William Joughin, Chairman and Corporate Consultant (Rock Engineering) Ed Saunders, Principal Consultant – Mining Rock Mechanics Diane Walker, Principal (Geotechnical Engineering)
With the world’s most accessible mineral deposits already discovered and developed, extraction conditions are becoming progressively more difficult – making rock engineering more demanding. In open pits, slopes are often required to be steeper, with increased production pressures. As underground mines deepen, there is growing risk of mining-induced stresses and rock bursts. Orebodies also tend to be geologically more disturbed, making them harder to mine.
Orebodies are often exploited from surface using low-cost open pit mining, but as the orebody continues deeper, waste stripping becomes excessive and underground mining is considered. The transition from open pit to underground is challenging. Many factors, such as the shape and size of the orebody, rock mass characteristics, geological structure, economics, underground mining methods, environmental constraints, management of water, surface infrastructure and impact on local communities need to be considered.
The decision to leave a crown pillar to prevent pit slope failure and subsidence or remove the crown and manage the failure impacts needs to be made at the very beginning. This decision also significantly affects the design of the access to the underground operation, which in turn affects the timing and cost of the transition. Rock engineering plays a major role in all decisions.
Among the underground mining methods available, block caving is often favoured wherever it is feasible, as its cost-effectiveness makes it possible to mine even low-grade deposits economically. However, it does require higher capital costs, including intensive upfront investigation and analysis. While some other methods provide opportunities to learn lessons as mining progresses, block caving is less forgiving – the correct strategies must be adopted from the start.
All this points towards the growing importance of rock engineering design and the various technical inputs that contribute to this complex field. This applies not only from an economic perspective, but equally from the point of view of health and safety as well as operational risk. More geologically disturbed environments present a higher safety risk, requiring greater engineering effort to execute the mine plan.
A comprehensive, integrated approach using a multi-disciplinary team is required, taking into account geological, geotechnical, structural and hydrogeological data. The interpretation of structures and rock mass is vital to anticipating hazardous conditions and can be incorporated into the mining strategies we recommend. The significant impact of water on the stability of pit slopes and underground excavations, especially in shallower operations, highlights the role of the hydrogeologist.
Various methods of analysis are available. Complex numerical modelling can assist in quantifying failure mechanisms, for instance, while a quantitative risk evaluation approach can be used to estimate the impact of slope failure on a mine’s net present value.
The quality of the analysis is, of course, only as good as the quality of the data. The tools at our disposal to gather the necessary data are constantly improving. SRK makes the most of existing data to focus engineering works from an early stage.
Incorporating new technologies improves our investigation methods into rock mass conditions and allows data collection to be conducted remotely where access is unsafe or inaccessible. Some remote tools are also proving useful during the COVID-19 pandemic, when it is difficult to travel and to gain access to mine sites. LiDAR drone surveys have been employed to scan narrow-vein stopes before backfilling, for example. Bathymetric and three-dimensional sonar surveys have even been taken in a mine closed over 50 years ago, improving the spatial understanding of the mine workings themselves, as well as the quality of the rock mass and the stability of the excavation.
The data and analysis must lead to a practical solution, and here there is no substitute for experience. At the end of a complex analysis, the experienced engineer must understand the risks that have been quantified and mitigate these in a safe and cost-effective strategy. SRK can match the most appropriate team from its global network of consulting practices with the project deposit and operating conditions, including structural geologists, hydrogeologists and numerical modellers.