What's REALLY Causing Millennium Tower to Sink and Tilt - Is the Developer at Fault?

[Music] hello welcome back to building integrity i'm your host josh porter and today we are continuing our conversation on the millennium tower the sinking tower in san francisco now in this episode and in the previous episode we talked about um sort of the chronology the history of how things have gone along but it was sort of a very surficial sort of look at everything right we only we only looked at the surface of the situation and the problems now we're going to start getting into some of the fundamental engineering concepts and failures to understand concepts that i think not only were exhibited by the engineers who designed millennium towers uh particularly the foundation but even the engineers currently today which i will have to get into in a future video because that's just too much to cover today but there's some fundamental concepts that i think everybody's sort of missing i've dug into just a ton of research on this and uh from everything i can come up with it seems like everybody is really good at describing the the symptoms of the problem um okay the building's sinking the building is tilting um the sand is compressing it's consolidating these are all symptoms of the problem but what is the design problem well to understand that we need to understand a little bit about how friction piles work if you're if you're if you've been following the channel we've talked about this in previous videos we've touched on it but essentially a friction pile relies more on its interaction between the skin of the pile and the sand and soil around it to act and to resist with friction against that pile more so than it does on the bearing end capacity of that pile and engineers will oftentimes design multiple piles in a group together uh referred to as a pile group okay and then these piles will work at sort of as one but whatever calculated uh resistance you get from a single pile let's say you're driving the piles you you when you're driving these piles you test them in the field and you get a certain feedback into the machine telling you how much resistance you've gotten on that pile and if you drive a pile three or four feet away from that pile you will get that resistance as well but when you add all the piles up in a group you don't actually end up with the cumulative effect and that's because the closer the piles get to each other the more that they exert a force essentially on each other and there's just not enough sand not enough soil there to provide all of the friction resistance that these piles require because you got to think the skin of the pile when it exerts a force on the surrounding soil it's exerting it on those on those particles of sand and those particles of soil that are in direct contact with the with the piling skin well those particles have to exert a force on the adjacent particles and so on and so forth but if the piles are very close to each other they will actually end up exerting a force on each other and sort of negating some of the effect and benefit you get of friction piles so a friction group will still have a lot of friction resistance but as a group it will rely on some end bearing and if end bearing is not to be relied upon in the math then essentially you have to design uh your pilings for a capacity lower than what they individually would would technically test at all so with millennium tower in san francisco they had a 10 foot thick foundation mat so instead of doing individual pile groups and piling caps they have one large cap and they have about 950 friction piles that they installed in close proximity to each other i mean these piles are so close you can barely get to two men to walk side by side between them now what's going to happen is because of the sheer volume number of friction piles and because of how close they are together you go you're going to get some skin friction benefit but it's going to be greatly reduced by the fact that these piles are interacting with each other and they're negating each other's friction forces by being in close proximity and being such a high volume of them so you have to start thinking of these these 950 friction piles more like one big uh um one big foundation mat if you will that goes approximately 70 to 90 feet into the ground some of the piles were driven a little deeper but that but on average that's about it about 80 90 feet deep so if they're acting as one large pile and you have minimum friction resistance now you're back to relying on end bearing of the entire group of 950 piles and what's really interesting about the way they designed these piles and the depths that they drove them to is most of the piles almost the entire group stops right above this layer called the old bay clay so let's start talking about the layers a little bit and i'm gonna and i'm gonna start talking about how all of this goes together at the very top in this diagram you'll see that the top two layers are generally just fill and sand so fill is soil that has compactable soil that has been brought off site but this is the kind of the top layer right below the roads and sidewalks and stuff as you're filling sand this was placed on top of what is known as young bay mud okay and then below that there's a layer of sand and then below that there's another layer called the old bay clay now you've probably heard old bay clay referred to in news articles and on television and uh newscasts and stuff like that um but essentially that's going to be the crux of what we're going to be talking about here today below the old bay clay you have alameda clays and then below that you have the franciscan bedrock now the pile the 950 friction piles acting as a large piling group are going to exert a force that if each pile exerts a force let's look at each pile individually each pile exerts a force sort of a you can think of the end bearing yes there is friction resistance but for now let's set that aside and let's look at how it would bear and bearing it it bears sort of like in a cone okay the load exerted by that individual friction pile is distributed in a cone now if you look at that cone shape load being distributed from all of the piles as one you can see that there is a triangular shaped concentration pointing towards the center so these forces are cumulative and they add up towards the center well what this exerted force actually looks like is shown in this in this diagram here showing the force arrows pushing down and you can see that it has a curvature to this uh so it is referred to as sort of like a force bulb uh if you will and so essentially the center area compresses or pushes or has a greater exerted load on the soils below it than do the outside areas of of the pile group and so it's interesting that when you consider that most of the load would be exerted to the old bay clay because the the end bearing of this pile group is right above the old bay clay there's a couple interesting things that come out here one i calculate the load from at the center of this uh a pile group to be anywhere between ten and fifteen thousand pounds per square foot okay now the uh the the old bay clay there's a study that was done uh by a group of engineers and scientists um who who used uh other sites but mainly they got a lot of their core samples from the neighboring trans bay authority project and they did a lot of studying of the old bay clay one of the things i found that was really interesting in there is they tested the what what we usually typically call the yield capacity of the old bay clay so in other words how much compressive force can it take before it deforms in a plastic manner doesn't bounce back in other words um and will super compress right or over consolidate and and from from what they have shown in their math and from just looking at my rough math of the mat it looks like this this structure would have exceeded the old bay clay's capacity by by quite a bit i mean it's pretty easy math pretty easy conceptual math you don't need finite element element analysis you don't need advanced software 3d modeling and all this stuff to kind of figure this stuff out it's actually pretty basic stuff and so it's no surprise then if you look at the at the forces in the middle exceeding the old bay clay's capacity but the forces at the skin being close to just or you know right at what the old bay clay's capacity is or or maybe slightly over but not as much over as it is in the middle it begins to make sense as to why the 10 foot thick foundation mat is actually dishing and if you think about the shape of the dishing and you think about the shape of the load pattern that i showed you before you'll realize that these two mimic each other now i've drawn the dishing of the of the slab to be symmetric and simple but it's it's not quite symmetric it's actually biased to one side but you still get the idea that that that that essentially this building is thrusting itself down into the old bay clay but because the force the magnitude of the force at the center is greater than at the perimeter the center is sinking faster than is the perimeter which is giving that dishing effect to the uh thick fountain the 10-foot thick foundation mat and i think this is something that when the geotechnical engineers and structural engineers who designed the building they they designed the building and from everything i can tell and i don't have those design drawings but just looking at like interviews that they've that that have been done and and reports that they've done and there was a 2017 report 2011 reports and just looking at people who have looked back at these original engineering documents other engineers and investigators it doesn't appear to me that this this end bearing result of this dense uh pile grouping that this was really considered it appears that they really just calculated the the capacity of the piles relying solely on their friction their skin friction uh strength as opposed to considering them as an entire group and what that entire group would do to the old bay clay and i think i think that was the conceptual error that was missed from those original designers was this understanding of how this would act as one unit and how that one unit would would work on the layers below so this begs the question then well you know why wasn't it built to bedrock in the first place are they are you correct josh in just saying that they missed this conceptual they had this engineering conceptual error and trust me this is not rare there's many projects i could cite tons and tons of projects historically and contemporary that engineers have missed basic engineering core concepts and structures are having problems failing or need retrofitting and repairs but i'm not going to get into all those right now but this is not an atypical occurrence but the question comes into why wasn't it built bedrock in the first place and i think part of that you have to kind of understand a little bit of the politics that were involved around the time the building was built right before millennium towers was built the transbay authority had a deal with another neighboring building project that the transway authority knew they were going to be retrofitting and upgrading their their system there in this area this corridor of san francisco and they wanted to have sort of a deeper foundation area uh excavated out and so they knew it would affect this other building that was going to go up well they had worked with the designers of that building and said hey listen we understand that your building doesn't require going down to the bedrock but we would like to pay for that upgrade to to make sure that our foundation isn't or that we don't affect you when we go to build our foundation well that deal never ended up happening in that in that way but then the millennium tower comes along a couple years later and they approach the trans bay authority and they say hey we want that same deal we want you guys to pay for the upgrade to go from friction piles to piles down to bedrock and i find this kind of interesting and unique because the the developer was trying to get trans bay authority to foot some of the bill for the foundation to get it to go down to the franciscan bedrock but but if the developers engineers had said hey we've got to go down to the franciscan bedrock anyway in order to get this building to stay upright and not sink too much then they wouldn't be able to go to the trans bay authority and ask for that money it wouldn't make sense because you have to do it anyway in order to build this massively heavy structure anywho so then it sort of begs the question well what what if and i'm just you know thinking out loud here theorizing but what if it was a situation where in order to get the money from the trans bay authority or attempt to get the money from the transbay authority they had devised a plan where they designed it to say oh look it does work with friction piles uh and then therefore if you want us to upgrade it you've got to pay the extra money which is estimated to be about four million dollars well as it turned out the transbay authority actually rejected the offer and did not offer to pay or the request for the offer so they rejected that request and they did not offer to pay the four million dollars to have the foundation brought down to the bedrock well then what happens is the developer now has in his hands something from the engineer that says well friction piles will work and so the developer goes ahead and chooses to just proceed with the friction piles now this raises a couple interesting questions because in in uh depositions and in interviews with the design engineers they tried to push the blame back to the developer and they said well we told we recommended to the developer that we go down to bedrock but the developer trying to save money and being cheap chose the friction piles now we can blame the developer all we want but in this particular instance i think they're actually innocent and the reason why is because the developer if the developer is given two options and the engineer says both options will work but we recommend this option what the developer hears is both options will work and one's cheaper than the other and the developer is not an engineer themselves they don't know they weren't they were told that the building would sink six inches so they planned for that they designed for that that's what the developer moved forward with and so i think you know in my opinion a lot of the blame falls down to the original design team even giving the option of friction piles to the developer as a viable option i think if they had done their research and done their homework and they had um they had really analyzed the way that these engineering concepts were going to play out and how they were going to affect the old bay clay that friction piles should have never been an option for this project and had the developer been told from day one you have to use uh pilings that go down to the bedrock there's no doubt in my mind that the developer would have done that so because you gotta remember at the end of the day this building could not have got permitted and could not have got built unless some engineers somewhere signed and sealed the documents saying that these friction piles not only met code but were going to be safe for the long term life of this building well i hope you enjoyed this video thanks for joining me take care

Channel: Building Integrity

Views: 151,158

Keywords: Champlain Towers South, Surfside Collapse, Engineering, Lawsuit, Surfside, Champlain, San Fransisco, California, Leaning Tower, Sinking, Millennium Tower

Id: gIxhpP7hQu4

Length: 16min 23sec (983 seconds)

Published: Fri Feb 11 2022