 Welcome everyone to our next installment of our committee on geotechnical and geologic engineering webinar series. Happy to have you here. My name is Marty McCann. I'm the current chair of the CAGA committee, which is a standing committee of the National Research Council at the National Academies. Before we get started, just wanted to pass along a few, a few messages. One is I wanted to thank our staff. Sam Maxino, who's the director of CAGA. She's a senior staffer at the National Research Council, and Remy Shepeta, who is a staff assistant for the committee. They take care of all of the organization and logistics that makes this happen. So Sam and Remy, thank you for your, for your time and efforts in putting this together today. I'd like to now turn the webinar over to one of our CAGA members, Professor Pedro Argrino from the University of Washington. He will, he will moderate the session and go through a few logistics about asking questions and then introduce our speaker. So Pedro with that, I'll turn it over to you. Thank you, Marty. Good morning and good afternoon everybody and welcome. A lot for joining us today. As Marty mentioned, my name is Pedro Argrino and I am a member of the Committee on Geological and Geotechnical Engineering, also known as CAGA. I have the pleasure of serving as your moderator for today's webinar on mind-telling stamps, together with John Stamatakis, who will help me collecting and organizing some of the questions. We are certainly delighted to have Dr. Alan Mark can join us. Before turning the microphone over to Alan, I want to review a few things on how to provide questions. To ask questions, you can use the question and answer box, which you can locate by hovering your mouse near the bottom of your screen. You should see a bottom label Q&A. Please type in and send your questions there and not in the chat feature. Please send those at any point during the webinar. We are pleased to have so many of you interested in this webinar, but I will ask you please to realize that we won't be able to answer all the questions if there are many of them, but we will try to do our best. Should you have any technical issues, please use the question and answer feature. Additionally, any conclusions or recommendations provided by Dr. Mar are his own and should not be thought of as recommendations from the National Academies or the Congress. So this is just a disclaimer. With that said, let me introduce Dr. Mar. Dr. Mar, I am very pleased to do this very well. By the way, Dr. Mar 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 and measurement of mechanical properties of earthen materials, designing earth structures, determining the cause of poor performance of geotechnical structures, developing cost affected remedial measures for trouble projects and risk management. He's an elected member of the US National Academy of Engineering and of the malls. He has published widely and has given invited keynote lectures around the world. Dr. Mar has extensive experience evaluating the stability of tailing dams and other waste retention facilities and monitoring their performance. I am very pleased to introduce you Dr. Mar so Alan please the floor is yours. Thank you Pedro. I appreciate those kind words. Just double check here you can see my screen. Hopefully that's okay. The topic today is focusing on why do what I calling here mind waste impoundments also by many as tailings dams experience stability failures. This is a particular geotechnical focus in what I'll talk about today. I will try to deal at a somewhat high level to set the scene and then get into some specific deal technical issues toward the end. I'll try to go for about 35 minutes to leave as much time for questions as we can. And we we've extended the time a bit to try to get in as many questions as possible. Let's see for some reason. Sorry I just had a little trouble going forward. I will cover a brief review of some of the significant failures that have occurred to tailings dams around the world over my lifetime. And I've chosen ones that had a specific particular impact that I'll describe. I'll then draw from that some characteristics of tailings dam failures that seem to be common and that should really influence how we approach tailings dam safety. I'll review some of the key aspects of geotechnical failures in tailings dams. And a lot of this gets into sheer strength behavior of the materials comprising the dam and of the retained tailings. So I'll review some of the sheer strength concepts for granite materials. And some design goals that ought to be on one's to do list for if they're responsible for the design of the tailings dam or for the review of its safety certification of its safety. And I'll summarize with a slide. Again, these are my views and not those in any way related to the Academy. Just a scope. What do we mean by tailings dam might refer to them more as barriers that are made of earthen or waste materials that retain particulate waste upstream of the barrier. And that are placed by where those waste are placed by sluicing at high water content. So I'm dealing mostly with hydraulically placed waste materials behind some form of barrier. Barriers are referred to in the industry by different parts or different groups as dams or impoundments or ponds. All of these share some of the geotechnical characteristics that we're going to talk about. The particulate materials themselves vary in how they're called or referred to as well as in their properties. They're referred to as tailings or washings or ashes or slags chemical byproducts like gypsum produced in the phosphate mining industry. The placement varies by industry and the mechanical and physical properties can vary widely even within one tailings dam. They may vary vertically and horizontally very significantly, which makes it a much more complicated problem to try to deal with than many other typical engineering situations. So let's review some of the failures that have occurred. I chose this one because it was the first one that I had the opportunity to work on when I was still a young engineer in 1971. It was common in Florida in the phosphate mining industry to build tailings dams of sand that were produced as a byproduct of the phosphate mining. So you you minded the ore, the phosphate, you got sand and you got slimes clay slimes. And so we would use the sand to build a dam and we put the clay slimes behind the dam. This this particular failure occurred suddenly no warning 45 meter 45 feet high. The clay slimes flow downstream and had a lot of bad consequences. The failure was caused by a geotechnical stability failure of the homogeneous dam due to the fact that the flow of water through the dam was not controlled in any way. We had a condition of higher poor pressures that destabilize the dam. The clay tailings went 120 kilometers down the river with a very large fish kill, no life loss, but the significance was in Florida became the first use of the courts of a strict liability doctrine for hazardous use of land. And so this widely were greatly broadened the potential liability of an operator of a tailings dam first in Florida and then expanded out into the US. It also led to the implementation of design regulations for tailings dams in Florida that were developed by the engineering profession became a part of the laws and one key part of that in fact was I think the first use of a regulatory requirement for minimum factor of safety on a tailings dam. So it really changed the rule of the day and how we do these things in the United States kind of in a separate industry but also very important was a failure of another type of impoundment in West Virginia in 1972. This was just basically a an impoundment placed across the creek to create a sedimentation basin on which to catch the washings off of mind cold so they didn't go downstream include the river. It rained heavily. The water got up and was flowing through this homogeneous dam caused it to burst suddenly the water and the tailings of the washings all go downstream. The town of Sanders is only about a mile or so downstream and everything was wiped out there as well as many people killed. The same time you talk about catastrophe this pile of coal ash just down or waste just downstream of that was on fire and when I was at the site three days later looking at this failed mass and destruction down the valley. There was also this fire still burning despite there having been a massive flood things. It's amazing what happens to us sometimes this occurred suddenly no warning. It was a collapse caused by slumping and sliding of the downstream portion of the dam after a heavy rain that the impoundment itself was sitting on cold tailings. So just overall not an engineered impoundment facility at all 125 people died within 30 minutes or so of this because the water went downstream in a very narrow valley and big wall of water waste debris and everything just just washed everything out of the valley. This was significant not only for the terrible damage but it led to the US Dam Safety Act which gave the Corps of Engineers responsibility to conduct a nationwide program for safety inspections of dams. And this was the first time we really discovered how many dams we had in the United States and how many of them were in such poor condition. First really step up in recognition of the importance of dam safety and the big job we had ahead of us in the US. Another one I worked on in 1980 then getting into some of the or iron or the metal business was a 66 meter high dam constructed by the upstream method large area used for copper tailings in a copper mine. It occurred suddenly again no warning was we determined that it was a stability failure through the dam and the tailings that resulted because these tailings are low permeability materials and when you add new weight on top of them. They developed they don't have time for the poor pressures to adjust to that new weight and so they are they can't gain enough strength so you're adding more weight than your gaining strength. And that just caused an untrained stability failure, classic geotechnical failure, but once that containment let go the contained tailings basically turned to mud and flowed down the valley. No one killed in this but a very significant environmental consequence. It took years to try to clean up. And in Italy it's not all the United States this was 1985 I'm following these and time sequence in which that tragic tragic failure of a tailings dam. Unundated the village of Stavia. This again was sudden no warning tailings dam made by the upstream method, which I'll describe in a minute. In this case, though the failure occurred through the silk foundation. Again, a geotechnical stability failure and then that let loose the tailings that developed into a mud flow of liquefied tailings floated a very rapid rate downstream killed 268 people destroyed a bunch of property. Back to the US in 2008. There was a major failure at Kingston, Tennessee on a TV a facility. This is coal ash in the blue shown in my slide here is hydraulically placed. This is the residue from burning coal to produce power. And so we start with a starter bike down here on the right side of compacted earth materials. And then we start using some of the ashes as compacted ash as construction material and then pound the hydraulically placed tailings behind this barrier structure. And so you keep building. They got to a certain point. This was an active facility still adding weight to it and it experienced a sudden slide mass movement that released the ash that then liquefied and flowed miles actually upriver in some instances. Fortunately, no one was killed here, but it has been a major billions of dollars costs to TV a to deal with the consequences of this. It's also led in this case to you EPA regulations, much more strict on building constructing operating these types of it and found it. A Expert assessment of this found that there was a very thin layer of mud like the plastic debris left in the bottom on which this dam was actually constructed. And it was failure. The failure was triggered in an undrained stability of that very thin six inch layer of clay slime like material. This occurred suddenly with no warning, even though it had been inspected the day before. It was an undrained failure through the thin week foundation layer over four million cubic meters of liquefied ash flowed quite a long distance, a lot of property damage, no life lost billions of dollars in costs to TV a Just two more a couple of three more I guess your Mount Polly up in British Columbia was a major tailings dam that let go in 2014. You can see here on the upper right the breach that developed the tailings are up in the upper right corner of that diagram. The dam itself was an engineered and construct designing constructed facility. This shows a cross section of Mount Polly where the red material is compact rock fill. Then we see transition zones and the core. This is an engineered dam section this as the pieces the features we like to see in a well engineered dam, and yet it's still failed. It failed due to a weak play layer here in the foundation that got overstressed for the weight that was being added to it. A case where more weight was being added faster than the foundation soils could adjust. So we had an undrained stability fire through the foundation that consisted of our play layer. This led to liquid fashion of the stored tailings followed by a mud flow that went into a pristine lake and down the Fraser River for as much as 600 kilometers. It's one of the biggest environmental disasters in modern Canadian history. There weren't life loss I don't believe in this. And you know the monies are less than say TVA experience. But it was kind of interesting to me here that this is a case now where another consequence of these appears and that is the design engineering firms being found responsible for negligence or unprofessional conduct in this and that had a big hit to them and their, their standing. Couple more fendale in Brazil in 2015, the largest environmental disaster up at that until that time in Brazil. This is an iron ore tailings facility. Let go and mud and water wiped out several villages below. This is a look by the expert team post mortem, what caused the failure, and they were able to determine that it was a stability failure on one section of the dam where using appropriate geotechnical concepts and analyses. They back figured a factor safety less than one for the conditions that existed at the time that the dam failed. This was attributed to high pore pressures being generated by adding tailings to the top of the dam at a rate faster than the tailings could dissipate filling too fast. There was no warning sudden failure it had been inspected shortly before the failure. Again, it was a case of an undrained failure of the containment dam, followed by liquefaction of the stored tailings. There were 19 people killed here and a lot of property damage and billions of dollars paid out by the owners. The last one is very recent in Brazil, the brimadinho tailing stand. There's a video of this online. I didn't not trying to show it here take too long, but it's for those of you in this involved in this area. I strongly urge you to go look brimadinho tailing stand failure and watch this video. It's a real live shot of several frames a second of the development of the failure. I've picked out something here that I think is about six seconds after the failure started. The tailings are up here behind. This is the dam portion you can see where my cursor is going. This dark area across here is the exposed scarf from the slip that is developing. At this point it's dropped maybe some 15 meters or so. The dam itself is 86 meters high. The front part of this is coming out at us out of the screen. So it's the back parts going down and the front parts coming out. So that looks to me like a geotechnical stability failure. And then the stored tailings, which is the black part in the background here, let go and liquefied and went downstream. 248 people were killed. There's still 22 missing damages that I know I can trace right now or at least $5 billion and no one knows yet where that's going to go. This was a sudden failure. No warning. The dam had been inspected the day before. 12 million cubic meters left this site. And again, I urge you to watch the video. So what are some of the characteristics that we're seeing here? I think you've seen me try to emphasize that failure can occur quite suddenly with little to no warning. This just seems to be a theme over and over again and something that ought to be at the back of our minds if we're in responsible charge for these facilities. In my view, generally from my review and from my knowledge of these things, generally something triggers a failure within the barrier itself. That is that outer part that's holding the creating the containment or within the foundation. And then this triggers a loss of containment of the loose tailings. That then results in the tail, the store tailings liquefying. And when they liquefy, they have no shear strength. They flow like molasses. So the key here is to keep that outer barrier portion stable and safe. And so we contain contain the tailings. I like to think of it when I'm designing is think of the tailings as though we're storing liquid just like an earth retention dam. I mean, sorry, a water storage dam and and then make sure we can hold that in place. Then we can avoid these liquefaction type static liquefaction type problems liquefied tailings can flow very fast for very long distances, and they can present great risk to downstream people in the environment. There's little time to warn and evacuate people if they're located with directly below the dam, as was the case of remedino, or even within a few kilometers of the dam. Visual inspections may not reveal the threat of an end of failure. You noticed in the cases I reviewed I mentioned several of those had been inspected visually the day before or recent just prior to the failure itself. Most monitoring systems as they've been put in the place in the past will not give us adequate warning. And I'll explain that a little bit more toward the end here. These above characteristics should be strongly considered in the design and operation of any tailing stand. They should govern our decision making and our efforts to do the right thing and try to keep these facilities safe. Where are we in this overall waste storage industry? And I think it's helpful to realize that these are generally waste materials. So anything we do with them costs money. And so a part of any prudent management system is trying to minimize your costs. So waste storage is always driven a lot by how can we do it for the least cost. But that has a consequence. Here is a diagram that Professor Becker and I put together many years ago for a client in the petroleum industry where we're trying to figure out how much risk is acceptable if there's no government standards or guidelines. So we went out and looked at all different types of facilities and tried to assess what was the accepted average rate of failure. And if they failed, what was a typical consequence of that in dollars? Dollars is across the bottom axis, annual probability failures on the vertical axis. So you see a case like mine pit slopes. The consequences there can be managed. And so there's not, if we get a slope failure, there's not necessarily on average a lot of loss there. So we typically mine with steep side slopes and accept failures. On the other hand, something like a large earthen dam could potentially take out hundreds of lives. And so we designed those with a lot more, a lot more probability of failure. Typically the data would suggest that our well modern modern engineered dam, earthen dam, storing water has a statistical average rate of failure about one in 10,000 per year. And you'll see that shown here and kind of the yellow on this slide. We put all this together and Greg and I kind of drew a couple of lines through this stuff saying, you know, here's kind of an envelope of accepted risk. Because risk is really probability times consequence. So, and then we had marginally accepted risks and our particular client in Japan at the time had a risk right up here. So this was a very useful chart to help say it's too high, we need to get it down. So recently I'm thinking about these tailings dams failures I got interested in well where do they sit in this this chart which is kind of a concept idea chart to just give relative comparisons. You could argue with the specific numbers as not being more than maybe an order 100% or so off rough numbers. The data suggests that the rate of tailings dams failures is about one in 1000. We have we have on average about two significant tailing dam failures a year. And there have been studies by folks kind of looking at frequency of losses and one in 1000 is a reasonably representative number and I've sketched that on here with the blue dashed lines. That's average a particular tailings dam might be anywhere here it could it could have a higher annual probability of failure. It could have a higher consequence of failure. Every one of these you almost have to think about it in terms of what is the potential consequence. And then if that's high to shoot to try to reduce the chance of failure by having a more stable or a stronger facility. I think in some ways one pot shot at this as to where are we as an industry today is I just kind of threw out here for concept. You know we're probably up in that pink zone with a lot of our tailings dams that the consequences can be pretty significant in the billions of dollars. And the rates of failure are in some cases for some specific cases potentially higher than what the average is. So I think that raises a cause of concern. You know, are we too high with the probabilities of these things failing and do we need to do something about it to review a little bit about the methodologies here and why does this happen. But let's look at a typical concept of an upstream method of constructing a tailings dam. It starts with a starter dam which is usually a compacted earthen embankment of some type or other built with some good control. And then we start putting tailings behind that they will try to attempt to put the coarser granular materials out in this so called structural zone, and then the fine grain materials out here. And if we can get the coarser materials here and if we can keep the water out of them then they're strong enough that they can help make up this barrier. As we raise the dam we might take material out of the structural zone and pile it up here to make these smaller outer dikes that are used to perform to provide the containment for raises. The problem with this methodology is there's not good control over what is going on in this structural zone. This red line, we don't know exactly where it is and in fact hydraulically placed, it's not a line like that at all but it's a zigzag pattern of fine grain materials and are woven with some of the coarser ones. So we wind up not really having much control over what materials get placed in the structural zone. And yet what goes in that zone is critical to the stability and safety of this overall dam barrier. There are other ways of doing this. I just described the upstream method. There's something called the downstream method in which you start with a starter dike, then you take competent, as you raise it you take competent materials and you raise that, which means you also add a barrier or you increase your barrier, your burn downstream. Your dam gets to be bigger and bigger and looks a lot like what we would use as a conventional earthen dam to store water. Clearly this takes a lot more earth work and therefore costs more to achieve than what one would do in the upstream method. And then there's something that's somewhat halfway in between in which we, the center line method in which we build a downstream half of what would look like an engineered dam and then the upstream portion is more like uncompacted tailings. For those of you who are in, I used to teach dam design years ago and one of the fundamental things I taught at the very beginning is you have to have positive seepage control in every dam. These simplified sections don't show any seepage control. And I realized the diagram was put together by my colleague Steve Vicks many years ago to just demonstrate, just to illustrate the different methods of construction. But this diagram unfortunately has been reproduced over and over again and has led I think to some erroneous beliefs that we can build these things without positive seepage control. What do I mean there? A simple thing like a drainage blanket that is constructed at the beginning that then pulls the water down into that drainage blanket. So we can keep the downstream portion of that dam unsaturated without positive pour water pressures and working as a structural barrier to help hold the stored materials upstream. This is one critical shortcoming in many of our facilities. We do not have these positive seepage control measures. The second part of it is that the waste materials that are used to construct the barrier portion and that we're also trying to contain their complicated materials. And some of the things that make them complicated are summarized here. They're placed in a loose state without compaction and without very few controls on their placement. Many of them are hydraulically sluiced into place and left there. They have a high water content. A large fraction is fully saturated, which makes them incompressible. That means when if they're saturated and incompressible, if we have anything like a shock or a shear stress applied to them, they will typically try to decrease in volume. And that causes them to turn to a fluid like material and I'll come back to that in a minute. They may be highly variable in composition with alternating layers of different gradations and plasticity. You may have thin layers of clay like material and over that a course like material, but from a stability standpoint that thin clay layer may be dominant in and whether the section is going to remain stable or not. Many may be chemically altered or weathered or aged, which can alter their strength characteristics considerably. These have some true cohesion to them, but then that may be may disappear under certain conditions. Once something triggers shear strain in many of these materials, they switch from the so-called drained mode, which can be safe to an undrained shear behavior in which they lose strength. And that's where they're turning to that liquid like material. In this situation, almost no strain occurs before they switch to undrained behavior, lose strength and liquefy. These are described as brittle like materials. My colleague Andrew Fori down at the University of Western Australia has done a lot of work on this and talks about when you have a brittle material and you don't have redundancy in the design, you're inherently dealing with an unsafe situation. So I've talked a little bit about this stability being the important thing. What are the key geotechnical factors that do affect stability of tailings stands? Geometry is key. How steep is that outer slope? The subsurface profile that is what's in the foundation? What is the layering of the tailings? Do we have weak layers that are going to dominate? The stability is always dominated by the weakest material. Have we found all of those? Poor pressures within the dam and the foundation and the stored tailings. This is one of the more complicated parts of dam stability. What are these water pressures? Because they directly affect the strength of the materials. Getting those poor pressures is complicated. We have computer programs that can calculate them, but how good are they? Because the materials themselves are very complex, heterogeneous. They're anisotropic, non-homogeneous. We get poor pressures if we're still building and filling things. We get changes. We get changes from weather. So poor pressures, really to know what they are, you have to have a reliable measuring system in the field to measure what's actually developing in the field. So you have a better understanding of what your true stability is. Then the strength of the materials. There are key words that are played here, whether the materials are drained or undrained, contracted or dilated, cohesive or non-cohesive. I'm going to take this up on the next slide a little bit more. Drained means that the soil, the materials are loaded slow enough so that as they try to change in volume, any water that needs to flow in or out to adjust to that volume change can occur and no excess poor pressures can occur. This drained case is something we understand. We can measure drain strength pretty well. Our students learn this and know how to do these calculations pretty straightforwardly. Most people think of sands as being drained because they have a high permeability. Water can flow out easily. Contrast that to undrained materials where the rate of loading is fast so that water cannot flow in or out of the element as we're shearing it, and this causes excess poor pressures to develop. It also causes them to fail in a mode we call undrained, the strength of which is quite different than the drained strength. In some cases it may be more, in some cases it may be less. It's a very difficult concept for many people to understand, and many people who do undrained stability analyses fail at assigning a proper undrained shear strength for those cases. That undrained shear strength is affected a great deal by whether the soil is so-called contractive or dilative. Contractive means that when we take an element and we try to strain it in some way or other, it wants to decrease in volume, which if the element is undrained, this causes positive excess poor pressures and a decrease in strength. So this key word in tailings is, are the materials contractive? Because they can become brittle. If they're contractive, they can exhibit this brittle behavior, lose strength and statically liquefy. On the other hand, the opposite of that is dilative, which where if I have an element, I strain it, it wants to increase in volume. It wants to spread out. And in this case, if it's undrained, that will create negative poor pressures, which increase the strength of the material. So if I could build my dam, my barrier of all dilated materials, which means mostly things that have been compacted so that they're at higher relative densities, then I don't have to worry because it's just going to get strong. If it's drained, I know what its strength is. If it's undrained, it's just going to get stronger on shear. So the real challenge for us is dealing with contractive materials that are going to strain in an undrained mode. We have to find a way to avoid that. And I would add a big strong caution here. Most textbook classifications of sands as always being a case of drain stability is terribly misleading. Because in many of these tailings dams, we analyze them as though they're drained because they're sand, they're draining our materials. But if they're saturated and if they develop this tendency to want to start to strain and develop positive poor pressures that can't drain off fast enough, you actually have a sand that's now failing in undrained mode. And that can be catastrophic. This slide shows some sources of geotechnical problems where failures have occurred. I don't have time to go through all of this. I just tried to pull out an example from some of the many cases I've looked at. There's no one simple triggering action in these things. We have to look at foundations. We have to look at the construction. We have to look at how they're operated. We have to look at environmental conditions. And so if you get a chance that the video will be posted and if you want, you can come back and go through these a little more care. Factor of safety is a standard number that we're using. Many of you have worked in this say we need to get a factor of safety of 1.5. What is the factor of safety? What does that mean? It's the chance that a massive soil is going to let go or waste is going to let go and slide away in an uncontrolled way. It's defined as the ratio of strength, sheer strength of the soil to the sheer stress that is created by gravity. So it only exists where we have sloping ground. So what's going to alter factor of safety? You can see if we decrease the numerator here, if we decrease the sheer strength, factor of safety goes down. Or if we increase the sheer stress, factor of safety goes down. So how would we decrease sheer strength? In the tailings dam, this mostly comes due to increases in pore pressure from water flowing through the dam. We raise the reservoir level. We raise the height of water in the reservoir. We get an adjustment in pore pressure throughout the cross section that generally is an increase in pore pressure. That's resulting in a slope that was stable up to now. We in fact could be decreasing factor of safety. An increase in sheer stress. This occurs if we're adding load that is adding tailings to the top. We're taking away from the tow area in effect, trying to make the slope a little steeper. Increasing pore pressure also will increase sheer stress because the water flowing through the dam creates a seepage force that adds to the destabilizing forces. We could have external forces such as earthquakes, blasting, loads collapse from piping or desolution of contained materials. All can result in increases in sheer stress. So these are things. And if we have a contracted material that's subject to liquid static liquefaction, these would be so-called triggering, could be triggering actions that could be just enough to set off an instability failure. Getting close here to the end, a couple more key concepts. Going back to drained and undrained strength, I just want to emphasize the importance here of getting this part right. Drain strength we refer to as the friction angle of typically say 30 to 40 degrees. And coefficient of friction would be the tangent of that. So the tangent, the coefficient of friction for drain strength would be about 0.6 to 0.8 as I show on the slide here. That exact same material, if it was sheared in undrained mode would have an undrained strength ratio of about 0.2 to 0.3. So that says if I have a condition where I've designed with drain strength, or I think I have drain strength, but something could switch the material in a way that it would now behave undrained, it would only have 1.3 to 1.5 the shear strength that I used in that drained calculation. I hope you can see that this is a critical piece for the safety of a dam and for its design. Again, if the materials are contractive, then they have the potential to shear in undrained mode, and their shear strength is going to be for that undrained condition much, much less than it would have been had I assumed that the failures would be drained. I think as a guide, a comprehensive stability assessment should consider both drained and undrained loading. Don't take shortcuts. You know, if the material is dilative, the drain condition will usually be the more critical. If the material is contractive, undrained condition will usually be more critical. But why in today's modern world, computer programs and everything, why take the risk? Do like our structural engineering friends do, consider all load cases and prove that each is in an acceptable condition. Coming kind of a stepping back out from that GeoSpeak kind of slide or two and looking at what are the most important design requirements for all dams. And that is we start do not lose containment of the contents of the dam. You pick out a book on design of water retention reservoirs. You know, this is the primary rule that we say, and that means we don't have, we don't allow stability failures and no piping failures, no over topping, no uncontrolled erosion, and no washouts around hard structures. This is just as important in tailings dams as it is to water retention structures. So I kind of put that in a simple slide. What does that really mean? Make sure our foundation has enough strength for all possible load cases. Remember, two of those cases I showed you were three actually were failures through foundations. Have all the materials that comprise the barrier portion of the dam, the structural part of it be dilative materials. And then we don't have this risk of static liquefaction contributing to a stability failure. We control the internal water pressures with internal drains to keep the factor of safety for drains and unconstrained conditions greater than 1.5 for all possible load cases. That's a safe way to go about this work. We prevent over topping. We control internal external erosion. And then there are other requirements for seismically active regions, which we don't touch on in today's presentation. Many existing tailings and palments do not meet these goals. And that raises the question, what do we do about that as a society and as an industry? Just a slide, quick slide on monitoring. There's been a lot of thought that that we could improve this situation a great deal with real time monitoring. There's actually some countries have gone to mandated required real time monitoring. I think we have to be careful and I'll give this caution at the end. What can we monitor that works in tailings stands particularly? We can detect lateral movements in the foundation beneath the outer slope of the dam. So there was three cases I showed you that failed through the foundation. A good monitoring system could have detected those. We can detect changes in flow rates from internal seepage that might be precursors of piping failures or failures along hard structures. We could have sufficient locations with poor pressure measurements to establish the flow pattern through the dam at several sections so that we do a calculation of factor of safety. We're doing it with confidence that our poor pressures, which are key to stability, are meaningful. But that means that you've got to have measured poor pressures along the critical failure surfaces that you're analyzing. And I see an awful lot of dam sections where the poor pressure measurements are made in the wrong place and they're not made with sufficient accuracy, sufficient frequency to be meaningful. Instrumentation should be reliable and redundant. I prefer these days to see automated readings. The technology is there. It can be done without a lot of added cost and we do that several times a day. Dams can change. Surprisingly, they can behave and look like they're the same thing, but undergo significant changes quickly. Any instrumentation monitoring program should be complemented by visual inspections using trained people who know what to look for. And then have a team available to help evaluate that measured data and interpret it. What does it mean? Have an action plan that gets triggered when certain measurements exceed pre-selected action levels. So just a warning for tailings dams made of contracted materials. Monitoring deformations most likely will not provide warning because these materials are brittle. They can lose strength suddenly with little to no strain occurring before that event occurs. You can show this in the laboratory testing. You can see this from some of the failures that have occurred. So summing up, we know how to design and build tailings dams that are safe. The geotechnical profession knows how to do this. We're not challenged by reshort shortages there. We could use some better tools to help us more closely define, separate, contractive and dilated behavior. There are always some improvements we can make, but the fundamentals necessary to do safe design we know. The problem enters when the designers don't use what we know and the builders don't build what the designers design. And this is particularly a problem in the mining industries where there's a separation many times between those who design the facilities and those who actually construct them. I'm not harping on anybody. I'm just trying to make some observations of where are potential areas for improvement. The problem we have is compounded by many existing tailings dams that were not designed or built using what we know is required for the dam to be safe. Tailings dams can fail after they are retired from service. Rumidinho had been out of service for three years. Water was being taken off the top of the dam slowly over time. Its safety should have been going up. Its factor of safety should have been going up by simplistic assessments. So the idea that once we take it out of service it's no longer a threat is up for serious argument now. The average failure rate of tailings dams is too high in my opinion. I showed you about one in a thousand compared to water retention dams. And half the dams are even higher than that rate. I think that raises a question for us as to is this acceptable and if not what do we do to cost effectively bring that rate down. Risk from failure of tailings dams are higher than that from many other industries as I showed in that one plot of different industry risks. And so that's something we need to be aware of. It seems logical that steps are needed to reduce these risks by at least an order of magnitude below the present state. So the challenge I serve on Cognate we're looking at ways we can improve things for society. And so one big picture view maybe can we can we help identify ways that that this can be done efficiently and cost effectively. And that is by reducing probability of failure and reducing potential consequences of failures. So I thank you very much. It's really strange to me to be sitting here speaking to 576 people and I don't see any faces. But I hope I was able to keep you all the way. OK. Thank you. Thank you for a great presentation. And you have created a problem for me because now I have a ton of questions. That we could try to answer. So remember if you have not already sent a question you can still use the question and answer a box which you can locate at the bottom of the screen. Please type and send the questions and we will try to answer them as they arrive. So we have a can you listen me Alan here. Yes I can. Can you see also the question and answer just in case the box. I will I will be asking the questions but maybe you can also look at them. No they we purposely kill that so I don't get confused. So you don't get perfect. That's your job. So I will be even mentioning the name of the person's just in case. One is by Edgar Sanjan is one of the first one is a simple one. He has are you considering slimes as a granular or particular material in particular because of the case at the beginning that you showed. Yeah that's a good question Ed and I would say no most of my comments are more dealing with granular materials. Slimes typically have plasticity to them and I think most people involved in most slimes are actually behind an engineered embankment. So so sure strength of them is less of a concern but I'm not considering slimes in my presentation today. Okay, so there is another one here by Steven Emberman and he said you seem and actually I kind of know this one but maybe you can answer. You seem to be talking about like a faction as a phenomenon that occurs after failure of the dam rather than as the cost of the failure. Let's say a little bit more about that. Is it is like a faction in use because of the failure or like a faction was the reason for this. So this is a really good interesting question and I think probably one open for further debate and study but my I spent a fair amount of time looking at this and going back and some of these cases I tried to review this morning, and I think if you if you look at this really carefully, it appears that that we get a stability failure first, which then releases containment of the tailings and then, then they they're subjected to shear strains, they collect they liquefy and then flow out. I know if you some papers out there they taught they describe this whole process is just failure by liquefaction, but I think there's there's a first step that where where there is a stability failure that occurs, and then let's let's the tailings forces the tailings to start to shear and liquefy. The exception of that would be if we have seismic shaking where potentially there you shake you liquefy the tailings. And if the barrier hasn't been adequately designed that could increase the load on the barrier and force a stability failure. But that's the only case that I can identify that where liquefaction occurs first and then the stability failure follows. Yeah, there is a question here by Jaime Castro that is a little bit more technical like but he's asking. It seems that there has been some studies on the effect on shear strength by principle stress rotation and it seems that there is some work that has been done related to the stability of dams. Do you have any comments on the effect of principle stress rotations on their reduction of strength. No, I'm not I'm not an expert in that. I worry a lot more about the more fundamental principle causes of drain versus undrained strength. You know, and then I guess the other thing we I worry about is you're making sure the tests that we are doing are representative of the conditions in the field. So maybe indirectly I take care of this problem in that a lot of the shear strength testing that I do and want to see done for these kinds of problems is more like direct simple shear test conditions, which more closely capture the stress pass in the field. Then what we might get in a triaxial compression test. So I kind of avoid this stress rotation question he's he's bringing up I think. Yeah, and so. Yeah, I don't know that I avoided I just I address by running a test that tries to try to mimic what happens in the field. Yeah, I think the question was some some people have been asking also about possible stress path that are followed by the material. Before the failure of cures and there may be due to the loading may be a stress path that puts you in in a point where failure is more imminent. Just going closer to a P equal to zero, which is the effect of for what the pressure that you are mentioned. Yes, here there is a question by Jorge Macedo what is approximately the number of existing upstream dams being operated in the US. And what percentage represents them in terms of the total numbers of dams that exist. That's an approximate numbers. Yeah, I don't know that I. I'm sure that's in the literature somewhere I just don't have it. Follow the moment. And let me see some other questions. And so here it says by Lindsay Newland boker. And she says she's a long question in his memo lecture this year, Dr. Morgan stand estimated that at least 50% of all catastrophic failures in history were due to static look fashion. And state that this year was to the level of this was to the level of expertise and mastery industry in stability analysis, the case records of these dams support is his observation and witnesses misuse of parameters in the case to be as you have a Bernardino case. So it seems that what she's implying is that there were a lot of signs in some of these tailing dams that even though that the failure was imminent and very fast. There was evidence that this was going to happen. And she's asking if this is common in others that even though that they occur very quickly. But there are evidences that people don't consider. So she has several questions related to me. Let me try to see if I can take that apart. Partly, you know, Dr. Morgan stern making a statement that 50% of catastrophic failures in history are due to static look of action. I don't recall it being stated that way. I'll go back and check again. He did a great job in two or three of these failures. He looked at it from Dale, for example, in that Mount Polly. And, you know, I think in both of those cases, static look of action occurred, but it did not cause the failure something else triggered it. And then static look of action was a consequence of that initial failure. I would read very carefully what he said, and I will. It's a great lecture. All of you should go read it. Dr. Morgan is one of the experts in this field and he writes very clearly. So I strongly recommend that it's available on the web. I think the question knows that there are, the second part of the question was, you know, there's lots of evidences out there that we miss. What I'm trying to say is when we do visual inspections of dams, what are we looking for? We're looking for cracks, we're looking for slumps, we're looking for water that's exiting in an uncontrolled way. And so when I say people go do inspections, that's what they're looking for. And you usually don't see in these tailings dams failures changes in those visual kinds of things. But what is she referring to? I think it's, if you're a very knowledgeable person in this field, you can go back and look at postmortems of failures. You say, oh, obviously this material was contractive and obviously this stability analysis was in at, obviously there were high pore pressures in this dam and that's what caused it to fail. But those are things you look at in backward. It's hard to walk the surface of the dam and come to the conclusion that the material is contractive or that the pore pressures are too high. So when I say, you know, visual observations, visual inspections that we do can't tell us a lot about these inward things that really drive the safety of the dam. Here I have a kind of a continuing question by Bruce Catter, and he says, rightly, you emphasize the importance of the dam stability, but is there a reliable way to predict the run out distance of the tailings given that failures will happen? Do you agree that entrainment and mixing of water with tailings flow continually, softens the stuff to slurry or a suspension that allow the material to travel several kilometers. Hence to predict run out, we need to account for entrainment. Oh, absolutely. Professor Cutter, nice to hear from you. I totally agree. And I often wondered if we didn't have water stored on top of these things, would they still liquefy and flow? And I think they will, but not anywhere as severely as what happens when we have a fair amount of water stored and then it all gets entrain. I even suspect that we get failures in which we have a lot of solid like material riding on a film of air and water that can go quite long distances. I've looked a little bit at the literature about trying to predict run out distances. You know, this is a really complicated problem and my hats off to those people trying to model this. As a design engineer, I guess I give up and say, I just want to make sure my dam never gets to that stage. So I focus on how can I prevent this tragedy from ever occurring. And I have another question here that goes back to the material properties by Murali Teran. How do you determine whether a material is constructive or dilating in a highly variable material? What kind of lab or in-city testing do you recommend, CPT? There were several questions related to that. Yeah, no, that's a great question. Alan, before Mr. Smarty, before you answer that question, I just wanted to let everybody know that we have a number of questions and we'll stay live for a number of more minutes so we can squeeze in as many as possible. And then at some point, of course, we'll have to pull the plug. But thanks everyone for all of your questions. We'll get to as many as we can here. Yeah, I see that we got up to something like 750 at one point and it's dropped off to 570 so there's still a lot of interest in this. I'll go as long as we can. I said that we know how to do these things but we have certain challenges. And one of the challenges is exactly this question. How do we determine whether the material is constructive or dilated with any degree of reliability? And so, you know, one technique that people are using and it's growing in its use is the CPT, comb penetration test. And there are some great work that's been done out there where it's pretty simple. You take the cone resistance and you plug it into a equation and you draw a nice chart that says, you know, this part of the material appears to be contractive. This part appears to be dilative. In fact, it's a nice simple line that's plotted with depth to tell you at what depths materials are dilated and contractive. And I see a lot of people using that and they seem to be using it with a high degree of confidence. I caution you in concept that can work, but it's not that simple of a break of a solid line drawn with depth between contractive and dilated. That really should have a zone there of uncertainty where you can't be really sure. And if you read the papers that talk about that methodology, there's always a sentence in there somewhere that says, you know, this is an approximate method. And for site specific conditions, you really need to calibrate and if it's important, you really need to calibrate this methodology to your own situation. That's kind of tough to do because how do you do your own CPT calibration to determine whether materials dilated or contractive? That's a big research project. So what do I do? You know, I typically want to see CPT work. I want to try my best to get samples. I know people say you can't sample granular materials, but you know, we do a reasonably good successful job at that. I will try to do sure way velocity measurements in situ. I'm looking for two or three different sources of information. I'll get samples so that I can classify the materials. I'm looking for multiple pointers to tell me how is this material likely to behave as a contractive or dilated material. And then the other part of her question or his question is kind of where you have high variability. How do you deal with that variability? Do you kind of go for, since it's a stability problem, the lowest values? And this is a challenge. You as a designer really kind of have to figure out what do you think is the best characteristic representation of this field conditions. And it certainly isn't the average, which I see some people using in some published methodology suggest the average. I use the word characteristic, which kind of falls back on some of the work by the Corps of Engineers many years ago. Take the average minus one standard deviation or so to kind of say, well, maybe that's a safer representative strength. But don't try to be overly conservative because you're going to cause your client money. So there were a couple of questions here, but if you think that the regulatory agencies and methods are appropriate, if they should go more into probabilistic methods or should continue to use safety factors. What are your comments? I am trying to bring together several questions here. I'm going to elaborate on what are the recommendations that should be followed? Are there regulatory agencies doing a good work? Should be pushing for probabilistic methods? What are your comments on that? Oh, yeah. This is kind of a general question across all different types of facilities, isn't it? We're trying to get away from kind of deterministic methodologies to more risk-based stuff. I do a lot of risk work. Of course, as a decision kind of tool, putting the problem in terms of probabilities and consequences to me is very satisfying. But I think it's kind of hard to carry that out in the regulatory framework. Regulators have to keep things pretty simple and for a number of reasons. I think if they just simply wrote some good rules or guidelines that were really clear about what we should be doing for deterministic methods, we would probably be better served right now. Around the world, you look at some of the written requirements for the stability of a dam. It says, you know, thou shalt have a minimum factor of safety of 1.5. But then there are variations. You can get into all kinds of situations where it's not really clear when you can deviate from that or whether it's possible to deviate from that. In a temporary condition, and I'm working to improve it, what's acceptable in that circumstance? So, and I think really more of the emphasis right now ought to be on making sure that we do the deterministic methods right, that we do the site characterization right, that we understand the concepts of fluid flow right without unnecessarily complicating it by adding in the probabilistic approaches. That's not to say they aren't useful, but I think from just a design standpoint, we don't want to overly complicate our work. And in that constant, do you believe that the federal guidelines for dam safety should be applied or can be applied to tailing dams? I think so. I don't know why not. The consequences, increasingly now the consequences, well, let me modify that. If the tailings dam is one in which a failure will get off site, then I think yes. It's not going to impact people and I don't really see the difference in a tailings dam failing or a water retention dam failing. If it's a situation where the consequences can be contained within the site, that to me is the, you know, that's up to the owner and an operator of the facility. So with that, I think that we are already 10 minutes over the hour. And I am getting some comments here from the bosses in Coggy that maybe we should start wrapping up this event. Do you have any additional comment that you want or something they want to say, Alan, before we finish here? Well, I think, you know, thank you for your attention and for this opportunity to talk about this subject. I'm hoping that we can somehow find a way that we get enough focused positive energy towards how do we improve safety of tailings dams. They don't need to have a black eye. They don't need to be viewed as a bad thing. They're necessary for our society to produce metals and other materials. Can we figure out a way that we can can go positive on this and find cost effective ways to do a better job? Okay, so I think that we are reaching the end of this meeting. We have plenty of more questions that we can really ask to Alan. I want to thank Alan for their, for his presentation and the great audience for the engagement in this event. And I want to say that if you have questions about Coggy, including ideas for topics you would like to seek over in our ongoing webinar series. Please, you can reach out some Maxino with there is an email that you can follow in the screen. I will also note that the presentation and the audio recording from today's webinar will be posted within seven to 10 days in our website. Please watch your email for announcements about future webinars and events. And again, thank you very much Alan, great presentation and goodbye to all of you. Thank you very much. Hello, hello everybody and welcome. I am Pedro Arduino. I am a member of the National Academy of Science, Engineering and Medicine Committee on Geological and Geotechnical Engineering, known as Coggy. Coggy hosted a webinar on September 5th of this year in which Dr. Alan Maher gave an informative presentation on technical aspects of tailing dams and their failures. After which we spent a few minutes responding to questions from our audience. Alan had a 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 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, and 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 that described as liquefaction failures are actually the result of stability failures of the dam that then where the 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 seismic is a separate special case. Okay. And 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 and close 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% sun 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 3000 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. 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 use 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 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 tailings dams 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 Polly 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 to 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 Polly, 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 seam in the foundation. I believe the instrumentation would have given an earlier indication of movements, unexpected movements in that foundation. Something like a sloping conometer, 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 clay seam 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 pore pressure. And so trying to put in pore pressure monitoring devices called piezometers at enough points that we get meaningful data to tell us what are the real pore pressures on the critical failure surface is a very useful tool. In the case of Brumadinho, this 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 involve a buildup of pore pressure. I don't know if that's the case in Brumadinho, but generally these kinds of failures are builds up in pore 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 pore 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's 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 trade-off 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 of 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. A follow-up question here is who should be doing the monitoring? Who is the authority in the US 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. 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 someone specialized in the monitoring area who knows what types of instruments are appropriate, what measurements are significant or meaningful to us. And then 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. Do you believe that the federal guidelines for dam safety are the ones that should be applied for tailing dams? 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 and 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, 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, why 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 to Pedro in that. Unfortunately, a lot of regulators, you know, they're thinly staffed that he 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 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 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 dance and make sure that they're all dealt with properly and that dam safety is kept in control. So, so you mentioned is a complex soil behavior problem that we are facing here. So we have a couple of questions related without so behavior soil testing, and a lot of them are related to what tests can be used to estimate contracted behavior, can we use a lab test or CPT or other in situ 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 situ testing, do you recommend. It's a real challenge. You know, I, 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 tests that very, very reliably discriminate between those two behaviors is hard. Our tools have a lot of uncertainty in them. And, and so while conceptually we know what needs to be done to actually achieve it on a on a tailing stamp can be quite difficult. So that is a preface techniques involve field tests, particularly comb penetration testing. And there's some semi empirical methodologies that have been developed that you can take your, 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. We move around based on material type. So we have, we, 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 having to make some some call at some point as to where is that dividing line between contractive and dilated. And that would 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. 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 know and feel comfortable with. There's no simple answer here. So 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 the samples, we 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. You go within a given tailings deposit that's been around for many years and stuff varies all over the place within that singular deposit. Why? Well, the source of the orders 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've got to look for 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 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 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 contamination. And so, leaving that out as well so now we're 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. Let's transition now to some other questions that are not just related to soy 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. A good question. Of course, tailing stands can fail during construction. Many of the fairs 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 stand that were very actively operating the facility, and they started adding tailings at a much faster rate. And 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 stands 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, 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 C-Page-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 the, 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 tailings dams don't really have adequate provisions for. 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. Morgenstern was referring to an industry-wide shortage of adequate skill levels in stability analysis and design. Are we missing something in our courses? Are we not teaching something? Maybe we are not concentrating 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. This is my impression. When I go to hire a new engineer just graduated out of the university and I start asking them these specific situations, how do you select strength for this case? Many of them are really puzzled and befuddled. They're full of knowledge and information on all kinds of things. We don't talk about probabilistic methods, but the basic fundamental stuff don't have a detailed working knowledge of. Basic assumptions and limitations in stability analysis. Many of them don't just really 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. 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 pore pressures to put into that program or how to take measured pore pressures and adequately describe those within that stability program. Or how to choose the appropriate strength parameters. For example, I see a lot of people taking triaxial 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 specialists in the field understand, but the typical student coming on the university doesn't really have a full grasp of. Yeah, so maybe doing more internships or calls before or during a master program or bringing more industry into our program would be a good solution. This is something that I was trying to add. So we are running a little bit out of time, but let me show a couple of more questions here. And one that appeared to me interesting is one that says, are dry stack tailings dams a substantial improvement? What are the problems of those? Yes, just so everyone understands, most tailings dams 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. So 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 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 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 or certainly avoid 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. Unfortunately not. 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 with the ongoing changes in climate, are other considerations and design parameters needed to make dams 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. 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 adds some challenges to what we already do. 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 over topping. 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 got to make sure that we're taking that into account. And, and then we're potentially creating more wet conditions during construct construction that make compaction difficult and leave weaker loose zones that can substitute, can later on contract and lose strength. So that, you know, if climate change means more, 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. Okay, so we better take a look at that. So I think that we have reached a 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 5. And for the time he took today to respond to more of the technical community's questions. Sure. 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 think if you have questions about Kogi 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. Bye bye everybody. Thank you Alan.