Shallow Pile Skin Friction - Frost Depth Penalty 2

[AlohaBob]

It seemed prudent to disregard possible support benefit from skin friction in shallow piles within the frost depth, since this soil is unstable.

I am curious of other opinions.

 

[VoyageofDiscovery]

I agree.

Regards

VOD

 

[slipstick50]

we always neglect pile skin friction in the predicted frost depth

r.

 

[Focht3]

Do you ever worry about uplift forces on the pile shaft due to ice lens formation?



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[AlohaBob]

Thank you.

 

[Hookem]

Do not merely ignore skin friction. Must consider uplift due to freezing soils!

 

[Focht3]

Hmmm,

Do not merely ignore skin friction. Must consider uplift due to freezing soils!

That's what I would expect; after all, the uplift forces can be substantial...



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[AlohaBob]

Around here for shallow foundations, so long as the bottom of footing extends below frost, the upper level frost effects are ignored. Frost wreaks havoc on foundations above frost level. Fences, Sign posts, pavements, can heave up out of the ground even 12 inches in one year.

The question came because I saw someone used skin friction in the frost layer for support in a light foundation. I never saw this before. He wasn't from here. It was a mistake. The owners of that house will likely have problems in the future.

 

[Focht3]

The question came because I saw someone used skin friction in the frost layer for support in a light foundation. I never saw this before. He wasn't from here. It was a mistake. The owners of that house will likely have problems in the future.

Do you have the designer's breakdown of the design pile capacity by end bearing and skin friction?

The structure may indeed have problems - particularly if the foundation terminated in the frost zone. But if it extends below the frost zone - and has significant end bearing - I would argue that the mistake may not be particularly important. (Of course, the devil is in the details...)

Keep in mind that skin friction in silts and sands are proportional to the overburden pressure, so that the available skin friction at shallow depths probably doesn't amount to much. In the big scheme of things, the skin friction probably contributed less than 10 percent to the total pile capacity - well within customary 30 percent margin of error for geotechnical design parameters.

And, of course, we have the issue of whether the piles will ever see the design loading during their useful life...



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[AlohaBob]

I can appreciate that. He had nearly enough capacity in end bearing, but neglected it. The code says for this foundation use one or the other, not both bearing and skin friction. I'm sure it will be okay anyhow, it just caught my attention as I'd never seen this done here. If this design were fair game, I'm sure I would find some use for it.

The piles terminated below frost depth.

 

[Focht3]

The code says for this foundation use one or the other, not both bearing and skin friction.

Which code?

Personally, I find the kinds of proscriptive codes to be ill-considered. They don't allow the geotechnical engineer to use his judgment and provide the most cost-effective design for the site. I understand that Chicago has a code of this type; I don't understand why the local geotechs haven't worked to have that provision changed.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[VAD]

Interesting dialogue. I have often said that to neglect the loads in the design of deep foundations, the understanding of how load is transferred and resulting settlement are some of the problems facing foundation designers.

I guess when a structural engineer designs a foundation he looks at the geotech recommendations which generally provide skin and end bearing and then goes to the code and uses one or the other based on the code available. This is probably okay for lightly to medium loaded structures. For heavily loaded structures where ground conditions would require very long piles etc I feel that the geotec and structural engineer have to huddle.

Very often settlement is given and based on the endbearing capacity which may never be invoked by the design, but this is often provided in geotech reports but not clearly understood by the foundation designer unless he understands the transfer mechanism etc.

There is genearlly some reasoning behind the choice of skin friction, end bearing or both.

For end bearing to be mobilized fully the pile base has to move. How much movement is at best empirical at this time.

We stll cannot determine realistically by the simple methods how much capacity in end bearing is invoked by varying magnitudes of movement of the base. We can assume that end bearing at least tells you what reserve capacity the pile has since as best as we know skin friction is first mobilized as it take about 2 mm of movement to invoke this.

Generally, for drilled shafts depending on the size of the base and the length of the pile some advocate end bearing only. The Brits do this. This results from the perceived mechanism of tranfer of load and the fact that the base is properly constructed and piles may be short.

On the other hand, for a straight shaft pile if the base is not constructed well, some like to stick with skin friction only as they do not want to invoke settlement by including loads to be taken by the base. No one does any calculations however but stick by this established rule in many jurisdictions based on local exerience, perhaps.

Before I decide on what the design should be based on I must obtain the loads, hence I do not particular like the approach to geotechnical recommendations that are generally provided. However, this is the practice that prevails and two camps exist, plus the codes which generally do offer not very good explanations.

Codes are course compiled by people like ourselves who are still struggling to understand how some of this stuff really works. As well on some of the code committees, if one of the experts is on the committee then he or she gets the upper hand.

Unless we can come closer to looking at the deep and shallow foundation designs as both a structural and a geotechnical issue we will design but we may never really know what we have done, except we perhaps feel good that the structure stands. Well on many occasions hats off to mother nature asshe takes care of some of that.


In closing I do not believe that we can ever come near in understanding these designs unless we can appreciate both the structural and geotechnical aspects. They cannot be separated. Put simply a load causes an action which has to be reacted by the earth. How the earth reacts is the intrigue and how we decide on how the earth will respond is based on its characteristics and how well we understand them. Likewise, we must understand the loads and how they are invoked and by what mechanisms etc. Sadly this is not our practice today in all quarters.

I have typed this rapidy and some may be out of context. If it is too long my apologies.


Regards

 

[Focht3]

In closing I do not believe that we can ever come near in understanding these designs unless we can appreciate both the structural and geotechnical aspects. They cannot be separated.

Truer words were never spoken -

My experience has been that skin friction fully mobilizes at 5 or 6 mm, but your basic point is on target: skin friction and end bearing seldom, if ever, mobilize at the same rate or time. This aspect of geotechnical engineering is still poorly understood by too many practicing engineers.

And, as a side note, the fact that skin friction and end bearing resistance develop at different levels of foundation movement represents a basic problem with applying LRFD to soils. LRFD deals with the "quality" of the load-carrying capacity of a structural element, but has no effective mechanism to deal with the variation in how the resistance to load actually develops. I see this as the fundamental flaw that will ultimately derail LRFD - unless owners are willing to foot the bill, either in design or construction.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[AlohaBob]

Thank you gentlemen. I have a much greater appreciation of your knowledge in reading your posts.