Effect of pre-drilling on the capacity of skin friction piles? 1

[abusementpark]

I'm looking to find any information available on the effect of pre-drilling on the capacity of skin friction piles.

How do you quantify how much the pre-drill will affect the capacity? Are there any rules of thumb of regarding how small a pre-drill needs to be for the effects to be negligible? Are there any good papers discussing this topic?

Any information is greatly appreciated!

 

[a7x1984]

If it is for non-transportation structures, check the IBC 2012 18.10.4.4 to see if any of the pre-excavation requirements meets your type of "pile". It will depend on the type of soil (the most obviously generic answer for all geotech problems). A soil strata with a lot of positive pressure in the region of the pile can cause the drilled shaft to contract and lose dimension, and thus create the potential for additional normal force on the pile. (good effect for driven friction piles)

There could also be adverse affects of soil disturbance that reduces the positive pressure.

I believe the IBC requires load tests to be performed on driven friction piles, specifically due to pre-excavation; I always specify it. For design, I assume that there are adverse effects, and I do the following "ad-libbing": I multiply the friction coefficient between the soil and the pile by a ratio of the at-rest pressure coefficient divided by the passive coefficient. For example, if the at-rest coefficient is 0.5 and the passive is 2.0, you have 0.25, so I assume that the friction pile coefficient is 25% of the value used if there were no pre-excavation.

This is what happens when we were not given a budget for geotech analysis. Again, the important part is to have the piles load tested per IBC. Hope that helps a bit.



If I got paid for every hour I worked, I'd be a wealthy man.

 

[oldestguy]

Question: Why pre-drill? Perhaps to get past obstructions? To be safe, in estimating the probable pile length, I'd neglect the support that may come from the pre-drill length. I also suspect some form of verification of the pile capacity will be used. If that shows a noticeable affect, certainly the experience with the first few piles will then govern whether of not to continue the procedure or equipment.

My experience is mainly for by passing obstructions and also to get a minimum penetration into very hard material such as rock. In neither case is side friction an issue.

 

[grabens]

I agree with oldest guy. I would neglect the skin friction in the zone that was pre-drilled. There will obviously be friction within this zone, but it will be reduced when compared to that of a non pre-drilled layer. I would install a test pile with pre-drilling and load it to see if there will be any significant reduction in the anticipated pile capacity. I wouild not recommend going cheap on the geotechnical scope of work, especially since it is the portion of the project with the most risk and unknown variables.

 

[Ron]

Agree with OG and grabens...neglect the skin friction in the pre-drill area. Unless you have a thixotropic material that will significantly remold around the pile, skin friction will continue to be negligible in the pre-drill area.

 

[abusementpark]

Agree with OG and grabens...neglect the skin friction in the pre-drill area. Unless you have a thixotropic material that will significantly remold around the pile, skin friction will continue to be negligible in the pre-drill area.

Seems like at some point, if the pre-drill is small enough, it's effect on capacity would be negligible. No?

 

[Ron]

abp...that's true. Keeping in mind that skin friction is a function of lateral soil stress, the skin friction in the upper portion of the pile is less critical than the lower portions anyway.