End Bearing or Skin Friction / Rock
End Bearing or Skin Friction / Rock
(OP)
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?
My understanding would be to only use one or the other?





RE: End Bearing or Skin Friction / Rock
RE: End Bearing or Skin Friction / Rock
RE: End Bearing or Skin Friction / Rock
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.
RE: End Bearing or Skin Friction / Rock
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
RE: End Bearing or Skin Friction / Rock
"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.
RE: End Bearing or Skin Friction / Rock
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.
RE: End Bearing or Skin Friction / Rock
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.
RE: End Bearing or Skin Friction / Rock
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: End Bearing or Skin Friction / Rock
....now if the owner only wants a certain amount of settlement, it locks in 1 of your parameters
RE: End Bearing or Skin Friction / Rock
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.
RE: End Bearing or Skin Friction / Rock
RE: End Bearing or Skin Friction / Rock
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.