Basic Foundation Caisson Pointers Needed

[iaw4]

I am wondering whether caissons are expensive more because of the material, or because of the cost of drilling into bedrock.

would it be more efficient to place one giant fat caisson that then forks above ground, or many thinner caissons that all have to be placed far down?

where would I find information about typical prices for caissons these days in CA? any pointers would be appreciated. most of the pricing info I found on this website is >20 years old.

(I want to build on an unstable slope in Los Angeles, so I know I will have to go 80' down, because bedrock is 40' down.)

 

[BulbTheBuilder]

Contact a local engineer in your area.

The prices vary depending on location, material, soil type, and a whole lot more.

You can never tell if one big system is better than multiple smaller systems. why? because it needs to be engineers to determine the effective system especially for an unstable slope in a location such LA

 

[dik]

Bulb's correct, a local geotekkie should be consulted. Also, a good foundation contractor might be a good source of info. It depends on the soil type, bearing capacity, loads. and distance to bearing strata. In my locale, bedrock is about 40' down and is sound limestone with few fissures. Igneous rock is better.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[BridgeSmith]

Larger diameter shafts require a larger drill rig. Small shafts (36" and below) can typically be drilled with a truck-mounted rig, which has a low mobilization cost. 48" diameter up 60" or 72", (depending on how hard the material is) can typically be drilled with a commonly available track-mounted rig. Beyond that size, you're into the larger, more powerful track-mounted rigs that may have to be partially disassembled to be transported, and may have to be transported from a long distance away.

Most of the cost of the drilled shafts, is in the mobilization and the time needed to drill. Consequently, there is a significant economy of scale. Concrete is a couple dollars per CY and reinforcing may add $50 to $100 per CY.

That said, the best resources will be local - drilling companies who do that work, foundation engineers, and geotechs.

 

[iaw4]

Thank you.

I am trying to figure out basic economics --- when does it make sense to build on one giant pillar with a steel platform on top vs. when does it make sense to build on 4 pillars on the corners. Intuitively, the deeper the bedrock, the more I should prefer the single fat pillar. I believe skyscrapers tend to go by the single-column construction. of course, I will consult a real geo engineer for design and real quotes for pricing. I may be crazy, but I am not that crazy. I want to just get a rough idea.

I am guessing that a 3000 sqft house should weigh about 70 tons, so I want to balance (rounded) about 100 tons, think 40' x 40' x 2 stories. nothing is exact. it's just for understanding.

now, I am thinking that one 80' column at 60" diameter displaces about 60 cubic yards. Four 32" columns displace about the same. I believe a heavily reinforced 32" column is about $40k in my area (SoCal). if a 60" can hold 100 tons, and one can do a 60" for $80,000, plus the steel grid on top of it, say another $20k, I'd come out way ahead. if a 60" can't hold 100 tons or it would be $200k, then this is obviously not even worth thinking about.

so, really, I want just a basic way of thinking about it.

 

[BridgeSmith]

For axial load only, you can use whatever size makes the most sense overall, but if there will be lateral load, you have to consider bending capacity and the potential for reduced buckling capacity if it's too slender due to the depth to fixity.

When supporting something like a 3000 sqft house, you also have to consider how the loads will get to the shafts. Sure you can do it with 4 shafts, or technically even one, but the beams needed to get the distributed loads of the house to the shafts would very large.

Another thing to consider is whether the shafts will carry the loads in end bearing or side friction. A larger number smaller shafts will have more side face surface area for the same volume as a few larger shafts, so for shafts carrying loads in side friction, more smaller shafts are better.

 

[iaw4]

thank you!

darn earthquakes (for the lateral). ;-) so, my engineer will have to figure out the stiffness of the reinforced concrete to prevent buckling. A Y-shape should be able to nicely drop the load onto the single pillar.

fortunately, going into the bedrock, I won't worry about needing friction. but come to think of it, maybe four short side-friction beams not reaching down to the bedrock could act as a supportive auxiliary feature for a central pillar, thus, not having to go down very far. I will ask my engineers.

thanks again. I am beginning to understand the considerations better, though not yet the economics --- by which I mean if a 36" caisson costs X, a 60" caisson is likely to cost f*X and I want to know **f**.

 

[BridgeSmith]

...if a 36" caisson costs X, a 60" caisson is likely to cost f*X and I want to know **f**.

In that specific example, in my area and for the scale of the projects we typically do (150' to 300' total for a site) f would be approximately 2.5.

Probably somewhat higher for a smaller project.

My guess would be for a house, even a 3000 sqft one, 24" to 30" shafts would be the most economical overall.

You have to be fairly careful with how they are reinforced, though. Because the concrete beyond the first 5-10 feet at the top cannot be vibrated, the clearance between the reinforcing bars should be 5 times the maximum aggregate size in the concrete. If the spacing is much tighter than that, the concrete may not flow to outside the rebar cage.

You definitely need a structural or geotech engineer to design this, anyway, so probably the best thing is to let them tell you what's going to work best. If you try to 'help' them, you'll likely have a tough time finding someone to take on the design. Amateur clients who think they know what should be used are a big red flag to most engineers.