Friction angle estimation for WOH/24" sand below 30' of ground level 1

[Shakta]

I have got a soil profile that has WOH/24" blow sand from 30' to 60' below the ground level. Usually min of 28 degree friction angle is being used for very loose sand in the area where I practice.
Would like to hear what friction angle can be used for this layer? Should we consider zero skin friction for this layer during pile tip determination? The lab test is saying moisture content is around 40% to 50%. Thanks!

 

[EireChch]

I presume WOH is mean weight of hammer? and 24" means one blow went 24 inches??

It would be very conservative to assume zero skin friction. I think even with SPT of 0 would provide a phi of 28 deg, and prob 30.

Have you done PSD tests, what percentage fines are there?

Do you have any disturbed samples that you can run a shear box test on ?

40-50% water content does however sound unusual. I would think it would be around the 20% mark or less.

 

[Shakta]

EireChch-Your assumption is correct. Fine content (passing #200) is 18%. No sample for shear test. Thanks!

 

[PEinc]

Be careful that vibration from installation of the piles does not cause consolidation of the loose sand resulting in settlement of any adjacent structures or utilities. I was involved in a project and lawsuit recently where pile driving vibrations caused a building to differentially settle 1.5 inches, one column at a time, as the driving moved along the building. The claim was for $3.5 million.

The building was being monitored for typical vibrations that might crack the building rather than considering what soil consolidation would be caused by the pile vibration. The building did not crack but it went along for the downward ride, one column at a time!

http://www.PeirceEngineering.com

 

[Shakta]

PEinc, Very good point. Thanks for sharing.

 

[GeoEnvGuy]

N value of zero for sand, you should assume the material has a high liquefaction potential. If you look at the chart from Peck which correlates N value to friction angle there is no correlation below an n value of 4.

 

[EireChch]

I think SPTs in very loose soil are useless. The SPT is too crude and it is essentially overpowering the soil, its not sensitive enough to capture to looseness of the material.

A CPT would is the best too for the job in this regard, and for liquefaction assessment too.

 

[MTNClimber]

My gut feel is that a seismic event may be your limiting design case. The negative skin friction from settlement of 30' of sand plus whatever is on top of that will be substantial. If that's the case, 28 degrees would be acceptable in my book for calculating your negative skin friction.

If somehow the seismic case is not the limiting factor, zero skin friction would be my first approach. Then compare that to a small amount of skin friction and see if that really effects your pile lengths. If it does, it could be worth sending out a CPT rig to sharpen your pencil for the pile design.

Also, beware of battered piles in this material. If you really need them, run a Plaxis model to check for overstressing the battered piles in bending.