Capacity of a bored pier in clay

[civeng80]

Im designing a tilt panel building and looking at using bored piers with or without strip footings.
Im using Bowles Foundation analysis and design (5th edition) as a reference text and have come up with the computations in the sheet attached.
Although Bowles is a pretty good reference text Im finding bored piers section to be a little confusing and alot of material sort hidden and in fragments.
Would anyone like to comment on my computations and SF and results.
Also any ideas as to how to estimate settlement from these comps?

 

[jayrod12]

Generally when we are looking at CIP piles you use either skin friction or bearing as it takes a lot to activate the bearing capacity at the tip. I may be wrong too but I do not believe that cu can be used directly as the skin friction (we're typically just given a skin friction number from a geotech).

As for settlement we've generally assumed it is negligible for friction piles.

If it is end bearing then the settlement calculations are the similar to shallow foundation designs.

Just my opinion,

 

[Mccoy]

The method used by cveng has been proposed by Reese et al., 1976, according to Bowles (I have the 4th edition, where some constants are a little different). Bowles explains that, even though the current practice in America is to adopt either only the friction resistance or the tip resistance, Reese considers both.
In my place most professional will follow the American approach to consider only skin friction in soft-medium soils and end bearing in stiff ones.
Many years ago I remember in one particular project flat jacks under the tip have been used to allow full mobilization of end bearing together with skin, in that case nobody would object maybe to use both in design.
My favourite quick reference in relation to drilled shaft is Kulhawy, 1991, in Fang's Foundation engineering Handbook.
Here undrained side resistance has the same equation as Reese 1976 and Kulhawy plots the adhesion factor versus Su (Cu) which turns out to be about 0.4 for Su=150 kPa, same as cveng used. I would not use end bearing in this case unless a conservative value of settlement equal to one inch is considered. The equations for undrained bearing capacity are more complex. I also used the FHWA design method and the LCPC method, choice is usually governed by experience and results of load tests.
cveng, I wonder why you used a Fs of 2, 2.5 would perhaps a more common choice.
The NCHRP report 507, LRFD for deep foundations, for skin friction in drilled piles with an FHWA design method, suggests a resistance factor ranging from 0.52 (minimum reliability index) to 0.36 (maximum reliability) so an FS=2 would mean lower reliability hence higher probability of failure.

 

[civeng80]

Thanks Mccoy.
Very useful comments.
Ive been reading Terzaghi and Peck and it says that friction is fully mobilized with settlement at about 10mm . In my case ultimate skin friction capacity is 254 kN (see computation). Bearing capacity needs about 5% of diameter of pile settlememt for it to start mobilizing, this is about 23mm in my case (as you said about 1 inch). The capacity of the pile from bearing is 214kN.

My working load is about 120kN.

So factor of safety = 254/120 = 2.12 If settlement of no more than 10 to 20mm is required.

or

Factor of safety = 469/120 = 3.9 If settlement of 25mm is tolerated.

So if I want a safety factor of 2.5 is the settlement obtained by interpolation ?

 

[AndreBC]

civeng80 :
I think factor of safety has nothing to do with the settlement.
Let say that because of 120 kN load, your foundation settled for 20 mm.
For 120 kN load action, the foundation with 469 kN capacity will give a reaction force of 120 kN, not 469 kN.
So whether you divide the capacity of 469 kN with safety factor of 2, 2.5, or 3, the settlement that occur will not exceed 20 mm, since it's a reaction to load, not to capacity.

 

[Mccoy]

AndreBC, I thing civeng's meaning was another, based on the treshold settlement which mobilizes shear strenght. Before the treshold, end bearing is not active, after the treshold end Bering is mobilized, it is summed to skin bearing so FS doubles.

So if I want a safety factor of 2.5 is the settlement obtained by interpolation ?

I wouldn't say so since we don't know exactly where is triggered the treshold settlement which mobilizes end bearing in the adopted hypothesis and when end bearing is fully mobilized and which is exactly the shape of the mobilization function (unless the pile is instrumented).

I'd rather adopt a reasonable treshold for full tip mobilization, Always making sure that the shaft bottom has been cleaned, and consider two distinct scenarios:
1)Settlement below treshold, FS=2
2)settlement above treshold, FS=4
Of course you should ascertain that in the second scenario you don't surpass serviceability.

Since the above reasonings maybe tricky, simplest of all, in lieu of load tests, would be to increase skin friction to reach a 2.5 FS, by increasing pile diameter, I wonder if there is a minimum number of load tests for project by regulations of contract in your area.

 

[civeng80]

Thanks Mccoy.
I think I will just design for friction and use a Factor of safety of 2.5 and ignore bearing pressure, mainly because I think the base will always have loose soil and maybe more which would be very difficult to remove from a practical point. Also limit settlement to about 10mm.

I do however think its tricky in evaluating safety factors if bearing is taken in account, because full friction must be mobilized before bearing comes into play. Putting a FS on the Qs + Qp is a bit simplistic.

Thanks again.

 

[AndreBC]

civeng80 and Mccoy :
Oh I see. Sorry, my fault. I agree if it is about threshold.

 

[Mccoy]

I do however think its tricky in evaluating safety factors if bearing is taken in account, because full friction must be mobilized before bearing comes into play. Putting a FS on the Qs + Qp is a bit simplistic.

Actually a procedure which was used years ago was to assign different values of safety factors to end bearing and skin friction, so we would have a Qs/FS[sub]1[/sub] + Qp/FS[sub]2[/sub] where FS[sub]2[/sub]>FS[sub]1[/sub]
This was a well accepted procedure 20-30 years ago, today it appears that things are changing in practice after the studies with instrumented piles.
Anyway going back to the conditions of the bottomhole, if you think that removing debris is going to be hard and not a reliable issue (especially if below water table) it's definitely wiser to adopt skin friction only.
Good luck!

 

[civeng80]

Again Thanks Mccoy.

Regards

 

[civeng80]

Does anyone know if the Spreadsheet (INFLUENCE) in the above paper actually exists, or do you write your own ?

 

[civeng80]

Sorry wrong thread. Look in foundations engineering section