Negative Skin Friction

[JohnnnyBoy]

Were in the middle of designing a sanitary lift station. the lift station is built down approximately 38 ft. For the sump pit they excavated down and sloped down at a normal 1/1 ratio. Originally we were going to use concrete piles although because they are back filing the area that we are piling it introduces a negative skin friction force. Our piles are required to resist 65 kips which is regularly not a large load.

Now from my research/understanding, negative skin friction occurs when the settlement of the soil is greater that the settlement of the pile inducing a down-drag force on the pile. Me and another engineer at my firm determined that if the soil was properly compacted to 98% SPT then there should be no negative skin friction on the pile. We further talked about this with the geotechnical engineer that has been dealing with the soils and were told that absolutely negative skin friction cannot be ignored whether compacted or not. We have now changed the design to screw piles to reduce the diameter of the pile therefore reducing the skin friction.

Soil testing was done of the soil (not compacted) and we were given -50psf for the negative skin friction. For the force required we ended up designing a pile that could withstand 242kips (total) while most of its capacity was taken from negative skin friction.

Is there something more efficient than this that can be done/ could anyone comment on the geotechnicals stance on the matter.

 

[SlideRuleEra]

I don't want this to sound silly, but could you increase the number of piles? Once a Contractor is mobilized for pile driving, the incremental cost for additional piling is relatively low.

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[oldestguy]

Where is your water table? Remember the effect on a buried box is up.

Your geotech may be right, PROVIDED the soil under that compacted fill can carry the load (presumably that lower soil had been under that load before).

I can't remember any lift station that I had any thing to do with had any concern about negative friction due to the backfill. Most worried about floating and needed that for the uplift resistance.

 

[Panars]

Are you saying that the backfill which will create the negative skin friction is just replacing excavated material? Kind of like shown below?
If so, then there should not be any settlement of the soil to cause negative skin friction. The backfill is only replacing soil that was excavated so there is no net increase in the overburden pressure.

Or are you increasing the grade around the sump? If so, then yes you have to consider negative skin friction. It would help if you could provide a diagram. I agree with the other responders that I don't usually encounter negative skin friction for this type of structure.

 

[JohnnnyBoy]

I've attached a quick sketch of the lift station. Thanks for all the responses.

As for adding more piles the capacity of the piles are only 65 kips, while the negative skin friction im calculating will give a pile an extra force of -230kips to reach down to the 38ft below grade. Now I also know that not every pile needs to go down 38ft to reach native soil but because the geotechs on site did not properly document where the grade started/ended or if the slope was 1/1 they will not recommend us assuming anything less than 38ft. I'm not willing to risk my reputation on something that the people on site won't comment on either.

I've never really looked into floating of the piles, but would this still come into effect with the sketch I've shown.

Thanks

 

[oldestguy]

Please clarify. Isn't this lift station on an area that was native soil? I can't imagine that this is fill placed into a lake and the area is deep esxsting fill to start with.

Also, I never intended to say piles will float. It is your big box of a lift station that will float. That's a major factor to consider.

Anyhow your situation is a service building resting partly on a lift station (maybe not if not actually connected) and also on piles where differential support is likely. Also that lift station is a big boat that will receive significant uplift due to the high water table. So you have a building needing support from below somehow, probably p8les. Then you need some form of anchorage to keep the lift station from floating. That resistance to floating may be partly from the backfill and negative friction against the lift station walls and probably also from some added feature, such as an extension of the left station floor out into the adjacent earth.

In summary, it would appear to me that you should not look to the folks here on the WEB site for final design help, but should pose this situation to more experienced persons in your office or an outside consultant. Your comments appear to be those of a very inexperienced person, sorry.

 

[Panars]

So you are not changing grade much, so only the consolidation of the backfill under its own weight is the concern. You are not changing the overburden pressure on the natural soils, so they should not settle. Is the backfill material granular or fine grained (clay)? If it is granular, then the consolidation happens relatively quickly and after a week or two it has settled enough that negative skin friction won't be a problem. If it is a clay, then consolidation will take longer. But you may still be able to place a settlement platform in it and measure it to see if it is settling.

On another note, how are you getting 230 kips of negative skin friction when your unit skin resistance is only 50 psf? You didn't say what size piles, but if I assume 16 inch square concrete piles and 38 feet long, I get 38*4*(16/12)*0.05 = 10.13 kips. And this is worse case because you won't have negative skin friction along the entire 38 feet (only above the neutral plane where the pile settlement and soil settlement are equal).

 

[ATSE]

1. Conventional compaction of the soil will have little or no effect on your pile capacity.
2. 230 kips of negative skin friction? Something is very unusual here. Not sure, but it seems like what the geotechnical engineer is saying is that a short pile is worse than no pile at all - this makes no sense.
3. Think skin resistance, which can work in directions (be positive or negative). When you say negative skin friction, are you referring the pile capacity to resist buoyancy ("floating" forces)?
4. You need to get a load capacity vs vertical deflection curve for the piles. You need to be talking about displacements, not just forces in the absence of displacements. If you can tolerate "reasonable" displacements - say, about 1" - then this discussion might go away. Once you have a full picture of total load vs displacement and total system settlement, then drag force penalties can be disregarded.
5. oldestguy last comment is a little harsh, but I concur. You need to get a very senior engineer involved. Buoyancy is an uncommon but very damaging condition.