🎓 Lesson 8
D5
Beyond Square Footage: Advanced Apportionment Bases
Choosing the best way to split overhead costs—like electricity, maintenance, or supervision—across different blasting operations based on what actually drives those costs, not just area or time.
🎯 Learning Objectives
- ✓ Calculate overhead allocation using detonator count and blast energy as apportionment bases
- ✓ Analyze cost distortion when switching from machine-hours to tonnage-blaster-hour bases
- ✓ Design an apportionment scheme compliant with SME Guideline 4.2 for underground development blasts
- ✓ Explain why burden-to-spacing ratio is a superior base for fragmentation-related overhead than bench height alone
📖 Why This Matters
In mining, misallocating overhead—like explosive storage, blast design software licenses, or vibration monitoring—can distort unit costs by 15–30%, leading to flawed capital decisions, inaccurate reserve valuations, and non-compliant cost reporting under SEC Industry Guide 7. Square footage or generic machine hours ignore that a 10-ton ANFO blast in competent granite consumes vastly different overhead than a 50-ton emulsion blast in fractured shale—even if both use the same drill rig for the same hours. This lesson teaches how to match overhead to its real drivers.
📘 Core Principles
Traditional overhead allocation assumes uniform resource consumption per time or area—invalid in blasting, where energy release, rock mass rating (RMR), and initiation complexity dominate cost drivers. Advanced bases fall into three categories: (1) *Output-based* (e.g., tonnes blasted, cubic meters fragmented), (2) *Input-based* (e.g., detonator count, kg of high-explosive equivalent), and (3) *Impact-based* (e.g., peak particle velocity [PPV] hours monitored, number of pre-blast surveys). The optimal base satisfies four criteria: causality, measurability, stability across cycles, and auditability. For example, detonator count correlates strongly with blast design labor, QA/QC effort, and initiation system depreciation—making it superior to drill-meterage for overhead tied to initiation reliability.
📐 Tonnage-Weighted Overhead Rate (TWOR)
TWOR allocates shared overhead (e.g., blast vibration analysis, regulatory reporting) proportionally to actual tonnes fragmented per blast round—correcting for scale effects ignored by fixed-hour rates. It is especially critical for mixed-operation sites (open-pit + underground) where unit overhead varies nonlinearly with scale.
💡 Worked Example
Problem: A copper mine incurred $285,000 in annual blast-related overhead (vibration modeling, regulatory filings, electronic initiation QA). In Q1, Blast A fragmented 12,500 t, Blast B fragmented 8,200 t, and Blast C fragmented 15,300 t. Calculate TWOR and allocate overhead to Blast B.
1.
Step 1: Total tonnes = 12,500 + 8,200 + 15,300 = 36,000 t
2.
Step 2: TWOR = Total overhead / Total tonnes = $285,000 / 36,000 t = $7.92/t
3.
Step 3: Allocation to Blast B = 8,200 t × $7.92/t = $64,944
Answer:
The allocated overhead for Blast B is $64,944, which reflects its 22.8% share of total fragmented tonnage—more accurate than allocating equally ($95,000 each) or by drill hours (which would under-allocate to high-efficiency blasts).
🏗️ Real-World Application
At Newmont’s Boddington Mine (WA), engineers replaced machine-hour-based overhead allocation for blast design with a hybrid base: 60% detonator count + 40% RMR-adjusted tonnes. This reduced cost variance between similar-grade blasts by 41% and aligned internal charge-out rates with external blast consulting fees (per SME Blast Cost Benchmark Report, 2022). The change also flagged under-resourced low-RMR blast zones—where prior hour-based allocation masked excessive design effort per tonne.