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Introduction
If you’ve spent much time in open cut coal, you’ve probably seen a plan that “worked on paper” right up until the dump ran out of capacity early, an access was delayed, the landform fell short or the truck loads didn’t reconcile.
In lots of cases, the common denominator is swell.
Swell is one of those topics that is easy to oversimplify in long term planning, which carries quickly through the medium term to the short term planning team. Namely, calculate a rough number, quickly sanity check it, plug it into the model then move on. But swell is not a constant. It’s material-specific and method-dependent, varying across lithology profiles, blasting styles and differing moisture conditions.
Commonly confused, the Bulking Factor is the multiplier that turns in-situ volume into operational reality, while Swell is the percentage increase in volume from bank cubic metres to loose cubic metres. If you get either wrong, you don’t just get a wrong dump capacity, you get incorrect truck hours, incorrect dump sequencing, missed opportunities and a reconciliation headache.
Definitions that keep the team aligned
Before talking about causes and impacts, it’s worth locking down definitions, mainly because swell is not a topic that is well understood by all.
Volume States
- BCM (Bank Cubic Metres): material in its natural, undisturbed in situ condition.
- LCM (Loose Cubic Metres): material that has expanded due to introduced voids between rock masses due to mining or blasting.
- CCM (Compacted Cubic Metres): material after placement and compaction or traffic densification.
Figure 1: Volume States - Flow
These terms are used widely in earthworks and equipment productivity references (and they matter because the same volume can be measured as different quantities depending on the unit being referenced; bank, loose, or compacted).
Note: CCM is a reporting state (volume after initial placement and compaction/traffic). Consolidation is a time-dependent geotechnical settlement mechanism. The wording can sound similar, but they describe different things.
How are these volumes measured?
A clean way to describe swell is:
- Swell (%) = how much volume increases when material goes from bank to loose.
- LCM = BCM × (1 + Swell%/100 ) or BCM x (1+Swell)
- Bulking factor (dimensionless) = LCM / BCM
Volumes are measured differently depending on where it is in the mining process:
- BCM is typically measured through the modelling of topographical surfaces and dig designs
- LCM is generally measured through the application of a truck factor (a combination of tray volume and fill factor) and quantity of loads
- CCM is measured from dump surveys (as-builts) to determine how much waste was actually placed
Why swell occurs
Swell is the void space, created when you break in-situ material up. Rock masses are created that can’t pack perfectly back together, creating gaps. The “gaps” between them reduce bulk density while increasing volume for the same mass.
Figure 2: In-situ face vs blasted waste
Why is swell so variable
Swell is driven by a mix of geology, mining method, and physical handling.
Lithology and weathering
Vertically, interburdens can quickly change between competent sandstone to weakened siltstone or claystone layers. When you add weathering and moisture sensitivity into the mix your ‘site average’ swell number can become a poor fit very quickly.
Mining method: blast vs rip vs free dig
The selected mining method has a notable impact on swell because it materially affects fragmentation and void space.
- Drill and blast: would typically produce a fragmented muck pile with meaningful void content.
- Ripping/free dig: can produce coarser and blockier material depending on what the waste type is. This can sometimes be associated with a reduced swell at the dig face.
It is also important to remember that rehandle changes particle size distribution and moisture interaction. Depending on the material, handling can increase fines, change tray packing density, and either increase or reduce apparent looseness. The practical impact is usually increased variability through the value chain (loading, hauling, dozing and final dump performance).
Understanding that there is an economic trade-off between; blasting costs, diggability, increased truck hours; having a goal of increasing or decreasing a swell number is something that requires adequate planning and is not something that should be readily modified.
Blast design and fragmentation
In practice, no shot is the same as another, regardless of whether they are designed identically. Hence, swell profiles can vary noticeably due to blasting for a variety of reasons:
- Burden/spacing and stemming effectiveness
- Confinement and geological structures
- Water, timing, depth, and powder factor
- Desired diggability vs wall control vs heave profile
These variations practically can present as:
- Faster loading and better bucket fill on a good blast
- Reduced complaints about blocky dig
- Varied dozer effort at the dump due to particle size mix and mobility
Moisture and fines
Moisture is a quiet factor that changes how material behaves:
- Fines & moisture can pack differently and change tray loading density
- Wet conditions can increase stickiness and reduce effective tray volume utilisation (even if swell is unchanged)
- Dump trafficability and compaction can change the “loose → compacted” part of the story
Where swell appears in the value chain
Swell isn’t just a technical services detail. It shows up everywhere.
Planning and scheduling
Your geological model and schedules are normally BCM-based. But the production team has to:
- Load trucks by loose volume and tray geometry
- Build ramps & dumps based on geotechnically available space
- Maintain dump geometry based on initial compacted behaviour
Production and fleet performance
Swell can influence whether you’re volume-limited or payload-limited in your truck fleet:
- Light, fluffy material: you “cube out” before you hit payload
- Dense, competent material: you hit payload first
That directly changes:
- Effective truck productivity
- Pass matching
- Haulage KPIs interpretation (especially if teams report in different units)
This is widely understood when comparing coal versus waste, but it is often underappreciated within waste types. Lithology, weathering and fragmentation can swing tray utilisation and dump performance even when the BCM looks similar.
It cannot be overstated how important accurate swell values are used when completing fleet sizing and pass matching assessments prior to purchasing plant. Incorrect trays (design and capacity) can cost millions in lost efficiencies.
Drill and blast, dig, and downstream effort
A blast that improves diggability might:
- Increase swell (more voids)
- Improve bucket fill and cycle time
- Shift effort to dozing/spreading at the dump
The “best” outcome is site-specific. The important take-away is that the plan accounts for reality.
A simple rule that saves arguments: always state the unit (BCM, LCM, CCM) in reports, and schedules. The vast majority of the time, mining professionals will refer to volumes in BCM, if this is not the case, make sure it is highlighted!
Worked example
The goal of this example is to show how a (small) change in swell can multiply into fleet hours and dump capacity pressure.
Example A: Converting planned BCM to operational volumes
Planned waste movement: 100,000 BCM
Assumed swell: 30% (bulking factor 1.30)
Loose volume hauled/placed: 100,000 × 1.30 = 130,000 LCM
If average effective tray capacity is 40 LCM/load, that is approximately 3,250 loads.
Example B: The cost of being wrong
If actual swell is 40% (bulking factor 1.40):
- Actual loose volume = 100,000 × 1.40 = 140,000 LCM
- Difference vs plan = 10,000 LCM
- At 40 LCM per truck, that is roughly +250 loads
At around 45-minute cycles at $500/truck hour that is just over $90,000 in unplanned haulage effort.
Unit consistency in practice; equivalating to BCM
Many operations standardise reporting and reconciliation in equivalent BCM (eq.BCM) because it aligns geology, planning, and production on an equalised basis. Operationally, trucks move LCM and dumps consume CCM, but these are commonly converted back to eq.BCM using agreed site factors. This process minimises the risk of volume being ‘lost’ due to mathematical errors and minimises confusion between stakeholders.
Applying swell to dump capacity
As dump designs and survey volumes are reported back in eq.BCM, we must be able to readily work between BCM, LCM and CCM. A simple way to convert is to use two site factors:
BF = Bulking Factor (BCM → LCM)
PF = Placement Factor (LCM → CCM), typically measured as the difference between surveys. Note this number is typically <1 as construction traffic inherently compacts the waste material.
Where:
CCM = BCM x BF x PF
Or:
eq.BCM = CCM/(BF x PF)
How to estimate swell at your mine
A practical workflow to follow when you want swell factors that stand up to scrutiny. The aim is not to find one “true” number, but to understand the range by material domain and keep it current as the mine changes.
Step 1 - Domain your material
Split swell assumptions by the things that actually change behaviour:
- Lithology and weathering (e.g., oxidised vs transitional vs fresh)
- Method (blast vs rip vs free dig)
- Moisture regime (dry vs wet season behaviour)
- Rehandle (primary movement vs multiple handles)
Step 2 - Assign starter factors
Use historical site data where available. If you are starting from scratch, use a conservative reference range and sanity-check against neighbouring operations with similar geology (then plan a calibration campaign).
Step 3 - Measure and calibrate
Use at least two independent lenses:
- Survey-based (preferred for calibration): shot-by-shot pre and post surfaces for a dig area, including any cast.
- Production-based (operational reality): truck counts x truck factor, validated periodically.
- Reconcile across several events and build a distribution (not just one number) for each domain. The key here is bulk data analysis (a great Six Sigma investigation).
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- To generate comparable data profiles, it may be easier to graph all of the interburdens on the same graphs to determine how appropriate a site average could be.
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Step 4 - Maintain a swell register
Keep a living record of your assumptions and evidence. At minimum include:
- Domain definition (what qualifies material into the bucket)
- BCM→LCM factor (and BCM→CCM if you manage dump capacity that way)
- Evidence source and date range (survey window, reconciliation period, trial blast)
- Confidence rating and review trigger (new pit, new pattern, wet season onset)
The process for estimating Placement Factors is similar and just as important in creating accurate dump schedules.
Why getting swell wrong hurts
Unplanned swell is not just extra volume; it shows up as:
- Extra digger/truck/dozer hours you did not plan for
- Earlier or later dump completion (both are problematic)
- Schedule pressure you only notice after the fact
Similarly, if you assume an overly conservative swell, you are at risk of:
- Not having enough waste to achieve landforms
- Reducing productivity through unnecessary rehandle or longer haul routes because milestone connections are missed
- Finishing digs too early (yes, there is such a thing)
Conclusion
Swell looks like a minor input until you follow it downstream. In open cut mining, it’s one of the most powerful hidden multipliers we deal with, quietly shaping fleet productivity, dump life, strip schedules, and reconciliation.
Whilst a small variation in swell may not appear to have a large impact, if it causes the net time for a dig to take longer, immediately the schedule is out. This may have no material impact, but it also might push targets out of the month because the required dig time has increased.
In mining, ‘swings and roundabouts’ exist, while most just accept them for what they are, the best teams understand the drivers, consequences and when enough is enough. Swell is often ignored as a rounding factor, but this factor can throw out landform dump balances by significant quantities if left unchecked for a long period of time.
When was the last time you validated your swell values? If you need a hand making sure you are on the right path moving forward, reach out to the team at Atlantech.