skin friction question with caisson 1

[snap]

Typically, I see that belled caissons are generally end bearing caissons and neglect skin friction. Correct me if I am wrong but can I design a belled caisson for primarily skin friction? (of course providing I have the appropriate soil profile) Please comment

 

[Ginger]

Caissons are usually sunk under a kentledge load or by hydraulick jacks applying jacking pressure to the caisson rings at the surface. In order to limit these loads and make jacking and steering easier, the caisson outer skin is lubricated (usually with bentonite or similar).

Therefore, the lubricant which is helping you get the caisson down into the ground will also prevent you using skin friction to resist any subsequent uplift. You can't have your cake and eat it !!!!

The only way around this is to grout up the annulus around the outside of the caisson when you have reached your design level. Do this using lances or other methods which ensure that the grout displaces all of the lubricant. Then you can use skin friction.

 

[BigH]

I think that there might be a misunderstanding about the term caisson. I believe that snap is talking about drilled-in caissons, or bored piles, or drilled shafts - a lot of names have been applied to 500mm to 3m diameter drilled shafts throughout the literature. Belled caissons are typically used for end bearing although some advocate being able to use skin friction at a nominal height above the bell up the shaft. Tomlinson, I believe indicates that many now forgo bells for deeper straight shafts in light of difficulties in doing bells - vs the time expended for straight shafts. Remember that there is a marked difference in the strains to mobilize skin friction than end-bearing. I noted in another thread that a learned prof that I know suggests to plot movement/diameter vs load (non-dimensionalized) and the break in the curve is the adheasion to end-bearing handover (about 1 to 2%). So, the answer is yes, you can design mainly for adhesion but then, why would you want to provide a bell??

Ginger is correct in the concept of very large diameter caissons (say 6 to 9m or more. These, here in India, are also known as well foundations. India uses a lot of these for major bridges even though drilled shafts would be a much more efficient choice (given time). A well, some 40m deep might take as long as 4 months or more to sink from inception of cutting edge to sinking (steining in 2m lift thicknesses). With such sinking, always have to worry about uneven sinking. Anyone interested in some references, etc. regarding well foundations as used here in India, give a e-mail address and I will get back to you.

Best regards to all.

 

[JAE]

Drilled Piers (caissons) in Texas are used for buildings situated on expansive clay soils that subject the building to shrink/swell actions.

The bells are used to not only take the vertical gravity loads, but also to anchor the pier in the ground to resist the uplift caused by swelling soils grabbing onto the pier and lifting. Thus, the bell takes the vertical gravity load but also serves as an anchor.

So you can get uplift skin friction in a pier with the bell...makes opposite sense that you can get the same skin friction over some of the pier as well....just how much, and over what length is what BigH refers to above.

 

[Focht3]

Please, let's call them drilled piers for the sake of clarity! (BigH is right about the various names given to this foundation type.)

Stress overlap from the loads on the bearing area generally precludes the use of skin friction for the first two to three bell diameters upward from the bottom of the bell. And shaft resistance usually develops with a shaft movement of only about 5 or 6 mm (about 0.2 to 0.25 inch), while end bearing won't reach its maximum resistance until the bearing area has moved 3 to 5 percent of the width of the bell. The difference in the amount of movement required to mobiize the ultimate shaft resistance and bearing resistance can be huge, even without a bell, for large diameter shafts. This is why I generally will use a larger factor of safety for end bearing when I have a pier (or pile) that has been designed assuming both end bearing and skin friction resistance.

Why would you underream (The proper name for the "belling" process) a drilled pier? In my experience, three reasons are generally given:

1. The designer has no confidence in (read: experience with) skin friction;
2. The shallow layers are much weaker than the bearing level soils; and/or
3. The designer is using the bell (underream) as an anchor to keep the foundation from moving with seasonal shrink/swell of expansive soils. The bell is constructed so that the top of the bell is below the depth at which soil moisture is not affected by seasonal effects. (This is the predominant use of underreamed piers in central Texas, where we have very active expansive clay soils.) This may require heavy shaft reinforcement, particularly for piers with small dead loads. Reinforcement in the range of 1 to 2 percent steel (by cross-sectional area) is common under these conditions.

 

[JAE]

One other note on the "name" of piers/caissons/piles.

The new IBC 2000 actually defines piling vs. piers based upon the length/diameter ratio rather than on the actual construction method used to install them. (See Chapter 18)

 

[Focht3]

JAE -

Very interesting - haven't had an IBC-related project since the 2000 IBC came out, so I wasn't aware of the code-based distinction in naming the foundation type. Could you give us the abbreviated version of the IBC definitions?

 

[JAE]

Per IBC 2000, Section 1807:

PIER FOUNDATIONS. Pier foundations consist of isolated masonry or cast-in-place concrete structural elements extending into firm materials. Piers are relatively short in comparison with their width, with lengths less than or equal to 12 times the least horizontal dimension of the pier. Piers derive their load-carrying capacity through skin friction, through end bearing, or a combination of both.

PILE FOUNDATIONS. Pile foundations consist of concrete, wood, or steel structural elements either driven into the ground or cast-in-place. Piles are relatively slender in comparison to their length, with lengths exceeding 12 times the least horizontal dimension. Piles derive their load-carrying capacity through skin friction, through end bearing, or a combination of both.

 

[Focht3]

Argh! I had a bad feeling when I posed the question!

This is a new definition for both piles and piers. It just muddies the waters even more. What a shame.

 

[JAE]

I can see some logic in defining the "entities" ...whatever they are based upon their length/diameter ratio as that may affect the design behavior of the two types to some degree.

However, I just can't believe that the behavior of the two realms of ratios are really that much different. And now, you can have a driller on site, using the same bit on his crane/kelly bar arrangement, and he would technically be installling both piling and piers while using the exact same construction process.

I agree Focht3, it stinks.