With the current spate of mergers and acquisitions, mining companies are acquiring portfolios that often include worked out or abandoned properties. Many companies often don’t know the extent of the legacy problems that they may have in their portfolio.
Often, old and abandoned properties can pose real public safety hazards, and clean-ups can generate a significant financial liability and a potential risk burden–witness the collapse of older workings in Timmins, Ont. in 2004, likely due to changes in groundwater levels. This type of collapse is a reminder that even though old mines may have been abandoned, their closure might not have been properly engineered and monitored. Collapses through to surface can have severe consequences for roads and buildings. Even if the collapse occurs in an uninhabited area, environmental problems can still develop.
How real are the risks and how well can they be quantified?
Closure bonds: part of the answer
Canada is a leader in managing closure and legislating the ongoing monitoring of underground mines. Miners are now required in all provinces to file closure bonds for an amount appropriate to the perceived risk level.
Each mining company that owns legacy workings must put aside “sufficient” funds to cover remediation and monitoring to the extent considered realistic in the closure report lodged with the government. This report must be signed off by a professional engineer qualified in this field.
The challenge for any firm preparing a closure report is to balance the contradictory requirements of (a) determining the appropriate level of risk to satisfy government requirements, and (b) not inflating the potential costs for the assumed remediation measures. Often, right up to the stage of submittal of a closure report, mining companies are reluctant to make significant outlays for any physical investigation of closure hazard risks, as all such expenditure is on the “wrong side of the balance sheet”. In consequence often hazards are ill-defined, and thus a consultant’s impulse must be to err on the conservative side in estimating the risk. This means that the mining company must set aside more money than perhaps might have been the case if a more refined investigation had been done.
The question is always, “How much study and investigation is enough?”
Assessing closure risk
The worldwide need to deal with mine hazards and develop reasonably accurate costs for closure and monitoring is creating an increased demand for better ways to estimate potential collapse risk from active and/or legacy mine workings. Better definition helps develop better risk management plans that are more cost effective because these plans not only better satisfy government legislators but they also tend to be more in line with the gut feel of experienced mining executives.
Much of the precedent data on collapses, which is the most important information for quantifying risk, is confidential. Companies are understandably reluctant to let this information out of their hands. Fortunately, in the mid-1980s, largely as a result of a series of highly publicized collapses, both industry and government recognized the need for better prediction and assessment methods. A Canadian initiative, funded by industry and both levels of government, was launched to create a database of stable and failed crown pillar geometries, as a basis for developing better analysis methods. It is to the credit of the mining companies that they allowed access to this closely-held information.
Under this CANMET initiative, Golder Associates developed a database of over 200 cases with 30 failures (Golder Associates, 1989; Carter, 1990). Over the last 15 years this database has been extended to include more than 500 records with over 50 failure cases. For a copy of the database, see www.golder.comcrownpillarfailure.
One of the most important insights gained from examining this database relates to the time history of failure. Like household appliances, mine structures tend either to fail when they are new, or not to fail for decades or even generations. This knowledge is important in planning for mine closure, because crown behaviour is rockmass-dependent. Crown pillars in degradable rock masses are the ones that tend to become less stable over time, and as such these crowns pose the greatest long-term threat for collapse.
Determining if a crown pillar will collapse, and deciding how great is the risk, depend on being able to properly characterize the rock geometry of the mined opening and assess the quality of the crown rockmass. Initial analysis by simple spreadsheet methods (see www.golder.comcrownpillarcollapse) can provide a ready means for rapid assessment of the level of potential risk. This can then lead to consequence assessment and to prioritizing actions. If, for example, the probability of failure is analyzed for a given crown as 22%, then based on the table below (Carter and Miller, 1995), actions could include total prohibition of public land use over this zone.
Studies: a little can mean a lot
Without due diligence, mining companies acquiring property portfolios may uncover some unpleasant surprises. These can often, however, be mitigated by approaches that minimize risks, which do not have to start with a major, expensive, in-depth assessment. Rather, examining the crown pillar scaled span guidelines from the existing mine plans can quickly allow insight into possible risks. Based on such preliminary assessments, decisions can be readily reached on which properties pose the most risk, and where the company should invest in further study and drilling investigations.
Even simple assessment procedures can lead to a more accurate understanding of the risks. This in turn can lead to better predictions, and hence to more effective planning, costing, and accuracy in defining an appropriate amount to be set aside as the mandatory closure bond. The result will be a healthier balance sheet with fewer hidden surprises. Even more important, the process will enhance the public’s confidence in the mining sector.
Trevor Carter is a principal in the Toronto office of Golder Associates Ltd., where he is a specialist geological engineer in the Rock Mechanics group. He holds a PhD in Engineering Geology. Tel. 905-567-4444; [email protected].