## AASHTO 2012 Side resistance for drilled shafts into rock

## AASHTO 2012 Side resistance for drilled shafts into rock

(OP)

To resist uplift to a footing that sits directly on solid granite my customer plans on drilling a 5.5" diameter hole, 10 feet deep with 2 #8 rebars embedded into the hole with 4000 psi concrete.I came accross equation 10.8.3.5.4b-1 on page 10-137 of 2012 AASHTO book to calculate to calculate the side friction. I found an examples within a presentation that raised a few questions. I would appreciate any comments or feedback on the 3 items listed below.

qs=0.65*∝

I am using ∝

For compressive strength of granite I am using q

Atmospheric pressure p

1. Should I use the lower value of f

2. The formula doesnt mention it but do I also need to multiple by the resistance factor of 0.4 from table 10.5.5.2.4-1?

3. is the 7.8 in the 7.8*p

qs=0.65*∝

_{E}*p_{a}*(q_{u}/p_{a})^{1/2}< 7.8*p_{a}(f^{'}_{c}/p_{a})^{1/2}______Formula 10.8.3.5.4b-1I am using ∝

_{E}=0.45 from table 10.8.3.5.4b-1 to be conservativeFor compressive strength of granite I am using q

_{u}=2736ksfAtmospheric pressure p

_{a}=2.12ksf1. Should I use the lower value of f

^{'}_{c}and compressive strength of the rock for q_{u}within the formula?2. The formula doesnt mention it but do I also need to multiple by the resistance factor of 0.4 from table 10.5.5.2.4-1?

3. is the 7.8 in the 7.8*p

_{a}(f^{'}_{c}/p_{a})^{1/2}formula a conversion and truly need to use ksi units for f^{'}_{c}& ksf units for p_{a}?
## RE: AASHTO 2012 Side resistance for drilled shafts into rock

As a matter of curiosity, how did you ever come up with qu = 2736 ksf? That seems unduly accurate.

## RE: AASHTO 2012 Side resistance for drilled shafts into rock

The q

_{u}is actually just = 19000 psi. Thanks for the attachment! What reference does the attachment come from?## RE: AASHTO 2012 Side resistance for drilled shafts into rock

I was making a jab that your "exact" approach to the qu value needs to be tempered. 19,000 is to 2 significant figures meaning that, if there is no "fuzziness" in the value - it could range from 18,500 to 19,500. You have, in ksf, given the accuracy to 4 significant figures which, in my opinion, is a bit unreasonable - almost bizarre! The accuracy of these values are no where close to that precise.

## RE: AASHTO 2012 Side resistance for drilled shafts into rock

Thanks for the references! There was no relevance for me to post the q

_{u}or p_{a}values not sure what I was thinking!