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.
> 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.
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.
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.
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!
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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.
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.
https://www.google.com/search?q=gabion+wall
What is going on here? This is a totally bizarre architectural element.
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.
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)
Others here https://scholar.google.com/scholar?q=galvanized+soil
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.
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.
Interesting look though.
https://dpw.lacounty.gov/bsd/lib/fp/Building/Residential%20C...
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.
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.
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. :)
1: https://practical.engineering/blog/2021/10/28/why-spacex-car...
2: https://practical.engineering/blog/2021/10/14/what-really-ha...
3: https://practical.engineering/blog/2020/6/1/why-does-road-co...
https://practical.engineering/blog/2016/5/15/mechanically-st...
https://practical.engineering/blog/2017/6/28/how-do-sinkhole...
https://practical.engineering/blog/2018/7/1/how-soil-destroy...
https://practical.engineering/blog/2018/7/16/how-quicksand-c...
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.
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
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.
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
Either way, you can't really get to an angle that would be considered a "wall" without mechanical reinforcements, can you?
[1] https://www.youtube.com/watch?v=--DKkzWVh-E
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.
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.
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.
Inefficiency doesnt work in packed urban environments that require to fit in a budget.
Deleted Comment
These exact topics are covered in paragraphs 12, 13, and 14, respectively. 9:14 to 10:41 in the video
[0] https://www.cpr.org/2021/08/12/us-highway-36-collapse-poor-d...
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...
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.
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."
He needs a second youtube channel or something where he can show way more detail about these things.
Every time I watch his videos I'm always left feeling kind of empty...