 m
2.
Step 2: Calculate Bench B-7 allocation = OAR × Bench B-7’s drill meters = ($0.80/m) × 42,800 m
3.
Step 3: Verify result against typical range: $34,240 falls within expected $25k–$50k per medium-sized bench (12–15 m bench height, 3–4 blast rounds)
Answer:
The allocated overhead is $34,240, which falls within the safe range of $25,000–$50,000 per bench.
🏗️ Real-World Application
At Newmont’s Boddington Mine (Western Australia), overhead for blast optimization software (MineSite™) and blast seismograph calibration was allocated across BOM levels using a three-tier hierarchy: (1) Corporate-level ERP cost pool ($820k/year), (2) Pit-section level (driver: number of blast designs submitted/month), (3) Bench-level (driver: drill meters + number of monitored holes). This replaced prior tonnage-based allocation, reducing unit-cost variance by 37% and enabling accurate comparison of fragmentation efficiency between north and south pit sectors—leading to revised burden-spacing targets in weathered granite zones.