Skin Resistance for a soil profile composed of silty sand, sandy lean clay, clayey sand, & sandy

[oengineer]

I am designing drilled pier foundations and the first five layers of the soil profile are silty sand, sandy lean clay, clayey sand, & sandy silty clay? The bottom of the pier rest in the silty sand layer. My question is for such a soil profile would it be accurate to consider at least 60% of skin resistance to be considered. I am designing these foundation mainly for bid purposes. I have 2 soil reports. One just gives the boring log with the soil profiles shown. The other mentions the type of soil for each layer and provides compressive strength, shear strengths, and passive resistance for different depths(all in psf). Both soil report are in the location of the project site.

 

[GeoPaveTraffic]

What you are proposing might work just fine and it might be very wrong. Depends on the specifics of the soils and the proposed loads.

I take it that you do not have a geotechnical engineer for this specific project. I suggest that you get one on board.

Mike Lambert

 

[oengineer]

The thing is that this design is for bidding purposes. The client has provided geotech reports in the bid specifications that go back several years. I am trying to design a foundation for estimating. If we are awarded the project then an actual geotech report will be required. Would you recommend having skin resistance applied for sandy soil? I am considering just using end bearing capacity.

 

[oldestguy]

My practice in the past was to use a SF of 3. That is working shear strength was 1/3 of ultimate. Never had a complaint.

 

[GeoPaveTraffic]

To answer you question, yes I use skin friction in sandy soil. Your original plan with an appropriate FOS will likely work. But, as has been pointed out in the structural form, there are many ways to get in trouble with what you are doing.

Mike Lambert

 

[BigH]

It might be interesting to take a look at the value of the skin friction and end bearing and the strains at which they apply - end bearing needs greater settlement to start to "bear". In some calculations I've done in the past, usually using a SF of 2.5 (sometimes 3) it seems that the allowable pile capacity applying this factor to both the skin friction/adhesion and the end bearing ends up with the value being pretty much the skin friction (fully developed without SF) and the end bearing is the safety factor . . .