Civ Eng graduate that ended up in IT here. For home projects, please consider gabions as well as the usual suspects when you are building retaining walls.
A gabion is a galvanised steel wire cage say: 4' x 4' x 2' (HWD). They have a hinged lid and you fill them with stones and then wire the lid shut. Sounds stupidly simple, and it is but they have some rather useful properties. Each unit is an easy one man lift, place and fill. Once filled, each one nominally becomes a large single block with great drainage properties. You can wire these things together into long rows. They work very well with water courses because they are easy to fix in place and once filled, won't move. Pouring conc. into formwork is a right old pain in a fast flowing river and it is all too easy to lose the finer particulates before the stuff has gone off (set and cured).
You can finish the exposed surfaces in various ways. You can pour soil on top and grass them, pour a bit of low grade conc and gravel for a solid "path". The cages are not particularly pretty but neither are they particularly ugly.
If a single cell fails then it generally won't cause much surrounding failure and is easy to replace or repair. You can embed fancy anchors inside them if you have a lateral thrust to resist that can't be dealt with by sheer mass.
Sleepers and the like are quite convenient but you must consider drainage otherwise they will rot within a few decades. Block backed brickwork needs a decent brickie to lay them and if they fail it is usually rather bad. I'm no brickie but I've just repaired a broken 3' retaining wall at home and it looks a bit shit. I will be hiring a professional to sort it all out in spring. Here a gabion wall is overkill!
I did say home projects above but these things are used everywhere and that includes some pretty huge retaining structures. If you are not a Civil Engineer and need to build a decent sized retaining structure then I highly recommend that you consider gabions first because you are far more likely to get it right first time.
They seem to have become vogue for decorative purposes in the last few years, for stuff like bbq pit walls, windbreaks, and those faux gateway entrances that have no fenceline around them. Personally I think they look very ugly, and the trend is a bit of a mystery.
Ugly - probably but they are rather clever in my opinion. They are modular and can be deployed by one bod nearly anywhere.
I have no idea what windbreaks and faux wotsits are let alone "decorative" means. I'm a bodger/Engineer.
If you need to stop a vast amount of stuff moving from A to B then gabions are a very decent solution with the added bonus that you don't need to be an Engineer. Civilians will generally get it right first time just by looking at the problem.
Fair enough, many are ugly, but I generally quite like the way they look. Lots of scope for different shapes and colours and textures. They can be blended in with the local geology via the choice of rocks. You can put benches or pathways on them, or soil on them and grow plants, or even grow mosses and lichens.
That sounds like a retaining wall failure waiting to happen. Thin galvanized cages are providing short term structure, but they have relatively short lifespan underground. Fine for knee high walls, but if your covering it with soil it’s not obvious these these things are going to become unstable.
I'm 51 years old. I built one to stop bank erosion in a stream at a change in level and direction at my parent's house about 30 years ago. All units are fine. They sit at least 1/3 height in water all the time and have soil/subsoil behind. They have a thin pour of mortar with gravel on top to make a path and have integrated into the bank and grass.
Galvanized steel is surprisingly rust resistant and that's the key. These gabions do get quite a bashing when it rains hard. The stream in question is in Devon, Woodbury (Salterton), near Exeter (UK), which is popple land. Popples are rounded stones found in the streams thereabouts. They make ideal gabion fill. They also destroy things downstream if the flow is fast enough to rouse them!
That's one case study that I've chosen deliberately to show that they work for DIY.
Apparently the answer is "it depends on the soil chemistry."
DURABILITY OF REINFORCED EARTH STRUCTURES: THE RESULTS OF A LONG-TERM STUDY CONDUCTED ON GALVANIZED STEEL.
M DARBIN, JM JAILLOUX, J MONTUELLE, and ROMANOFF
Proceedings of the Institution of Civil Engineers 1988 84:5, 1029-1057 https://www.icevirtuallibrary.com/doi/abs/10.1680/iicep.1988... (pay article)
The following appear to be true: (1) HDG (or equivalent) has an order of magnitude greater resistance to corrosion in soil than uncoated steel, (2) corrosion rates are highly influenced by presence (or lack) of chlorides, sulfate, and citric acid in the soil, as well as temperature.
Given the above, the suitability of galvanized steel in soil, at standard steel diameters and coating thicknesses, is either suitable (for low aggressivity soils) or non-suitable (for high aggressivity soils) over an example span of 20 years. Tl;dr: test and characterize the soil it's going to be in.
How long to they last if they are not underwater but moist all the time? I would think that if you are retaining soil, it will be always moist in many/most climates or where one is watering landscape above.
See my comment above. I built one 30 odd years ago at my parent's house. It is still fine and looks as good as when I built it. The key thing is: galvanized.
Not sure these are permitted in LA Country. Having tangled with the permit office before, I know just how ugly it can get (brutally ugly!) to try to do something they don't have a checkbox for.
I can see them for decorative purposes (a firepit or bbq island). Even then, one of the problems we have out here is that you are guaranteed to have all kinds of insects make a home out of a pile of rocks. We have nice ones, like black widows, that you really don't want anywhere you are going to be walking around in flip-flops.
I would add that gabions being "loose" have an additional usefulness as they are very flexible and allowing differential land movements (which are often the cause of cracks and instabilities in more rigid structures) and are thus a good anti-sismic structure.
About duration, at least here (Italy) they are usually considered in the 100 years duration range, not unlike many other kinds of structures.
The inherent draining is the clearest advantage when compared to other solutions (make sure to use some geotextile behind them to "filter" small particles) , though - to be fair - they do need more space than other kind of walls and they rapidly become unusable when you go over 3-4-5 meters height.
They can be made a bit prettier by using flat sided rocks and line the front of the wall with rocks that seems to interlock a bit. Any round rocks that will not make a flat wall go in the back.
I helped build earthen structures with sharper slopes than 25 degrees 30 some years ago that are still holding back ponds today. "Soil compaction" is the magic that can turn native earth into a real wall that will hold load for quite a long time. The surface treatments, bricks or vegetation or etc are not load bearing, they're a skin to prevent erosion, much like paint on steel.
I'm slightly puzzled it doesn't get mentioned here. Has this knowledge been lost? I've not been observing construction work first hand for a while but I don't recall the last time i saw a sheep's foot roller in use.
> I'm slightly puzzled it doesn't get mentioned here. Has this knowledge been lost?
Clearly not. He talked about soil compaction in earlier articles:
In "Why SpaceX Cares About Dirt"[1] he talks about soil compaction through surcharge loading.
In "What Really Happened At Edenville and Sanford Dams?"[2] he talks about how the lack of proper soil compaction was one reason behind the dam failures.
In "Why Does Road Construction Take So Long?" he identifies soil compaction as one of the most time consuming parts of road construction.
If anything he didn't talk about soil compaction in this article to avoid repeating himself. :)
Soil compaction has been covered in a couple of other Practical Engineering videos.
It seems several people are complaining that a transcript of a ~10 minute video isn't an entire engineering education in all possible details of how to create retaining walls. I think that's asking for an awful lot. But at the very least let's credit the things already discussed elsewhere.
It's kind of surprising to learn that this is a transcript! The "play" icon was really easy to miss for me and almost entirely blended in to what I now realize is a video thumbnail (I thought it was just a header image!).
Given that this is a transcript of a video, and that video has pictures and diagrams to help illustrate what is being discussed, I would have really liked to be able to see those pictures and diagrams interspersed with the text while I was reading.
Show me a homebuilding article or video that doesn't discuss essential tools - that's kind of my issue with not mentioning Proctor Tests in this context, as they are fundamental at the pro geotech level. Its not too much to ask.
>discuss essential tools - that's kind of my issue with not mentioning Proctor Tests in this context, as they are fundamental at the pro geotech level.
The top 2 videos from the following Youtube search results about "retaining walls" do not mention Proctor Tests.
And halfway down those search results is a retaining wall video presented by geotechnical engineer Andrew Lees that's longer in duration than Grady's video and he doesn't mention Proctor Test either:
https://www.youtube.com/watch?v=HGuX7rmzlzA
And after reading the wikipedia article about Proctor Tests, I think Grady made the right editorial judgement to omit that topic from a 10-minute video targeted at a general audience.
Or put another way, if cf100clunk made a video about retaining walls and was constrained to 10-minutes, you would also be forced to leave out some essential topic that other geotechnical engineers would criticize. You can't please everyone when you have time constraints.
EDIT add: thanks to other's links, I noticed that Grady already mentioned Proctor test in a previous June 2020 video "Why Does Road Construction Take So Long?"
No need to call me out personally for my observations. It isn't an HN thing to do. As I've said in another post, glad the website and video are out there, and glad to inform folks about the Proctor Test.
How many other equally important things are also not mentioned?
I'm going to find "zero" very hard to believe, what with this being a 10 minute general audience video. I seriously doubt I'm just three additional minutes away from being an expert on the topic ready to take on any geoengineering task I could desire.
Edit: Let me put it a different way. While I watch and enjoy his video series, I've been showing them to my 10-year-old and 13-year-old kids, and I'm pretty sure they're not that far out of the target audience.
Well I won't belabour this, as the definition of "valuable" information can be as amorphous and/or personal as that of "quality". Glad that website and its videos are out there anyways, and glad somebody may have learned about Proctor Testing.
> I helped build earthen structures with sharper slopes than 25 degrees 30 some years ago that are still holding back ponds today.
I think you may be mistaken - note that a "25 degree slope" is just a different way to say a 2 to 1 slope (2 feet across, 1 foot up). I've seen 2:1 slopes used for highway embankments, but earth fill usually specified as a 3:1 slope (18 degrees) - eg when I worked with an earth fill dam (holding back water, same as yours). I've only seen anything steeper (1:1, 45 deg) used a as a temporary (during construction) condition.
Sheepsfoot rollers are good for packing very fine material (silt, clay) but not very good for packing larger granular material (i.e. crushed stone/gravel). Silt and clay are very water sensitive materials: each has a very specific moisture content where it can be packed properly, if your material is outside of this narrow range it will not get to maximum compaction (and therefore it will eventually settle). Water moves very slowly through clay so if it is too wet or too dry it's very difficult to get it back into the proper moisture range, and if there's a bit too much sun or some rain between excavation and placement it will not get packed well. The only reason I've seen clay-ey earth intentionally used is for inhibiting water movement, IE an earth fill dam - and even there, it was a secondary barrier if anything happened to leak through the barrier membrane.
Crushed stone is (relatively) very easy to get to maximum compaction, and if it sits out for a while and gets too dry you can just hit it with a water truck before placing. It packs quickly and easily, and is stable at the ssame side slopes as earth fill
Yes my experience is dated and my memory none too good, thank you for expanding.
My experience is all with "perfect" high clay soils and I'm sure many of our jobs were "under-engineered" to put it politely.
We had some crazy operators who would do things like chaining the dozer to a trackhoe at the top of the hill so it would not roll over, to do the final grade of the slope. There's more laws now, and/or fewer fools with earth moving equipment.
> Yes my experience is dated and my memory none too good, thank you for expanding
No worries, when I heard 25 degrees my gut reaction was "that can't be right". Then I did the calculation and saw that the common ratios used are a lot fewer degrees than I expected :) I was thinking of grades in percentage (rise divided by run expressed as a percentage - so 50% for a 2:1 slope, 33% for a 3:1, etc)
> There's more laws now, and/or fewer fools with earth moving equipment
I think stricter rules (or enforcement thereof) has led to the proliferation of long-reach excavators.
In my experience "normal" slopes (without using any kind of reoinforced earth) are 1:1 (base/height i.e. 50 % slope degrees) for excavation and 3:2 (base/height i.e. 66% slope), with reinforced earth (depending on types, soils, etc.) you can usually go to 1:3 or 1:4 or even 1:5.
Yeah, I acknowledged 1:1 for excavation in a comment up a couple levels. That said, where are you that 3:2 is a standard slope for unreinforced permanent slopes? That's 34deg, which is outside the angle of repose for many materials, and doesn't leave much factor of safety even for a crushed stone.
Reinforced earth is a different beast altogether for sure.
Yep, it is the the typical slope for earth moving, Italy here, thousands of km of roads are made with this slope, exceptionally it is lowered up to 4:1, a reference:
What I mean by 3:2 (base divided by height) is often referred to as 2/3 (height divided by base) and is roughly the "natural" angle of repose of - say - gravel.
Thanks for the reference. It's interesting that they work that close to the stability limits without just adding some slope reinforcement. It doesn't leave much room for error (execution error, erosion over time, unexpected surcharge load, stability changes with soil saturation, or earthquake).
I towed my sheepsfoot roller around with a big Italian Same tractor in the
1980's, in West TN. We had those for the ground speed PTOs but they
climb things like mountain goats, unique to the market at the
time, which enabled some of the stupid tricks we built.
By comparison to their source, our hills are pleasant soft
sandcastles, all rubbery clays with veins of sand or gravel and never
any excuse for explosives.
I can assure you surface compaction is still a thing. Never seen sheep's foot rollers, but I have seen really big square rollers. A square doesn't roll very well, but that is the point: every tumble it makes it crashes into to soil. Problem is it is extremely uncomfortably for the operator, even if pulled by a really big tractor. These rollers are called impact rollers.
Also in use are rapid impact compactors, basically a crane pounding away. Vibro compaction, where 30m long vibrating needles are driven into to soil. And best of all, Dynamic impact compaction: lift a big chunk of concrete 50m in the air and let it free fall. Then do it again and again... See https://vimeo.com/415927984 @ 3:15
I'm a civil engineer, soil compaction is still critical for any grading fill and will continue to be important. One of the biggest issues though is that it can be really hard to compact native material to the required spec. It has to be perfectly within a narrow window of moisture content +/- just a few perce t to achieve full compaction.
If you click the image at the top, it will load the youtube video[1] (which is the better source in my opinion - the blog is essentially closed captioning for the video, without any of the graphics). The only indicator is a small play triangle in the centre of the image/link - the cursor doesn't change on mouseover.
I realized something was fishy when I read "I’m Grady and this is Practical Engineering" at the end of the first paragraph... but still, I appreciate these articles, easier to skim through than a video. And if you're really interested, you can still watch the video...
I agree. I already watched the video and was surprised that I couldn't find a link from this page. The fact that the header image is clickable is very non-intuitive. At the very least the cursor should change on hover.
The Proctor Compaction Test and its related procedures are absolutely vital to understanding retaining wall capabilities. The article oddly seems to miss such an essential cornerstone of geotechnical engineering. Does an amateur need to know about the Proctor when doing a low retaining wall at home? No, of course not. Does a website called "Practical Engineering" get to miss out on such a fundamental design prerequisite? Not IMHO. Great article and video, nonetheless.
I think the point of this blog and YouTube channel is to make these concepts approachable to the lay person, and introducing lots of technical jargon (relevant though it may be to actual geotechnical engineers) is not the best way to accomplish that.
Time to rename it from "Practical Engineering" to "Popular Engineering"? To me, the Practical handle is significant, so taking a few seconds to explain why soil compaction tests are vital makes practical sense.
The practical in his videos probably refer to the fact that he always explains things in physical models. It's very rare to just be theory. Thus, practical engineering demonstrations.
I salute you. My late father-in-law was a slide rule wrangler of a pipeline engineer who often did the same. By the time I got started in geotech computing support (Unix, Apollo Domain, VMS) we had the numbers stuff readily available on a CRT screen for guys like him.
"The Proctor Compaction Test establishes the maximum unit weight that a particular type of soil can be compacted to using a controlled compactive force at an optimum water content."
That looks like geotechnics to me (UK). It's also not too useful here.
Compacting soil is for certain parts of paths and roads construction, not retaining walls.
For thousands of years retaining walls were successfully constructed without more than a cursory understanding of soil compaction and they didn't have rebar or geotextile to help them.
Nobody needs to understand soil compaction if they're willing to move and expend way more material than the bare minimum in order to solve the the problem. This is true in a lot of subject areas. You don't need to understand a lot of things if you're willing to copy what is tried and true and can tolerate some inefficiency.
For almost all personal and commercial projects the material is going to be cheaper than paying a real engineer to poke the soil with a calibrated poker and plugging the numbers into a spreadsheet that has some formulas.
This is a decent very high-level approach, but doesn't go into the practical applications for most people. If you're a civilian looking at small to moderate-sized retaining walls on your personal property (4 feet or less in height) rather than a civil engineer designing massive projects for infrastructure, your retaining walls are almost certainly failing due to issues with drainage (not enough drainage material, incorrect drainage material) or possibly a heavy surcharge rather than the forces described here.
> If you're a civilian looking at small to moderate-sized retaining walls on your personal property (4 feet or less in height) rather than a civil engineer designing massive projects for infrastructure, your retaining walls are almost certainly failing due to issues with drainage (not enough drainage material, incorrect drainage material) or possibly a heavy surcharge rather than the forces described here.
These exact topics are covered in paragraphs 12, 13, and 14, respectively. 9:14 to 10:41 in the video
Way I heard it as an undergrad in mechanical engineering is anyone can make a bridge that stands up but only an engineer can make a bridge that barely stands up.
Anyone can do anything, if they spend a bunch of time studying the ins and outs of it and maybe had an an experienced mentor, but someone who thinks they understand enough to just 'run the numbers' is more dangerous than someone who knows they don't know.
And thats why running numbers has nothing to do with experience and judgement, something you don't seem understand. If you've never built a bridge you don't really understand anything about building a bridge, even if you've been to engineering school.
It's like putting out the firmware for someone's pacemaker because you can 'run the numbers'. You sure can, if you don't mind killing a few people and spending time in prison.
Did you have a professional install a dimmer switch, connect your gas stove, install your washer drain, change your flat tire? Where does it end? Not everything needs to be done by a credentialed professional. The average bridge is a relatively simple I-beam and footing structure spanning a stream or similiar and is not deserving of more than the "plug your max load in here and consult the table in appendix A for footings" treatment that similiar structures get.
I see you've edited out the comment about not speaking out about putting me in a boxcar because I think people who design bridges should be certified.
I recommend you take some time off the internet and do some mental self care. Whatever it is you think you're doing, it isn't good for you or anyone you interact with, throwaway or not.
I didn't say I'd put you in a box car, just that I wouldn't extend my neck or burn my political capital vouching for someone like you to keep you out. There's a big difference.
If you're a normal person looking to build a retaining wall without an engineer, 10 minute youtube videos are not where you should be getting your information.
> Depending on the steepness, it’s either inconvenient, or entirely impossible to use sloped areas for building things, walking, driving, or even as open spaces like parks. In dense urban areas, real estate comes at a premium, so it doesn’t make sense to waste valuable land on slopes. Where space is limited, it often makes sense to avoid this disadvantage by using a retaining wall to support soil vertically.
As someone born in a completely flat city and now living in another completely flat city (600 m higher, but still flat), I always kinda liked slopes, especially sloped house plots - they force architects to come up with creative solutions instead of cookie-cutter boredom. But I didn't realise how far people's dislike for slopes can go until I saw this monstrosity near Nice (France) while on holiday there: https://goo.gl/maps/zf5H1jSA855bSa4XA (you can take a better look in the 3D view). That's right, they must have excavated a whole lot of rock there, and are putting up with a ~ 20 m sheer rock face right next to their houses, just so they can have nice flat plots of land! Ok, it's rock, so probably more stable than a retaining wall holding back dirt, but I would still be worried living next to that precipice (either above or below) - if not for my immediate safety, then for the long term value of my property...
I live on a hill, and the driveway ends below the house, so when we need to haul up materials (gravel, lumber, etc) for various projects, it's walking up a bunch of steps and slippery/muddy hills. I don't mind slopes on their own, and in fact quite like the exercise for every day use, but when you're hauling buckets of gravel or a cord of firewood, it sure would be nice if you could just load up a utility cart and walk it over to where it needs to be (or hell, drive your truck across the yard). It's not possible where we live because of the hill. So I can absolutely relate to why people don't like to live on a hill. And for hanging out outside with friends, you can't beat flat areas.
That said, the view is really incredible (we live in the woods) and we don't get water pooling in our place or flooding or anything like that thanks to some well-designed drainage, so, you know, pros and cons.
If you look at the area in the 3D view in Google Maps, it's pretty obvious - the plots left and right of this small neighbourhood are on a slope, just there the terrain is almost horizontal...
I watch a lot of Grady's videos and there's a comment I've always wanted to make but I don't leave comments on YouTube. I'm going to leave that comment here: it would be great if he spent more time going over the models that he builds and really showing what they're representing. Show it from different angles, show it in slow motion; really explain what's happening and what we're seeing. Note: I haven't watched this particular video yet.
He clearly spends a lot of time building high quality small-scale versions of things to show how they work, but more often than not he just shows those things while the voiceover isn't actually talking about what's being shown. Or when he is talking about the thing being shown, it's a very brief comment and then he moves on.
I love his videos, but I very often finish them and think "I could have learned more if he spent some more time explaining in detail what's happening with the model, and replaying some component of it several times over as he explains it in more detail."
Just want to mention that his email is on his blog, and he's surprisingly responsive for having such a big following. I once wrote him with some unrelated questions and he gave me a detailed response, which I really appreciated!
If you look carefully at the failed retaining wall it is obvious that it failed at the bottom which is where lateral (horizontal) pressure would be at a maximum. For my money, insufficient drainage was the cause.
Pretend that the retaining wall is a dam. Most of the time it has to only deal with soil/subsoil/rock pressure. The lateral force on a soil dam is not the same as water - most of the force is downwards (gravity) which does not affect a dam which worries about lateral forces. However there is a certain lateral pressure anyway. Think about a pile of sand being poured. The sand tries to move sideways as well as building upwards - the sideways bit is the lateral forces on our dam. Water always moves sideways and that's the main difference. However water is less dense than sand.
If we don't allow water to drain effectively then we have both a soil and a water dam.
Retaining walls that survive a day (ie can deal with initial loads) generally collapse later due to shit drainage or inappropriate "solutions". In my opinion - mostly drainage.
Yes, lack of drainage is one of the main causes of failures of retaining walls in my experience also, and understandably as the amount of soil pushing agains the wall increases its weight when wet, often more than 20% and water is also a good lubricant to allow for slides to happen.
Perhaps this is usual, but when Caltrain was building the new approaches for the elevated Hillsdale station I noticed they were just pouring concrete on top of plants.
Maybe not the biggest deal, but I’m pretty sure the engineering algorithms assume a solid mass of concrete. Hopefully there’s enough safety margin that this doesn’t matter.
Not usual - what sort of plants? What did the concrete look like it was doing?
Organic material would be fine if it was underneath eg, a temporary pathway or something, but that's about it. Actually it might also be fine for secant pile cap (talked about in this video[1]): when you drill a secant pile, you generally pour a concrete cap (that then gets drilled away).
I think this is a great article about something that I didn't understand. However, as feedback, what separates this from top tier articles is a lack of pictures and diagrams. Just adding a few of those to illustrate the different types of walls would make this content more engaging and link it to real world examples. I'm not likely to google "soil nail", but I'm def open to spending more time on your page to check out an example.
Grady's web site seems to be experiencing the HN hug of death right now, but I watched the video last night on Nebula, and there definitely are visuals and they definitely clarify the lecture. And as usual, Grady has built a practical demonstration of the forces and it also helps.
I thought the article was in a weird place. It read like a very long introduction. If its the first in a series, its great. If its a standalone article, it lacks needed depth.
Great little video explaining all the hidden engineering and technology that we don't see. As with any good presentation, nie we have questions. For example, how do you prevent that slump from happening, that he showed at the end? How did engineers fix those broken highways? Then part two? I assume, would be to show how soil, ground analysis works, with drilling, sampling etc. to define stability, and what engineering used for that.
The article mentions several construction techniques that rely on steel for structural support. (I don't mean just during construction; I mean in the finished product.)
But my understanding is that steel always deteriorates over time, especially when in direct contact with moisture or cement.
So does that imply all such structures have a lifespan that's counted in decades, and that maintenance basically requires major surgery to replace the steel?
Using these as “permanent” support is usually not done for corrosion reasons as you point out. They can be made into long term support elements but it’s costly and only done when footprint is essentially like in the middle of a city in between buildings. The buildings basement walls are designed for the entire lateral load as if the support of excavation system is fully corroded and gone, so most basements are very over engineered against earth load type failures.
Long term is generally 30 to 100 years in civil engineering, depending on when the structure was built. So beyond that life yes it’s a tear down on paper to rehab below grade.
I think they did it all with non-reinforced concrete, which can last more or less forever. It has good compressive strength but hardly any tensile strength, so they'll have had to design the dam in a way that doesn't depend on that.
Yes, 2.5 million cubic meters of unreinforced concrete. According to Wikipedia they took some core samples in 1995 and found the concrete to be stronger than when it was poured. "Last more or less forever" sounds about right.
There is some steel involved, in embedded cooling pipes that were used to speed the concrete curing process and later filled with grout.
Large concrete dams like Hoover are almost invariably arch dams which are shaped so that the entire dam goes into compression as the water load comes on, so there is no real need for tensile steel. These dams can take decades to cure and just get stronger over time.
The connections to the rock are usually reinforced - steel that's encased in concrete is generally thought to be relatively immune to most corrosion effects for the design life of any particular structure (plus a lot longer if it's maintained).
>article that could really use some diagrams, photos, or other illustrations.
The image at the top of the article is a clickable url to the Youtube video which has the diagrams/illustrations/etc.
The page submitted to HN is really a pre-written script that Grady reads to narrate the video. The intended content for integrating visuals is really the video and not the script.
Was involved in replacing a large failing rail-tie retaining wall, roughly seven foot tall and 100 feet long. Had one engineer suggest unilock with geogrid but after watching a lot of youtubes of failed unilock walls we went with redirock. Eleven years so far including snowy winters and doesn't look like it will ever move an inch.
Can someone explain to me why the brickwork in the picture in this example is just separated columns, using the cap to hold them together, rather than interlocking the bricks? I'm sure that's not the reason the structure collapsed, but once there's movement in the wall, it seems like that would contribute to loss of cohesion.
He covered this type of construction in an earlier video, and I think this is the key line:
> Gravity walls and mechanically stabilized earth are effective retaining walls when you’re building up or out. In other words, they’re constructed from the ground up.
The soil behind this type of wall is supposed to have been stabilized by layering a tension material at intervals to keep it from moving. There's a demo of compacting sand with layers of cloth and then using the cube of 'sand' to support one wheel of a car. The material sags slightly and then holds.
My understanding is that those concrete puzzle pieces aren't predominantly load bearing. Mostly they are for erosion prevention and perhaps moisture control (not just keeping water out but keeping the hydration consistent over time and distance).
Yes, he goes into it in more detail on the video about mechanically stabilized soil, but the techniques used to stabilize the soil mean the wall is entirely stable without the concrete face, and the face is there to keep the soil from getting eroded away and to look nicer.
Saw the video on YouTube and thought about posting in on HN, but somehow I felt Video content generally dont belong to HN, so I am glad there is blog version and getting some attention.
I am much more interested though to know why it was already 4 years behind schedule.
I live in a very flat part of the US. I always assumed that the opposing retaining walls were tied to each other. I'm surprised there's no real anchor to this stuff.
Don't make fun of people's names, man. People don't usually choose their own names. If you're going to make fun of someone, make fun of something they choose to do, instead of the things they don't choose.
Is that making fun of his name? Or is it simply pointing out / wondering whether he a) chose a profession aligned with his given name, or b) chose a name aligned with his profession?
Lots of people have never heard of X for almost any value of X. Even if one had, it can be surprising to encounter an actual case of the thing in the wild.
The Grady Hill House? Why do you want to know about that, no one ever goes up there since the...incident. Say, you're not from around here are ya? Well, I'd stay far away from there.
Grady Hillhouse does seem like an ironic name for a civil engineer. As far as I can tell, that is indeed his real name. FWIW he uses this name on his LinkedIn profile as well: https://www.linkedin.com/in/gradyhillhouse
IMHO, Grady's YouTube channel is one of the best things on YouTube. He really is fantastic.
When I saw the title, I thought that Grady Hillhouse was a house that was in the news in the US that might have had a retaining wall collapse recently.
Retaining walls (generally speaking) are tough enough and do not fail.
Those that fail were either poorly engineered/calculated or badly built (or something else in the soils/terrain behind the was overlooked or external actions - like loads and/or vibrations from trafic were underestimated).
That is beyond and besides the actual shape and orientation of the wall.
Let's say Macbook keyboards fail much often than mechanical keyboards. Why is that?
One could say, the reason they are failing because "they were poorly engineered/calculated or badly built" and that might be true.
But another way to state this is the reason they are failing is because we like thin laptops. And making laptops thin pushes the engineering into territory where it is much more difficult to make the keyboard be designed and function reliably.
Just like our love of vertical walls makes it much more difficult to engineer retaining walls that do not topple.
You see, both of these do not necessarily conflict with each other. They are just different points of view on the same problem.
I think that blaming the problem only on engineering is shallow thinking. It is obviously right and at the same time it gives absolutely no understanding of what is happening.
It is the same kind of thinking that, for decades, blamed drivers for all accident. Obviously, it is the driver that caused the accident, but it gives no further useful understanding of the problem.
Once people understood there are other factors at play some countries started introducing solutions that make the traffic much safer without messing with the driver side of equation.
Your analogy ends short because noone wants thin retaining walls, or - better said - the only one that decides how thick they should be is the calculating civil engineer (that normally does try to optimize thickness of the wall, in order to optimize costs).
An owner/road department might decide to put some limits to the amount of land needed to build the wall and of course there may be particular cases of "narrow" areas, but we are talking of meters (and in my experience there is not that much difference in space needed for excavation between a "basic" and an "exceptionally heavy/thick" retaining wall, the difference may be at the most 1 or 1.5 meter in the base slab), and in these cases one could use gravity walls (that use much more concrete, though with less rebar and usually need less space "behind" for the excavation) or as the original article explaines anchored walls or walls on piles, or even temporary structures (micropiles and anchors usually) only to allow the building of the retaining wall.
The retaining walls (under road) in the first minute of the video are not conventional cantilever retaining walls, BTW, they are one of the common ways "reinforced earth" is made (the typical rebar concrete wall is sketched, as an example, at 10:26) compare these:
https://en.wikipedia.org/wiki/Mechanically_stabilized_earth
they work in a completely different manner, the MSE is more similar to an anchored wall, the "anchors" are in this case pieces of steel strips that resist the pull because of the weight and friction of the earth/soil, if the soil is not suitable or not properly compacted or you introduce a lubricant (water)there is the possibility of these strips to lose adherence, the thickness of the reinforced concrete outer slabs is irrelevant and usually fixed, independent from height of the wall or other local conditions, if I recall correctly some 14 cm.
Anecdotally, I have built both kinds of walls, and actually also once "saved" a piece (some 40 or 50 mt long) of MSE-like wall that was failing (though due to other reasons, the foundation was underdimensioned), the huge difference is that traditional retaining walls are "independent" from the soil you put behind it (as long as it is within a normal weight of 1,600-2,100 kg/m3) while the MSE stability depends greatly on the quality of the soil and the way it is compacted (besides a valid calculation of the length of the strips).
As a side note, MSE-like walls are usually perfectly vertical (and actually do seem like falling inward optically when they are high).
The beginning of the article sounds so modernist mid-20 century, I couldn't stand that.
Old roads that formed on old paths are much more stable because people went where the area was dry, and then dirt roads were stable and didn't move down or cause rain erosion. The generalizing phrase, that terrain is just an obstacle that should be plowed through, is laughable. People lived very well without retaining walls and without such huge excavations, until transport engineers decided to please cars, and not slow them down, or not make them go too steeply.
Same for the sentence about buildings. There are plenty places where slopes are taken adavtage of. In Stockholm, there are some houses that have +2 storeys on one side, and entrances to 3 storeys from different sides. In my town, a mall built in 1960s has 2 storeys, the top one can be entered without steps at all. In Finland, they used sloped ground to build stadiums, and even to make basements with windows.
People like parks with slopes, children like to ride sleighs in winter on them. The most stupid thing you can do near housing is a flat surface and a concrete retaining wall. That generates more problems than benefits: the wall may float, it may create new concentraded areas of water running on the surface, and mini-waterfalls in rains. This never happens with natural slopes, because with grass they're very stable at 3-10 degrees, unless you concentrate water runoff on them.
Finally, the place becomes completely uncomfortable to stay at. It's pretty normal to lay down on a grass on a slope. But unthinkable if there's a retaining wall above you.
Unfortunately, nowadays I see the architects and clients prefer not to think and adapt to terrain, but just bulldoze the ground.
Otherwise, yes, there are engineering solutions to making retaining walls and making them stable.
There are more people now, and more people means pushing into more marginal areas. And, I'm sure people built roads that sank and eroded in the past too, they just aren't around any more.
Personally, I care a lot more about hills when I'm riding a bike or walking. When I'm driving a car, hills don't matter.
At the same time, building on slopes can be problematic. It depends on the local geology whether it is possible to do that or not.
FWIW, I'm a bit tired of "cranky old guy thinks everyone else is an idiot" posts on HN.
Ok, I may have put it in annoying tone, will try better next time.
As for buildings on slopes. Swedes and Finns enjoy rocky ground, to which bulidings stick perfectly, and they make all the nice things I mentioned. Here's an example: a mall & offices complex, the elevation difference is 5 m (15 ft): https://www.google.com/maps/@60.1687254,24.9389851,295m/data...
If there's rock under the ground, you bore and put the building on pillars. It won't matter if the surface around the buliding is flat or not.
If you have to build on a sedimentary (sandy) slope then a retaining wall won't fix this at all. You'll need many long piles to hold it to the ground by friction. Retaining wall -- no matter which end of the constuction site -- would have to be a huge and heavy structure to hold to the ground, otherwise it'll be slipping couple of cm/inches per year, which is enough to cause problems.
Even if you level the site on a slope and sedimentary ground, a building will still need long frictional piles to be stable.
A slope at a park is nice for the occasional sled, but slopes aren't that great when you're trying to play organized sports, or if you want to set up temporary structures or host events at your park (kind of difficult to set up a tent at an angle). Other than natural areas, such as national parks or forests, these are the primary reasons I have been to parks in recent years.
You're right that playgrounds can't be steeply sloped.
But, first, many playgrounds are sloped up to 3%, people just don't notice that, and put tents on 3-5% slope without issues. (But if you mean tent for sleeping, I slept on 10% slope, without problems.)
Secondly, for playgrounds, you can put the pitch along level lines, level only the pitch itself and leave a steeper slope next to it to serve as a grandstand. Example from Finland: https://goo.gl/maps/CssidLvnTYCeZ6Qx8
Thirdly, a playground still does not require the entire lot to be leveled, which I see nowadays a lot. Earlier architects, if they wanted to build just a copy&paste standard project (rather than design something new with entries on different levels), they'd just put buildings and playgrounds along the level lines, and minimize levelling works. The lot would have 2-3 small slopes of 15-20% with grass on them between buildings or playgrounds. Nowadays, with digging machinery available, even if the same can be done, clients (mostly public agencies) order to level everything and build retaining walls at both ends.
My point is there's a plenty of ways to work around slopes in parks and land lots.
A gabion is a galvanised steel wire cage say: 4' x 4' x 2' (HWD). They have a hinged lid and you fill them with stones and then wire the lid shut. Sounds stupidly simple, and it is but they have some rather useful properties. Each unit is an easy one man lift, place and fill. Once filled, each one nominally becomes a large single block with great drainage properties. You can wire these things together into long rows. They work very well with water courses because they are easy to fix in place and once filled, won't move. Pouring conc. into formwork is a right old pain in a fast flowing river and it is all too easy to lose the finer particulates before the stuff has gone off (set and cured).
You can finish the exposed surfaces in various ways. You can pour soil on top and grass them, pour a bit of low grade conc and gravel for a solid "path". The cages are not particularly pretty but neither are they particularly ugly.
If a single cell fails then it generally won't cause much surrounding failure and is easy to replace or repair. You can embed fancy anchors inside them if you have a lateral thrust to resist that can't be dealt with by sheer mass.
Sleepers and the like are quite convenient but you must consider drainage otherwise they will rot within a few decades. Block backed brickwork needs a decent brickie to lay them and if they fail it is usually rather bad. I'm no brickie but I've just repaired a broken 3' retaining wall at home and it looks a bit shit. I will be hiring a professional to sort it all out in spring. Here a gabion wall is overkill!
I did say home projects above but these things are used everywhere and that includes some pretty huge retaining structures. If you are not a Civil Engineer and need to build a decent sized retaining structure then I highly recommend that you consider gabions first because you are far more likely to get it right first time.