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Home / About / News / RIC2021 - Lifetime Achievement Award Dr. Evert Hoek Session Transcript (Part 2)


RIC2021 - Lifetime Achievement Award Dr. Evert Hoek Session Transcript (Part 2)

This is Part 2 of the transcript for the YouTube video "RIC2021 - Lifetime Achievement Award - Dr. Evert Hoek"

Slideshow presentation finishes.

Transitions to Reginald Hammah on screen.

Reginald Hammah and Evert Hoek begin the question-and-answer portion of the event.

Reginald Hammah: Thank you very much Dr. Hoek for taking us through this journey of rock engineering developments over your long and distinguished career. Once again, thank you very much for sharing these thoughts with us. I want to remind everyone that on the conference chat, there is an option to enter questions. So, while we start this question-and-answer session, please feel free to go to that chat section and post any questions you have. We will try and cover as many questions as we can but if for some reason, we can't cover all of them, what Dr. Hoek has graciously agreed to do is we can collate those questions type them up in an email and he will answer all of the questions and then we'll send them out to all participants in an email. So please post your questions and we'll pose them to Dr. Hoek. Now just to get us started and Dr. Hoek you’ve already spoken about this a little bit towards the end of your presentation, given your experience, given what you see rock mechanics or rock engineering achieving on the path it is traveling along, what advice would you have for young engineers who want to become well-grounded in this discipline? Who really want to become masters of the discipline?

Evert Hoek: I was able to hear the question and indeed as you say I did discuss it at the end of the of the presentation because in my opinion, it's essential for a young engineer or geologist just starting in this field to gain a link between the practical reality of site conditions and the abundance of theoretical and numerical analyses which they've been taught in their courses. Assuming that they've already done a first at least a first degree. But even if somebody is proposing to go into the field just from high school it's worth considering getting into the field for a one or two year period, getting your hands dirty and learning to understand what are the functions, how does a site in civil engineering or mining actually work, where does rock mechanics fit into all of this, and so that that period in industry to me is a critical part of the education required in starting in rock engineering.

Reginald Hammah: Alright thank you very much Evert. Then my second question to you is here we are going a bit almost mythical or mystical, but if you had a magic wand to solve problems which of the current rock engineering problems would you attempt would you tackle first? Well, when you look at how rock engineering has developed, and I’ll concentrate really on the properties of rock masses and the analysis of these masses.

Evert Hoek: You'll be aware that over the last 60 years and as described to a certain extent in the presentation you've just heard, there's been a significant development of the understanding of intact rock properties. We can carry out very well controlled tests giving high quality results in many laboratories around the world and so we're not short of information, particularly on intact rock properties. In parallel to that there's been a good development of methods for obtaining information from rock masses in terms of their nature character geometry and number of joints, faults, shears that break the rock intact rock up into a rock mass and in general most of our problems in practical rock engineering are in fact related to the rock mass behavior and not to the intact behavior. So the question is how do you put those two bodies of knowledge together? How do you take the intake properties we've determined in the laboratory and the properties that have been mapped in the field and put them together into a meaningful rock mass model? And that's a very complex problem because there are many many parameters at issue and the methods for dealing with those is not too obvious. Fortunately, during the last 10 or 15 years there's been a development of the so-called synthetic rock mass which comes from work done originally under the direction of Peter Cundall in a research project carried out by Itasca to develop a model in which the behavior of a jointed rock mass could be adequately modeled and this has come a long way and you saw a very good example of it in the last example in the presentation of the Palabora open pit to block caving transition and the quite remarkable ability of the program to predict caving induced by excavation underground and the control of the interaction between the open pit failure and the underground block cave. Obviously, you don't want that process to get out of control otherwise you end up drawing waste material from the from the surface. And this is a very exciting development. We have a long way to go because the operation of the programs is not a trivial process at all. It requires somebody with a very solid understanding of the origins and the operation of the synthetic rock mass model which is run through the Itasca process using generally 3D programs and so it requires a very good understanding of the process of the model and a very good intuitive feel of how you put the parameters into the model to generate a real rock mass behavior. The model itself is very demanding of computing time. It runs quite slowly but this is not unusual. We've been there before with the earlier development of models like finite element analyses of Slide or RS3. In the early days these went through the same birth pains as it were but I’m very hopeful that eventually the synthetic rock mass will provide us with an extremely powerful tool for studying real rock mass behavior.

Reginald Hammah: Thank you very much Dr. Hoek. Now I’m going to give you a few questions from the participants. One of the participants would like to know what are the biggest technical and professional challenges you face in your career? That's the first question.

Evert Hoek: I would say in general; you'll have noticed from the examples I’ve given, the size and complexity of some of these projects that I’ve worked on has been enormous. If you take the 670-kilometer long Egnatia highway running across Greece which I worked on for about six years, obviously this is not simply a rock mechanics question you're talking about a very complex interaction between a vast number of contractors, planning authorities, route choice based on government allocations of land value and so that the geotechnical component is actually in total terms quite a small one. Nevertheless, a very important one because the worst thing you can have on a project like that is a failure of some kind. Failure of tunnels or failure of slopes or whatever. So that it's necessary to be extremely careful in working with that and clearly this can't be done by one person. There were I would say probably a hundred people related to the geotechnical operations on that total project which was built with simultaneous work on maybe 10 tunnels at a time. and Paul Marinos and I were members of the consulting board, and we visited the site about every three months or so and that was as complex and as interesting a project as I worked on but many of the others have the same type of characteristics.

Reginald Hammah: Excellent Dr. Hoek. Another question, this might be a quick one to answer but would you recommend junior engineers to get master's degrees after a few years of field experience?

Evert Hoek: I would say yes because the bachelor's degree that you've that you've done whether it be in engineering geology or civil engineering or mining engineering, the component of the rock mechanics in those mainly around the world is quite small. So that the knowledge you need to deal with complex problems in the field of rock engineering, if that's going to be your career, is probably not adequate.

Finding a master's degree might not be that easy because they're not that many around the world but they're all good ones and they're well worth participating in and getting a master's degree. But as I’ve said, at some point you've got to get into the into the field. Whether that's before you do the master's degree in other words between the two degrees or immediately after the master's degree, I think that's an essential component of the educational process.

Reginald Hammah: Thank you very much Dr. Hoek. Another person is asking have you adapted the Hoek-Brown criterion to bearing capacity applications? For example, when you have massive foundations, these new wind turbines have massive foundations sitting on the rock, can you adapt the Hoek brown criterion to a problem like that?

Evert Hoek: I have done relatively little work in foundations. I've done some. For example, I did a study of a cable anchor for a very high large bridge in Greece and the cable anchor is in effect a foundation of sorts that the forces are natural rather than vertical but it's still a foundation problem and the Hoek-Brown criterion is used in exactly the same way that you would use it for any other purpose. So, there's no need for a specific adaptation towards foundation problems. I would say that there hasn't been much published on how practical examples of foundation analyses so to a certain extent if that's your field you're pretty much on your own. As I say the Hoek-Brown criterion doesn't change but how you use it is something that's still got a long way to develop.

Reginald Hammah: Alright, thank you very much Evert and yet another question. What are the challenges to professors of rock engineering? Throughout your career, what would you say that some of the challenges they face and how best can they help students master the discipline?

Evert Hoek: That's an awkward one to answer because I’m probably talking to a number of them but from my own experience and obviously there's a lot of practical reality in my background. If you can make the coursework that is presented as practical as possible, that I would say is a very high priority because this is above all a practical field and what I used to do was to try and for example at the master's degree level with a significant number of students form teams of students who would then be given an assignment to study and solve a fairly realistically defined problem. Carefully defined but with enough unknowns that they had to use their imagination and experience and education to try and figure out what information was necessary, how was it then used to arrive at a solution, whether it be a slope stability analysis, or a tunnel design, or a cavern design or in some cases a foundation design, not many of those. So, relating the end use of the knowledge that they're being taught and that is available today in abundance. If you read the literature, it's absolutely saturated with the solutions of a numerical mathematical style and it's necessary to balance that with a link to the real world. It’s not simple, unless you have a lot of practical experience to teach practice practical problems directly by lecturing but if you do that by setting projects and there are plenty of these available in the literature that you can use, so that the student is if you like artificially involved in dealing with real problems excuse me that's a link that is missing in my view in many courses.

Reginald Hammah: Okay well thank you very much Evert. There is a question on, so you describe some of these new you know large open pits that are converting to block caving situations. Somebody is asking so how do you construct or design good field investigation for such projects? How do you measure the stresses in particular if you're dealing with projects like that?

Evert Hoek: Well, you have to realize that before you can go into block caving, there's a lot of work you have to do to develop the underground infrastructure. The shafts, the haulage routes and so on that you need to get to the block caving operation, which would probably occupy two, three, five years. And in many cases, if you're working as in the case of Palabora or Chuquicamata under an existing open pit, that might already have been running as in the case of Chuquicamata for 100 years or so. So, there's a huge amount of information available to you either from previous work on the site itself or from work that you have to do to develop the access. Now the critical part of the of the operation is the design of the actual draw points. Their spacing, their layout, their elevation, their capacity. But that that can come, you can afford to wait a little bit to finalize that. That design, you might have to choose the area in which you plan to induce caving but the actual detail design of the draw points and so on can be delayed a little bit until you've got enough information on the rock mass properties. In most cases you're dealing with relatively hard jointed rock you're not dealing with soils or very soft materials because that is a real problem if you get a major fault or so in deep underground mines, that's a special problem. So, in general you're working in relatively high-quality rock masses and the information you need for that is quite easily available by mapping the shaft as you go down the access roads as you develop them. So that it's almost natural that this evolves as you as you work your way through the project because it's going to be a long time in getting from surface to your first active block cave. I mean that's probably a four- or five-year process and there's a lot of time in there to learn what you need to learn. And also, I should also add if you can find somebody with real experience in block cave mining in the geotechnical side of it, it's worth getting some consulting advice from somebody who's been there and done that before. They're not too many of them around so they're not easy to find but it's worth going that route if you can.

Reginald Hammah: Okay. Well thanks a lot Evert. There was a question that came not on a platform but through an email that said in all your publications with Dr. Ted Brown, you state that the Hoek brown failure criterion should not be used until the potential for structurally controlled failures have been eliminated. It is apparent that many practitioners either do not understand this or choose to ignore it and use other aspects of the criterion such as the D factor to get the modeling results, they seek. That is, they ignore geological structure. Do you have any suggestions on how to overcome this industry gap?

Evert Hoek: Yes, I can, and I can illustrate it by an example it was quoted in the at the tail end of the paper where we were looking at the east face of the Chuquicamata mine and the analysis of the rock around the entrance to a tunnel that goes to a conveyor transfer chamber. And in that case, let me first of all deal with the question of the blast damage factor D. This was a late comer into the Hoek-Brown criterion, and it was involved literally to deal with blasting and nothing else. Blasting and to a certain extent relaxation due to movement of the of the rock mass in the crest but it's very local. I've seen it applied to entire rock masses and that is totally incorrect. That is not what it's intended to do, and it does not work because it's really intended to look at a blast where the D factor at the blast face itself would be one and would grade down to zero at a distance of depending on the size of the slope, maybe a fifth or a tenth of the height of the slope behind the face. So, it's a thin strip of rock behind the slope all behind the walls of a cavern that has been damaged by blasting. Don't use it as a general tool for trying to scale down the rock mass properties that's much better done by the GSI number that you choose to use. So that what you want to do then where you have a situation like we had at Chuquicamata, that was illustrated in the in the example just before the end of the presentation, we had a thousand-meter-high slope that we were looking at. We were looking at perhaps 300 meters in detail 300 meters high. That's a big slope. So that in general in that slope the rock mass although the blocks might be one or two meters in size it's still like a granular material, so it's justified to treat that as a Hoek-Brown material characterized by GSI. However, running across that slope are several major shear zones or faults which cannot be hidden in the GSI system. So, what we did there was we built a model and it was done by Pedro Verona of Itasca and Felipe Duran of the mine geotechnical staff. This was, I think it was done in FLAC3D and with the rock mass determined by a Hoek-Brown criterion with GSI numbers determined from mapping and the features which were shown in blue on those slides if you remember seeing those lines were put in explicitly. So those were overlaying on the rock mass and that's relatively typical of how I would do that. The extent to which one dominates the other depends obviously on the problem you could have a rock back mass which is as I and I worked in some of them where it's almost intact and where there is no rock mass failure involved. It's all either intact failure or structurally controlled wedges and blocks. The other end would be where you have a rock mass that's heavily broken up and there are in fact no major structural features where you can treat it as a Hoek-Brown material exclusively. So somewhere in the middle lay most of the problems that we deal with.

Reginald Hammah: Okay, well thank you very much Evert and we still have a few questions that we could have gone through but like all good things. Nice things, they come to an end we are um at the end of this session, however we promised that questions that have been asked and if you have any extra ones, you can please email them to us. We'll send them to Dr. Hoek who has as I said agreed to answer them and then we'll disseminate them to all participants in the conference. But again, thank you so much and at this point I would like to invite Dr. Thamer Yacoub to take over the rest of this session.

Evert Hoek: Could I just interject before you do that and say something that I should have said at the beginning of my discussion with you, but you didn't give me a chance and that is to thank Rocscience immensely for the honor that you've provided for me. For John Curran's excellent presentation at the beginning, telling you a bit about my history and for the award of the of the medal that you've given to me. So, thank you very much Rocscience for that.

Reginald Hammah: And thank you very much Dr. Hoek for your time Dr. Yacoub.

Transition to Thamer Yacoub on screen.

Thamer Yacoub: That's great. On behalf of Rocscience family, I really want to thank you Dr. Hoek for sharing your time and knowledge with us. Your many contributions to rock engineering have proven to be extremely beneficial and not only for Rocscience but for the entire rock engineer community and you've seen from the questions, you are a mentor, and you did an amazing job throughout the 60 years of your career. With that being said we want to again extend a big congratulations on the lifetime achievement medal. Congratulations Evert. I also want to thank everyone who will be contributing to the conference over the next two days. The theme of this year is “The Evolution of Geotech: 25 Years of Innovation.” We hope events like this will provide opportunities for many conversations and learning. And spark new ideas that will help move the industry forward. We are very excited about what will be coming over the next two days and I look forward to welcoming you to the Rocscience International Conference 2021. Enjoy the rest of the day and see you all tomorrow. Once again, thanks Evert for your precious time. Thanks everyone, we'll see you tomorrow. Bye.

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