Development of Rock Engineering Transcript
This transcript describes the YouTube video "Development of Rock Engineering - Dr. Evert Hoek Lecture Series"
Text appears on a black screen reading “Rocscience presents.” It fades to black.
Text fades in “Practical Rock Engineering Lecture Series by Dr. Evert Hoek Copyright 2014.” It fades to black.
Text fades in “Development of Rock Engineering Lecture 1.” After a moment a title slide appears showing an image of a large open pit mine. The title “Development of Rock Engineering” is shown. The bottom of the slide, text reads “Practical Rock Engineering Lecture No. 1. Evert Hoek, January 2014.”
Dr. Hoek: The aim of this lecture is to try and present a brief summary of the historical evolution of the subject we know today as rock engineering,
This screen fades to reveal Dr. Evert Hoek standing in a large room in front of a fireplace with green walls and gold accents. He is facing an audience seated in front of him.
Dr. Hoek: which is the building of structures in or on rock masses. I'm going to take you through a series of slides that illustrate some of the key development points in this history.
A slide appears with an image of the Eupalinos tunnel on the left. It is 1036m long and referred to as the “double mouthed tunnel” by Herodotus because it was started from both ends to meet in the middle. After a moment, the slide moves to the top right corner to reveal Dr. Hoek on screen.
Dr. Hoek: This slide shows a tunnel in Greece that was constructed 2,500 years ago to bring water to the city known today as Pythagoreion on the Greek island of Samos. So, rock engineering has been with us for a long time. What's remarkable about this tunnel is that it was started at both ends and met in the middle, so it's really more a success story about surveying than about rock engineering. But nevertheless, there you see that the subject has been around for a very long time. In the early days it was generally soil that was a problem, so if you were building a building and the foundation was on soil, you tried to go down until you hit rock and when you hit rock, the problem was at an end, and you assumed there were no difficulties with the foundation. And it was not unusual to assume that rock was infinitely strong, as you see in this slide taken in BC in the 1920's.
A slide appears in the top right corner. On the left is an old image of a roadway with an old car beside a body of water. A large rock mass overhangs the road. Text on the right reads “In the early 1900’s it was frequently assumed that rock was infinitely strong.” After a moment, the slide becomes fullscreen and then returns to Dr. Hoek.
Dr. Hoek: There was one area where there was a serious problem with rock, and that was in very deep level mines in the Kolar gold fields in India and in the deep level gold mines in South Africa, where they were mining at depths of in excess of 2 kilometers, sometimes 3, and where the rock stresses are so high that the rock effectively implodes into the excavation you've created.
An image is shown on screen of a miner in an underground excavation looking at a large number of rock pieces. Text reads “the results of a rockburst in a mine.” Transitions back to Dr. Hoek on screen.
Dr. Hoek: And that's known as a "rockburst." Not a lot was written about it, because this was an embarrassment to the mining companies. A lot of miners were killed in these events and so relatively small groups were assembled to try and sort out the problem. And the evolution of that was a very long one and it still goes on today, because we still really don't have a means of predicting accurately or preventing these damaging events. The one thing that was unique about them is that they were generally in very hard good rock. The discontinuities in the rock were tight and so the whole subject was treated from an elastic point of view. The role of the discontinuities, or the joints, or the faults, in the rock were not really an issue. So, all of the early literature of the subject dealt with rock as an engineering material, rather like steel or concrete, which is a little bit artificial, but that was they was the history. Although we learned a lot, we had to supplement that in later years with the introduction of discontinuities. So, there were tests done in a lot of laboratories around the world, particularly geophysics labs in the United States, where a lot of excellent work was done on the properties of intact rock in their case for the purpose of earthquake prediction, in our case in rock engineering, to gain an understanding of the behavior of the rock.
A slide appears in the top right corner. A drawing of a triaxial cell for testing rock specimens is shown on the left. An image on the right shows hands opening a triaxial cell. After a moment, the slide increases to full screen then returns to Dr. Hoek.
Dr. Hoek: So, this is an illustration of one of the tri-axial cells that was used to test as you see there, a sample of core, diamond drilled core of rock, which has tested under confinement with various gauges attached to it. We had no computers in those days, personal computers really only became available in the 1980's and in the late 50's, 60's, 70's, we had to make do with other tools. One of the most powerful of those was photoelasticity, in which if you view a stressed transparent body in polarized light, very often you'll see colored fringes. And you see this quite frequently in the windscreen of a car in certain lighting conditions. But this is a very precise tool,
A slide appears in the top right corner. The image on the left of a slide shows a man looking at a high-capacity biaxial loading frame. The image on the right is a black and white illustration with wavy black and white shapes showing the photoelastic study of tensile cracking from a hole in a glass plate. It becomes fullscreen and alternates between Dr. Hoek and the slide being shown on screen.
Dr. Hoek: and this slide illustrates capacity frame in which we were able to load glass plates of about 15 centimeters squared under very high loading and by ultrasonically drilling cavities into the glass plates you could study the behavior of cracks or tunnels or whatever you were interested in. And on the right, there you see a photoelastic pattern showing stress distribution on a circular hole and you'll notice that there are vertical cracks in the top and bottom, the roof and floor, of the of that simulated tunnel. And this technique played an important role in understanding the mechanics of how cavities behaved in rock masses. The rock masses, remember, were in those days considered elastic. We didn't have the tools to do anything else. One of the things that did concern us a great deal was the difficulty of incorporating gravity into our theoretical models. We had theoretical models which were solved by traditional methods of log tables and calculators, slide rules. A very tedious process. But it was very difficult to include gravity in those calculations and so the idea arose that we could use a centrifuge to simulate gravity.
A slide is first shown in the top right corner and then expands to full screen. A photo of a man looking at a 3m diameter 1000 g centrifuge to simulate gravity loading in a mine model. The centrifuge is a large silver and grey machine. Below it are two photos of cracks, the image on the left showing uniform loading which the cracks are more linear. Gravity loading is shown on the right with less uniform cracking.
Dr. Hoek: And this is a centrifuge built in about 1960, 3 meters diameter, 1000 G capacity, so you could increase the gravity on a model by a factor of 1000. And you see in the lower part of the slide a plaster of Paris model of a mining sequence with uniform loading on the left, and gravity loading on the right, and the failure patterns are quite different, as you would expect.
Transitions back to Dr. Hoek on screen.
Dr. Hoek: So, these kind of tools helped us a great deal in understanding how failures propagate in intact materials. Centrifuges are not really used in rock mechanics these days, because even with a 1000 G, you can't break intact rock and if you're dealing with a discontinuous rock, it incredibly difficult to construct a model that's going to survive a centrifuge test in any meaningful way. They are very heavily used in soil mechanics for testing all manner of structures and simulating failure and behavior of soils.
A slide appears on screen. In the top left, an engineer stands near a model of orange spherical pieces of material behind glass forming a v shape. Text below it reads “Model of slope failure in jointed rock (PhD study by N. Barton, Imperial College of Science and Technology, London.” The lower right image shows a physical model of a coal mine roadway with text in brackets (Australian Coal Industries Research Laboratory). Alternates between showing this slide and Dr. Hoek on screen.
Dr. Hoek: There were other kinds of models used in those days, and the one on the left is a large model that was built at Imperial College in London for testing jointed rock slopes. Gravity was simulated in this model by constructing the model on a glass plate horizontally and then putting another glass plate on top of it and turning it into a vertical position. So, gravity was turned on by simply rotating the model vertically. And, as you can see in the slide, failure of the jointed rock slope was created. The lower right is a model looking at the behavior of coal mining roadways. And you'll see the coal pillar right and left of an opening and the timber struts very realistically constructed at a relatively small scale in the model. So, we went along for many years really playing, if you like, with the peripheries of the of the subject but learning a great deal about the mechanics on the way. Until the 1960's, where this was really a wake-up decade for rock engineering in both mining and civil engineering.
An image appears of a dam failure. The slope is on the left side with greenery on it. The dam comes down from the slope and across a running body of water through the middle of the image. Text reads “Photograph by M. Diederichs.” After a moment, the slide moves to the top right corner revealing Dr. Hoek on screen.
Dr. Hoek: And this slide illustrates a dam in France, the Malpasset dam, which failed in December 1959, during filling of the dam. It's a concrete arch dam, so you're looking upstream at the reservoir, you're standing downstream of the dam. And the failure was caused by a fault within the reservoir, which seemed to be safe when you looked at the mechanics, but the loading of the water pressure created as the reservoir was filled, caused that fault to slip and the dam to fail. A more serious failure occurred at Vajont in Italy in 1963.
A slide is shown in the top right corner. In the slide there are two images. The image in the top left shows The Vajont arch dam, with the dam taking a curved shape and the reservoir behind it to the left with mountains behind. The image in the lower right shows the slope beside the reservoir failing. After a moment, the slide expands to fullscreen.
Dr. Hoek: And here you see the dam on the upper left, the dam's concrete arch dam being impounded and the reservoir behind it accumulating. And in the background, you see a white scar on the mountain up here. This mountain is called Mount Toc, and you see a white scar there, which illustrates instability. And the general consensus at that time is that this would not be a problem in the filling of the of the reservoir. That proved to be wrong and the lower right you see the consequences of a massive slope failure where the reservoir was almost completely filled with debris. It caused the water to overtop the dam by 100 meters. And miraculously, the dam remained, but downstream of the dam,
A new slide is shown with two images. The image in the top right shows the buildings of Longarone from a hillside. The bottom image shows the remains of the town after the failure where debris covers the lowland of the valley and only the houses on the hillsides remain.
Dr. Hoek: the town of Longarone, which you see before the failure on the upper left and the lower right shows the complete destruction of the town. And 2,400 residents were killed in that flood that resulted from the overtopping of the dam.
Transitions back to Dr. Hoek.
Dr. Hoek: In the mining field, a lot of coal mining, for example, was done by room and pillar mining, where the coal seam, which would be horizontal, is extracted by removing a cross pattern of tunnels, leaving a pillar at each intersection. And the tendency is to rob those pillars because that's a valuable resource you're leaving in place, and in many cases the estimates are wrong, and the pillars failed. I don't have a photograph of the failure that occurred in South Africa. In about 1963, a coal mine collapsed and instantaneously killed 430 miners, who were underground at the time. I don't have a photograph of that, but I have a photograph of a mine elsewhere,
An image is shown of three miners standing in an underground excavation with a large pillar on the left-hand side. It then transitions to an image of the surface after the failure. The image is overlooking aslope that has failed and debris runs down towards a road. In the background are hills and mountains.
Dr. Hoek: metal mine, and this is what it looked like. There are the pillars and that's what it looked like at surface after the failure, which actually occurred while I was underground.
Transitions back to show Dr. Hoek on screen. Alternates between the photo of the failure and Dr. Hoek once more.
Dr. Hoek: We had driven in in the morning, parked on the paved area that you see there, overlooked the valley, then we changed and gone underground. And while underground, it became increasingly evident that things were not going well. So, we were all evacuated and when we got to the surface, the parking lot was gone and there was a big hole in the ground. So that kind of failure is a very serious event. And it occurs more frequently than we like to admit. But these events, all of them, the dam failures, the underground pillar failures, were a wake-up call that we were really pushing rock to its limits, and we had to do something more serious than playing with the toys that we'd used up to that point. At that time most of the problems, as I said earlier, where soil-related so when you drilled typically for a foundation, you drill down until you got half a meter into the rock and you've said, "that's it, we're not going to do any more drilling, we've got enough information." So, the techniques for drilling in rock were very poorly developed. The machines were hopelessly inadequate. There was no real method for storing a core securely.
A slide is shown on screen. The image on the left shows a man looking at an orange inadequate drilling machine from the 1970’s. The image on the right is of a man looking down at broken core samples in the dirt after vandalism of an insecure core storage. Transitions to show Dr. Hoek on screen again.
Dr. Hoek: And you see on the right there, a core in Sri Lanka on a dam project which had been vandalized and is of very little use for gathering information. Fortunately, over the years, due to a lot of pressure from clients and consultants, the drilling industry improved enormously and today you can demand core in almost any location and expect and receive almost 100% core recovery. So that problem has been largely overcome.
A slide is shown in the top right corner. After a moment it becomes fullscreen. The top left image shows a large number of boxes of drill core laid out for inspection. In the bottom right image, engineers are standing near logging drill core from a mining project. After a moment, Dr. Hoek returns on screen.
Dr. Hoek: So today on visiting a large project, you would very often see the core laid out. In the lower right there, you see probably several kilometers of core laid out. A very high-quality core and that is map then to determine the characteristics of the intact and other discontinuities that occur in the rock mass as you drill through it. When we look at the impact of discontinuities on the behavior of a rock mass, which is almost exclusively the topic that we deal with today, so we have the intact rock behavior there as the core, but the behavior of most engineering structures in rock are controlled by the discontinuities. We have to consider the issue of scale. And I'll come to some large-scale issues in a moment.
A slide is shown with an image of two engineers with their hands pressed on a rock face. Text to the right reads “In many cases heavily jointed rock masses can be treated as homogeneous engineering materials.” After a moment, the slide disappears and Dr. Hoek is shown on screen.
Dr. Hoek: Starting at this sort of pattern, where you have a very densely populated joint pattern in a rock mass, this is a volcanic area, where there's been a lot of fracturing of the rock due to tectonic activity and a very large number of joints closely spaced. So, when you stand back from that and you consider building a tunnel or a slope in that, it almost looks like a homogeneous rock mass, like a sort like a sandy soil with very big grains. And so, in the early days, we were able to do quite a lot of work by assuming that rock masses were homogeneous, uniform in their behavior. And this then led to the evolution of a number of classifications.
A slide with a list of Rock Mass Characterization and Classifications. They will be described verbally by Dr. Hoek. The screen alternates between this slide and Dr. Hoek while he explains them.
Dr. Hoek: Remember we’re still talking about today's when computers weren't available. We're still looking for techniques for gathering and interpreting information that have to be rather simplistic in their nature. One of the best classifications is a qualitative description of rock masses by Terzaghi in 1946, and it's still worth looking at the words that he wrote to describe what he visualized different rock masses would do, how they would behave in tunneling. Then the classifications tended to be become a little more quantitative. In 1958, Laufer in Germany, produced a classification for the stand-up time, how long do you have to work in a tunnel before it falls on your head. And then the most quantitative effort was by Deere in 1963, where he produced the Rock Quality Designation, which was a measure of the number of intact core pieces longer than 10 centimeters that you get in diamond drilling a core. So, if you had 40 centimeters of pieces longer than 10 centimeters in a 1-meter core, the RQD of that core would be 40%. And then followed a number of classifications designed primarily for the estimation of support in tunnels: 1972 Wickham & Company, 1973 Bieniawski, in 1974 Barton, 1977 Laubscher. And they all followed a similar pattern with an increasing number of parameters, all of which were estimated or measured from core or from observations in the field. 1980, there was the introduction of a different type of classification or characterization by myself and Brown and this aimed to provide from qualitative descriptions of the appearance of the rock mass, a table or a chart which enabled you to estimate the rock mass strength and deformation characteristics. One of the best constructed classifications was in 1990 by Pierson & Company in the United States for rockfall hazards in highways. It includes a number of factors, other than geological, including line-of-sight and speed of vehicles and so on, so it's a very comprehensive classification used for designing against rockfalls. And finally, an additional one by Palmström in 1995. These were all very important developments and enabled us to move things ahead, to estimate parameters, to estimate tunnel support, and are still used extensively today. One of the things they did do, was to give us a feel for what the properties of a jointed rock mass might be.
In the top right corner, a slide appears. The top left plot shows the estimation of reduction of laboratory rock strength due to the presence of discontinuities. In the bottom right, a plot of the estimation of deformation modulus based on characterization of the rock mass.
Dr. Hoek: And so here are a number of plots from the different classifications. The top one showing the ratio of the rock mass strength to the laboratory strength, in other words, how much is the intact specimen you test in a laboratory degraded by the presence of discontinuities. So, they would go up to 100% on the right-hand side, meaning intact rock, and as the rock mass rating, or the geological strength index, or Barton's Q rating decreases in value, the strength decreases. And similarly, the lower right is for the deformation modulus, how much does that decrease in terms of rock mass quality.
Transitions back to Dr. Hoek on screen.
Dr. Hoek: These then for the first time, gave us together with the evolution of computer software and hardware, the ability to do some very serious stability analyses.
An image appears in the top right corner. It shows a plot with a bell curve, with red underneath the left portion up to 1 and purple above 1 up to 1.21 for FOS. Text to the right reads “probabilistic analysis of the factor of safety and probability of failure of a slope – both measures of the hazard of slope.” A model of a slope is on the bottom right with different material layers and a blue line running through the middle. Alternates between Dr. Hoek and the slide shown on screen.
Dr. Hoek: This is a fairly recent example, but it illustrates the principle of where we were in the, I would say, the early 80’s and from then on. This is a large rock slope shown in the lower right there, which is actually a flow of andesitic lava from a volcano inland on an island and that lava flowed over a weak marine sediment. So, you have quite a strong jointed rock mass resting on a very weak marine sediment base. And so, this represents, the slide you see there, represents a probabilistic analysis of the factor of safety of that slope for different conditions. So, there were five thousand attempts made varying the properties of each of the component materials within a distribution and producing a factor of safety diagram seen on the upper left. A factor of safety one implies failure, and so that gives you them the red part to the of the distribution curve. If you take the area of that, divided by the total area, you get the probability of failure, which in this case is 18 percent. So, this introduced us to A) the ability to do serious stability analysis, and B) the beginnings of what today is very commonly done in rock engineering design: the use of risk analysis.
The Risk= Hazard x Consequence is shown and will be described in the table below.
Red= Very High Risk Orange= High Risk Yellow = Moderate Risk Green = Low Risk | Very High Hazard | High Hazard | Moderate Hazard | Low Hazard |
Very High Consequence | Red | Red | Orange | Orange |
High Consequence | Red | Orange | Yellow | Yellow |
Moderate Consequence | Orange | Yellow | Yellow | Green |
Low Consequence | Orange | Yellow | Green | Green |
After a moment, the table is shown in fullscreen and then returns to the top right corner.
Dr. Hoek: Risk, as shown in this matrix, is a product of hazard, which is the probability of failure, and consequence. It's no good calculating the probability of failure if you don't know what the consequences of that failure will be. So, you could have for example, as the case we've just analyzed, a moderate hazard as an 18%, would be a moderate hazard but very high consequence, and that means you'd be in this box of risk. Similarly, you could have a very low hazard with low consequence that you don't have to worry about.
A slide is shown in the top right corner. It is a triangle that is red at the top (intolerable; risk can only be accepted in exceptional circumstances) and green at the bottom (tolerable; negligible risk) which has a gradient from one colour to the other. There are two blue lines going through the triangle. There is an arrow pointing towards the red which is labeled increasing risk.
Dr. Hoek: This has been a summed up in this diagram, which is very often called the A-L-A-R-P diagram. A-L-A-R-P meaning as low as reasonably practical. So, if you look at that diagram, the increasing risk is on the vertical scale, and you have negligible risk at the bottom, where you'd be in very good rock, shallow depth, easy construction, and then as you go deeper and deeper, the rockburst problem we talked about earlier on, would be at the very top end there, where you don't go there unless you really have to. And in the middle is the ALARP area, and the question that you have to then assess as you're designing a structure is “how much money do I have to spend in order to make the risk of that structure acceptable?”
A new slide is shown on screen of a plot. Text beside it reads Australian Nation Committee on Large Dams (ANCOLD) recommendations on acceptable risk, 1998. After Ashby, 2006. The points to note are explained verbally by Dr. Hoek. The slide alternates between fullscreen and being showed in the top right corner so that Evert is visible.
Dr. Hoek: This is a plot, and there are many, many versions of this, but this one was put together by the Australian National Committee on Large Dams, and it shows in the red plot at the bottom there, the ALARP risk that is considered acceptable for major structures today. This point here is an interesting one because that is the risk of an individual, you, or I, living a normal life, not doing skydiving, or extreme sports but living a normal life. That is the risk of being hit by a bus or having a heart attack or whatever one in 10,000 per annum. And so that's considered the pivot point for most of these diagrams and as the number of fatalities increase here, so this is annual probability of occurrence of n fatalities plotted against n fatalities. So obviously, you want to stay if you can, within that diagram or below it. The squiggly lines here are built on historical data and refer to fatalities that have occurred in fires, in landslides, explosions, as they've been identified on the captions. And clearly all of those are unacceptable, so you try and if you can, stay below that. There are some circumstances where you might want to go above the upper red line there. For example, in an open pit mine, you have very tightly controlled limited access. Only people who have gone through rigorous training are allowed to enter the property. There is continuous monitoring of displacements of many things, there are evacuation plans. And so, under those circumstances, it's justifiable to go as high as you can justifiably go for the risks involved, but certainly over the red band there. So, the risk process is becoming increasingly important and will become a dominant part of rock engineering in the future. The big issue is where do we get the data from? Because we're not dealing with man-made materials. We're not dealing with steel or concrete; we're dealing with what you're given in a rock mass and the factors that have influenced it over geological time. The earthquakes, and the plate movements, and so on. And so, it's a highly variable material and we have to accept that. So, we're always going to be data deficient in our construction of these kind of diagrams when applying them to rock engineering. And then we come to the question of scale. I mentioned earlier on, so far, we've just looked at rock masses that we can treat as homogeneous. But if you have very large structures like faults, shear zones, or continuous joint planes, these can dominate the whole behavior.
A new slide is shown in the top right corner. The image on the left of the slide is a black in white image with a miner standing in a large mine. Text below reads “Cut and fill stope in the Mount Isa lead/zinc mine in Australia, 1967.” The photo on the right is of a man in a suit standing on top of a bedding plane. Text below it reads “Bedding planes in the foundation of the Meadowbank arch dam in Tasmania.” After a moment, the slide becomes fullscreen and then alternates with Dr. Hoek shown on screen.
Dr. Hoek: And in the upper left, you see a cut called a cut and fill stope, which is an excavation created to mine an ore body and its shape is controlled entirely by the two major parallel structures you see there, the bedding planes top and bottom, and the cross jointing. So that's not a mistake, that's how it is mined to take advantage of the natural control of those features in the rock mass. And on the right-hand side, this is in the foundation just below a major arch dam in Tasmania and you can see that the horizontal bedding planes there, would have been critical in evaluating the stability of the foundation. So, what do we do in order to incorporate this into our geotechnical models?
A new slide is shown with an isometric drawing showing intersecting joints in rock with a variety of different colour planes. The bottom right is a complex Discrete Fracture Network model for the Chuquicamata open pit mine in Chile.
Dr. Hoek: We call this model a discrete fracture network and in simplistic terms, in the upper left you see, an isometric drawing of a number of intersecting discontinuity features, joints or faults or shear zones. And in the real world this is what it would look like. This is a large open pit mine in Chile, which I'll show you in a moment and you can see the myriad of discontinuities crisscrossing there. And that is the joint fracture network that we have to contend with. The dark-blue fault running, trace running right through there is the major fault that runs right the length of Chile and is the source of most of the of the mineral. This is a copper mine.
A new slide is shown in the top right corner of an open pit mine with blue lines tracing a number of different lines throughout the mine. Text reads “traces of important structural features in the Chuquicamata open pit mine.” It becomes fullscreen and then transitions back to Dr. Hoek.
Dr. Hoek: Looking down on an isometric drawing of the open pit as it is today you can see the traces, that means where these features intersect the surface, of the myriad discontinuities that occur in that rock mass. This mine is four kilometers long in the vertical axis, three kilometers wide, and one kilometer deep. And so, there's a lot of rock there that has to be dealt with.
A new image is shown in the top right corner of a side profile of an open pit wall with more blue lines scattered throughout. It becomes fullscreen. A red arrow highlights Bloque1_19 which is in the middle of the wall.
Dr. Hoek: And one of the examples I'd like to quote in this lecture, and to illustrate how its dealt with today is this one here which concerns the stability of this block of rock here. That slope that you're looking at there is about 300 meters, the top 300 meters of a 1000-meter pit, and the concern is how stable is that block. The process that is used here is to map the structures from either mapping along the benches or from core and to build up a discrete fracture network and pictures such as this one, where you know the features are.
A new slide is shown with three models. The top left shows the 3D discrete element model. The middle shows the distribution of rock types, and the bottom right shows the incorporation of Discrete Fracture Network in model. Transitions back to Dr. Hoek.
Dr. Hoek: And then to incorporate those into three-dimensional discrete element models, of which this is one. So, the top left is the mesh network used to create the three-dimensional model of that slope. The middle one is the distribution of rock types because there many rock types that occur in that rock mass in that model. And the lower right is the superimposition of the major discontinuities, the ones you saw as long blue lines in the previous slide, into that model. And that model, which is a very complicated one construct and to run, produces then the behavior of the rock mass.
A new image is shown in the top right corner of a Computed horizontal displacements in 3D numerical model. It is blue but shows green, yellow, orange, and red, in the center along a certain portion of the slope benches. It becomes fullscreen and then transitions back to Dr. Hoek being shown on screen.
Dr. Hoek: And what is illustrated here in the colored contours are the horizontal displacements in that part of the slope. In order to calibrate that, which you have to do with any numerical model this particular mine Chuquicamata in Chile has been in operation for a century, and for the last 20 years has had a very extensive network of monitoring displacements.
An image on the left of an olive-green machine on a table with an open pit mine in the background. Text on the right reads “Automatic displacement monitoring of the pit slopes on which over 1000 targets have been installed. It becomes fullscreen and then alternates between Dr. Hoek and the slide.
Dr. Hoek: So, you have two systems in place: one is a distance measuring device based on laser beam which is shot to a mirror target, and the travel time is measured between its exit and return, and there are a thousand monitoring points around the slope and there are six monitoring stations with automatic distance-measuring devices and those scan the pit on a regular interval, so it's under constant surveillance. We're a zone of instability is shown in the general picture that can then be focused on in much greater detail by radar equipment.
A slide is shown in the top right corner with two images. The top left image shows radar equipment used for displacement monitoring on a trailer with two engineers overlooking an open pit mine. The image on the bottom right shows displacement contours compiled from optical and radar measurements. Red, yellow, and orange are overlaid onto a benched slope wall. It becomes fullscreen and then returns to its position in the top right.
Dr. Hoek: And there are a number of these devices which are radar transmitters, which send out a radar beam and which look at the rock itself, not a target, and then measure the return time for movement of the rock face. And you see here contours of displacement which match those predicted in the numerical model. So, over the years, the technique for monitoring and modeling have gone hand in hand and the net result of that is a process where if you know that there is instability you can plan to do something about it. You can plan to divert your mining activities to take the failure out or do various things. You can't reinforce it on this scale because the slope is simply too large.
An image of a top-down view of the Chuquicamata mine is shown so you can see the scale of this large open pit mine. It becomes fullscreen and then transitions to Dr. Hoek.
Dr. Hoek: So, this is the Chuquicamata mine today, probably the world's biggest open pit mine. As I say, it's four kilometers long on this axis, as you're looking at it, three kilometers wide, and a kilometer deep. And the little dots that you see on the highways there, that one there for example, is a 400-ton capacity truck. So that gives you a feel for the scale of the whole operation, about 500,000 tons of rock are mined every day in that mine. So where do we go from here? In the early 2000’s, the concept was introduced of what is now called a synthetic rock mass.
A slide is shown on screen. On the left is a cylinder of yellow spheres with text reading intact rock representation (including brittle fracture) with an arrow pointing towards the center colourful cube. A colourful cube with jagged shapes representing fracture representation – 3D discrete fracture network with an arrow also pointing to the middle cube. The middle cube shows the bonded-particle assembly intersected with fractures. Synthetic Rock Mass (after Cundall, 2008). This slide alternates between fullscreen and top right corner to reveal Dr. Hoek.
Dr. Hoek: And this has been evolving in many centers around the world, but the most intense development has probably followed from the work of a man called Cundall in the US. And what that does is it looks at a numerical model where you can take a discrete fracture network, shown in the upper right there, integrate it with the blocks in between the discontinuities, which are then modeled separately, as illustrated on the left-hand picture of a compression specimen, and produce a model which looks at the behavior of both the discontinuities and of the intact material. We have a great distance to go on this, but it opens up huge possibilities of much more sophisticated modeling than we've done in the past and much more productive and interactive design processes. The big problem is data deficiency, where do you get the data to populate these. And the other problem is computer capacity, because as you can imagine, you need a huge computer power and very sophisticated software to do these kind of models.
A final slide is shown on screen with black and white illustrations in the top left showing discrete element model including individual properties of individual grains and grain size distribution. The image on the bottom right shows the predicted and observed tensile and shear failure in the walls of a large diameter borehole due to thermal loading. Modified from Lan et al, 2010. Transitions to alternate between Dr. Hoek and this slide being shown on screen.
Dr. Hoek: And just as an example of where we might be going, this comes from work done under the direction of Professor Derek Martin in Alberta in Canada, where he's modeling down to grain scale, so the upper left shows pictures of grains in a granitic rock mass, up down to one millimeter in size, and these grains have individual properties and are the sizes are distributed according to the size distribution in the rock itself. The contact strength can be modeled and that then produces results, which are increasingly approaching those that we actually measure or detect in the field. So the lower right there shows some three sketches, three pictures from an experiment in southern Sweden, where in the nuclear waste industry there they are looking at the disposal of nuclear waste in crystalline rocks, deep underground and one of the big issues there is the fact that when you put nuclear waste in the ground in the first place it's very hot, and so you induce thermal loading in the rock from the waste before it decays in radioactivity. And you can see the first slide there model, from this model up here, you can see little marks on the screen there, which indicate either tensile or shear failure. And those then match pretty well the observations which are sketched in the middle slide and the photographic evidence that you see on the right.
Transitions back to Dr. Hoek on screen.
Dr. Hoek: We're not there yet, but it's indicative of where we will go as our skills in gathering data, of building software, and hopefully the hardware industry will meet our needs for much, much greater power than we have today. Thank you.
Fades to black.
Text appears “Practical Rock Engineering Lecture Series is sponsored by Rocscience. Software tools for rock and soil.” Fades to black.
Text appears “Filmed on location at the University Club, Toronto, Ontario, Canada, January 2014. Technical production by Harvey Montana Production. An image of an abstract painting is shown. Text below reads www.seanharveydp.com.
This ends the video.
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