Moving from open pit to underground operations is fraught with complications. North American Mining spoke to three experts about the considerations and risks involved. 

By Jonathan Rowland

A majority of mining operations begin life as open pits; however, as mining progresses, ore reserves may be proved to extend to greater depths than originally anticipated. To continue exploiting these reserves, the pit must either be extended to access these deeper deposits, or the mine must switch to underground mining methods (or quite possibly a mix of both).

“Typically, the decision to transition from open pit to underground operations is an economic one based on the cost of mining – but there is no industry standard approach,” Andrew Hall, director/executive lead – Advisory at AMC Consultants, told North American Mining. “Some believe that you should mine the biggest open pit you can to take advantage of potential higher production rates, as well as lower operating and capital costs. Others take a strict cost-analysis approach to determine the depth at which the transition occurs. However, the ‘optimal’ depth of an open pit lies somewhere between these two limits.”

Surface-to-underground transitions are complex undertakings, agreed Peter Terbrugge, corporate consultant and expert in mining engineering at SRK, who has worked on a number of such projects. To make the transition as efficient as possible, “requires extensive planning. For a large mining operation, this planning process will be measured in years, so it’s important to begin as soon as possible to avoid any production gap between the end of open pit mining and the start of underground mining.”

It is never too soon to start planning 
“Ideally planning for an underground transition should start at the early planning stage – conceptual to feasibility – of the project,” said AMC’s Hall. However, underground mining is often not on the cards at these early stages: it may only be considered an optional extension, if it is considered at all. In these cases, “the planning process is often deferred, particularly if the level of knowledge about the orebody at depth is low, or the company does not have the in-house skills to plan an underground mine.”

Ideally, mines should begin to plan for the possibility of an underground transition at the preliminary feasibility study (PFS) stage, agreed Joe Luxford, managing director at Luxford Mine Management Services. “The PFS may flag when the transition is likely, if at all, and identify the future drilling program needed to determine the orebody extensions below the open pit and the likely feasibility of an underground operation.”

Unfortunately, mining companies still underestimate the timeframes involved in planning an underground transition. SKR’s Terbrugge offered a couple of examples of large diamond mines in South Africa where planning took much longer than anticipated and left the mining companies involved scrabbling to cover production gaps.

“The CEO of one mine asked if we could steepen the slopes of the pit to bridge production until the underground operation was ready,” said Terbrugge. “We managed to do so with some fancy drilling and by putting additional support in the kimberlite. The operation became more expensive and it was a logistical challenge, as we had to work around development of the underground workings, but we managed to extend the pit life for the three years they were looking for.”

Extending the pit is not always possible, however. Terbrugge cited the example of another major diamond mine that was left to treat low-grade leftovers of the surface operation until underground operations were ready – some three years behind schedule. 

Echoing Terbrugge’s experience, AMC’s Hall agreed that “the timeframe for an underground transition is typically underestimated,” adding that it is a particular issue when additional knowledge about the orebody is needed to support detailed designs and approvals. “When you consider the time needed to complete the required drilling programs, test work, technical studies, approvals, construction, commissioning, and production ramp-up, a transition timeframe of five plus years is not uncommon.”

And timeframes can be much longer. Drawing on his experience, Terbrugge notes planning periods can extend beyond 15 years and up to 20 years for large mining operations. This not only raises the ore gap issue (as mentioned above), but may also impact the quality of the underground operation. According to Hall, if the transition timeframe is underestimated, “shortcuts may be taken to assure production continuity. And with every shortcut comes added risk that the design is a little less optimal, compromising the longer-term future of the mine.”

Establishing the transition zone limits
According to AMC’s Hall, when beginning to plan a transition, it is “good practice” to first establish the upper and lower limits of the transition zone. “The lower limit is set by the Lerchs-Grossman ultimate pit limit: the deepest you can economically mine using open pit methods without considering an underground operation. The upper limit it set by the minimum depth you can mine using underground methods, which is typically at the base of the weather profile.”

When these limits have been determined, the Lerchs-Grossman pit “can be restricted using the opportunity cost of each block to determine the pit limit (instead of the profit), where the opportunity cost is the difference in the value of the block when mined by open pit or by underground methods,” continued Hall. “The underlying assumption here is that, if a block can be mined by either method, if it is not mined in the open pit, it will be mined underground.”

These two pit depths – that of the Lerchs-Grossman pit and of the restricted pit – provide the theoretical upper and lower positions for the base of the pit. “The optimal position lies somewhere in this transition zone,” concluded Hall, “but the actual ‘sweet spot’ depends on a wide range of factors, both technical and non-technical.” 

There is never too much data
The success of any surface-to-underground transition – indeed the success of any mining operation – relies on “the extent and quality of the technical data available to the project team, which will largely determine the technical and economic feasibility of any potential underground operation. This, in turn, determines the position of the transition from open pit to underground mining,” said AMC’s Hall. 

According to Peter Terbrugge of SRK, this data includes: 

  • Definition of the orebody – its shape and footprint, depth extent and variability, 3D geometry, and maximum potential mining depth. 
  • Rock mass characteristics – major geological structures, jointing in the rock mass, rock types and rock mass quality, and groundwater conditions. These will all “affect the stability of any proposed underground excavations,” said the SRK expert. 
  • Definition of boundary conditions including in situ stresses, topography, and superloading (both positive or negative), presence of groundwater, and blasting and seismicity. 

Obtaining such data is not easy, however. Focus should therefore be on “obtaining as much geological and geotechnical information from the open pit as possible to apply to underground development and operation,” said Luxford. 

Exploration drilling is another main source of data. But here there are challenges. “As depth below surface increases, the density of drilling is typically lower, as costs increase and drill site availability declines due to ongoing surface mining operations,” said Hall. “This means there is less drill core available for logging or samples for test work, reducing confidence in a wide range of key design parameters, including geology, mineral resource estimates, ground conditions, selection of the mining method, capital and operating costs, potential production rates, mined grade, and recovery rates.”

As a result, at the conceptual design stage, “assumptions will need to be made based on existing data, much of which will be interpolated or inferred from the open pit experience,” concluded the AMC expert. “But as the project matures and access to the deeper parts of the orebody improves, continued data collection is critical to reduce the risks associated with developing the project.”

On top, down below
Other factors that require consideration include the interaction between the open pit and underground mining development and operations. For example, what effect will surface blasting have on the development of underground operations? Conversely, what impact might the underground workings have on pit stability? There may also be potential dilution of underground reserves from the open pit, especially after it has been abandoned, as well as health and safety issues, such as mudrushes (more on them later).

There are also issues around infrastructure, both on the surface and underground. “Surface installations, such as tailings dams, rock dumps, mineral processing plant, roads, rail lines and pipelines, should be planned to ensure they are not impacted by underground operations, while underground openings must be secure from any surface effects,” explained SRK’s Terbrugge. Decisions must also be made about the location and size of underground shafts, ore passes, crushers, silos, chambers, ramps and declines, pump chambers, and conveyor excavations, etc.

“Cave angles and cave crack locations will affect the location of surface infrastructure,” continued Terbrugge, “while the location of cave cracks underground will impact the location of shafts and the effect on the pit.”

“The depth and severity of weathering will be a major consideration when locating decline portals and shaft collars, while you must also consider how ore will be handled from portal or shaft to the mineral processing plant,” added Luxford. “At the same time, the size of the orebody and planned production rate from the underground workings may require significant changes to ore treatment processes, especially crushing, grinding, flotation, and filter dewatering circuits.”

This latter point was also picked up by AMC’s Andrew Hall. “Providing appropriate feed volumes and material types (e.g., oxidation state) to a process plant that has been sized for open pit volumes is a common challenge when moving underground. This requires strategic metallurgical plant reviews to balance underground feed volumes – which are typically lower, albeit with higher grades – to achieve acceptable project economics.” This could require throughput reductions (turning off a parallel stream), reduced plant utilization (campaigned milling time), or other geometallurgical considerations (increased hardness with less throughput), which may be counter-intuitive in open pit operations where unit-volume is king.

Administrative locations should also not be overlooked, as Luxford pointed out. “Will the underground operations gradually take over the open pit offices, workshops, etc., as happened at Palabora in the early 2000s, or will they require new facilities?” And then there are the workers. Will underground development and/or operation be conducted by the mine company’s own technical and supervisory staff or contractors – and will this raise any union issues? 

Underground mining – but how? 
Of course, decisions will also need to be made about the method of mining method. Considerations here include fragmentation, layout geometry, draw point sizes and spacing, drifts, and the location, geometry, dimensions, and support requirement of undercuts.

Meanwhile, the method of mining selected “can have a very important influence on existing mine surface and underground infrastructure,” said SRK’s Terbrugge. “For example, caving methods will result in the formation of cave craters, which may grow in size – and influence – as mining depth increases. Conversely, cut-and-fill and open-stoping methods preserve a stable rock mass and thus any mine infrastructure; however, should an open stope collapse, it may lead to the development of further failures in the rock mass. All of these potential behaviors should be analyzed when evaluating mining methods.”

“Another common challenge is the separation of open pit and underground activities during transition, especially if underground access is from within the pit environment,” added AMC’s Andrew Hall.

“To ensure continuity of production, it is often necessary to develop the underground mine and ramp-up production concurrently with continuing open pit operations,” the AMC expert continued. “This can impact performance, especially if mine access is not well planned and demarcated. Early identification and development of underground portals within the open pit may therefore ensure more efficient development of both the underground workings and the open pit. Other key interactions that need to be managed include ventilation (dust from pit, location of raises), materials handling (using the pit fleet for surface movements), ore accounting/reconciliation/measurement, and the impact of blast vibrations and fumes ingress.”

Health and safety 
Underground mining obviously presents different health and safety risks than open pit mining – with a range of different hazards and challenges. There are however a number of specific risks that must be considered when transitioning from surface to underground mining, or when underground workings is taking place beneath a working or abandoned open pit.

According to SRK’s Peter Terbrugge, “when a shaft has been sunk in preparation for underground mining, but open pit operations continue beyond the originally-planned depth, deformation of the shaft may occur as a result of a relaxation of or increase in stresses caused by the deepening on the pit, and consequent encroachment of the pit rim.”

The surface-to-underground transition “can also introduce the risk of mudrushes from within the rock mass – when mud-forming minerals are present – or from sumps and surface dams,” continued the SRK expert. These incredibly dangerous events can travel at speeds of 30-40 kph (18.5-35 mph) and occur at the coincidence of four factors: mud-forming materials, water, disturbance in the form of mining or drawing, and a discharge point. 

“Water ingress into the underground workings from the open pit therefore needs to be carefully managed, particularly when open pit operations have ceased. Some pit dewatering may need to continue through the life of the underground mine.”

A new factor? ESG and the underground transition
As noted at the start, the timing of any surface-to-underground transition is traditionally based on economics; however, increased focus on ESG factors within the mining industry could bring a new variable into the equation – and perhaps see mines move underground earlier.

“Underground mining has many environmental advantages compared to an open pit operation,” said AMC’s Andrew Hall. “When this is coupled with strong ESG commitments, it creates a positive image for investors and could bring forward the transition, irrespective of what direct cost analysis tells us. At the same time, open pit mines may provide an ideal access point for underground mines.”

This only emphasizes the importance of planning any potential shift to underground working as early as possible. Yet underground mining is generally also much more technically challenging, with higher upfront costs. “It is therefore often an easier decision to mine an additional cut-back and delay developing underground operations,” concluded Hall. “Unfortunately, feasibility evaluation is usually not undertaken early or comprehensively enough, and the optimal transition point is missed and the opportunity lost.”

And so we return to where we began – and the message emphasized by all contributors. When it comes to transitioning from an open pit to underground working, it is never too soon to start planning! 

Note: For a more detailed discussion of many of the topics covered by this article, see: Stacey, T.R, and Terbrugge, P.J., ‘Open Pit to Underground – Transition and Interaction’, in proceedings of MassMin 2000, pp. 97-104; and Luxford, J., ‘Surface to Underground – Making the Transition’, in proceedings of the International Conference on Mine Development (Mindev 97), Sydney 1997.

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