ension capacity of auger cast piles in limestone

huangyhg

tension capacity of auger cast piles in limestone
wright and reese (1978) ( mcvay et al., 1994) gives an equation for calculating average skin  friction on auger cast piles in sand. the equation reads:
fs = po'ks tan (phi) =< 1.6 tsf
fs= average skin frictin on pile
ks= 1.1
tan(phi)= tangent of the internal angle of friction of soil
po'= average effective stress along length of pile
since my case conforms to a case where a auger cast pile is to be embedded in limestone (non-weathered), whose angle of friction could fluctuate from 35 to 45 degrees, does the equation given by wright and reese could correctly approximate the tensional capacity of such a pile? or the fact that the augering process by itself would tend to smooth out the rock surface to such an extent, that the frictional angle of the rock does not represent the reality at the interface between concrete and rock?
thanks
francisco j. fuentes p.e.
miami-dade water and sewer department.   
have the driller put a bell at the bottom of the hole. this will give you something to pull against. what kind of tension are you putting on the piling?
richard a. cornelius, p.e.
you need a geotechnical engineer familiar with the area.  although the equation itself is generally correct, there are more limitations than <1.6 tsf.  ks varies for different n-values(or more accurate strength parameters), and po should be limited to 20*pile diameter.  the approach works well in sands and sandy clays, but limestone acts differently than its n-value or phi value would indicate.  the frictional angle of the rock, as well as the overburden stress, are not the same as for soil.  if you are doing preliminary calcs, use a lower bound.  will the gear box for the auger be strong enough to auger through non-weathered limestone?
i understand the "limestone" in the southern florida area is not what others consider to be true limestone.  it's best to contact a geotech from your area.
i understand that the "limestone" in your area is very porous and can be drilled into with auger-cast rigs.  in fact, the frictional capacity of this type of "limestone" is very high because the grout from auger-cast operations actually infiltrates the "limestone" and a very high bond strength is developed.
talk to a local, auger cast pile contractor.