End Bearing or Skin Friction / Rock

[nbr1]

Would it be appropriate to reference a skin friction and end bearing value in rock (compentent shale; RQD 50%+).

My understanding would be to only use one or the other?

 

[tadeng]

This has been a topic of debate in our office. Technically skin friction cannot begin working until the end bearing is overcome. Some structural engineers use both skin friction in combination with end bearing when designing drilled shafts. In my opinion if the concrete makes good bond with the rock, then you can add skin friction (at least some) in with end bearing.

 

[nbr1]

I have always thought the reverse true. End bearing after significant mobilization of skin friction in rock??

 

[ishvaaag]

I think the answer is that they can be added. As nbr1 suggest I think that in rock vertical loads are inmediately diverted through interlock (shear at the interface, more than friction, really); i.e., the axial stiffness path is shorter with a number of inclined struts to the sides of the pile that to the bottom of the same. Then if the interlock fails we have some kind of friction and I see then secondarily axial stiffness to the bottom developing. Nut I think can be addes at least for a final strength if what you are considering is really skin friction and not shearing the rock (or concrete) at the interface, for some level of friction will be developing as soon as the interlock breaks and load is starting to be transferred to the bottom, and to make fail the pile you will have in the end both tip resistance and side friction.

For service level loads, the ideal situation would be values that do not break the interlock a situation where you (for homogeneous rock) would be more or less compressing an elastic rock. To make use of the full process, some characterization by test would be very convenient, you will have somewhere to read appled loads and settlements atop the tip till considered failed.

 

[ishvaaag]

errata post above

line 2 ... THAN to the bottome o the same.

line 3 ... BUT I think it can be ADDED at least ...

line 7 ... interlock, a situation ...

line 8 ... , you would have somewhere to read APPLIED

 

[nbr1]

The reason I bring this up is a text reference by Das.

"In the design process to be recommended (Reese and O'Neil, 1988, 1989, it is assumed that there is either side resistance between the shaft and rock or point resistance at the bottom, but not both."

If settlement is below 0.4 inches, uses skin friction.
If settlem over 0.4 inches, uses end bearing value.

 

[InDepth]

This sounds like a displacement and stiffness compatibility question. I believe if you plotted out a curve for soil it would be different that that for rock. I believe it also depends on the technique used to drill the hole (or drive the pile) that will affect the amount of side friction stiffness/strength and end bearing stiffness/strength.

For a long pile in rock, in my mind, as you load the pile this is what the load displacement curve may look like

1. The load will start being resisted by skin friction at the top of the rock. (elastic linear with pile compression in the load shedding zone)

2. As load is increased additional length of skin friction resists the load (elastic linear with pile compression in the load shedding zone)

3. As the load increases again the skin friction starts to yield near the top of rock and then transition back to elastic as the load is transferred to the rock. The yield zone may find a limit value (plastic) i.e. maximum force that can be developed in skin friction.

4. Once the skin friction zone intersects the bottom of pile, compression of the pile and movement in the skin friction zone starts to activate the end bearing (first elastic then yielding). During this time there probably is some redistribution of force along the pile length based on the relative stiffness of end bearing versus skin friction. End bearing essentially provides "strain hardening".

5. If you continue loading you either find a maximum limit of force which then transitions to a drop in load and or "plowing."

My point is, the question of wether skin friction and end bearing is applicable is based on relative stiffness and ultimate values as well as the relativity of the magnitude of force to the length of pile. ex. Imagine a 100 ft pile with 1 kip of force....will it ever develop end bearing? I doubt it. Everything relative.

 

[nbr1]

It sounds like the Reese and O'Neil approach is valid. Is anyone in concurrence?

I have seen reports reference skin friction and end bearing values for drilled shafts in rock.

Based on the above approach, it would seem to me that you would reference end bearing or skin friction based on estimated settlement, but not both. In other words, it does not seem appropriate to give the structural both skin and end bearing in rock w/o some discussion of settlement.

 

[paddingtongreen]

My take; generally, a report should give both friction and bearing values provided they are meaningful. The design should only use one unless the other is a fall back for an extreme load case and it can be shown that one does not have to fail before the other kicks in.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[InDepth]

nbr1, I absolutely agree. A geotech should provide a plot of settlement (pile top movement) versus available force resistance. i.e. don't give the structural the skin & end bearing, just provide total force resistance. You could create a series of plots for different length piles and different diameters....a suite of options that the engineer can then select from.

....now if the owner only wants a certain amount of settlement, it locks in 1 of your parameters

 

[blgeo]

There is an excerpt in Reese's 'Analysis and Design of Shallow and Deep Foundations' which directly addresses your concerns. I will summarize the points below for everyone's reference:

1.) The decision to add skin and end bearing resistance for drilled shafts in rock needs to be made on a case by case basis using engineering judgment.

2.) If there are no uplift loads, and the rock is massive. The shafts shouldn't be socketed (or only partially socketed). Therefore capacity should be derived entirely in endbearing.

3.) When the shaft is rock socketed (to resist uplift usually) 1) If the rock is brittle, most of the skin resistance may be lost as settlement increases. Therefore only consider endbearing resistance. 2.) If the rock is ductile in shear (and strain softening does not occur) then both may skin and end bearing resistance may be considered.

4.) When end bearing resistance of the rock is quesitonable (e.g. poor quality rock) the rock socket will usually extend long enough into the rock that a majority of the resistance is developed in skin resistance. Both end bearing and skin resistances may be used.

5.) Run a load test with Osterberg Cell to verify.

 

[csd72]

skin friction in soil definately not but skin friction in rock would require only a very small movement to mobilise so...maybe...

 

[nbr1]

blgeo,

Thanks for your response. This clarifies a lot of my questions. Now I just need to find out what is brittle rock and what is ductile rock??

Thanks Again.