Correction of cone resistance for pore pressure at the cone shoulder 1

[ChrisPriess]

Hello to everybody!
I´m actually performing a comparative study about the applicability of those pile capacity prediction methods based on CPT-Data. Some methods do use the measured cone resistance qc instead of the corrected one qt. Since I just have CPT-Data-Plots where just qt is put out, I need to calculate qc. There is a relation given by Robertson et al.(1986): qt=qc+u2(1-a). Unfortunately I can´t figure out how to calculate the factor "a". It is defined as the "ratio between shoulder area (cone base) unaffected by the pore water pressure to total shoulder area". Does someone know how to compute this ratio for an 15cm²-Type 2 piezocone penetrometer?

Thanks a lot
Chris

 

[BigH]

Might I suggest that you google Paul Mayne at Georgia Tech. He has an extensive list of references on his web site and most are downloadable. This is a good place to start. Of course, I am assuming that you are a practicing engineer and not a student. Cheers

 

[ChrisPriess]

Hello BigH,
thanks for your reply, but I already browsed nearly every web site and there is NO Information about my mentioned "a"-ratio.
The German standard 4094-1 from June 2002 calls it "a quotient from the net area of the cone to the area of the cone base An/Ac=a. Ac in my case is 15cm², but what is the net area? I figured out that is must be the cross section area of the rod, but there is no info about the diameter of this rod.
Somewhere else I found a diameter of 36mm for a rod of a CPT-cone. But is it the same for a cone-type like mine?
Chris

 

[moe333]

"a" is the ratio of the cross-sectional area of the load cell or shaft (An), divided by the projected area of the cone (Ac). You have the cone area, you need the cross-sectional area of the load cell or shaft (An) in order to calculate "a".

This form of the correction is only for pore pressure measurements taken at the u2 location, directly above the cone tip.

 

[dgillette]

This refers to the difference between the gross area of the base of the cone (10 or 15 cm^2) and the area of the cone base that is unaffected by the pore pressure. The pore pressure acts upward on the face of the cone, and downward on the small area (a few mm wide) around the perimeter that is outside of the O-ring seal. The net area is the total minus the part outside of the seal. On most modern cones, the net area is pretty close to the gross area because the O-ring seals are very near the perimeter of the base of the cone, which makes the exposed area on the base of the cone small. The rod area is irrelevant.

The value of a should be on the order of 13/15 or 14/15. The mfr. should be able to provide that info.

Clear as mud?

 

[coneboy]

The "a" area ratio you speak of is totally dependent on the electronic cone manufacturer. Different cones have slightly differnt internal dimensions. Just ask the CPT operating Engineer who performed the tests for you. They should be able to tell you in an instant.

For example:
Most (not all?) Hogentogler cones have an a = 0.8
ARA/Vertek = ?
Conetec = 0.85 (I think?)
Fugro = ?

Who did you use for your test data?

Hope this helps

Coneboy

 

[coneboy]

Hi Chris,

Just wondering......what procedure did you find works best for predicting pile capacity using the CPT data?
I have found the LCPC method to be quite accurate after a little tweaking of the correction factors.
I have some friends that like to rely on the API method.

Just curious

Coneboy

 

[Mccoy]

Just wondering......what procedure did you find works best for predicting pile capacity using the CPT data?
I have found the LCPC method to be quite accurate after a little tweaking of the correction factors.

As a matter of fact, the following thorough study:

Titi Hani H., Abu-Farsakh Murad Y, Evaluation of bearing capacity of piles from cone
penetration test data, Louisiana Transportation Research Center, LTRC project No. 98-3GT,
1999

has compared some methods and found the best ones are the LCPC and the European method.

The publication is available online, don't have the link right now but it's enough to google it.