Side Friction on Caisson
Side Friction on Caisson
(OP)
Hi everyone, this is a relatively basic question but i am not specialising in geotechical issues. The question is:
You are using a cylindrical caisson for a footing to a column and you want to take the load both in bearing on the end of the caisson and friction on the sides of the caisson. The allowable end bearing on the founcation material is 500kPa, but in the absence of any further information what value would you use for side friction in your preliminary design?
Thanks everyone.. :)
You are using a cylindrical caisson for a footing to a column and you want to take the load both in bearing on the end of the caisson and friction on the sides of the caisson. The allowable end bearing on the founcation material is 500kPa, but in the absence of any further information what value would you use for side friction in your preliminary design?
Thanks everyone.. :)





RE: Side Friction on Caisson
RE: Side Friction on Caisson
I consulted some other person about this and he told me that he used a friction coefficient of 0.45 however i am unsure as to what i do with this value.
Your help will be much appreciated.
Adam
RE: Side Friction on Caisson
The skin friction can actually be negative(i.e. the soil is dragging down ward),.
RE: Side Friction on Caisson
f-d
¡papá gordo ain’t no madre flaca!
RE: Side Friction on Caisson
"just because skin friction is potentially available, does not mean that is is actually available (i.e., if the caisson is sitting on rock)."
I assume the description "caisson" is here being used to describe a bored pile (or in USA sometimes called a drilled pier)
A bored pile can be designed to be end bearing or for shaft resistance, or a combination or the two. If the bored pile is sitting on rock, then skin friction does not come into it. On the other hand there are many cases where deep bored piles in the range 60 - 100m long are designed mainly in consideration of the skin friction with end bearing capacity being negligible.
RE: Side Friction on Caisson
Just a matter of strain compatibility. It requires less strain to fully mobilize end bearing and more strain to fully mobilize shaft resistance. For the case of end bearing on rock, there may not be enough strain to fully mobilize the shaft resistance.
Caisson - referring to drilled shaft.
I don't disagree with anything you've posted.
f-d
¡papá gordo ain’t no madre flaca!
RE: Side Friction on Caisson
I 100% agree with fattdad on this statement. Soil needs to give a bit before it resists(e.g compaction) even in shear.
Rock resists straight away.
This is an often overlooked aspect of soils.
Caisson is the east coast USA term for what is also called a bored pile or a drilled pier elsewhere.
RE: Side Friction on Caisson
RE: Side Friction on Caisson
As for the caisson I just find the term, when used in place of "bored pile", confusing because to me a caisson is a shaft with a void in the middle.
RE: Side Friction on Caisson
But it has always been my understanding that for a pile, it takes more strain (vertical movement) to mobilize the end bearing than that for side friction - in other words side friction (adhesion) is almost wholly mobilized before the end bearing comes into play - the reason why in piles in sand, the allowable capacity after applying the factor of safety is, in effect, nearly the same as for the piles frictional capacity. Tomlinson explains this in his foundation design book (figure 7.23 of latest). For sockets into rock, usually it is typical to use one or the other. Osterberg and Gill (9th CDN Symp on Rock Mech) shows a curve of % of applied load carried by side wall shear for various degrees of L/B in rock.
RE: Side Friction on Caisson
RE: Side Friction on Caisson
If the caisson is bearing on a dense sand and penetrates a stiff clay and if sufficient time has elapsed for excess pore pressures to be relieved, then conditions may be different. Not sure what I'd do in practice (well, I'd probably make it a belled caisson in the stiff clay, eh?).
f-d
¡papá gordo ain’t no madre flaca!
RE: Side Friction on Caisson
what you say is correct for a pile, which is typically long and narrow.
But what is usually referred to as a caisson is much shorter (20 times diameter at most).
The behaviour of the two is quite different.
RE: Side Friction on Caisson
There is a difference, granted, if the pile is end bearing "on rock" - and you are looking at adhesion/friction in a soil - but the original post said nothing about end-bearing "on rock". I believe, in rock, the current thoughts (see Canadian Geo Journals of say 4 years or so ago), are either to design for end bearing or for shaft grip - not both although the Canadian Foundation Engineering Manual says that both can be used - but you need to take into account the shear transfer. When "on or into (more likely), you don't really take into account the soil contributions.
I don't see how "if the thing is supposedly going to be end bearing, the frictional part will engage at nearly the same time the end bearing component engages as the pile doesn't know it is "supposed" to be end bearing. If you want it truly end-bearing, the negate the side friction/adhesion. I am not sure the soil is aware of your design assumption.
Good discussion - though.
RE: Side Friction on Caisson
RE: Side Friction on Caisson
Look at the vertical movements necessary to mobilize the side friction/adhesion and the end-bearing - see the link to Tomlinson's figure. The side friction/adhesion takes less overall vertical movement (strain) to reach peak than does the end-bearing. Look, too, at curves showing load carrying capacity down the length of pile as the load is applied. You will see that the load reaching the tip is small until the side friction/adhesion is overcome - then the load carried at the tip will increase.
RE: Side Friction on Caisson
You raise a good question that I wanted to think about before answering and then was overtaken by the holidays.
I am assuming that since it is a cassion, it is a) drilled in and b) of fairly good size diameter, giving it significant bearing capacity and axial stiffness.
So lts say it has an ultimate bearing capacity of 600 kips, using an FS of 3 it might be assigned an allowable bearing of 200 kips. If it is of good size, say 24 inch, you may not see significant movement until about this value. Therefore if your load is only 150 kips, you have suficent capacity to cary the load regardless of the side friction, and at this point there may not be enough settlement to engage the side friction. However, at some point the cassion will move, the side friction will be engaged and the friction capacity will be available.
RE: Side Friction on Caisson
Fellenius has some very good papers which are freely downloadable showing load transfer.
RE: Side Friction on Caisson
RE: Side Friction on Caisson
See the FHWA drilled shaft design and construction manual. The strain-capacity curves shown indicate that side friction mobilizes faster (at less strain) than end bearing. By the time that end bearing is fully mobilized, the mobilized side friction is very significantly reduced as a percentage of ultimate capacity.
Naturally, the rates of capacity mobilization will vary depending on the size of the foundation element and the soils involved.
This makes the design of very slender elements such as ACIP piles problematic - the strain-capacity curves for side friction do not extend into the range of strain experienced by these elements. For example, my firm installs ACIP piles up to 24 inches in nominal diameter. If you assume a 1-inch allowable settlement - by no means unusual - then the strain over diameter is 1/24 = 4%. These curves go up to only 2% for side friction. Yet the most recent design guidance for ACIP piles (FHWA GEC No. 8) recommends using the drilled shaft design procedure for preliminary sizing!
Jeff
RE: Side Friction on Caisson
Back to the original question: if end bearing equals 500 kPA, what would be the skin friction for preliminary design. From an EXAM standpoint and NOT from a CONSULTING standpoint,
Here is my answer: Q=Qend + Qskin
given Qend = 500 kPA = 10.4 KSF. I would assume soil is very firm to dense SAND. I would assume that the soil is constant from the ground surface to the bearing level. From Terzaghi and Peck, observed skin friction for caissons in a dense sand is between .7 KSF and 1.4 KSF. I would select lower value and go to the next problem. Feels good to be back in class.