Negative Skin Friction
Negative Skin Friction
(OP)
Hello All,
I've got a project that has about 45 feet of granular soil underlain by overconsolidated clay from 45 to 80, and then normally consolidated clay from 80 to 165 feet (rock at 165 feet). The project will include placement of about 3 feet of fill over the site, and then a new building.
Given the 3 feet of new fill, I'm estimating the negative skin friction on two pile types (steel H piles and precast concrete piles). I have load test data from an H pile project directly across the street.
My question is: is there any technical reason not to use the available load test data to determine "site specific" skin friction, and then apply that total skin friction load to my project as negative skin friction? In other words, use the static pile load test results that will provide me with positive skin friction and apply it as a negative to my site? Given that they would be piles end bearing on rock, the negative skin friction would be applied to the full length of the pile.
Additionally, intuitively it would seem excessive to apply the NSF to the full length of the pile (165 feet) given only 3 feet of fill (if using the nearby load test data, we would be talking about approximately 125 tons of negative skin friction), but given the overall area of the new fill and looking at stress distribution with depth, I expect there to be some amount of consolidation the full depth of the clay layer.
Any insight would be much appreciated.
I've got a project that has about 45 feet of granular soil underlain by overconsolidated clay from 45 to 80, and then normally consolidated clay from 80 to 165 feet (rock at 165 feet). The project will include placement of about 3 feet of fill over the site, and then a new building.
Given the 3 feet of new fill, I'm estimating the negative skin friction on two pile types (steel H piles and precast concrete piles). I have load test data from an H pile project directly across the street.
My question is: is there any technical reason not to use the available load test data to determine "site specific" skin friction, and then apply that total skin friction load to my project as negative skin friction? In other words, use the static pile load test results that will provide me with positive skin friction and apply it as a negative to my site? Given that they would be piles end bearing on rock, the negative skin friction would be applied to the full length of the pile.
Additionally, intuitively it would seem excessive to apply the NSF to the full length of the pile (165 feet) given only 3 feet of fill (if using the nearby load test data, we would be talking about approximately 125 tons of negative skin friction), but given the overall area of the new fill and looking at stress distribution with depth, I expect there to be some amount of consolidation the full depth of the clay layer.
Any insight would be much appreciated.





RE: Negative Skin Friction
If you place the 3ft of fill, the granular soils would settle elastically during the placement of fill. And by the time the stress distribution reaches the OC clay the net increase in pressure would be minimal which would result in minimal consolidation plus as its already OC it i would expect it to be minimal.
I dont think you need to consider NSF as the 'downward' forces have occurred prior to you piling.
RE: Negative Skin Friction
Given the extents of the fill (for reference, let's say it's a 350' x 350' area with a building of about 150' x 150' near the center), I was considering the fill as a flexible load at ground surface. For that extent of fill, there is still some applied load down at the deeper depths (say, 0.4 x the fill load ballpark).
Regardless of that, do you have an opinion as to the use of a pile load test result for application to NSF in general?
RE: Negative Skin Friction
The results of a static load test is more or less a graph of load vs settlement ? how to you propose to split the capacity up into end bearing and shaft capacity?
RE: Negative Skin Friction
The load test results are a graph of load v. strain. If you plot the load v. strain of the steel on the same plot, you can determine at what point all the load is transferred to the tip of the pile (i.e., when the deflection measured at the pile top is equal to the theoretical steel strain over the length of pile, then all the load above that point has been carried by the soil friction and any load after that point is carried by the pile tip).
I also have dynamic pile test results for the same pile that separates the side friction and tip bearing capacities.
I guess my simplified question is "is it technically correct to apply the positive skin friction value to negative skin friction." Per studies, for an end bearing pile the negative skin friction would be applied to the full length of the pile within or above the consolidating soil layer.
RE: Negative Skin Friction
RE: Negative Skin Friction
165 feet long H-piles... maybe with welded slices as driving progresses. What is the proposed size of these piles, and what hammer specs?
165 feet long precast concrete piles... I don't think so.
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RE: Negative Skin Friction
We haven't determined the pile sizes yet - that will depend on whether we need to account for high negative skin friction load. But a nearby building is supported on HP14x117s. That size may be a bit larger than we need for this building.
RE: Negative Skin Friction
I think if you did that you would be grossly overestimating the amount of negative skin friction.
You need to get some geotechnical advice. Given your soil profile I would not be surprised if the level of NSF would be small enough to be ignored. I agree with EireChch's initial reply.
RE: Negative Skin Friction
RE: Negative Skin Friction
How did they arrive at this value of 20 tons?
RE: Negative Skin Friction
RE: Negative Skin Friction
As far as the clay, the upper OC layer has N values around 5 bpf. The lower NC clay is WOH or N=1 bpf. Triax testing was performed.
RE: Negative Skin Friction
Dik
RE: Negative Skin Friction
RE: Negative Skin Friction
Not being a geotechnical engineer I have always wondered how you could get a normally-consolidated clay layer under an over-consolidated clay???
RE: Negative Skin Friction
RE: Negative Skin Friction
If the client doesn't want it, then go all out... it's his money. But, we don't want another San Francisco tower that is slowly sinking into the sunset...
Dik
RE: Negative Skin Friction
http://www.fellenius.net/papers/095%20Singapore_19...
RE: Negative Skin Friction
"For reasons of simplicity, the shear stress along the pile is assumed to be independent of the direction of the displacement, i.e., the negative skin friction, qn, is equal to the unit positive shaft resistance, rs."
RE: Negative Skin Friction
Yes but you do not apply the NSF along the entire length of the pile. You need to calculate the location of the neutral plane.
Elsewhere in the report it is stated that the magnitude of NSF is normally around 20-30% of the vertical effective stress. I imagine that there is an upper limit to this in that the NFS cannot exceed the shaft resistance.
Also, remember that NSF does not affect the geotechnical capacity of a pile. It will increase the structural load in the pile and will increase settlement.
RE: Negative Skin Friction
RE: Negative Skin Friction
Only if you assume the rock is infinitely stiff; however, depending on what value of base resistance you have chosen it can take 10-20% of the equivalent pile diameter worth of settlement to mobilise the full base resistance.
RE: Negative Skin Friction
RE: Negative Skin Friction
At the end of the day it's really only 3 ft of fill on a deep NC clay layer. Worst case either have them import/compact fill before they drive piles (surcharge) or give the piles a bitumen coating (whale grease) :)
RE: Negative Skin Friction