Best practice management of non-use management areas

Video transcript

Good morning everyone and welcome to this morning's seminar on geotechnical stability of final voids as non-use management areas hosted by the office of the Queensland Mine Rehabilitation Commissioner. It's great to have so many of you calling in from across Queensland and further a field. To start, I would like to acknowledge the traditional owners of the lands across which we're meeting today and pay my respect to the elders past and present. Also have a couple of housekeeping items to cover before I hand over to today's MC. Today's session is being recorded so and will be made available on the OQMRC website in the coming days. So if you do have any colleagues who couldn't make it along today, uh please feel free to share a link to that recording. During today's session, we're going to have plenty of time to um ask the presenters questions. So, please use the Q&A function in the top of your toolbar to send through questions throughout today's session. They may disappear from your screen. However, our team will receive all of those and we'll get to as many as we can. With that now out of the way, it's my pleasure to hand to today's MC, our lead technical adviser, Louisa Nicholson. Over to you, Louisa.

Thanks Sam and welcome everyone. It's my pleasure to be MC for this event this morning. Um and part of the role here at the office of the Queensland Mine Rehabilitation Commissioner is to engage with our stakeholders and community and we raise awareness about mine rehab matters as well as producing the technical reports on leading practice rehab of land and that's why we're here today. It's um obviously you're aware that we published a practice note late last year and the purpose of that practice note was essentially to summarize the components of a leading practice assessment regarding um of long-term geotechnical stability particularly of voids that was published in a very detailed study um part of an ACP project uh by John Simmons and also Greg and Sue who are with us today.

And so the role of the practice note was also to promote a consistent understanding of what's required for our mine operators, regulators and technical reviewers when it comes to these geotechnical assessments as well as clarifying the required assessment components and the role of professional judgment in this assessment and the limitations of prediction which is quite important. this um a process that is talked about in the practice note is a certifiable process accepted by industry and regulators. Um and so we're very excited to have been able to bring this work to you. There are a few um definitions that will be handy as we start to get into the weeds this morning and so I just wanted to cover those quickly before I introduce our speakers to you. Firstly, what is a final or residual void? Well, in the context of this work, it's we're talking about excavations that were made in order to carry out the mining activity and that have not been backfilled to at least the post mining ground level and so they remain in the landscape. And in Queensland, we have NUMAs.

For those who aren't aware, a NUMA is a non-use management area and is defined in the legislation as an area that cannot be rehabilitated to a stable condition. And stable condition is also defined in our legislation. And it means the land is safe and structurally stable, that there's no harm being caused by anything on or in the land, and that the land can sustain a post mine land use. And so when um the residual voids that we have after mining can't sustain a post mine land use in Queensland, they likely meet the definition of a non-use management area or a NUMA. Now NUMAS are approved under particular conditions, but that doesn't mean they don't require some sort of rehabilitation activity associated with them. and a NUMA must achieve best practice management and minimize environmental risk which is another reason why this work is so important because geotechnical stability is a key element of NUMA management and so with those definitions um um out and we are aware of what we're all talking about together and we're on the same page it's my pleasure now to introduce um our authors for this practice note who are going to present to us today so it's wonderful to have Greg join us. Greg is a principal engineering geologist at PSM. He has over 16 years experience working on various domestic and international mining and civil engineering projects. Greg's worked on projects across Australia, Asia and the Pacific. And interestingly, this include covering as the geotechnical superintendent um role at Octetti mine in Papa New Guinea. I suspect Greg that was quite interesting but challenging all at the same time. Greg's an accredited RPQ in civil and mining in Queensland and a slope risk assessor under the New South Wales um RMS RTA slope risk assessment program which is great because that means we've got some good practical local knowledge here in the team. Dr. Sue Henderson is a civil and geotechnical engineer at Hendo Geotech and Sue has more than 35 years experience in consulting and research. Many of us know Sue and um she's been a large figure in the industry for quite a while. Sue has been mainly involved in um surface ge mechanic mechanics sorry and has been a key researcher on several AAR projects and has also served on subcommittees negotiating with the government on dam conditions, water conditions, rehabilitation and closure. So without any further ado, welcome Greg and Sue and I'm looking forward to the presentation. Thank you.

Okay, thank you everyone for turning up to the process itself. We've got some more concepts or definitions to set the stage for geotech stability. So if we commit to the first the difference between geotechnical and erosion stability sometimes gets geotech is the lack of excessive ground movement caused by loads or stresses within the ground. Erosion really by contrast is the lack of excessive loss of ground material caused by water usually at the surface but definitely at the interface with those. So, we've got the example that you probably all recognize as geotech inability. Something down near the toe of that that wall obviously wasn't quite strong enough to hold up. You have the material falling out quite a large amount and that scarp shape. Erational stability also very familiar but quite different in shape. uh this one no argument that it's it's big and ugly but it's a completely different mechanism. They need to sets and professional pain assess. They can interact but they're not the same. You can imagine cases where a scar like that geotechnical instability could lead to greater erosion. There are also limited situations but they do occur where can geotech instability for example uh if it's at an angle and undercuts um a bed or something like that. So in long-term geotech stability assessment one must consider the potential for erosion to impact but it's not the same thing. So geotechnical assessment is not an assessment. Next one that we need to hammer home and we will be for some time in this talk is  that geotech stability is not a certain thing. We can't quote it to three decimal places or if we do a kid in your sites. Some of the reason profile has not been designed by us.

Whoever put it there is a bit whacker at times. So there's random layers and pockets. Soil and rocks are not manufactured materials. So they don't have a a predictable QA band of properties. The properties they do have can vary with the low geomorphology also moisture content, time and of course spatial variation. The models that we use to analyze stability are approximations. And how how accurate a particular model is really depends on how well it captures the important aspects in this situation.

So the upshot of that is there is no single correct answer for stability. Instead, what we as geotechnical engineers have to do is define a threshold or an acceptable outcome that's based on both the confidence we have in our particular model and method of analysis, consequences of instability that requires judgment. It's not something you can dial up out of a recipe. So that's where the professional training experience comes in.

Given the uncertainties, the load history, moisture effects and things like that, a an appropriately qualified person can't guarantee stability for a significant time into the future. Only all we can certify is that we have evaluated stability using sound and accepted procedures. Next idea here to put out is that it's not about maximum or minimizing risk of instability because what you do to improve stability can make other aspects of your NUMA worse. First example, flatter slope usually improves stability and it certainly can reduce uncertainty. However, it also increases your void footprint. It may increase thement which leads to high level which in turn might saturate more of your low wall and change your stability. Uh the longer slope may actually be subject to increased erosion and flattening the slopes particularly where you've got a dump uh will change reduce your dump space that way so that what you would have put there if you could have justified a steeper slope you got to find somewhere else to put that material. Another example, partial backfill of voids. That can improve stability, usually by just reducing the exposed height of your walls, but it can lead to pit lake and that might turn what would have been a void that is a sink for groundwater into a source of contamination to aquifers.

So once again we come back to this rather than maximizing stability aiming for an acceptable level of stability that depends on consequences and considering that particularly in the context of this practice note. We've got a NUMA but it's got usually adjoining postmining land use areas. So while this practice note is about the numeral, one does have to give consideration the adjacent land. Ah so if all of these things and bad news about what we can and can't do hasn't come home, the basis is we can't certify in the long term. And if we're uncertain now, the uncertainty can only multiply the further into the future we go. Trying to in the assess what's a reasonable frame. If you think about living memory, 60 to 80 years, there are photographic records of land forms of uh Queensland in our area of about 160 years that you could use to gain some insight in the long-term changes and civil design life is about 60 years. Even your critical infrastructure like uh hospitals is only about a hundred. So they set the time frames on which uh accepted understanding is that we could reasonably make some forecasts. So on that basis we're suggesting that the re for long term is in the order of 60 to 100 years to be decided at each assessment based on the information you have the complexity of the model how confident you are. Quick pause for any comments or questions on that stuff. We haven't any um questions across into the chat or the question answer yet. too. So, we might just keep going, I think. Thank you.

Oh, okay. Now, we're on to the elements of the assessment process that are in the practice. In the rest of this presentation, go through these in uh detail and with examples. The point of formalizing this is to provide confidence in the process but still allow this professional judgment that is the core of geotech engineering. What I want to stress here is that none of these elements are new to the Geotech engineering or stability assessment. Any competent Geotech would be doing all of these in their assessment already. The difference here is that we set out a framework that you can formally document and in a couple of places provided a way to compare and report uniformly across different assessments. So the first component statement of design intent is really where the the geeky engineer just communicates out in the report to the audience what the point of it is. So typical scope and limitations to the introduction to a report obviously the purpose of the NUMA given the number of PRCP revisions I've done that becomes very important the time period that you have assessed that where in the 50 60 200 years you feel confident to forecast and importantly any changes that would mean you need a reassessment. I'm not going to go through this. It's just an example of a statement to that I put in. So it'll be in the U slide pack and presentation, but it's it's not a difficult thing to cover, just important to put it there to hold the whole assessment together. Now we get on to some of your engineering stuff and I'm going to pass over to Greg to carry on. Thank you.

Thanks S. Unfortunately I'm going to start with a definition as well. So the definition of the geotechnical model the um Australian standard gives a gives a great one. So um it's the interpretation of ground conditions in a form useful for engineering design or assessment. It may contain a subsurface model detailing geological and engineering characteristics of various materials and groundwater. So obviously um ground conditions especially over mindsight scale are often complex and highly variable. Um and one of the most important tasks for the geotechnical practitioner is to review and simplify these variable ground conditions as much as possible into something useful. So i.e. a clear and logical framework that can be confidently used for stability analysis and design. The geotechnical model um obviously the ground is in 3D. It presents and communicates the ground conditions. Models can be 3D or 2D. Many of the components of today's geotechnical models are created in 3D.

But stability analysis can be either 3D or 2D depending on the mechanism and most often the model is illustrated in reports in 2D cross-sections and plans. The closure geotechnical model should include information on the landform design. So that's anything like the pit design, um the surface topography, um mining, environmental, cultural heritage, or public waste boundaries, and any infrastructure such as buildings, drains or flood levies. It also includes the geological model that will include information on the deposit type because that's quite important to understand the geotechnical conditions, the strategraphy, lithology, weathering, alteration and distribution of geological units. uh the soil rock mass model that provides information on engineering properties such as unit weights, intact and rock mass strengths, defect strengths, um and things like defamation properties if numerical modeling was to be undertaken.

We've also got the structural model. Um this provides information on the orientation, location and character of geological defects such as faulting, folding, bedding, jointing or any fabric and the groundwater model or the hydro geologology model. So that will cover information on groundwater surfaces, permeability of the rock mass, pore pressures and poor pressure responses to things like rainfall events.

There's also other factors which could be important. So the climate is one. So um a mine site located in the tropics for example will perform differently from a geotechnical perspective to a mine site located in the Arabic desert.

So just due to the amount of rainfall um a high rainfall environment is going to advance your rates of weathering erosion um and result in a lot more um poor pressure loading events on the slope which can have an adverse impact on stability. The level of detail in a model typically increases throughout the project cycle. Obviously a model developed during the planning stages of a project which is likely to only be based off a small number of bore holes is going to have a much lower confidence or higher uncertainty than a model based on excavated slopes for an operating mine with approaching the closure stage. The level of detail that's required in each of those components really depends on its influence on stability within uh a soiled or highly weathered rock mass deposit. The material strength governs the stabilities and pore pressures also play an important role. Within certain types of deposits such as residual soils um like saplites, the relic structure of the parent rock mass can also influence stability. So that also needs to be considered within soft rock which typically includes uh Queensland coal mines. All aspects are important but intact and rock mass strength and structure are typically the most critical and poor pressures um do play an important role in certain situations such as foot walls or where the stability of the wall is already marginal. Within certain mines there's certain technical features which deserve particular attention. So within coal mines um bin orientation bedding strengths whether she or not and the presence of any weak layers such as tacious units um are very important to identify as they they often play a big have a big influence on the stability hard rock stability is entirely controlled by structure um where the structural stability is marginal pore pressures and in particular transient pore pressures um play a big role on influencing the stability. So during high and prolonged rainfall events the transient pore pressures can load those structures which may be marginal and and cause movement and defamation in the slope within variable deposits. So these things like copper pfrey deposits all aspects are critical. The key is to understand what controls stability in each rock mass domain and how each rock mass domain interacts and influences the stability of other domains within the same area of the pit.

The model for closure is similar to an operational design but it needs the additional focus on the long-term effects on material properties and performance. So things like weathering, degradation, durability and erosion over time. Um this slope here provides a good case study of of why that is important. This slope here was excavated within tertiary sediments. So ligignites, clays, some reworked bassalts which have since weathered into clays. And these were all capped by an over cap of fresh bassalt which continuously recharges groundwater into the underlying clay units.

The the slope was excavated within a strip mine. So pit walls were sort of turned over every 9 months. the material was relatively stable for those short time frames required for a strip mine. But then mining unexpectedly stopped in the strip and as the material was prone to desiccation and strain softening over long time periods the slope began to uh deteriorate and experiencing progressive failure. So this is 18 months after the slope was first excavated and more recently 6 years after and you can see the effects of this progressive failure um on on the slope here. So this serves as a good example of why closure models need to include potential long-term effects on on material properties and the failing mechanisms. We're going to go through a couple of case studies now to illustrate um geotechnical models and how the different important components are presented. We've got a one case study hard rock which I'll go through and then Sue's going to go through a coal example. Um so for this example, this is pretty simple geological conditions.

It's an intrusive deposit um with sheer hosted mineralization. has various types of granite lithologies with pretty gradational contact boundaries between each. Um, but they're all very similar from uh strength and durability perspective. So, the geology model doesn't actually have a huge impact on stability. For the rock mass model, there are some localized breiated zones around falting, but typically it's all high strength. It's all blocky rock mass. Um and it's not a critical control of inter ramp or overall scale stability. It does control the bench scale to a certain degree. The hydro geological model um the VWPs installed in the slope show a depressurized slope. Um so as M progresses the slope quickly depressurized but this mine is within the tropics. Um so transient pore pressures from the wet season from those high prolonged duration and high rainfall events do have an impact on stability wherever the structural stability may be marginal. But not surprisingly as you can tell from this image um the stability is entirely controlled by structure. So that's where we focused our efforts to try and get that part of the geotechnical model as accurate as possible.

To do that we did pretty extensive photoggramometry and field mapping. Um so this on the left is all the faults that have been mapped using photoggramometry techniques and on the right all the joints. We also reviewed a lot of acoustic tele uh teleer ball holes beyond the pit slope to try and get a better understanding of the conditions beyond the pit walls. Um for any geotech and geologists there you'll see there's quite a bit of scatter um within all these these bore holes.

So it this is a very complex um structurally complex system. We then used all that data to create a detailed structural model covering the key influences on stability such as we created 3D surfaces of a major falting that were intercale or greater. We also come up with structural domains. So these areas of similar structural conditions which really helpto I guess remove the scatter from a particular area and make sure you're only making the assessment based on the data that's actually within that spatial area. And we also did a lot of work looking at defect strengths so separated by defect type then infill and um we wereable to do that from the field mapping exercises and drilling and laboratory testing.

So this model then allowed us to identify and quantify the um credible failure mechanisms for each area of the pit which I'll go over a little later in the presentation. Okay. I'm here to reassure everyone that we don't always have to have it quite that complex. This is an example from the other coal low wall. The void was about 100 meters deep, had a relatively steep floor dip, steep coal seams. Two strips were mined out by a truck and shovel and as is typical in strip mines. Some of the spoil was dumped back into the pit against the box cut. A creek overflowed into the pit uh while they were still mining caused large slips in the low wall and that was when mining had to stop. In the 10 years since there's been a bit of continuing movement at the cracks but no further large slips. So in this case strip M 2D analysis is fine. You can see that it's a very simple strategraphy not much structure which is typical of spoil. The key driver to stability in this issue this case is the strength change when that spore becomes saturated. So for the long term stability assessment we have look at as the pit lake is now and the predicted long-term pit lake level and carry the analysis out on that. Another example, this was a high wall, coal highwall for a PRCP final void. I haven't got a photo of the wall because it won't be mine for another decades. So, it's really a planning stage remembering that the PRCP planning part is to demonstrate that you can achieve

the requirements of rehabilitation. So the geological model was used to develop the the profile the different layers. Hydro geo geological modelling from the PR and the void water balance modeling gave us the boundary conditions for pore pressures that were analyzed between the two. And being central Queensland, there's a fairly good mass of published strength properties for your tertiary weathered perian, fresh perian. Those are simple model, but for the long-term planning at this stage, perfectly open.

Hopefully, we've hammered this enough that the model we cannot be certain. This is there's a high level of uncertainty in developing the model. So instead of trying to ignore it or spend an unreasonable amount of effort minimizing it, the approach is to address it and admit it. And this is probably one of the key things in both the AAP and the practice note is that we've developed an uncertainty rating sheet for the geotechnical model. There are eight model elements identified and there's a score assigned to each that reflects their relative importance and there are different scores for the hard rock and soft rock examples. uh four uncertainty levels for each of those ranging from what is nicely called introductory to a low uncertainty which would be when we've done a lot of testing and probably have the war there with observation description for each combination of model element and level and simply you sum the the score for the element times the waiting for the level at all of those together for the different elements to get an uncertainty ranking.

I won't go through this whole one, but as an example, the top line was geological model. It's a hard rock example, so that's got a score of 10 out of the 100. It was rated as medium uncertainty. So the contribution of that one is 10 times six. the an example the last component which is I think observation and monitoring it's given quite a lot of weight and we'll pursue that again later 25 and in this case it's been there there's been quite extensive monitoring so the score for that one's 25 add them all up gave a total of 67 which then comes into medium uncertainty now I want to stress that high uncertainty or medium are not in themselves bad things if it's appropriate to the stage that you're at.

What you do is take account of that level of uncertainty in selecting the design acceptance criterion which we'll get on to a bit later. And yeah, I I just want to highlight so this was um the Hard Rock example and you might notice there's two um uncertainty waitings given to the structural model. So, one of them had a medium uncertainty. We were sort of within 100 meters of the the pit walls because we could actually map the structures in the pit walls, project them with some confidence. Um but then we had a high uncertainty anywhere where it was sort of 100 meters or so away from exposures just because of the scatter in the bore hole data, the limitations on being able to know what sort of persistence um and termination relationships those structures had. Different areas of the pit can have different um model uncertainties.

Okay. So, thank you everyone for starting to put some questions through. That was a really important session I think Greg and Sue because um firstly it was important for us that this work could be broadened out to discuss hard rock mines and um the differences for an assessment that are relevant there and then also the the model uncertainty statement I think is really invaluable to this process into helping us understand the approach and um that's been great. So thank you for that. We have a question here about closure model best practice. Should parameters around weathering, degradation, etc. be site based and lab tested or regional database or are regional database inputs sufficient? Not so sure who wants to like throw that question to someone in particular or do we draw straws? I'll start. Thanks S. I'll start. One of the problems well causes for courses it depends on how important things are in the model and in the anticipated mechanisms. Weathering in particular always looks worse than it is or most of the time looks worse than it is. It tends to be a very shallow effect. And so except for particularly particular cases where you have persistent uh wheat layers, it doesn't penetrate very far. Remember reading one of the the highest rate of soil development measured is about 8 million year. So what we get on the surface is a degradation and a raveling which is a an aspect of weathering but it's only physical but it tends to look worse than it really is. the times that it will matter will be uh particularly with your residual clays where long-term properties are different and I would think in most cases and particularly for planning judgment based use of parameters is fine but there will always be a few cases where it becomes so absolutely critical that it warrants testing and this taking a lot of time here, but it's part of the the value that we haven't stressed here in that uncertainty model for the geotech engineer is that if it shows up something that we really are uncertain about, but it's important, it gives us some leverage to go back to the client and say, "Hey, we need to invest some more money in getting this bit better understood because if we do, we can improve our uncertainty rating and justify a different design acceptance criterion. So I would say most of the time general assessment certainly in the co but it's always got to be confirmed that that is a situation because there's a small proportion of cases where it really could matter. Greg, did you have anything you wanted to add? No, I think Sue answered that pretty well, but yeah, I mean it's case by case really. You know, the hard rock example um that granite's not going to weather very quickly. We can see a lot of, you know, exposures um just in natural landscapes where we know that doesn't happen.

So, you know, just I guess some justification of of why using regional analoges would be fine. But yeah you know areas like the tertiary sediment example um that I had like you know it's desiccation of the of the material is obviously big big factor. So um yeah it's it's really just case by case and whether weathering could control it does probably need that localized sort of testing.

Okay that's great. Thank you for the question. I think we might continue on because there's lots more to cover. So thanks Okay, I'm up. Um, so credible failing mechanisms. Um, these must be identified, um, and justified for each landform element or zone. So that's your crest, um, your slope and your toe. Um and I've just provided some illustrations here of of of typical mechanisms such as um toppling wedge planer wedge player with with non-daylighting and circular and rotational.

Um the practitioner will use a geotechnical model and engineering judgment to assess the credible fing mechanism. So they need to be appropriately qualified to be able to do so. Um, importantly, there's quite often multiple potential mechanisms within the same pit uh pit slope. So, you might have um the up few benches, for example, excavated in deep soil or um highly weathered rock that's going to be controlled by more of a circular style mechanism. Whereas glo globally that slope might actually be controlled by a larger structural failure um style mechanism within the fresh rock mass below. And both of those mechanisms have the potential to fail beyond the crest and therefore they both need to be quantified.

The mechanisms identified will determine the method of analysis undertaken. And again that's up to the practitioner's judgment to decide what method of analysis is appropriate and the assessment of the failing mechanisms needs to account for these changing conditions over time. So like we were saying just before the weathering um erosion um changes in groundwater is an important one for closure because you've got to not only assess the ground water conditions when you know the day mining finishes but also the final pit lake level um and you know potentially any any stages in between.

And for numerous um small-cale instabilities may be acceptable if the consequence of failure can be proved to be neg negligible or low. The consequence assessment um this aims to identify the likely harm if instability was to occur based on the identified critical failing mechanisms. This table uh table is being taken from the AUP study report and can be helped uh can be used to help guide the assessment. The um assessment ranks the potential consequence of failure from ne negligible to high consequence against three harm categories. So harm to humans, environmental harm, and property loss and damage. And the table provides some qualifiers to help guide the practitioner's selection for each. Obviously, the higher the consequence of failure, typically the higher the design acceptance criteria value should be for that failing me mechanism. Uh this is an example from our hard rock case study of the consequence assessment that we did. So what we did for each area of the pit, we defined the extent of instability and the exposure within the extent based on the critical failing mechanisms and infrastructure across that final landform. We did this for the crest zone, the body of slope and the toe of the slope. uh we then assigned a consequence category through here for each mechanism against each element of harm and provided commentary in the table justifying that selection.

We also provided a map illustrating the different maximum consequence areas across the pit. Uh this was often design uh defined by what infrastructure was um within the plausible instability zone beyond the crest. You can see this red polygon here. Uh that's an area of heritage listed significance. So obviously a high consequence for property loss and damage which we've got circled in this um red box up here. This orange zone, some of you may notice there is an out of pit waste dump quite close to the crest. Um that's covered that orange zone is covered in its own separate assessment table. Um and that's been assessed to have a moderate consequence of environmental harm just due to the fact that if the site failure was to occur in there within extending into the waste dump. Um the failure would mean there's some moderate rehabilitation u remediation earthworks required um if the failure was to extend in there just to be able to remediate that failure. Um the rest of the pit didn't really have any um any infrastructure or or anything near the crest and access was going to be excluded on enclosure and therefore the consequence of failure was assigned negligible or low for those areas.

Um the design acceptance criteria that defines a threshold for acceptable stability. So if a landform achieves the design acceptance criteria selected then it can uh can be considered geotechnically stable. There's a lot of different methods um of stability analysis and each of these different methods can produce a different parameter value for the design acceptance criteria. Um, for example, a factor of safety. The factor of safety is the ratio of available shear strength of the soil or rock mass to the sheer stress required to cause failure along the potential slip surface. A strength reduction factor or SRF. Um, that's a factor by what the sheer strength parameters are progressively reduced until non-con convergence occurs within a numerical model i.e. failure. Um, and SRF and factor of safety are typically taken to be the same. a kinematic probability of failure or PF. Um these are used for structurally controlled designs and is the likelihood uh expresses a percentage or probability that defects will intersect to produce an unstable volume of rock mass for any given slope orientation. There may also be other design acceptance criteria more appropriate for the situation. Um an example could be acceptable defamation limits u where minor movement may be tolerable. So all slopes do move. Um some mechanisms such as toppling mechanisms can move a long way before the slope progresses to collapse. Um I've personally been involved in a managing a slope which moved more than 10 meters and didn't didn't end up um progressing through to collapse. So definite defamation limits may be an alternative design acceptance criteria if small but manageable movement uh is anticipated. Again the appropriate value for the design acceptance um criteria is basically up to the the practitioner um to use their judgment or justification provided and the appropriate value um will be selected based on the model uncertainty and consequence category. So higher uncertainty higher consequence should mean a higher design acceptance criteria. So some suggested ranges for design acceptance criteria appended to the practice note and also in published literature such as the large open pit guidelines for mine closure and the practitioner should be aware of typical industry acceptance criteria for different methods of analysis and risk profiles.

um stability may be quantified using observation or observational or analytical methods. Observation methods they don't quantify stability directly but they do quantify the slope performance over time. So for example, if a slope has been standing stable for 30 years with little signs of degradation or instability, there's a good chance it will stand for another 30 years so long as the loading conditions do not change. Um Sue is going to provide an example of the observational assessment in her case study shortly. As far as analytical methods, they can include empirical methods. So these are experience-based correlations. Um benchmarking such as stability design charts. Um for example, these may show stable and failed case study slopes over different angles and heights within the same geotechnical conditions. Deterministic. So these include limit equilibrium which is very commonly used. Um limit equilibrium provides a factor of safety. It's typically applied to interramp and overall slopes where stability is controlled by soil and rock mass strength with or without some structural and isotropy numerical analysis. So that provides a strength reduction factor.

That analysis can allow for the impacts of stress redistribution within a slope as it moves. um good for quantifying progressive failing mechanisms and quantify defamation levels and probabilistic um methods for evaluating stability. So these provide a probability of failure and allows um for incorporating variability and uncertainty in in the parameters. Um these may be more useful for example with structurally controlled slope in the planning phase uh where you might only have a data set of structural orientations from B holes with a limited understanding of the spatial location and persistence of the defects and you've got to design the slope um using a statistical method to ensure only a small percentage of those defects are kinematically unstable which then becomes your probability of failure. And typically the the level of analytical sophistication will increase as the projects advance and the geotechnical model uncertainty decreases. So you wouldn't expect to see advanced numerical modeling within a project that's in the sort of scoping or prefeasibility stage because the model uncertainty is going to be so high that it's going to have a ma major impact on the reliability of the of the analysis. So I'm going to go through a case study um from our HUD rock case study now. So for that we used multiple analysis methods due to the complex conditions.

We made good use of the fact that this mine had been in operation for 40 odd years and I'd mined a series of cutbacks uh within the same rock mass at steeper slope angles that were that were actually planned um for actually being proposed for this final design. So you can see here they've successfully um mined interramp angles up to 64 degrees. The rockfall risk within that you can probably see there's not many bmss or anything like that. So that um that is something to take into account but on the overall um scale it did it did manage to stand up. To quantify this we first measured um the inter overall slope performance by assessing achieved probability of failure of the current and historic slopes. So we measured any areas of um the pit that had failed on on a multiple bench or greater scale and measured that as a percentage of the total pit wall area.

That confirmed that even when mining steeper pit wall angles had been used in the past, the inter overall slope performance was within acceptable design acceptance bounds um for mine closure. So the probability of failure overall was less than 5%. So that gave some confidence that the proposed design um angles were achievable within acceptable probability um limits so long as the structural conditions were consistent. Um so we needed to do some further analysis to quantify the conditions and make sure that there was no unstable intersections within these proposed final walls. So we did that first of all by just looking at um all the modeled 3D structures that we did. We did a basic discrete kinematic check um essentially on the major structure intersections um using all our mapped and modeled structures with respect to the pit wall aspect and location of the pit wall that they intersect. So that really helped to present and understand the failing mechanisms that we're dealing with. So the scale um you know whether they were a wedge or more of a complex mechanism um and that sort of helped feed into the more advanced analysis that we also did. This is one example of the advanced analysis that we did. So we did a deterministic 3D limit equilibrium analysis on the mapped and modeled faults um that checks for the potential kinematically unstable wedge and complex 3D mechanisms. The good thing about this particular type of analysis is that it also checks for successive block failure um i.e. a progressive failure. So within hard rock mines um often the loss of a key block in the slope or a failure of a small wedge in the middle of the slope can destabilize wedges further up the slope. So I triggers a progressive mechanism much larger than that original uh wedge failure.

So the result of that analysis was we identified areas um where there was larger failing mechanisms and in any areas where there was a high consequence of failure we modified the design to remove those larger failures from impacting the crest and keep the overall design within acceptable probability of failure limits. So as you can see this area here um is actually that area of the pit there. So, as you can see, there's still smaller, you know, bench scale instabilities, but because it's a NUMA, they're not impacting the crest. They've been assessed as as being acceptable. Um, and none of the wedges are um anticipated to impact beyond the crests there. So, that was um was a good good outcome. Now, I'll throw it back to Sue. Back to the easy stuff, the coal mines.

There's a reason I stick to the coal mines. I can tell you again this one it's a typical coal mine void in a strip mine. Uh in this case the high wall and low were on the same and the model reliability was considered high uncertainty. The consequence of instability within the new map however level is negligible as I said 2D analysis considering the uncertainty and consequence a design acceptance criterion of safety of 1.25 25 was selected for the NUMA and these show the results of the minimum factor of safety for both high wall and low wall and for both the end of mine when the pit was dry and the long-term pit lake level and just quickly there one of the things that a geotech is that you didn't get something unexpected in the analysis and in this case the minimum factor of safety slip surfaces all connected with a layer you can see under the floor which is the yabby tooth which is what we all know and hate.

So that's reassuring on it. Now, this was for the NUMA um and there's a bit of an awkwardness in that the um rehab commissioner's remit on this was specifically the NUMAs, but normally when you do avoid stability, you've also got to consider the impact not just within the interact with the joining PMLU. So in this case the stability was still pretty good but for example grazing PMLU you might uh consider that the appropriate factor of safety there was 1.5 rather than 1.25 25. Um, but the trick the the important thing is that is only assessed at the boundary of that post money land use at the grazing limit and your options are either if it's not. So, you set back the boundary of that to where you achieve it if you can or you can stabilize. So, you've got two different alternatives. If you don't meet your design sectors criteria, either change the consequences by changing where your more valuable features are if you can, for example, a road or a grazing boundary. Or if you can't do that, if it's a river, then you have to look at alternative measures to either stabilize or again you may have opportunity further investigation to improve your certainty in your model.

This observation determination it isn't respected I think as much as it should be. We do a lot. It's very normal in operations during mining to assess stability by a documented inspection. Once you've got your basic geotech model and you understand the mechanisms that could give you problem, just observing whether or not they are manifesting is the strongest indication of stability you can have. Also, if you're observing an existing wall, you've already taken away all the uncertainty between at your desk and designing in that point. If it's stable today, then it's only likely to become unstable in the future if there is a change in either to the ground stresses or strengths. So going forward for long term simplifies down to identifying potential change.

Any given observation is a point in time. Granted, but if you've got all your operational inspections and then going through the the period while rehab is being undertaken, you can build up a time and that does give you some good insight into the nature and rate of any of the changes that may occur. So to use observation as a method to evaluate long-term stability, you start off by identifying from your geotechnical model what are the credible potential future stress and strength changes be relaxation changes in moisture content those sort of things. Then give consideration to the likelihood rate and impacts of those changes and your time series observation is very helpful in that. And having made the assessment of course you continue to evaluate and check that you didn't that you everything's going as expected byation to be useful. The main thing to do is to capture uh repeated inspections or observations in a consistent form. so that you can compare this trend over time. So a performer inspection performer is the obvious way to do it. The the standard information about the the date location your existing geometry that you're looking at and then importantly observation of any stability related features. For example, is there evidence of uh tension cracking, rock falls, whether they're small or large, seepage at the face. Also, this is where you can better assess the extent of erosion and weathering and if they are occurring in any way that could trigger those ground stress changes that would lead to instability.

So this example is of a coal mine highwall photo was taken when it was 17 years old. It's now 25 years old and really there's blood oil change. The geotech model uh was quite shallow tertiary and weathered zones. You can see that the the gray the unweathered extends quite a way up. There were two sets of adversely oriented defects. They were at quite close tense spacing and that's actually a good thing because that means you tend if they intersect unfavorably you'll get small pieces of rock rather than large wedges coming out. Uh the high wall had a pretty small catchment of runoff draining out of it. The condition to date you can see there localized moderate erosion but it's all going down the face not causing any undercutting. There's no slumping or existing geotech failures. There's evidence of small localized rock falls size fragments falling off and a little bit of crest retreat but no big gouges.

So in that the anticipated future load changes is that the ongoing stress relief could open those defects those joints a little bit more. You can expect some continuing small rock falls or raveling as we call it. And of course in the long term lake level it's quite low at present. Uh and given that the base of weathering is a long way up the wall uh it's unlikely in the prediction water table wasn't going to get up there where the materials are susceptible to strength change on wetting uh no undercutting. So on the basis of all of that the determination by observation is that the geotech stability will be adequate for enuma continue that way for at least 50 years if it's been stable for 25 and there's no evidence of developing beyond that little surface rallying. uh it's reasonable to forecast that twice the existing time frame will continue in that way. Okay.

Okay. Yes. Everyone take a breath. We've got a couple of questions. Um well, we've got lots of questions but time for a couple. A question from Kane. How to account for slaking of saturated materials in coal mine voids and do you get a loss of strength over time that needs to be accounted for? I'm assuming Sue you might like to take that one. Okay, I can start on it. Again, slaking is part of this degradation aspect and it really is a fairly gen fairly shallow phenomenon. Uh you certainly are aware of it already in operationally. It tends by and large to cause a bit of surface raveling. So you certainly wouldn't promote this a particular void for water sport where someone was getting close to the level in a numa unless there's a particular and I can think of a couple but unless it's a case where a slakeable a tending to slake material is underneath a harder material and you can get the undercutting it doesn't usually cause much trouble. a little bit of retreat. Um, so it's more of an identifying if you've got it. It's really flaking. As I say, it's one of those ones that looks bad, but it usually only goes back the the wetting front only moves so far. And so, it's quite a slow thing. If you have those sort of circumstances where you're undo cutting another layer and you can then create a toppling type failure or a major wedge, they're the only cases where it's a biggie. Um there's a lot of ways of assessing slaking for the purpose of operational. They're all quite simplified because we want them quickly. It's I suppose I didn't mention with weather and one of the hardest things is how do you test something now for its condition in 50 years time. So it's more building on this uh existing but we've got high pie walls in the basin that have been there 50 years now. Now, that does they aren't always representative of all of them, but we've really got to use the field tests that we've got to draw those assessments.

Thank you. A question for Greg. How do you take the influence of mining activity to the rock stability into consideration? The hard rock model seems to be based on initial geotechnical investigations instead of postmining investigations. So the hard rock model that mine is in operation so a lot of the data is being I guess it is a progressive rehab like a really um it's a quintessential progressive rehab um closure plan the data the model confidence is such that we basically need to continue mapping continue collecting data all the way through mining for this one. Because as you saw in those um initial investigations, there's there's a lot of scatter. So we can't really reliably predict exactly what the final um hold conditions are going to be. Um we do control so as far as mining related um disturbances, we do try and control the blasting as much as possible to reduce the impacts of blast effects. Um we monitor the pit walls using prisms um and radars. So any areas that look like, you know, they're showing any subtle movements, we'll have a more detailed look at those and what the potential impacts on the the final closure are going to be because it's it's a it's a long mine life. There's going to be opportunities if there are any areas where the conditions were slightly different and maybe lower factor of safety than first predicted. We do have the ability to potentially back fill, step the design out, that sort of thing. But we are monitoring the slope very closely to be able to do that before it gets to the point where I guess it starts failing. Um yeah. Okay. Thank you. I think we might continue moving on because time is starting to catch up with us and we have a little bit more to to hear about. So we'll continue. Thank you.

We are on the homewood run. uh this component this element of the assessment a safety assessment uh those who are familiar with safety ALP as low as reasonably practical implies that even if your evaluation says that your slope has met the design acceptance criterion if there are additional measures you can apply with reasonable ease and costs that reduce your risk still further then good practice is to apply them. So to do this part of the assessment you basically use the same hazards you identified in the consequence category assessment in the added context that you now have the outcomes evaluation but it's still about okay what are the foreseeable changes going forward. This assessment is typically qualitative. The aim isn't to quantify the remaining risk but to identify any measures that can reduce it a bit more regardless what what it is. The typical measure that come out would be access exclusion particularly for nums you know your bum your fencing signs and there's potential to document any constraints in a land management plan that would be attached to the land tenure the title at close of mining.

In the practice note, we have deliberately not provided a method and left that for the Geotech AQP's discretion. The main thing we wanted to get in here at present is that that assessment is done. So you don't just finish the assessment with it's met the design acceptance criteria. It's that extra given that we know it's longterm, are there a few extra things that we can add? start. Uh last component is the the certification which really is is formalizing what is already our professional responsibility. Stability assessment is considered a professional engineering service and therefore it is governed by the professional engineers act that requires that the work be undertaken or directly supervised by competent geotechnical RPQ. It requires that our communications and our evaluations in the assessment report are specific and accurate. And this is important because it comes down that given the uncertainties going into the future if we were to write that we guarantee stability indefinitely that could be taken as a bridge because that's stating something that we can't be sure of and that's where this whole process of focusing on the reliable and repeatable process came from. As I say, we're already bound by those standards under the professional engineers act. Certification is a way to justide clarity and confidence to the reader be it the client or the regulator. So you state clearly that accepted practice is followed although as an RP I can't imagine what what you would have been doing following accepted practice. You get a chance to restate the outcome of the assessment away from all the words and and geeky stuff put in the middle of the report and reiterate your limitations and assumptions which may just be by reference back to the statement.

Uh we put in a quick bit of monitoring after closure just for some guidance. Obviously particularly when you've been designing nothing beats uh the proof in the pudding of observing it at the end uh it validates your assumptions confirms your performance one hopes or gives you an opportunity to modify if necessary. The typical way of monitoring are obviously visual inspection, movement measurement sometimes and groundwater. But which of those measures are justified or required and the frequency should be at the judgment of the ACP? One would expect them however to reduce over time and to largely cease before they surrender.

So takeways that we wanted to convey today is that long-term detailing can't be certified. However, we can certify that we have followed a reasonable process in the evaluation and that process needs to be riskbased but also depends heavily on the judgment of the GTH AQ. Next point is that the design acceptance criteria has to be based on consideration of uncertainty and consequence and I suppose the identified uh instability mechanisms. The upshot of that is that factor of safety is not always the appropriate measure and even if it is no magic about 1.5 that may not be the appropriate dome. Observation analysis and risk controls must all contribute to managing numvoids and the assessment report to be to be able to provide the confidence that the regulator requires.

It has to be transparent, defensible and reviewable by an independent. And I think that just about gets us to the end. It sure does. Thank you. So we have some questions um which I'll start to roll through so we can use our last 15 20 minutes wisely. Um so a question from Darren. Thanks Sue. In the world driven by processes that lead to penniplation that is the process by which a relatively flat land surface surface is produced through a long period of erosion. What is a realistic time frame for applying GA technical safety assessments post closure? And that sort of circles back to one of your earlier slides um that we had. Geotechnical stability is yeah it really will be somewhere depending on your particular model your mechanism your data something in the range of half a century to two centuries is about the limit. We just don't have verified information any further. we have indication geomorphology but we certainly can't quantify any further than that. Um yeah I came across this when in the early days working with a geologist or geologists on some of these stability assessments and they throw over their shoulders and say it's all going to fail. Well yes in geologic time but we're really not thousand years. So it's really and that's part of this process is being honest and upfront and saying this is the time frame over which we believe it's reasonable and that our that we have confidence in our evaluation and that's it. It's at best a couple hundred years depending on the material quite often less than. Thank you.

And that's a question that a lot of people have isn't it? So, it's good for us to be able to discuss that. Um, we have another question here and maybe Greg, you might be able to answer. I'm not sure. What factors should be considered when determining locations for PO installation and to monitor the geotechnical stability parameters? So, where should what should we think about when we're thinking about where to put our POS? So, I guess the short answer would be anywhere where the pore pressures are playing a pretty large role in the stability. So, um you may have an example where you've got a hard rock and you know you're worried about a certain um wedge configuration or something like that and where you don't want any high pore pressures acting on that wedge. Um therefore, you would look to install pometers either side of the fault um behind the fault. So, you know, to make sure that we're not getting high pore pressures behind that wedge and and essentially sort of pushing it pushing it out. Um other examples maybe there's there's been um mine sites where I've been involved at where to in order to achieve an acceptable factor of safety you need to achieve a certain amount of depressurization. So if you've got hydrostatic conditions the factor of safety may be close to one. If you've got um much more reduced pore pressures it the factor of safety is 1.5. It can be that extreme. So you'd obviously need a lot of pometers in in a rock mass or a soil mass like that to prove that you've achieved the pole pressures um which are required to meet that design acceptance criteria. So um so yeah quite often they are targeting certain certain stability situations. um if there is no you know nothing really to worry about it might just be a sort of general monitoring of you know making sure that the porpas aren't astronomical in the general rock mass but yeah they're normally targeted towards um a certain um requirement thank you.

A question here about 2D versus 3D modeling while 2D modeling is faster for specific details 3D offers comprehensive visualization My question is, how do you determine whether a 2D or 3D approach should be used for a specific project based on your professional experience? Thank you. I'll open the floor. I'll start with the easy. I'll start with the easy stuff. In the first case, it's about identifying your stability mechanism and is it can it be well enough approximated by 2D? social strip lines are often that ex except you hit particular features like faults in it 3DS 3D then you've got to go for it but we do have a problem particularly with limit equilibrium analysis even though it's a 3D situation we do not have uh with body of experience confidence in assessing in the appropriate design acceptance criteria. So quite often I think if you if you honestly believed 3D was your best, you'd put it there, but you may also do a 2D because we have a lot of experience of knowing that if we this factor of safety in a 2D, then we're probably going to be okay anyway. So we're still in the process with the 3D of building the body of knowledge that gives us confidence about what the appropriate factors of safety are. There is some guidance in the uh in the appendices to the note but it's quite awkward there. It's also a case that one can't really do through the analysis until one's got a quite detailed understanding of the profile and Greg may be able to talk now about that just briefly. Yeah. So our take our hard rock case study with you know the 2D analysis we sort of we we did it based on anything that's uh any faults which struck fairly parallel with the wall. Um but it didn't really give us a realistic um idea of the actual stability because all the instability mechanisms really were more 3D. So they more wedge they're more like complex wedge mechanisms. So they have to be analyzed in 3D um for those sort certain situations.

The other type of situation might be where the pit topography um has a big impact on the stability. So in the case study that I provided in the practice note as an example, so there was a uh a mine which had been already excavated in care and maintenance. Um there was a diversion drain back from the crest and we did 2D LE to start with in the end wall but it was a really sort of I guess narrow um circular pit and the 2D analysis was really quite conservative because it didn't take into account any of the effects the positive effects on stability of that 3D buttressing of the pit wall. So we also did 3D limit equilibrium as well um as that. So, so yeah, 3D really is required wherever it's a 3D problem with the pit wall topography or it's a really 3D problem with your failure mechanism.

Thank you both. That was good. Um, we have a question now that's a little bit longer. It's like a two-pronged version. So, how is the effectiveness of changes made to the model assessed when a model shows medium or high uncertainty? So, I'll read that again. How is the effectiveness of changes made to the model assessed when a model shows medium or high uncertainty? Additionally, what is the accepted level of residual risk for the numerous? Is there always going to be a degree of uncertainty? And is this determined through a standard framework or on a case-byase basis, which sort of brings us to the point of this work in some ways, doesn't it? Um, I'll just throw that across to you both. Okay. Um, so I I'll I'll have a go first, Sue. Um so the effectiveness of changes made to the model assessed. So um the table does give some um some guidance some qualifiers um of where you you know where you might be able to sort of move it from a medium uncertainty to low uncertainty and that might be more localized sort of testing further investigation. So if there is something that um you know if you look at your failure mechanism and for example in a coal mine it might be it really depends on the bedding strength. So whether the bedding strength is sheared or non-shared has a big impact on your factor of safety and final design acceptance criteria. So you may do targeted investigations to quantify whether that bedding sharing is present or not. And that will then help you move up through um move down sorry to a to a low uncertainty um in your model um in your model certainty um criteria.

Um what was the second part? What is the accepted level of residual risk for these new? the residual risk I guess comes into you know the design acceptance criteria um if there's a lot of uncertainty and the risks are high then you're going to have to take that higher design acceptance criteria which allows for that I guess the um you know the sort of I guess all the uncertainty within within your models um uh if there's going to be a degree of uncertainty determined by a standard framework or case by case basis. And yeah, I think I think everything the the thing I've learned about these closure studies is it's almost always case by case basis. I've I've been involved in probably about 10 different um PRCP closure projects and the acceptance criteria and the method of analysis and everything that we use is almost slightly different in each because it's all got different rock types, different failure consequences, different mechanisms and different I guess residual risk from um from a potential instability.

Hopefully that's answered some of it. Okay. So, I'll slip a little bit in about the residual risk which is uh there's an existing version. It's under redevelopment again. But the residual risk framework for the government is uh the basically identifying not the cost of necessary rectification if the identified hazard occurs um b context of the probability. So if you were uh I can't in a numa where there's not much around it just some grazing in woodland I can't see I I would be surprised if the residual risk quotient which is a repair times likelihood is would trigger the quotient and go to a a payment the sort of place it's very much different in towing stands and things. But if you had uh a high value feature near the void, for example, a major public road or a water course, that's where the residual risk takes account of that that even though it's an acceptably low probability, you still have to make allowance for it there. So there's a bit of scope for playing. The lower the lower the remaining the lower the the more conservative you are in your closure then the less likely you are to trigger a significant residual risk. But the way the regulation in the PRCP whole framework is now it it's chased down quite a long way. you know this the whole idea of selecting design acceptance criteria in the context of those for example if you had the river nearby has already driven it down quite a long way and I don't think there's a little bit of scope but not much for the the mine owner to take on a high or to impose a high level of residual risk at the end if you do it will cost you in the residual risk payment. So quite rightly the the combined mechanisms of the the PRCP closure and residual risk are drive uh a practical minimization. I mean you can't get down to zero risk but it is quite a low level if it passes through all those gates. Thank you.

And that was a really thoughtful question that made everybody think hard as we answered that. That was good. One final question for the morning. How do climate change considerations play into the assessment? So, um, we've got a surface water team, um, at PSM and I know that they do have to consider the impacts of climate change on certain rainfall modeling and flood modeling. So, that's probably, I guess, the biggest impact. So, um, that would be, yeah, your flood modeling, your levy design. Um, and then we would need to take that into account to work out um, if there's going to be any over topping events of the levy based on climate change scenarios. Um, and then what's the potential erosion effects of over topping of the levy and and that sort of thing. Um, I think based on my experience, that's probably the main impact um, on these closure assessments. And I'd point out that if the climate change assessment quite rightly impacts your levy, that's where you put the attention, not the NUMA. Um so your levy design rather than where it will catch in a a void or enuma is where under current assessment you do not require flood protection that you might under climate change and also that it can change your predicted long-term pit level. So they're the they're the two aspects. It doesn't climate change doesn't really in in the scale and in the time frame of climate change, it doesn't affect the geotechnical properties or model. It changes the water related loadings that you might have to consider. Great. Thank you. Well, I think we've come to the end.

It's been a very wholesome discussion and a busy morning. Um I do appreciate everyone's help in pulling this presentation together today and also to the folks who've joined um we'll have come up on the screen soon a feedback survey so a QR code um I would encourage you please to click on that with your phone your camera on your phone it is only a short survey just a couple of questions won't take long but it really does assist us as we um continue to develop our workshops uh thank you to Greg and Sue and Sam, it's um been very interesting. We have another workshop coming up next week actually on a technical paper we published last year on um creating alternate growth media if you have a top salt deficit outside. So if you haven't already um registered for one of those events, please do so. But in the meanwhile, thank you everyone for joining us. Um it's been a pleasure and we'll see you next time. Thank you.

Thanks.

From Queensland Mine Rehabilitation Commissioner