Skin Friction value for crushed stone

[Polecat]

Problem: A smooth round concrete pole embedded into an augered hole and backfilled with a compacted stone (using long-stemmed vibrators); more specifically, a #57 Florida DOT crushed limerock (essentially, the coarse aggregrate for a concrete mix).

Question: what is a conservative value to use for skin friction (psf) of the pole against the compacted rock to resist the vertical load?

http://www.spiraleng.com

 

[oldestguy]

Some here might allow as high as 1,000 psf. To be conservative maybe 1/4th of that.

However, that compacted filled hole then is against native earth which may have a much lower shear value value (but larger perimeter).

Also you need to think about the characteristics of the stuff you are pulling against, which is the nearby earth. In that case the chunk of anchoring land is an inverted cone of some unit density. The angle of the cone sides respect to that pole depends on the shear strength of that earth. If it is under water that makes a difference also.

I assume you knew that, sorry. More things to look at for these cases than what you asked for.

 

[Polecat]

Oldestguy,
Thanks for the info. I'm amazed that it could be so high. So, when it comes to crushed rock, I guess there isn't much of a relationship that you could draw between the skin friction and its friction angle as there would be with sand?

BTW, having been around since 1933, I might be able to challenge you for your title, but I'd still be willing to let you keep your adopted name.

http://www.spiraleng.com

 

[msucog]

may be off-topic a bit, but why not just use a lean concrete? that may help facilitate the construction process faster (and maybe better) while possibly improving the frictional capacity to some degree.

 

[Polecat]

msucog

Thanks for the response.

A concrete backfill is always better as far as performance and interaction with the soil is concerned, but when dozens of poles are to be placed in a line, the contractors have an aversion to holding each one with a crane long enough for the concrete to set.

http://www.spiraleng.com

 

[oldestguy]

Polecat:

How about 1928?

 

[Polecat]

Mr. Oldestguy,

I graciously concede. I've always have respect for my elders.

http://www.spiraleng.com

 

[BigH]

polecat - not sure how deep the holes are - but could you not use shotcrete with accelerator? I understand that there might have to be some modifications to shooting, etc. But it should set up relatively quickly - or if you are using clear stone for backfill, then grout it up afterwards. Just a few thoughts - damn, I am a youngster!

 

[Polecat]

Thanks to all for your helpful comments, but there is no debate about whether concrete or rock will be used. The client wants rock, period.

My mission is to come up with a reasonable value to use for skin friction of the compacted rock against a smooth concrete pole. The vertical resistance against the axial load is our concern here.

http://www.spiraleng.com

 

[FixedEarth]

Let us say your SPT value is 10. The allowable bearing is 250 psf x N or 2,500 psf. Friction is 50% of that, so 1250 psf. Realizing that there is no cohesion and all the aggregates are poorly graded use FS of 2.0 So 600 psf would be a reasonable value to use for friction.