Allowable Point Bearing Capacity of Piles Resting on Rock

[Lake06]

I am involved in a design for an H pile which is to support a 90 ton vertical load. The pile will be driven down to rock. The skin friction of the pile has been calculated and has been determined to be relatively small compared to the point bearing capacity. Two methods were used to calculate the point bearing capacity of the pile resting on rock and were found to be extremely different. My question is has any had experience using the following equation for Point Bearing Capacity of Piles Resting on Rock Qp=(qsubu*Nsubphi + 1)*Ap from Goodman 1980. This referenced formula was taken from Principles of Foundation Engineering 5E by Braja M. Das. Is this the correct formula to be using when trying to calculate the capacity? We are trying to compare our results to the CivilTech Software program which uses the following equation Qp= Ap*(Nsubq*Ssubv + Nsubc*C) to calculate the capacity of piles on rock.

 

[csd72]

Personally, I would not count on the skin friction if the end is bearing on rock. Skin friction needs more movement to mobilise than is needed for rock bearing.

 

[Lake06]

Thanks for the recommendation. I agree that most of the capacity will come from the pile resting on the rock. Based on the two equations listed above have you had any experience with the Goodman 1980 equation or have a recommendation of what equation to use for determining the capacity of the pile resting on rock?

 

[BigH]

Might I suggest you consider that, in a majority of the cases, it will be the structural capacity of the pile that might govern?

 

[Lake06]

BigH let me give you a little background on the project. The pile is a HP 13 x 73 with 50ksi steel and is going to be driven down to rock about 35 feet below the ground. The soils engineer has determined the phi of the rock is 31 degrees and has a compressive strength of 2,000 psi. The pile has been checked for axial stress for the 90 ton load in the pile and also checked for the driving stresses from a selected air hammer. Both of these stresses are within the allowable range. The 1980 goodman formula will yield an ultimate load of 89 tons. When utilizing the civiltech software program will yield an ultimate load of 204 tons. Assuming using a factor of safety of 2.0, this will produce 44.5 tons for the goodman equation and 102 tons when using the civiltech allpile program. My question is which formula should one be using when calculating allowable bearing capacity of piles resting on rock. It appears the one equation will fail and the civiltech software indicates its ok 102 > 90 tons. The two formulas have been referenced above. Looking for some advise, thanks.