Side Friction on Caisson

[CivilStruc86]

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..

 

[hokie66]

In the absence of further information, I would not guess. The 500 kPa end bearing must have come from somewhere, so refer back to this source for skin friction. There may be a number of strata involved, each with different skin friction values.

 

[CivilStruc86]

It was a question in an exam which was worth only 4 marks. The examiner told me that it was a relatively simple procedure to adopt in order to obtain the "side friction" as it is only for a "preliminary design" so it much be a very simple calculation.

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

 

[csd72]

I would take skin friction as zero, if this was the only information I recieved.

The skin friction can actually be negative(i.e. the soil is dragging down ward),.

 

[fattdad]

In practice (Richmond, Virginia) we would not account for any skin friction. The extent to which it is present would be attributed to the support required to offset the weight of the newly-placed concrete within the caisson. Bear in mind that the strain required to fully mobilize skin friction is different than the strain required to mobilize some end-bearing conditions. As such, just because skin friction is potentially available, does not mean that is is actually available (i.e., if the caisson is sitting on rock).

f-d

¡papá gordo ain’t no madre flaca!

 

[Zambo]

hmm, what does this mean

"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.

 

[fattdad]

hmm, what does this mean

"just because skin friction is potentially available, does not mean that is is actually available (i.e., if the caisson is sitting on rock)."

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!

 

[csd72]

Zambo,

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.

 

[hokie66]

With 500 kPa bearing, I consider the bearing strata soil rather than rock, although it is probably weathered rock. So I think in many cases skin friction could be used in conjunction with bearing, but I have no idea how much.

 

[Zambo]

I like fattdad's posts as well.

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.

 

[BigH]

This post is confusing me - perhaps because we are getting ready to kill an ox for a local custom thing or had a few too many rum and diet cokes last night.
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.

 

[msucog]

i'm not sure i follow you bigh. i agree that the thing must move to some degree to "engage" the frictional capacity (this would be assuming not end bearing on rock). however, 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. in my opinion, the end bearing portion (in rock) will engage "somewhat sooner" than the frictional component. realistically, i'd say both are more or less at the same point however the end bearing would probably be just slightly sooner. i'm no deep foundation expert but that's my opinion. i'd suppose i agree with fattdad's mindset.

 

[fattdad]

All I'm trying to say is if you have a caission stitting on rock, there will only be a PL/AE component to the shaft movement (if it's hard rock, that is). This shaft movement (or lack thereof) will not be sufficient to fully mobilize the shear strength between the newly-placed concrete and the adjacent soil. My deep foundation notes (yes I kept them. . . .) say that 0.1 to 0.3 in of shaft movement is needed to mobilize the shear along the shaft.

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!

 

[csd72]

bigH,

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.

 

[BigH]

csd72 - tell that to Bangkok foundation designers where 1.5 to 2 m bored piles are installed to in excess of 70 metres. (p.s., the graph showing the shaft and base resistances is actually for a bored caisson (drilled shaft, drilled-in shaft or whatever term wants to use or is used locally).
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.

 

[DRC1]

If the pile is end bearing, you don't need side friction. If end bearing is not sufficent, the pile will start move and engage side friction. There are many ways to calculate side friction but it is generally the cohesion on the side of a pile or the friction. Side friction is open to a wide range of variations on a similar theme. Basically it is some component of the overburden stress and the friction. I have seen values from .1 to 1 with .3 to .5 being common, so for a sand, .45 may not be a bad preliminary estimate. Break the pile into several sections, compute the effective stress times your factor and multiply by the area (circumferance times section height). Do this for each area and sum, That is your friction. Add to end bearing for total capacity

 

[BigH]

This is where I am having a problem in these discussions. You say - "if the pile is end bearing, you don't need side friction." The pile does not know it is end-bearing just because that is the model you have chosen. The problem is that the load, as it is applied, will be taken up by whatever side friction/adhesion you have first - when it's peak is reached, it will transfer the load to the end-bearing portion. The capacity of the side friction/adhesion may be very small - it may be significatnt.
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.

 

[DRC1]

Big H
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.

 

[moe333]

I agree with BigH, side friction is always mobilized before tip resistance. It may be large or small, and whether you want to consider it or not is up to you. It takes a lot of strain to fully mobilize the end bearing, at least in soil, which is why the end bearing is typically limited to a fraction of the diameter.

Fellenius has some very good papers which are freely downloadable showing load transfer.

 

[BigH]

Thank you moe333! Was wondering if I was the only one that thought side friction/adhesion is mobilized at smaller strains than end-bearing.