The Cost of Geotechnical Conservatism in Mining
In mining, safety is non-negotiable. But when caution turns into over-conservatism, the cost can quickly erode productivity, design efficiency, and project value. Whether in open pit or underground operations, geotechnical engineering plays a critical role in managing risk. Yet, conservative assumptions, often made due to limited data or time, can lead to over design and inefficiencies.
The Cost of Over-Conservatism
In open pit mining, conservative slope angles reduce pit depth and ore recovery, increasing strip ratios and lowering NPV. Underground, excessive ground support, constrained stope dimensions, and rigid sequencing can slow development and inflate costs. These decisions, while made in the name of safety, often divert resources from higher value activities and delay returns, especially in capital-sensitive environments.
Why Does It Happen?
Geotechnical conservatism stems from sparse or poor quality data, reliance on legacy methods, and a natural tendency toward risk aversion. Engineers often default to cautious designs when confidence is low. Communication gaps between technical teams and decision-makers can further amplify this conservatism, leading to missed opportunities for optimisation.
Optimising safety and efficiency
To balance safety and efficiency, mining operations must invest in early orebody knowledge via targeted drilling, mapping, and instrumentation. Advanced modelling techniques, when paired with practical experience, enable accurate interpretations of ground behaviour. Embedding geotechnical input into mine planning ensures ground conditions are considered from the outset.
Figure 1: Example of calibrated model results based on actual slope performance data.
Monitoring and the observational method offer dynamic, data-driven feedback. Real-time systems like radars and piezometers allow engineers to adjust designs based on actual ground performance, and not have to rely on worst case assumptions. This approach supports steeper slopes and leaner support systems, enhancing safety while unlocking economic value.
Figure 2: Example of slope stability radar monitoring data
Culture Matters
Ultimately, reducing conservatism requires a shift from risk avoidance to informed risk management. Open dialogue between geotechnical teams, planners, and leadership fosters shared understanding and informed decisions. When risk is qualified, designs become more strategic, efficient, and aligned with operational goals.
Case Study
At a long-standing strip mining operation in the New South Wales coalfields, conservative geotechnical design had long dictated slope angles and excavation methods for final strips. Despite benign ground conditions, characterised by shallow alluvium, stable sedimentary sequences, and minimal groundwater impact, final highwalls were traditionally mined at 65° to 70°, limiting coal recovery.
Faced with topographic and boundary constraints, CM&G engineers in collaboration with professionals at the mining operation in question proposed a bold alternative; mining final highwalls vertically at 90°. With pits typically backfilled within three months, the exposure time for unsupported walls was minimal. Each 100 m × 60 m block had the potential to yield an additional 100,000 tonnes of Run-of-Mine (ROM) coal, translating to approximately $10 million in added revenue per block. Across 50–70 strips, the financial upside was substantial.
The proposal was supported by photogrammetric mapping, which revealed short joint persistence (<2 m) and a low likelihood of instability. Kinematic and finite element analyses confirmed slope stability, with Stress Reduction Factor (SRF) values exceeding 1.5 and no signs of tensile or shear failure. Visual monitoring was deemed sufficient given the site's history and conditions.
A vertical wall was successfully mined in a final strip using standard practices, with no incidents reported. Minor coal loss at the toe, due to excavator reach limitations, was the only deviation from design. Following this success, the methodology was adopted site wide, demonstrating that data-driven design and the observational method can safely challenge conservative norms and unlock significant economic value.
Final Thoughts
Geotechnical conservatism is understandable, but not always justifiable. With better data, innovative tools, practical experience, and collaborative planning, mining operations and capital projects can achieve safer, more efficient outcomes.
Whether you're designing a highwall or a stope, the key is not to eliminate risk, but to understand it and manage it with accuracy.
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