Negative Skin Friction

[tq3610]

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.

 

[Retrograde]

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

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.

 

[tq3610]

Understood. For a pile bearing on rock as this site will be, the neutral plane is the toe of the pile. So, the full length of the pile would be included.

 

[Retrograde]

For a pile bearing on rock as this site will be, the neutral plane is the toe of the pile.

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.

 

[tq3610]

Yes. Potentially the neutral plane could be just above the pile toe, but likely just a matter of feet. The load test results indicated on the order of 90 percent end bearing.

 

[kmart30]

Your maximum NSF is going to be based on the soil adhesion to the pile. At 1/2" of settlement or more brings you over your max soil to pile adhesion and the soil will slip instead of grab. Did you do any CPT testing? I would def look at the pile data to see how it correlates to skin and NSF but I wouldn't base my design on it.

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)

 

[tq3610]

Thanks Kmart. Are you suggesting to use the depth of overburden for which there is 0.5" or greater settlement (soil with respect to pile) to calculate NSF? I know I've seen reference to using a 0.4" limit, but I've read more recent studies that indicate much less movement is needed to develop NSF.