 Good afternoon, everyone. Welcome to the Coggy webinar that we hold periodically. We appreciate your attendance. We got quite a lot of interest in today's presentation, and so I'm sure you're going to find it quite interesting. I'm Alan Maher. I am the chair of the Committee on Geological and Geotechnical Engineering, which is a standing committee of the National Academies of Science, Engineering and Medicine, and particularly the Board on Earth Sciences and Resources. This committee was established to be the focal point for the National Academies for Government Industry and Academia on technical and public policy issues related to earth processes and materials, soil and rock mechanics, responsible human development and mitigation of natural and human hazards. If you have questions about the committee, please contact Samantha Magsino, who acts as our staff director for the committee and does a very able job for us. This webinar is part of the quarterly series that is produced by the committee Coggy through the support of the National Science Foundation, which we greatly appreciate. Without their support, we couldn't do this, and the webinar will be posted on YouTube. An announcement will be sent out to all those of you who are registered as soon as it's available, and that's usually just a few days. You can open your chats to send messages and receive messages from us if necessary and to see the speaker's bio in more detail. Sam and Emily Bermudis set up this webinar, and Mandy Enriquez is producing it. Without their help, this would just not work. They usually make it go quite flawless, so thanks for their help in advance, because when this ends, it ends abruptly, and I won't have time to give or an opportunity to give thanks, except to our speaker. We will have time for Q&A after the presentation. You can submit your questions at any time using the Q&A tab on the Zoom panel that you'll find on your screen, and I will pose as many questions out of this those you present. I'll try to organize them and moderate them and ask as many as the time permits. We do try to end pretty close to the hour. I need to say that any opinions, conclusions, and recommendations expressed by the presenter or even by any of the questionnaires in this webinar are those of the individuals and do not represent conclusions or recommendations of the National Academies of Science, Engineering, and Medicine. So with all of that to begin with, I'd now like to introduce Lord Robert Meyer. He is the founding head of the Center for Smart Infrastructure and Construction at Cambridge University in Cambridge, England. He's also Professor Emeritus of Civil Engineering at Cambridge University. He worked in industry for 27 years where he had founded or was a key player in the what is called the Geotechnical Consulting Group, GCG, still a very active firm in England and doing worldwide projects. He, in 1998, was appointed Professor of Geotechnical Engineering at Cambridge and head of the Civil Engineering Group. He has had extensive experience in designing construction of a wide variety of civil engineering projects in many countries, particularly those involving geotechnical engineers in underground construction. He was President of the Institution of Civil Engineers from 2017 to 2018. He's a fellow in the Royal Academy of Engineering, the Royal Society, and the United States Academy of National Academy of Engineering. He was appointed as what's called an independent cross venture in the House of Lords in 2015, which means his title is literally Lord Meyer. He's a good friend of mine and I like to tease him a little bit about the only Lord I know in the world. And I feel it a real privilege to talk and work with Robert. We're very privileged to have him here present today to talk about some of his professional experiences, particularly on a major tunneling project in London that's been completed and opened called Crossrail. And so with that, we're delighted to have you and look forward to your comments, Robert. Alan, thank you very much for your introduction. It's a pleasure to be speaking to Coggy this afternoon. So my lecture is going to be about the Crossrail project in London, which has recently been renamed the Elizabeth line in honour of our late Queen, Queen Elizabeth. And so when I'm talking about Crossrail throughout this talk, it is in now in fact called the Elizabeth line. So I'm going to give you an outline of my talk. I'm just waiting for the slide to change here. Here we go. So I'm going to talk about the Crossrail project, the importance of geology. I'm going to say some some things about the tunneling techniques and some of the innovations. I'm going to then go on to talk about settlement effects of tunneling, because this was right underneath central London and prevention of damage to buildings. I'm going to be talking about some advances in the technique compensation grouting. And I'm then going to focus on some tunneling effects on pile foundations. Quite a number of buildings are on piles and some of the tunnels went very close beneath the piles and I'm going to describe describe what we learned from that. I'm going to finish with some innovative fiber optic monitoring of underground infrastructure, specifically on the Crossrail project. So it was Europe's largest infrastructure project. It involves 21 kilometers of new twin bore, seven meter diameter railway tunnels right under central London. And as you can see on this plan, the red part are all underground. And there are a number of stations there are eight new subsurface stations shown in the big red, the big red circles. It's part of a much bigger scheme that actually links all the way from Heathrow Airport, right into central London. And when it arrives at Paddington. It goes underground, and it splits towards the east. You can see in on two different lines. So what I'm going to show you now is a snapshot of the geology. And before I do that, I want to emphasize that the vertical scale is in tens of meters. The horizontal scale is in kilometers, in other words, hundreds of meters so very distorted. Now what you're seeing here is the, the line of the tunnel and the stations, what winding its way through. And I'm going to in a minute just scan right across the project, but this big brown, light brown stratum you're looking at here is London clay. And it's a very well known stiff over consolidated clay, which is highly appropriate for being able to tunnel in almost open face conditions it's extremely strong clay. However, as we go further to the right of this, you'll see that as I, as I get this moving, you'll see that the geology changes you follow the line of the tunnel. The first half of it is in London clay and then you'll see that the geology changes quite a lot. And you can see that now it's going down into sands and gravels on a fault. And it, it, it has a quite complex geological route once you get away from the west end. So what we're looking at here is going from west to east. And it ends up in fact, in chalk that light green is in fact chalk, so there's a very big variety of different geologists I'm just going to show you show you this one more time. So this is the west end, starting in London clay for the first 500, about five kilometers. And as we traverse to the east, you again, you will see the tunnel alignment, moving into different geologists, and typically the tunnels are about 40 meters below ground. And the reason they're that deep is that they have to go under a lot of existing subway tunnels. And just to give you a flavor of the complexity of the geology of the Crossrail project, which involves a huge amount of of geotechnical investigation, huge amount of boreholes. So one, most of the stations are in the London clay that very stiff material I told you about, with one exception. And this, this is a plan view of Faringham station, which was in a different, it's directly in that fault zone that I pointed out. And that means that it's, it's in a very, very mixed and complicated geology, involving a lot of sand lenses, all of which would be under high water pressure. Typically, the water level is very close to ground level. And I'll come back to that problem a little bit later. These are the earth pressure balance tunneling machines used for the project for the running tunnels between the stations, seven meters in diameter. And for those of you not familiar with the principle of the earth pressure balance tunneling machine, the EPB tunneling machine. I'll give you a very brief description here. Essentially they are completely closed face, as you could see from the previous photograph. They're closed with a wheel turning with cutters on it. And the principle of this machine is that there is a, an actually a bulkhead, a solid bulkhead, and on the, on the cutting side, there is water pressure and soil pressure. These are quite high pressures which are resisted by the machine as it's moving forward. And the high pressure water and soil is then removed by means of a screw conveyor from the high pressure right down to atmospheric pressure and onto a conveyor belt, and carried down the tunnel. And all the time as this is moving forward, there are jacks pushing against the concrete linings that have been erected. And when there is sufficient room for the next lining, the jacks are retracted and the next lining is put in. But the main point to convey here is that there is a pressure maintained on the face to resist earth pressures and also to resist water pressures. And these EPB tunneling machines are now very advanced in technology and they've enabled us to be able to tunnel through much more difficult ground conditions than in the past. Here's a view of one of those machines being lowered down into a shaft, 40 meters deep. You can get an idea of the scale by looking at the people on the site standing around the shaft watching this machine weighing 550 tons being lowered down into the shaft. This is a view a little later looking down that same shaft, and you can see both two machines, they've been lowered, and they've been assembled. It's quite a complicated machinery that goes on the back of all the machines. And those two machines are ready to start tunneling going to the left of the picture, towards central London. Stations, what I've just been describing are the running tunnels, machines that construct the tunnels between the stations. The stations themselves are much bigger, about 10 meters in diameter, and they're also, as you can see from the photograph here, quite elliptical in shape. And that's partly for for economy, in order to be able to optimize the shape rather than being circular. And this is made possible by the technique of sprayed concrete lining, which is a means of creating quite, quite different shapes as part of the final design but also for the temporary construction. And you can also see in the in the plan, the, the whole layout of the station. And the station is about 200 meters long, which is pretty long for for a normal subway station. This is not a normal subway this is train size, and the stations are much longer. And, and I get to just give you a snapshot here of of house, the technique of using sprayed concrete is very versatile. So if you're constructing a large cavern of a region of around 10 meters, you can construct smaller ones first. So on either side here, you see egg shaped on the right hand side you can see it more clearly an egg shaped tunnel that was constructed first. And there's a similar one on the left hand side, and then they then eventually get demolished. And the bigger one, the full size tunnel is then constructed. But the, the sprayed concrete linings are extremely versatile and being able to create different shapes and can and have different construction techniques, but you get an idea of the size by looking at the, at the size of the operatives down at the bottom of the photograph. One to really show you here is is a large, a very large cavern 15 meters in diameter, which is a crossover cavern to allow the trains to switch tracks. And so it's 15 meters from one side to the other, and it's about almost a little less than 15 meters from top to bottom. But the reason for highlighting this is that this was in a complicated geology, only the top two thirds were in that strong clay that I was describing earlier. That strong clay London clay means that you can do these sprayed concrete linings in free air without having to worry about unstable ground. But the lower third in the lambeth group was in very much more complicated and more treacherous ground with extensive sand layers on the high water pressures. So the way in which this was constructed was to divide it into these different types of tunnel, you can see the, the egg shaped one on the left, the egg shaped one on the right, and they were constructed first. And then from number one and number four, what was then done was that there were special inclined vacuum well points at four meter centers that were drilled in down into the lambeth group to depressurize all of those difficult sand layers full of water pressure. And when that was done, the ground was then converted from being very treacherous and very difficult to do in open, open mode to a much more manageable material. In other words, if you, if you deal with the water and you, you remove the water pressure, then sand is completely okay to deal with, but the crucial point here was the, the inclined vacuum well points that were installed first. And that was the end product of a very large cabin. I had the idea to scale by the by the people in the invert of the completed tunnel. Now I'm not going to just give you a flavor of the settlement effects that I said I would talk about. And the prevention of damage to buildings. At this point, I'm going to pay tribute to a mentor of mine, who I learned from whom I learned a huge amount. Sir Alan meal would, who was a very distinguished civil engineer and tunneling expert. He was a graduate Cambridge University. He was a president of the institution of civil engineers, and he made a very powerful statement. He said, it has been said that a tunnel is a long cylindrical hole through the ground. With a geologist at one end and a group of lawyers at the other. He then said, yet more dire is the present day phenomenon of lawyers at each end. Now, that's particularly relevant when it comes to tunneling under a whole lot of extremely valuable buildings in central London. There was a lot of sophisticated modeling and analysis done. Nowadays we, as many of you will know there are very. There's a lot of software around. One has to be pretty careful about the assumptions that one makes, but essentially three dimensional finite element analysis, if it's done well can be an incredibly powerful. Means of making these assessments involves complex modeling of nonlinear stress strain behavior of the soils. We often use models like the cam clay model. We need to know the soil stiffness properties we need to know the horizontal in situ stress and crucially we need to know the soil permeability. And that really dictates an awful lot of the of how successful a tunneling project is. And I'm this point going to talk about how we protect some building some of the predicted movements were going to be very large, which would have damaged the buildings. Many of you will be familiar with the principle of compensation grouting. But essentially, what this means is that if you're going to construct a tunnel beneath the building before you do it. You might be assessing that there would be severe settlement without compensation grouting. So before any tunneling a shaft is excavated you can see that on the left hand side of the slide and from that shaft. Tubes are drilled into the ground, known as to buy my shirts. These are sleeve tubes, and they're all installed in the ground prior to any tunnel being constructed. And then as the tunnel is constructed injections are made from those to buy my shirts in order to compensate for the movements that the tunnel is causing, and thereby you prevent those movements from getting up to the building itself. And you can see that there is a solid line, saying slight settlement with compensation grouting. So you reduce the very what would have been very severe settlement to much, much less damaging settlement that's the principle of compensation grouting. It was used extensively on this cross rail project. Bond Street station. Many of you who know London will know that Bond Street is in the middle of Mayfair. It's probably the most expensive real estate in the whole of London. And so, if you look carefully, you can see that there are little white circles, five of them. And these were all shafts. And from those were drilled before any tunneling. These tubes, you can see these little black lines going out from every one of those shafts protecting all those buildings shaded in blue, we're all being protected by compensation grouting. And you can also see in the same slide, the outline of the station construction, the two big platform tunnels on either side with the cross passages in between and various escalators as well. Now one thing that was particularly innovative on one of the stations. This is the slide I showed earlier, Liverpool Street station, which involved, it was constructed. This station was constructed underneath a number of very important buildings, and it wasn't possible to do the compensation grouting in the way I described. So a completely new and novel idea was put forward by the contractor. So this is a big, it's called, it's actually called Finsbury Circus. So this is it in fact, a series of very big office buildings on massive foundations, and actually that the contractor put a shaft down in the middle this is a cross section below here. So this is the actual tunnels being constructed for the station, but halfway down, they constructed some special tunnels shown in blue, specifically for the compensation grouting. So they were. This is pretty innovative. They actually elected to construct these small tunnels, four meters in diameter to, to actually install the compensation grouting. So these tunnels, you can see these operations going on. They were dedicated only for compensation grouting. And so you can see over here on the right hand bottom right hand side you can see holes. You can see on the left hand side, a grout tube going into another hole, and the top left is a guy who's controlling the grout injections. This is a very successful operation. But what was impressive was that from those tunnels, you can see all of the grout tubes that were drilled to protect the buildings that you can see in the photographs, the three big buildings there. This is unusual and very innovative to actually construct temporary tunnels specifically for the purpose of protecting the buildings from constructions of the much bigger tunnels beneath you can see the outline of the station, the two big platform tunnels shown in blue superimpose on here. So, that was one very big innovation. And now I'm going to talk about another issue, which is pile foundations. This is a great picture that killer killer produced. Many of you will know killer. And you just got to imagine that you are one of these three people standing down here, and you've got to imagine all the soil has been removed. And what you're left with is the myriad of other obstacles in the ground when you're contemplating a new tunnel project. And you can see the pile foundations supporting high rise buildings, you can see other tunnels. And what was very challenging for the cross rail project was that we were often tunneling extremely close, sometimes directly beneath pile foundations. So that takes me to some work that we did at my university in Cambridge, which was directly of value to the cross rail project. This is a big geotechnical centrifuge in Cambridge. This is 10 meters in diameter. It's an amazing piece of equipment, which enables us to model scale to take model tunnels, reduce scale typically 100th of full scale and then accelerate them to 100 times G. And then it stresses that you would actually get in in the real problem. So what we were specifically looking at was the effect of a tunnel being constructed underneath a loaded pile in clay. And on the right hand side, you can see a close up of a pile. It's a half. A circular pile cut in half up against a glass screen. And you can see the, the, the pie, the tunnel 62 millimeters in diameter. This was actually tested at 75 G. So that means it's, it's, it's typically modeling something like a five meter diameter tunnel at full scale involves a big package. On the left, that whole package that goes at the end of the centrifuge weighs about a ton. And so it's, it's got a lot of clay, it's got a lot of instrumentation, and it's can tell us a great deal about what happens when you construct a tunnel under control conditions beneath a heavily loaded pile in clay. And just at this point I want to just introduce you to the concept of volume loss. So normally when you construct a tunnel, you get a settlement trough shaped like this. And there is in fact a, it's like a Gaussian distribution. And we can very quickly established that there is a volume of ground in that settlement trough. The volume is sometimes expressed as a, as a proportion of pi D squared divided by four, which is the, the volume of the tunnel. And that's a very useful indicator as to how much deformation, the tunnel is causing, typically nowadays that volume loss is only around 1% for modern earth pressure balance machines, typically 1% sometimes even less. What I'm going to show you now are some images of a centrifuge test in progress so that the you will see the volume loss in this bottom right box here, increasing steadily. The image analysis means that we're able to get a complete picture of all the deformations being caused of the ground and the pile as the tunnel is being constructed with increasing volume loss. So basically it's now going up to quite high values to to up towards 3% 4% and on the right hand side is the is the measure of the movements the red being the higher movements. The point of all that is that we get an extremely accurate picture as to what actually happens when you put a when you construct a tunnel beneath a loaded foundation. I'm not going to look at all these graphs, but if you look at the, the one in blue on the left hand side, what is plotted there is the change in the load on the pile, because it has a load on it. As the volume loss increases, you can see that the pile the load in the pile starts to decrease because of effects a bit like negative skin friction as the ground is moving around the pile. It's actually tending to put the pile more into tension it actually stays in compression, but it is reducing the load in the pile. On the left hand side, there is a in black is the completely different response of a pile to one side of the tunnel, which actually increases the load quite significantly to compressive load in the pile. The message from this quite complicated slide is that the response of piles to tunnels constructed directly beneath them very much depends on the position of the pile in relation to the tunnel. And so, I'm now going to just show you a case in Crossrail of exactly that kind of problem. So this is a block of residential apartments. It was constructed in 1996 load bearing masonry. There was no basement for story buildings, and the building was piled. And this is a view of of this of the setup so the left hand picture shows you the layout of the building is a bit complicated geometry. The big tunnels going beneath it, the cross section on the right hand side of the slide shows the big 10 meter diameter tunnels, which you can see were constructed intersecting even some of the piles. And they were constructed with six meter diameter pilot tunnels first, and then enlarge to the full 10 meters. The length of the piles were within four meters of the tunnel crown. Some of them were even intersecting the tunnel crown. So some piles had to be cut. You can imagine that we had some interesting discussions with the owners and the residents of the building. And really, we learned from those centrifuge tests that we predicted there would not be a very big problem, and it turned out to be exactly that case. So, what we are looking at here are three different cross sections through the building, comparing the actual settlement of the building with the green field results. In other words, we were also measuring not just the building response but also the response of the ground, slightly away from the building. And the, and the big, the big message to take away from here is that the, the piled building responded almost identically to the surface of the ground. And if you, if you took away the building and the piles, the settlement that you would get was almost the same as the piled building so that despite the concerns that a lot of people had the actual response of the heavily loaded piles and the settlement that they were supporting was the same as the ground surface in the absence of any piles or building, which was very reassuring because it meant that nobody had to be evacuated from the building, which is what we had predicted. But the centrifuge tests were extremely instructive in giving us that insight. And finally, I'm just going to talk about some innovative fiber optic sensing. This is the same station I showed you earlier, Liverpool Street Station. And I'm going to focus on a very tricky area shown when that big red circle, which is where you have three big tunnels, very close to each other with cross passages between them. So, we're looking here at the cross the concourse tunnel. CH 5. And, and the westbound platform tunnel, the eastbound platform tunnel. And then what happens when you knock holes between them all, which are those cross passages, CP one, CP two. Now this involved a lot of work by the designers by mott McDonald the consulting engineers involved a lot of three dimensional and finite element analysis, very complicated, sprayed concrete properties, soil properties highly nonlinear. And they were able to conclude that when you knock a hole in between tunnels, you have to have a specially thickened areas of of concrete in order to allow for the stress concentrations. What we were able to do was to install fiber optics to actually assess how much that was really the case, and to validate the analysis and this is looking at the central tunnel, which is pretty much 11 meters in diameter. You can see two of my colleagues from my group in Cambridge, fixing fiber optic reinforcement fiber optic cables I should say. On the on the first pass so there was already one sprayed concrete layer, but they before the actual cross passages, we put fiber optics all over it to measure what happens. The, the cross passage positions are shown in red cross passage one and two. And what we were able to do was to assess to what extent the thickening of the concrete was really needed by measuring very accurately the strain with the optical fiber and these optical fibers have fantastic potential. If we launch light down an optical fiber 95% of it gets transmitted some of it gets back scattered shown in the red arrows. And if we then plot the power of the back scattered light against frequency, we get these special peaks called brilliant peaks and Rayleigh peaks, and the point, the important point is if it gets strained. If this fiber gets strained, these peaks shift in position. So you have a special analyzer shown on the bottom left, and that sends waves down the optical fiber. And if any part of it gets strained, then it moves. It actually, you can see the shift on the bottom right there. Actively, it means that the whole fiber optic is acting as one continuous strain gauge to very powerful technology. And it meant that we were able to put optical fiber all the way around prior to those two openings, CP one and CP two. And when they were actually not through, we were then able to see exactly what strain was induced in the sprayed concrete. So this is the end result. This is a plan view. You can see the the big, the big tunnel, the big 11 meter diameter tunnel with the two cross passages knocked through and I decide. And the, the green was the ordinary sprayed concrete, and then the, the, the blue showed to what extent the additional strain actually happened because of putting in those, those extra openings. But the finite element analysis and the complicated design that was done before any of construction reckoned that they had to thicken the whole of that red part. So it turns out that that's quite a lot of excessive concrete, and that's got quite a cost that goes with it. So improvements for future sprayed concrete openings like this, we can be enormously enhanced by such measurements. So we will need less material less excavation, less time, safer construction, but the, the power of putting fiber optics in to inform us on that kind of measurement is invaluable. So, I'm just going to summarize because I know time is running, running out and creating urban underground infrastructure is increasingly challenging, especially going beneath so many important buildings and in difficult geology and the cross rail project was an excellent example of many innovations. I hope I've emphasized in the short time, the key role of geotechnical engineering, you know, it's vital to understand the geology, the ground conditions, the soil properties and their behavior. It's not possible to control building settlement with compensation grouting in a very success in a very good way. And what I was able to show you was innovation to actually construct temporary small diameter tunnels specifically for compensation grouting was was a completely new development. And I hope also that you've seen that the effects of tunneling very close beneath power foundations is now much better understood. And finally, innovative fiber optic monitoring offers huge potential for the future. So with that, Alan, I will stop. And I'm very happy to answer any questions. And thank you for a very clear straightforward presentation but don't be fooled folks there's a lot of innovative and complicated work and thinking behind all that very impressed by it. I remind you to throw in some questions. If you have them. We will try to get to as many as we can. I'll start with a few and see how that question list fills up here. And again just before in the case we get cut off I just want to thank you and then fans. Robert for an excellent presentation. How many TBMs were used in the 42 kilometers of tunneling and what was their size. There were eight TBMs used. And they, they were seven meter diameter. They were, they were all heron connect machines. And some of the, the listeners this webinar will know heron connect a very, very successful German company that that is one of the world leaders in in tunnel boring machines, particularly pressure balance machines but also slurry shields. Yes, we have some experience with those in here in the United States and it's really interesting how we can actually use feedback now on those machines and limit ground move motions or movements to very small quantities is huge forward progress. Yeah, that's absolutely right. It's been a very big, very big development build to, frankly, in the Crossrail project. The settlement effects coming from the running tunnels, the, in other words, the TBM machine tunnels were of no, no consequence, no problem at all. The challenges were those big stations. Yeah, speaking of those stations, you know, was there some lessons learned comparing those stations as I understand it some of the stations were actually opened up by tunneling versus some that were cut and cover. And did you see differences in those two approaches. Very big differences. I mean, there are pros and cons. And I guess that's what lies behind your question. To do the stations as a cut and cover operation. In other words, starting from the ground surface and putting down slurry walls diaphragm walls and propping them all the way down to to a depth of maybe 40 meters is a is a big big undertaking, highly disruptive of course to the, to the infrastructure all around. And in many ways the mine stations is a is a much is a much smarter and more convenient and better way of constructing stations. It didn't always. It turned out there are only two of these stations were done by cutting cover. The rest were done by mind operations. And they and the mind tunnels where we're undertaken extremely well. And by and large, it's a lot. It's a lot more desirable than going by cut and cover. Especially in minimizing surface effects on traffic and pedestrians and other things right. Exactly. And services and all of the things that you know, they, I mean they services are in the engineers nightmare, as we all know, especially in the old city like London. So the cut and cover operations uncovered all sorts of history all sorts of old skeletons from hundreds of years ago. If you're doing a mine tunnel at a depth of 40 meters, you don't have that problem. Yeah, very good point. We have that here in our old cities like New York gold for us that just a nightmare you say for the engineers I'd say more for the contractors. Yes. What were the most difficult geological groundwater conditions that the project had to deal with. And were were these unanticipated any of these and anticipated. I would say they weren't unanticipated in that we had a very extensive program of site investigation. I'm pleased to say that this was a very enlightened client. And they really understood the point that it's worth paying the money for comprehensive site investigation and understanding the detailed geology, with lots with lots of pizza, pizza meters installed in boreholes. So the hydro the hydrology was well well understood. So I would say that there was nothing unexpected. Difficult. The, the one that I demonstrated briefly where you had the, the sand lenses on the high water pressure. And they had to be especially de watered with vacuum wells, but that was all anticipated. We knew that that would be needed. So lots been done researched on London clay, even us students study London clay in geotechnical engineering. So well known material but did you have any surprises did you learn something new about London clay from this massive project. I think we did. And we always learn more things about it. And sometimes you can get clay stones within London clay. So these are really very, very cemented hardened, not not a problem to excavate. But sometimes water can go with them. So London clay normally is an extremely low permeability. It's very strong clay. And then sometimes you can hit clay stones with quite a lot of water flowing through the clay stones. And so, there was a bit of that, and some of the cross rail tunneling but broadly, never no big surprises. What happened all the tunnel muck is enormous volume of material. And it was, it was something like 7 million tons of, of material, and it was stipulated at the outset because it was in central London that there wasn't allowed to be any trucks, removing any muck through the streets of London. They had a very well organized, well thought out system of the muck or going out to the portals, and some about half of that muck was taken off my train down outside to well outside London. The other half was taken by a river. The River Thames big river flowing right through through London, as many of you will know of course, and about 3 million tons of muck was taken by barge downriver to a bird sanctuary, which, which is on the estuary of the Thames about 15 miles towards the sea. The bird sanctuary is now one meter higher than it was. So the muck was spread throughout the bird sanctuary. And it was a very environmentally satisfying way of dealing with the muck. Interesting comment. We have a big project here in Boston called Bird, called Big Dig. Much of our muck went to a place called Bird Island Flats. It's a smalling friend, usually is where the birds go, right? Yeah, absolutely. The question here, you know, any comments relative to seismic effects on the design and impacts, you know, England's not that much seismic, but how did you consider that for the design? It's probably good. It's a very good question. But we are very fortunate, as you just said, we are very fortunate in that in that the UK is seismically very inactive, compared with the US and compared with many countries, in fact. The country is much closer to to us in the UK. In Europe, you know, southern side of France, Italy, I have to worry a lot about seismic design, but I can honestly say that the seismic considerations for cross rail were minimal, because the, the, the, the risk of seismic activity is extremely low. You know, was during the very interesting compensation grouting program there, did you have cases where you have the ground and how did you prevent that from happening? It's a really good question, because if grouting, we will, those of us that know a lot, have experienced a lot of grouting, it can, it can do things that you don't expect. There were very careful, there was a lot of measure, a lot of measurement, a lot of monitoring, and, and certainly there is a risk of heaping the ground if you get over enthusiastic about the grouting, that the, we didn't in fact experience that. So what you're aiming to do is to create probably a small amount of grout of heave, perhaps up to about 10 millimeters, but no more than that. And, but to do it in a very incremental way to be grouting a little bit, tunneling a bit more grouting a bit tunneling a bit more. You don't do anything violent, you don't violently heave it, you don't allow violent settlement, but you keep everything pretty much in a, in a, in a minimal amount of movement. And that really requires real time monitoring and a good instrumentation program to stay on top of it right. It certainly does. Certainly does. I'm sorry to be the one asking these questions, but we have tried to do this with the questioner asking the questions and it's just too much time consumed in technology so. So I'm, I appreciate the questions that are coming in forgive me for pulling them all off and asking but it helps it go smoother. That's fine. Are there, were there special considerations given in the fault areas you pointed out. Yes, they were indeed the fault areas that station that I briefly showed you early on in the presentation called Farringdon station was founded exactly in the fault area. It would, it would in an ideal world not be the greatest place to to found to site the station, but there were other constraints. It was difficult. It meant that the ground was very faulted. It was not London clay, just to make matters more complicated. So there was a, there was a lot of water, a lot of sand gravel. The way in which that was handled was that the TBM tunnels were constructed with earth pressure balance machines, right away through the station. So you had, you had before you did anything else you had a completed segmentally lined pair of tunnels. And then from within those I didn't have time to show this from within those. I was then drilling radially out from those segmentally lined tunnels to to depressurize all of the faulted ground and all of the water pressure, all of the permeable zones of sand and gravels, and then those TBM tunnels were then dismantled effectively, but they were used as the as the as the way of effectively making the ground stable and and and the right condition for for for bulk excavation for the main tunnel. The turn to a few questions about the instrumentation something you spend a lot of your career on and being the head of the seasick there the Cambridge Center for smart infrastructure, which I've had the pleasure of visiting very impressive work you all do. Could you talk about any other innovative monitoring work you did here. In addition to the fiber optic strain measurement. Yes, there was a lot of. Again, I didn't have time to in this brief presentation to show you other things I chose to focus on the fiber optic instrumentation. But in addition to the fiber optic which itself was very successful not just in that case I showed of the of the breaking out of cross passages from tunnels, but it was also used for slurry walls diaphragm walls. So we were able to put fiber optics into the reinforcement cages that went down into the into the into the diaphragm walls. So we were able for the first time, by putting the fiber optic on the intro DOS and the extra DOS, we're able to see exactly the full bending deformation profile of the diaphragm walls, but you asked the question about other forms of instrumentation. We also have a lot of wireless tilt meters that we use and this is a such a big advance to be able to not have lots and lots of wires as you will know very well Alan. So the, the, the advent of wireless technology has has made such a big difference to to be able to have a really well thought out monitoring program without just so many wires it just gets in everybody's way and the contractors just get upset by them and they get cut by machinery and all the rest. So wireless technology was a was a big innovation on this project. Yeah. One of the questions I like to ask is on these monitoring systems which are increasingly used in tunnels. You know, but can we really quantify the benefits of them. They seem like a good idea to geotechnical engineers but owners constantly question why do we have to do this. So was there been any effort to try to quantify or capture the benefits in some way like return on investment or a benefit cost ratio or something that we could communicate to non geotechnical people why this is so important. I think it's, I think it's a really good question you ask, I think it's not straightforward. It's very absolutely agree with you it's it's it's quite common for for particularly the kind of financial people on a project to say well why are we spending the money on on these instruments. If you have a an enlightened client as we did with Crossrail, who are almost certainly going to be the same client for the next project, rather similar to Crossrail, then they absolutely saw the value. They saw the value, if for no other reason than the learning for the next project. The example I showed of the cross passages. Well, you know the significant saving that could be made next time. Because of the measurements that were made this time work can certainly be quantified and can certainly show that there is a big saving. It's more difficult, often to make the case for the actual construction. Unless you are doing observational kind of construction and we did do that on Crossrail. So there were some good examples of deep excavations involving four levels of props. The fiber optics were put into the diaphragm walls on both sides of the excavation and also measuring prop loads. And it was decided very early on to dispense with one of the prop levels completely, which was a big saving, both in time and in cost, but it was it would never have been possible without problem monitoring. Hopefully you've documented that in ways that others can take advantage of that to help sell the benefits of these modern approaches. We're almost done. Two more questions if I can try to shove these in. Waterproofing is always a big deal. Did you have challenges here with waterproofing? How did that, how did you address that, how did that work out? Yes, it's a big deal. You're absolutely right. Fortunately, much of the tunneling is in that very low permeability London clay, which means that the issue of water is not as acute, but still needs to be waterproof. So there were waterproof membranes, there was a first pass of spray concrete, then there was waterproof membrane and then there was an in situ concrete permanent liner on top of that. Quite involved. Did you have a drainage net in somewhere in that sandwich too? So it's the belt and suspenders that we find we really have to have to do to get these things watertight, right? Yup. That's right. That's right. Final question. And this one, I love this question. Are there two or three places I can go on my next trip to London to see some of this work and see some of these practices? It's all covered up, right? It's all covered up, I was going to say. It's all covered up. But you have to go on the Elizabeth line as Crossroads is now called. I know I'm a little bit biased, but it is a fabulous bit of infrastructure. Everybody who's using it says it's wonderful, it's very spacious. The architecture of the stations, you know, the finished architecture is very well thought through. So it's delightful to travel on. But to see it under construction, we've got to have another project for you Alan. You're absolutely right. You know, I take my kids or family through something I've worked on and they see all this nice finishes. Dad, what's the big deal? This looks like anything else, you know. Well, we thank you so much. Very well presented and very clear, simple answers, direct answers. I really appreciate that. And I appreciate the attention of the audience. We've had about 200, I counted at the peak and there's still about 170. You held them on, Robert. I would like to thank everybody again. And I have to remind us that the opinions, conclusions, recommendations expressed here are those of the folks, me and Robert, I guess, the only ones that spoke and do not represent the conclusions, recommendations of the National Academy of Science, Engineering and Medicine. And with that, we'll stop. Thank you again for your participation. And again, I thank you, Robert, for a very nice presentation. It's been a pleasure, Alan. It's been a pleasure. Bye-bye, everyone. Bye.