As a WASMA Comitis Partner, Go Engineering plays an important role in strengthening connections between industry, alumni, and students. Read their case study to learn more about their business and industry expertise.
Taking the Brakes Off: Rethinking the Stoping Method at an Underground Gold Operation
By Alston D’Lima, Operations Manager, and Jose Grado, Senior Mining Consultant, Go Engineering
When backfill sets the pace of production, the answer may not be more fill. It may be a different way of mining. Go Engineering recently completed a desktop study testing whether an established underground gold operation could move from an Avoca-style method to longhole open stoping (LHOS) with pillars.
The challenge
The operation mines several orebodies using an Avoca-style method. Each stope is extracted and then backfilled with waste before the adjacent stope can be taken. It is a proven, selective method, but it ties the production rate directly to how quickly fill can be sourced and placed. Over the remaining life of mine, fill had become the constraint. The client asked a practical question: would LHOS with sill and rib pillars remove that dependency, and what would it cost in ore left behind?
The approach
The study began as an assessment of a single mine. Within three weeks, early design and geotechnical work gave the client enough confidence to expand the scope to the whole underground complex. The team delivered the full study in about eight weeks. Senior Mining Consultant Jose Grado led and executed the technical work, including all mine design, planning, and scheduling. Alston D’Lima (2012 WASM graduate and holder of a WA First Class Mine Manager’s Certificate) provided senior review and oversight. An independent specialist geotechnical consultancy completed the stope and pillar review.
- Baseline review. The client’s mine designs, schedules, block model, geotechnical data and cost model were reviewed, along with the existing auto-scheduling rules.
- Redesign for pillars. The supplied stope shapes were cut for sill and rib pillars, level by level, in Deswik. An early decision was made to drop long, vertically aligned stopes in favour of level-by-level assessment, so higher-grade lodes could be prioritised.
- Geotechnical check. A high-level span and pillar review found staggered, “diamond” rib pillars kept stope spans within stable limits better than stacked ribs. Sill pillars were placed at regular level intervals, with tighter spacing at depth.
- Two complete schedules. Full life-of-mine schedules were built for both methods on the same recovery and dilution basis, so the comparison was like-for-like.
- Comparison. Physicals were compared in absolute and per-tonne terms to isolate genuine efficiency gains from the effect of a smaller mine, and to identify the cost drivers for the economic evaluation.
What the study found
LHOS with pillars addressed the constraint it was meant to solve. Backfill placement fell by about 73% across the complex, and total material moved fell by about a third. Importantly, the gains were structural rather than a by-product of a smaller mine. Every haulage and materials-handling measure improved on a per-tonne and per-ounce basis:
| Efficiency measure | Change under LHOS with pillars |
| Total material moved | ~33% lower |
| Haulage per tonne of ore | ~28% lower |
| Gold recovered per tonne moved | ~33% higher |
The trade-off is ore left standing in pillars. Scheduled ore was about 12% lower and the producing life around a year shorter, but the monthly production rate held steady. The ore in pillars also remains an opportunity for a future recovery assessment. The geotechnical work added an important nuance. Where several ore lenses are mined together across a major shear structure, pillars alone may not be enough, and a hybrid of fill with rib and sill pillars is the likely answer in those zones. The study recommended LHOS with pillars as the basis for the complex, with a staged path to implementation: confirm the economics, then complete site-calibrated numerical modelling of pillar stability before any operational decision is made.
Lessons for mining engineers
- Challenge the constraint, not just the schedule. Re-sequencing an Avoca schedule would only have moved the fill bottleneck around. Changing the method removed most of it.
- Fix the basis before you compare. Holding modifying factors constant and comparing full mine lives meant the result reflected the method rather than the assumptions.
- Look at per-tonne numbers. A smaller total can hide a better mine. Normalising by tonne and ounce showed where the real gains were.
- Bring geotechnical engineers in early. Pillar layouts were being tested in the first two weeks, which shaped the design rather than checking it afterwards.
- Be clear about what a desktop study can tell you. The study set out what was proven, what was conditional and what validation had to come next.
Thanks again to Go Engineering for being a valued WASMA Comitis Partner! Your support helps us bring our community together, create valuable opportunities for students and alumni, and strengthen connections across the resources sector. Learn more about Go Engineering Managing Director, Cornelius Hattingh, his journey with Go Engineering and what led them to become a WASMA Comitis Partner here.
To learn more about Go Engineering, visit: goengineering.net.au