 Hello. Hello everybody and welcome. I am Pedro Arduino. I am a member of the National Academy of Science, Engineering and Medicine's Committee on Geological and Geotechnical Engineering, known as COGGI. COGGI hosted a webinar on September 5th of this year in which Dr. Alan Maher gave an informative presentation on geotechnical aspects of tailing dams and their failures. After which we spent a few minutes responding to questions from our audience. Alan had the time to respond only to a few of them that we received. So he has generously agreed to spend an additional amount of time with us responding to some of these other questions that were presented. So just to introduce Alan again, a quick bio of him. Dr. Alan founded and leads Geocom, one of the foremost providers in the United States of America, of real-time web-based performance monitoring of civil engineering structures, including dams, deep excavations and tunnels, among others. He also has extensive experience in testing to measure the mechanical properties of earthen materials, designing earth structures, determining the causes of poor performance of geotechnical structures, developing cost-effective remedial measures for travel projects and mismanagement. Dr. Maher has an extensive experience evaluating the stability of tailing dams, the topic that he of his presentation, and other ways to retention facilities and monitoring their performance. He's an elected member of the US National Academy of Engineering and of the modes. Please keep in mind here that any conclusions or recommendations provided by Dr. Maher are his own and should not be thought of recommendations from the National Academy. So with that intro, thank you, Alan, for being with us. I want just to mention that we have like 85 questions. We've responded very few during the webinar and maybe we can address some of those right now. So I will try to do this as informal as possible and trying to be as accurate to what the question was. So I will try to read them as they were presented. So the first one of the questions that we had or the first one was, you seem to be talking about liquefaction as a phenomenon that goes after failure of the dam rather than as the cause of failure. Could you please say a little bit more about this? Yes, I think that this is somewhat misunderstood in the industry. You know, my view is that almost all known liquefaction failures or described as liquefaction failures are actually the result of stability failures of the dam that then where liquefaction then follows. If we have a dam, a barrier that's containing the tailings designed properly with the proper factor of safety for undrained and drained failures, then it's the failure of that containment. If it stays in place, liquefaction really cannot occur in static situations. So generally liquefaction follows the loss of stability. Seismic cases might be a little different where the shaking can actually liquefy the tailings that can increase the pressures on a barrier that it has marginal stability and as a result create a failure, but the seismic is a separate special case. Okay. Here I have a kind of a long one to read a little bit, but I think it's a good one. And it reads in world mind tailing failures estimates that there are at least 18,000 enclosed tailing dams in the world, most of which are of the upstream construction type, which is generally a knowledge to be unsuitable for storage of material susceptible to static liquefaction. We've seen the failure record and now in the Church of England disclosures that upstream dispositions are continuing for type of materials known to be subject to static liquefaction. A big log goes that upstream dams shouldn't be used for any materials that are less than 60% sand and the granulometry curves of the failed dams clearly show that he got that right. We have all the science, though not the technical expertise in sufficient numbers. How can we go about identifying and prioritizing that the de-risking of recently at risk dams? I estimate that at least 3,000 worldwide require this in-depth analysis urgently. Can you comment on that? Yes. I think there are kind of two points out of that question that are relevant. One is, I think I heard a statement that in general upstream dams or tailings dams constructed by upstream methods are not advised. I think there's not consensus on that. There are some belief amongst people who understand how to design these properly that a properly designed upstream tailings dam can be just satisfactorily safe and perform quite well. So we need to educate people about how to do these properly and make sure they get built properly. In my view, there's no reason why the upstream method has to be abandoned. But there are many out there as the question implies upstream methodologies used that are not safe or have marginal stability. How do we sort through all of those? That's a challenge. And to me, it would be great if we could add to Steve Vick's screening recommendation where based on gradation we can determine whether the material might liquefy or not. If we could add a couple more screening methodologies to that so we can help narrow the number of risky facilities, that would be great. There needs to be some work done on that. I think I'm reminded in this of what the Corps of Engineers did here in the United States in the 70s when we had a recognized dam safety problem with water retention dams where they underwent what was called a phase one assessment where people did some screen had some screening guidelines and tried to rank dams mostly by risk and from that then went to a phase two investigations for those that were more risky. I think it would be good if we could identify something along those lines that get applied to these tailing stands, particularly those built by upstream method. Okay. Let's go a little bit on some questions on monitoring. So are there available inspection or monitoring methods that could have been used at the, for example, Mount Poly or even the recent facilities in Brazil that could have been identified, that could have identified the problems that led to the failure. What are the emerging techniques that could be implemented for future cases? Yeah, this is this is a very good and interesting question and interesting area. I think the first of all, we have to be really careful in designing monitoring programs that we're really looking at specific failure modes for a dam. Where are our uncertainties and what instruments can we use that will give meaningful measurements? There's a tendency to just throw out a bunch of different types of monitoring devices common across all dams and then hope for the best. I really urge people to spend some time and effort designing your monitoring program to get after the information you need. In the case of Mount Poly, I was not involved in that, but I have reviewed the excellent report that was done afterward by the experts that looked at it. And that was a failure through a soft clay scene in the foundation. I believe instrumentation would have given an earlier indication of movements unexpected movements in that foundation, something like a sloping thermometer, for example, which is a vertical pipe put down in the ground and then you take readings of tilt of that pipe, integrate those to tell you how lateral movement is occurring with time. I believe that that weak play scene would have developed shear strains in advance of the failure and would have given warning. There are also when we're looking at the stability of the barrier, it's of the barrier dam itself, a key part of all that is poor pressure. And so trying to put in poor pressure monitoring devices called piezometers at enough points that we get meaningful data to tell us what are the real poor pressures on the critical failure surface is a very useful tool. In the case of Brumadinho, this is a unique or is a case where with upstream method of construction containing very loose tailings that tend to fail in a brittle mode. It is really hard to get advanced notice out of the instrumentation of movements because the movements prior to failure are very small. And generally those failures are involved a build up a poor pressure. I don't know if that's the case in Brumadinho, but generally these kinds of failures are builds up in poor pressure, which is like an unloading and deformations and strains that occur there are quite small. It's very hard in many of these cases to expect that deformation monitoring of the tailings themselves is going to give you an adequate warning. But piezometers telling you what the poor pressures are can really help inform what the actual factor of safety is. And if that drops well below 1.5 then that's a good warning to me. This is good. Actually, you have already answered a few of the other questions that were here, but there is one that struck me. Suppose that you have now a limited budget. Is it better to spend more in-ground investigation to capture weak layers in the foundation or geotechnical monitoring by raping cores, piezometers and binometers? Oh, that's a tough question, isn't it? You know, the tradeoff we always have to wrestle with. You know, I personally favor knowing what is at the site, knowing what the conditions are, making sure I understand what is there that may drive stability. To me, if I don't have that, even if I go and put in instrumentation, it may be difficult to understand or interpret what the instrumentation is telling me. To best use instrumentation, I've got to have a good mental model of what that site is and how it may perform. So I guess I would prefer defining the site conditions and the parameters first and then still trying to get a little money for monitoring. But I also say don't get beat down by limited money. We have on these projects a potential on some of them for very significant risks. We need to inform our clients, the owners of what those risks are and what the value of exploration and monitoring is. So our clients can better understand their risks and can assign appropriate amounts of money to help gather information to manage those risks. And so a follow-up question here is who should be doing the monitoring? Who is the authority in the U.S. and maybe you can comment in other parts of the world that you may have seen. Who is in charge of doing this? Well, it's rarely authorities. I don't think that's the proper role of authorities. So we can cross that one off. It's ultimately it's the owner who's paying the money to do this. And so they have a vested interest, but many owners don't really have the technical skills and capabilities to carry out effective monitoring programs. Some do, but many most don't. And so it gets to a someone specialized in the monitoring area who knows what types of instruments are appropriate, what measurements are significant or meaningful to us. And then how do more importantly too is how to interpret the data that come out of these monitoring programs. I see a lot of failures in that last step. People get the data and they don't know what to do with it. So I think the answer, short answer to your question, Pedro, is somebody who has the knowledge and capability to do it and know what to do with the data. And in terms of the, now I have a question here that it was more on the regulations. And do you believe that the federal guidelines for dance safety are the ones that should be applied for tailing dance? And if not, what is the most significant factor that should be addressed first? Well, just speaking as an engineer, who understands how these things behave and work, the federal guidelines that have been put together recently under the joint efforts of FERC and the Army Corps of Engineers in a Bureau of Reclamation are commendable, I think. And if followed, produce dams that are safe. I'm of the personal belief that the technologies we use to design and construct and operate dams to store water are entirely relevant to dams storing tailings or any other material that the release of which could cause harm. So I think we have to wait for the industry and the regulators to try to figure out how they might best move towards something like those federal standards. They basically have been around and served us well for a long time in the water dam business. So to me, they're, if you have, if you need to go someplace, go to those and they'll give you a good background basis for designing a safe dam. Yeah, and from that perspective, from a regulatory perspective, what do they or the regulators or which where should we be asking for to ensure we are getting correct information to assess barrier safety for mining companies? Yeah, that's, that's a good question too, Pedro, in that unfortunately, a lot of regulators, you know, they're thinly staffed that they might not have the technical expertise to get into what is really some pretty interesting and complicated soil mechanics and dealing with tailings. And so it's quite a challenge. You know, as I think that the bet we could hope for on average a regulator to be able to read a report from a mine owner, prepared by a qualified geotechnical engineer and get a sense that whoever did this work knew what they were doing and that the results are reliable, believable and can be be used. Some regulators are capable of actually performing stability analysis, for example, but that's rare. I think we really have to rely on getting competent expertise and specialized consulting firms to address these failure modes and dams and make sure that they're all dealt with properly and that dam safety is kept in control. So, so you mentioned it's a complex soil behavior problem that we are facing here. So we have a couple of questions related without soil behavior, soil testing and a lot of them are related to what test can be used to estimate contracted behavior. Can we use a lab test or CPT or other in C2 test? But one that attracted all my attention is how do you even if you have that, how do you determine whether a material is contracted or dilated in a highly variable material? What kind of lab or in C2 testing do you recommend? Yeah, this is a real challenge. You know, I think in the presentation, I made the point that it's very important for us to be able to examine the different soils involved, potentially involved in stability and separate them into dilated behavior. That means they want to expand when we try to share them or contracted behavior, which means they want to decrease in volume when we try to share them and under certain circumstances, that volume decrease causes liquefaction. And I said, conceptually we understand those those points as specialist in soil mechanics, but to actually try to do test that very, very reliably discriminate between those two behaviors is hard. Our tools have a lot of uncertainty in them. And so while conceptually we know what needs to be done to actually achieve it on a tailings dam can be quite difficult. So that is a preface techniques involve field tests, particularly comb penetration testing. And there are some semi empirical methodologies that have been developed that you can take your penetration test resistance, throw it in a spreadsheet and get a curve that nicely divides the measurements into layers or soils that show contracted behavior from those that show dilated behavior. And you know, that looks that's very nice. It's very appealing. It's very straightforward and almost any engineer could do this simply taking the data file from CPT testing, but I caution people that that divider line is really not well defined. And it could move around based on material type. So we know what to do there, but it's just not as precise a delineation as we'd like to have. We wind up with a lot of uncertainty. The specific question is how to deal with the variability. You know, the cone penetration test is one of the best we have to reveal that variability because it can give us a measurement every couple of centimeters. But we're still going to be left with engineers as you know, having to make some some call at some point as to where is that dividing line between contractive and dilated. And we can do that with the help of lab testing. Some people believe we can't do lab testing because we have a hard time getting samples of tailings that are not undisturbed that are but with good techniques, you can improve the success right there. A lot of drilling is not done properly. And therefore your destined to not get good samples that can be overcome. Or you can try to get disturbed samples and reconstitute them in the laboratory to feel conditions and then run tests on that. The laboratory testing we have control over stress and drainage. So we tend to be able to better define the difference between contractive and dilated behavior. In my practice, we really do all these things. We do field testing. We do lab testing. It's all trying to help us get ourselves confident that what we have to have for density or void ratio in the field to give us dilated behavior is something we we know and feel comfortable with. It's no simple answer here. So there is there was a question that related to that because we talk about we have the tailing dams and then of course they are there. We look at them and get samples between city test and go to the lab. But the question was what do you recommend for determining tailings properties before the tailings are being produced? Oh, that's a tough one. That's a you know, you go within a given tailings deposit that's been around for for many years and stuff varies all over the place within that that singular deposit. Why will the source of the oars change over time? The milling processes change radically over time. The way the materials get deposited within the retention area can change quite a bit. So you just got a hodge podge of materials that may vary all the way from coarse grain to very fine grain both horizontally and vertically. So what are you going to do beforehand when you have no clue as to what that process what the ore in that process is going to produce for tailings? You got to look for you know some operation someplace maybe in the nearby area that has similar ore deposits and similar processing facilities of what you're using here. Take data from that situation. We've even gone on to other companies sites and taken samples and tested them to be able to try to characterize what our new site might might have for properties. And then in that case introduce a healthy dose of conservatism really and selecting design parameters for the beginning of the operation. Build in a design that might have some flexibility so that as you start producing tailings you can adapt the construction methods or maybe the design itself to what you're actually producing. But I think a key part of this question too is how do you track and monitor the characteristics of the tailings over time so that you are ensuring yourself that what you're getting is consistent with what you used in the design. And we fail in the industry is a lot to do that. We just we start out with the design and we start doing things and the milling processes change and we don't recognize or the placement processes change. We don't recognize how those things might be impacting the safety of the dam. And so somebody I was asking here when the tailing materials the mechanical response of the tailing materials is very similar very similar to that dose of sands. So with what we know about sands is can we infer more or less or they are different. So I think first of all I should make a distinction here between cohesive non-cohesive tailings. That's very important and most of my remarks here have been focused on non-cohesive non-plastic tailings because those are the ones that are really subject to potential liquefaction. And so if we continue along those lines of non-cohesive non-plastic tailings which is the majority of ores then and we have to recognize that in some cases some of these materials may develop some cementation and so leaving that out as well. So now we're down to a non- cohesive material that is primarily frictional in its sources then they behave very similarly to the loose sands and a lot of the studies of liquefaction behavior of materials are done with with sands because they're easier to work with and don't have some of the environmental contamination concerns. It's easier for students to work with sands and tailings and do this research. So let's transition now to some other questions that are not just related to soil behavior more with design and dam performance. So the question was I would like to know if tailing dams can fail in the short term during or immediately after construction or not. If they do what are the stability requirements for the short term? Yeah, good question. Of course tailing stamps can fail during construction. Many of the failures that have occurred have occurred while material was still being added. One of the examples I gave in the presentation was at Tyrone down in the Southwest United States which was a copper tailing stamp. They were very actively operating the facility and they started adding tailings at a much faster rate faster than how the tailings could respond to that added load and that drove the failure. So, you know, we absolutely have to be very concerned with stability during the so-called construction which in tailing stamps usually is we're constructing and we're filling at the same time and that's different than a water retention dam where we typically construct it, complete it and then fill it. The general ideally during stability during this phase ought to be in my mind the same as once it's completed stability requirements and so I'm generally pushing for a factor of safety of 1.5 myself. There are some guidelines that come out of the dams for water retention that says you can use a factor of safety of 1.3 during initial construction but that's keeping in mind that the consequence of failure during the construction of a water retention dam is quite small really to offsite facilities because there's no water in the dam. If we get a stability failure it's confined to the project itself. In the case of a tailings dam you know we're constructing the dam it's you know it may be partly torch its final design height and we're putting water and tailings behind it and so to me this is more like what we use in earth dam design where the required minimum factor of safety is 1.5 but there are some regulatory agencies which will permit and allow a factor of safety of 1.3 during this construction phase. There is a question here on C-page and it reads based on your experience which C-page control method would you recommend or have worked well before if you can comment on those? Yes and this is you know any as we design water retention dams you know a good dam designer knows that a key part of the design is to control how water flows through that dam and so one of the primary elements we use for that is a drain to capture water that gets through the dam and safely remove it from the dam so that we don't get poor pressure buildup in the downstream half of the dam and we don't get seepage induced fine migration or so-called piping or internal erosion so the common methods there would be drainage blankets across the bottom of the dam between that on top of the foundation reaching well back in from the toe and then perhaps having to supplement that with inclined or vertical drainage blankets to capture water that's coming through the dam at higher elevations these are to me are really important elements that many upstream many tailing stamps don't really have adequate provisions for and hopefully we're recognizing that in the industry and future dams will make better use of these drainage control features. So here there is a question where I will add my grain of salt here because the question was very short and it was mentioned several times and the question is imagine Dr. Morgan stand was referring to an industry wide shortage of adequate skill levels in stability analysis in design are we missing something in our courses are we not teaching something are maybe we are not concentrated on some things and not in some important topics would you recommend us to do something different? I think this is a challenge for our geotechnical discipline and that I this is my impression you know when when I go to hire a new engineer just graduated out of the university and I start asking them these specific case situations how do you select strength for this case many of them are really puzzled and be fuddled they're full of knowledge and information on all kinds of things they can talk about probabilistic methods but the basic fundamental stuff don't have a detailed working knowledge of basic assumptions and limitations and stability analysis many of them don't just really have it have it deep enough in them that they recognize how that's meaningful in practice that's something that comes with experience and with experience under mentoring and guidance of older or more experienced engineers I think we have to be careful you know a young engineer can pick up a stability program and make it really zing and zing in a matter of a few hours but they don't have any concept of how to get poor pressures to put into that program or how to take measured poor pressures and adequately describe those within that stability program or how to choose the appropriate strength parameters for example I see a lot people taking triaxial strength test results undrained strength test results and using them in undrained stability analysis that may not be the right strength I would prefer to use direct simple shear test which is about two thirds of what you get out of a triaxial test these are some important details that the specialist in the field understand but the typical student coming out of the university doesn't really have a full grasp of yeah so maybe doing more internships or calls before or during a master program for bringing more industry into our program would be a good solution this is something that I was I was trying to add and in the so we are running a little bit out of time but let me show a couple of more questions here and one that it appeared to me interesting is one that says a hard dry stack tailings dance a substantial improvement what are the problems of those yes just so everyone understands what most tailings dance historically have been filled by hydraulic means where we just mix the tailings with a lot of water they may have like 15% solids content you dump those out into the reservoir and let them settle out that leaves a very loose structure so over the last few years people have been trying to overcome this by somehow consolidating the tailings compressing them reducing their water content increasing their solids content to something that's more truly like a solid it's more compact it's more dense and ideally would not liquefy and then you haul those hydrically placing them by pipeline you haul them by truck or other piece of equipment and you put them into an area like a landfill that's called dry sacking and that's definitely an improvement in that we're getting a material that's not in a very loose state it's going to be more like medium to dense depending on how it gets placed and so there's an inherent it costs a lot more and there's an inherent belief that if we do this we're not going to have any problems but I would like to caution that that you know it's still they're not dry they're called dry stacking but the tailings aren't dry they still have water in them and so if that process hasn't been controlled carefully you can wind up with layers or zones within the dry stack that are higher water content they're going to have lower strength you will get changes in the degree of saturation as you add more material on top and cause these these early place materials to be compressed which means their degree of saturation is going to go up or there's a tendency to and if you're in an area with rainfall where you might get rain on a layer overnight and then it gets covered and now you're left with a high water content loose material in the dry stack that becomes a potential plane of weakness so dry stacking overall yes is the potential for us to significantly improve and or certainly avoid the the reduce the potential for liquefaction failures but they're not a total cure-all for our problems in geotechnical stability not the holy grail exactly yeah unfortunately not and and we're still I mean the industry is working hard to find ways of doing this that are not for optimal costs so one last question one last question here and then I will let you go so and with the ongoing changes in climate are other considerations and design parameters needed to make them safer what do you think about this yeah it's an interesting question you know all of us are kind of trying to wonder what all this you know what are the real engineering impacts of climate change you know we design things that some of them are to last in fertility so what how do you do that and so I don't think it introduces anything conceptually different than what we already do just add some challenges to what we already do I think climate change in terms of dam safety and dam stability you know are things like higher inflows into dams so that the like the potential for failure by over topping it's up that you know we haven't designed enough reservoir capacity in the dam to handle the inflows that come from these increasingly large storms so increasing probability of overtopping that also brings with the potential for increasing pore pressures within the stored tailings in the dam because you put more water on top so pore pressures within the dam are going to increase and that's going to decrease stability so we got to make sure that we're taking that into account and and then we're potentially creating more wet conditions during construction that make compaction difficult and then leave weak or loose zones that can later on contract and lose strength so that you know if climate change means more wet days a lot more rain a lot more difficult construction conditions then that's probably challenging our ability to get things done right and we're going to be left with some defects that show up later so we better take a look at that so I think that we can reach in more than 30 minutes that's what we allocated for this so I think that we will stop around here I just want to thank Alan for the first representation that he gave on September 5th and for the time he took today to respond to more of the technical communities questions so yeah any final word that you want to say no just let's let's all renew our efforts to do good engineering on these and try to improve overall safety for the good of our communities and the environment excellent so just want to mention that to the audience that if you if you have questions about COGGI or ideas for topics you would like to seek over in future webinars please reach out to Samantha and Maxino with the email that I think is provided in maybe in this screen so with that thank you again Alan very much and goodbye to all goodbye everybody thank you Alan