SKIN Frinction Help needed

[aayjaber]

I have the following Data.

Borehole in rock with visible serpentinite presense, other kinds of rock exist as well drill refusal at 5 ft.

The sample depth is 2ft to 2.5 ft
(Blows per ft*0.63/Blows per ft) is given as >50 (blow counts obtained from Modified California Sampler were multiplied by 0.63 to correlate with Terzaghi values(Martin&Lew,1999)
Dry Density 75.5 pcf
Moisture Content 31.3%
Angle of Internal friction (phi) 35
Unit Cohesion 0.2 Kilo Newton psf)
No Plasticity.

The design of the pier is recommended with 750 pfs skin frinction.

Now is not this too low for such a smaple? I did not see the calculation and the reason behind this number but I think it is too low.

 

[BigH]

Not sure that good it is correcting for different spoons when they are being driven in bedrock! Where did you get the phi' and c' values? Check out the "skin friction" based on the phi' / c' and using normal geotechnical formulations. Geotech might also be considering the fact that you have serpentine - and has made allowance for it. You might wish to check out some of the work that has been published in Cdn Geo Journal over the last decade - some good papers - also Horvath has written on the subject. The Canadian Foundation Engineering Manual might be referenced. Normally socketing into rock you (1) rely totally on end bearing, (2) totally on side friction/adhesion (which can be problematic and which is why the geotechnical engineer gave a "low ball" in your opinion (or) (3) combination of both. Strength of concrete vs strength/modulus of rock important.

 

[hokie66]

With refusal at such a shallow depth, why would you bother with side wall friction? Sounds to me like an end bearing pier.