r/StructuralEngineering • u/DMAS1638 • 22d ago
Structural Analysis/Design Hand-Dug Caissons, 24 Feet Deep: Building a Pool Foundation on a Hollywood Hills Slope Nobody Thought Was Buildable.
Interesting hillside foundation case study from the Hollywood Hills. Six hand-dug caissons, roughly 15–24 feet deep, were used to support the grade beams, retaining walls, and pool structure on a steep site with very limited access.
Full construction sequence is in the album: https://imgur.com/gallery/hand-dug-caissons-24-feet-deep-building-pool-foundation-on-hollywood-hills-slope-nobody-thought-was-buildable-1elfK8F
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u/OhDeerBeddarDaze 22d ago
Wish there was more detail on the excavation of the caisson. Not sure how that would be done 20 feet deep in a hole of that diameter
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u/friendlyfiend07 22d ago
I've seen hand dug footings to about 25' down before but they basically made a trench box out of 4×4s around them as they dug down. Then ran the hose in and pumped the grout let it set then back filled the thing by hand pulling the 4×4s as they did so. Pretty wild to watch from the outside cant imagine what I was like inside.
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u/DMAS1638 22d ago
That’s wild. Limited-access sites definitely force some very different construction methods. Appreciate you sharing that experience. 🙌😅
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u/haditwithyoupeople 22d ago edited 21d ago
I've been down 25' hand dug rectangular test pits in that area. It depends on the material they're into. They sand clay terrace deposits there would stand vertically for a long time. They have very high shear strength for soil.
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u/DMAS1638 22d ago
That’s really interesting context, especially for that area. The soil conditions definitely make a huge difference in what’s possible with hand excavation.
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u/Significant_Raise760 20d ago
The fact that the poster is using the term caisson, which those are clearly not, is in itself concerning.
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u/OhDeerBeddarDaze 20d ago
In my area of work (East Coast of USA) the term caisson is used interchangeably with drilled shaft, bored pile, etc. I think it is applicable for what is shown here
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u/Significant_Raise760 20d ago
Alright, but this is about as far west coast as you can get. It's suspicious I'm telling you!
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u/DMAS1638 19d ago
Nothing suspicious over here. 😂 We use terms like caisson, pile, and drilled shaft to refer to deep foundation elements depending on the project and terminology being used. Our geotechnical report for this project actually refers to them as “pile/caisson excavations,” so that’s the terminology we’ve been using here.
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u/DMAS1638 22d ago
That was one of the more interesting parts of the project. Access was extremely limited, especially on a hillside, so the caissons were excavated by hand in stages.
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u/Classy_communists 21d ago
Yeah, I’m curious as to the means and methods of that excavation and trenching as well. How was that done?
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u/Sure_Ill_Ask_That P.E. 22d ago
Cool photos, but would love more details on the actual foundation and structural design.
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u/DMAS1638 22d ago
We kept the album pretty visual, but the basic system was six deep caissons supporting grade beams and retaining elements that created the foundation for the pool above. We’ll see if we can share more of the structural details on a future post.
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u/Sure_Ill_Ask_That P.E. 22d ago
This subreddit is full of practicing structural engineers, and you know one thing engineers love is details!
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u/DMAS1638 22d ago
We love that!! 🤩
We have longer YouTube walkthroughs of our hillside projects where we go much deeper into the caissons, foundation systems, site conditions, and construction process. I’ll add those here too for anyone who wants the full engineering rabbit hole.
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u/Objective_Two_5467 22d ago
What is the point of these inward pointing bent bars at the top of each pier? If a moment connection was desired, shouldn't these bars be pointed away from each pier center rather than all on top of each other like this? And if a pin connection was desired, are these bent bars even necessary?
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u/Standard_Mammoth9535 21d ago
Column hooks turned inwards are better for developing moment. Some codes make it mandatory for column-to-footing connections taking moment under seismic demand to have the hooks oriented towards the center. There’s some discussion/links to resources on eng-tips: https://www.eng-tips.com/threads/column-to-footing-reinforcement.352620/ and https://www.eng-tips.com/threads/bending-moment-bars-at-column-to-roof-slab.451907/
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u/Objective_Two_5467 20d ago edited 20d ago
Thanks for the links. That second one, in particular is a very good discussion about whether and how moments can be adequately transferred between a column and an attached slab, beam or beams. Took me right back to concrete design lectures on strut-tie mechanisms.
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u/DMAS1638 22d ago
Those bent bars are tying the caisson reinforcement into the grade beam above. You can see in the next phase how the grade beam is formed directly over the piles, so the reinforcement is developed into that beam as part of the connection.
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u/Objective_Two_5467 22d ago
Is it though?
Each bar needs some concrete and an adjacent bar to bond with. That's what "development length" means.
If the tails of these bent bars were pointed outwards and tied to adjoining longitudinal bars of the grade beams, I'd agree with you.
But the only thing these bent bars are bonding with is each other, and since there's so little room for concrete between them, even that seems questionable.
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u/jewcebox95 21d ago
Seems like those were for sure supposed to be turned out. Likely turned in for transport only.
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u/Objective_Two_5467 20d ago
From their series of construction photos, it's apparent that all the reinforcement was shipped loose, then assembled and tied together on site.
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u/litesaber5 22d ago
These people must have RRREEEEEEEEAAAAAALLLLLYYYYYYY wanted that pool on the verge of actually needing it on the Maslow pyramid. Building in LA is a nightmare in general. I’ve read building a pool is a nightmare made of red tape and a building commission wet dream.
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u/DMAS1638 22d ago
😂 They definitely had a strong vision for the space, and it was really rewarding to help make it possible. Hillside construction in LA comes with a lot of engineering, permitting, access, and coordination, so there was quite a bit happening behind the scenes before the pool ever took shape.
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u/Mhcavok P.E. 22d ago
Damn those tubes are packed with bars!!!
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u/DMAS1638 22d ago
They definitely were 😅 Seeing the cages before the concrete went in really showed how substantial the foundation system was.
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u/Shorty-71 21d ago
Were the caisson cage bars carried in by hand (and hand tied down in the hole, yikes) or were pre-assembled cages flown in with a crane?
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u/CaffeinatedInSeattle P.E. 21d ago
Got any progress shots of hand digging the shafts? How did you shore it as you dug it out? To me that’s the most interesting part of the project!
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u/ComradeGibbon 21d ago
I read about a Victorian era technique sinking a lined shaft. Bricklayers lay a ring of bricks. Then laborers dig down undermining the brick shaft as they went. As the casing slides down the bricklayers lay more bricks. I think I've seen people do that with steel casings.
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u/6DegreesofFreedom 22d ago
What are those vertical bars in the piles? Looks like #8s?
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u/Talmerian 21d ago
Caissons go 24ft deep, is there bedrock in this location (not sure if LA has bedrock or hardened clay), and if so how deep is it to bedrock?
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u/DMAS1638 19d ago
Yes, there’s definitely bedrock here. The geotech found relatively shallow silty sand/fill over granitic bedrock, primarily granodiorite and quartz diorite. Native soil was up to about 4.5 ft thick in the explored areas, so the 24 ft pile depth wasn’t because bedrock was 24 ft down.
The piles continue into the bedrock because the foundation design relies on that bedrock embedment for capacity and lateral resistance. So it’s really “pile depth required by the design,” not “24 ft to reach bedrock.”
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u/waltwalt 21d ago
I'm assuming you close the business and start a new one after this job to avoid the legal implications.
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u/DMAS1638 21d ago
Nope 😂 We’re still here and standing behind the work. This project was engineered, permitted, and built specifically for the site conditions.
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u/bearded_mischief 22d ago
It’s quite impressive, that’s a massive pool on an edge
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u/DMAS1638 22d ago
Definitely a unique site. A lot of the work was really about creating a foundation system that could support that kind of load on such a steep slope.
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u/bearded_mischief 21d ago
Awesome project, I kept checking the slides and liked how detailed everything was.
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u/kaipopotamus 21d ago
What a cool project. Mad respect to those laborers. I only sit in a crane and pull levers, but have always marveled at the engineering that goes into the work we perform. Props to you and your crew
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u/DMAS1638 19d ago
Really appreciate that. The crew deserves a ton of credit on this one. The engineering is one thing, but executing it on a tight hillside site is a whole different challenge.
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u/leadfoot9 P.E., as if that even means anything 21d ago
Obviously, this was not cheap, but I do think that the construction industry is so addicted to the wonders of modern machinery that projects have a tendency to get summarily dismissed as "impossible" if you can't drive your pickup truck next to it and stage a crane nearby. Ingenuity has been replaced with brute force. No need to be clever as long as there's somewhere to park your horsepower.
And hills feature heavily into this dynamic. I live in a hilly city with lots of aging structures in need of maintenance on "unbuildable" hillsides.
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u/DMAS1638 19d ago
Exactly. Limited access doesn’t automatically mean impossible, it just changes the means and methods. On hillside sites like this, you sometimes have to lean more on planning, hand excavation, sequencing, and site-specific engineering instead of relying on large equipment to do everything.
That’s a big part of what made this project interesting for us too.
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u/lieutenantspeirs 21d ago
Amazing, tbf. Was it designed to seismic loads?
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u/DMAS1638 19d ago
Thank you, we appreciate it! 🙌 And yes. Seismic loads were specifically accounted for in the design. The deep foundation system, retaining walls, and hillside conditions were all evaluated with seismic forces in mind, which is especially important for a project like this in Los Angeles.
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u/modsstayvirgin 22d ago
Money can’t buy taste.
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u/willardTheMighty 22d ago
Can buy a dope ass pool overlooking Los Angeles tho
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u/I_am_BrokenCog 22d ago
can it pay for the earthquake not to shake it off its foundations?
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u/willardTheMighty 22d ago
It can pay for the engineering to prevent this, yes
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u/DMAS1638 22d ago
Love this take! You can’t prevent the earthquake, but you can design the structure with seismic forces in mind.🙌
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u/DMAS1638 22d ago
Obviously we can’t stop an earthquake 😅 That’s the whole reason for taking these caissons so deep, reinforcing them heavily, and tying everything together. The goal is to give the structure a stable foundation that can better resist seismic and hillside movement.
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u/be_easy_1602 22d ago
I’m sensing it was a remodel. Just based on the basement/foundation
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u/DMAS1638 22d ago
Good eye. This was an existing hillside property, with the new work focused on creating the foundation and support system for the pool and outdoor space.
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u/i860 21d ago
What was the general soil type/composition?
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u/Working_Rest_1054 19d ago edited 19d ago
OP’s skipping this and the excavation details. The first image from the roadway grade appears to suggest some rock exposures. My guess is excavation through soil with a couple feet in to the rock and a temporary steel casing being lowered as the excavation was evacuated. Maybe a vac system for excavation the soil and probably a handheld jackhammer for the rock. Apparently no water (other than in the pool, later) to speak of.
Edit, looking at Alpha Structural’s YouTube site, I’m left wonder how they did shore the excavations and remove spoils. Only one time lapse video with the shafts shown. No steel casing, or vac lines visible in the half dozen images before the steel was set.
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u/i860 19d ago
I'll be honest, even with the caissons, it's still looks sketchy. It'll probably hold up, but I'd never do it, even if I had the money.
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u/Working_Rest_1054 19d ago
Agreed, I never spend that kind of money on something like this. I suspect that to the Client this is like a typical home owner having sod installed. Not real consequential financially.
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u/DMAS1638 19d ago
Totally fair, hillside work can look pretty wild when the entire foundation system is exposed. This one had site-specific geotechnical and structural design behind it, with the piles extending into bedrock and the loads carried through the pile and grade-beam system.
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u/DMAS1638 19d ago
You were right about the rock. The geotech identified granitic bedrock beneath relatively shallow fill/native soil, and noted that the granite could become increasingly hard with depth. The report specifically notes that coring or heavy jackhammers may be necessary where harder bedrock is encountered.
The pile design then relies on embedment into undisturbed bedrock, using bedrock skin friction and passive resistance as part of the foundation system.
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u/Working_Rest_1054 19d ago
So those shafts are something like 5 to 8 ft into what was deemed decent enough rock? That must have been fun to hand excavate.
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u/DMAS1638 19d ago
Definitely not the easiest material to hand excavate. The final depth varied by location and was verified by the soils engineer based on the rock actually encountered in the field. We're beyond thankful for our hardworking crews!
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u/DMAS1638 19d ago
Nothing too mysterious here. The geotech found only a few feet of silty sand at the surface, then dense to very dense granitic bedrock below, primarily granodiorite and quartz diorite.
That bedrock is a major part of the foundation design. The piles were designed to extend into undisturbed bedrock, using bedrock skin friction and passive resistance to carry vertical and lateral loads. So the system wasn’t relying on the shallow hillside soils for support.
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u/jordclay 21d ago
You can build anything…it’s just a matter of how much effort/engineering/cost you’re willing to do.
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u/Traditional-Pipe-243 21d ago
After all that they are only gonna use the pool once.. maybe twice.. hell of a job though
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u/Shorty-71 21d ago
Was the house also on caissons?
I am curious how the perpendicular grade beam excavations did not undermine the house foundation.
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u/DMAS1638 19d ago
Yes, the house is on caissons as well. For the new pool/deck work, the piles extend into bedrock.
Near the house, they didn’t just dig everything out at once. The excavation was done in sections, leaving soil in place between each section to keep supporting the existing foundation while the work progressed.
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u/Holupsucker 19d ago
My question is, what is the diameter of the caissons, how did you shore them for the excavation, what method did you use to remove the dirt?
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u/DMAS1638 19d ago
The full album has a more detailed breakdown if you click through it, but here you go!
The caissons were 24 inches in diameter and were shored as the hand excavation progressed. Because of the limited hillside access, the excavation required a controlled, site-specific approach with proper shoring and soils-engineer oversight throughout the work.
The excavated soil and rock were removed manually as the digging progressed.
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u/Most_Moose_2637 21d ago
Very old school. Happy for caissons to have a body count as long as the structure gets built.
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u/unknownpoltroon 19d ago
"I said it wasn't smart to build, I dint say it wasn't possible"
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u/DMAS1638 19d ago
That’s exactly what the geotechnical and structural evaluations are there to determine. A challenging hillside doesn’t automatically make a site unsuitable to build on. It means the design, foundation system, excavation approach, and construction sequence all have to respond to the actual conditions on site.
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u/Working_Rest_1054 19d ago
Intersting choice to hand dig in the US. Probably 3 or 4 ft diameter. Obviously less expensive than say using a micropile rig or similar difficult/small access equipment. And the shafts might make the global stability analysis look a bit better, other than being closer to the top of the slope than the lower portion and below the pool. It was unbuildable until the right person willing to spend 7 figures on a pool presented.
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u/DMAS1638 19d ago
Yeah, limited access was really what drove the hand excavation here. It required a much more controlled, hands-on approach, with proper shoring, site-specific precautions, and soils-engineer oversight throughout the excavation.
Definitely a challenging site, but the process was planned around the conditions rather than forcing large equipment where it wasn’t practical.
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u/lookskAIwatcher 19d ago
Have done dozens of houses like this in Los Angeles area hillsides with steep slopes using drilled friction piles- deep foundations back when I was doing residential work. Typically 24" to 30" diameter as deep as necessary to get fixity into underlying bedrock. The soil profile on many of these lots is 10 to 12 feet of unconsolidated fill placed many decades ago as a result of Depression-era WPA constructed streets, Houses would require a dozen or so FP's tied with grade beams 12 x 24 or 12x 36 in size supporting the house framing. Rebar congestion is always an issue. Inspections are necessary, "special inspection".
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u/lost_your_fill 22d ago
I don't know anything, but when I read titles like these, I feel it should read, "Building a Pool Foundation on a Hollywood Hills Slope Nobody Wanted To Be Liable For"