Pile Installation

[shanksnorth]

Hello,

We are in the process of using steel pipe piles for a project and I have few questions on pile installation. I really appreciate if you guys can help me out:

1) Is it necessary to have a geotechnical Engineer at site during the pile installation program?
2) How is the pile refusal determined at site?
3) Can the capacity of pile be determined during pile driving?
3) If the pile reaches the designed embedment depth but still the number of blows is too low for the established settlement (I know this criteria is established by the geotech engineer at the beginning of the pile driving program based on the hammer), does it mean that:
a) The pile has not reached its capacity or the soil condition is too bad at that location?
b) The hammer energy used is too high?
c) If any of these is true, is it the responsibility of the geotechnical consultant or the pile driving people to find out the real reason and notify the engineer on record?

I am having these questions as the client wants the home office engineer to solve all the problems related to piling without the help of geotechnical enginneer and I am not sure how can I (structural engineer) answer it.

Thanks!!!

 

[motorbiketocrank]

So if you were a heart surgeon, would you do brain surgery because the patient didn't want to pay another doctor?

Seriously you need to educate your client and run away fast if he/she won't listen.

Yes, you need a geotechnical engineer. But not just for pile installation. You need them to perform an investigation, then a design, then construction support. How do you even know piles are required if there's been no design? The issues are way more complex than driving a pile and expecting the blows to tell you everything about capacity.

 

[shanksnorth]

We have a full geotech investigation for the project. The question now is whether we need to have an "Geotechnical Engineer" during pile installation. National building code says that the program has to be supervised by the "designer" or "a suitable person". Client is of the view that this can be an experienced person from the piling contractor side (not an engineer) and if there are any problems they it will come back to the home office for resolution.

Since I didn't have a great idea of the pile driving program, I asked the above questions. But I know that it is necessary to have geotech engineer at site. I need to make myself familiar with a few things so that it will be easy to convince the client.

 

[dgillette]

I think you should have a geotech on site, either the actual designer or somebody with the designer on speed dial, at least for startup. Thereafter, you probably don't need the designer there as long as you have someone without a conflict of interest tracking and reporting blow counts, refusal depths, etc. to the designer. This person should also have the designer on speed dial. It's necessary because the actual driving conditions, refusal depths, etc. may not be the same as what the designer expected, and there has to be someone who knows the issues, and the conditions can vary from point to point within the site. (I've seen funny things on two different jobs out of the pretty small number of pile projects that I've worked on, including one where the refusal criterion was almost met before the pile broke through into a weaker layer.) You might need to adjust pile type and section, depth, hammer size, etc.

As for capacity during driving, there is real-time wave-equation analysis that can be done to determine the resistance to penetration at the tip of the pile and along its shaft, based on strain transducers and accelerometers at the top. After each blow, the Pile Driving Analyzer machine spits out a value for the driving resistance, along with the energy transferred from hammer to pile, and other paramaters that I don't recall. I believe that's still considered a preliminary estimate (called the Case method, because it was developed at one of the finest, most superbly excellent institutions of higher learning in the world, Case Western Reserve University). The actual strain and acceleration data are then used in a program called CAPWAP that provides a more refined estimate. There are a couple of caveats: First, there can be setup or relaxation that affects the pile capacity, with the former being more likely, so it is common to do "restrikes" a day or so later. Second, pile capacity says little about settlement under load. The leader in the field is GRL in Cleveland. They send people all over the world to do this stuff.

http://www.grlengineers.com

If you've gotten the piles to design depth but the blowcounts don't come up, you probably don't have the necessary capacity, which goes back to my first sentence, about having the designer or a knowledgeable close associate on site at least at the beginning. A pile-driving contractor getting paid by the foot has a legitimate self interest in getting the piles to depth and getting off of them as quickly as possible. Very few would deliberately cheat, but time is money and hard driving is slower than easy driving. Specs need to have blow count criteria as well as expected depths.

That's my long-winded take on it; others with more pile-design experience than I have might disagree on the construction oversight requirements.

 

[JedClampett]

I'm no expert, but I've got a couple of pile jobs under my design belt.
Refusal is a number defined in the contract documents. In our projects, it was 20 blows per inch.
I'd make sure the Geotechnical Engineer was there, at a minimum, at the beginning of the pile driving operations. After a while, hopefully thing will be routine enough that he can hand off to the regular on site engineer. The pile guys can toss around a lot of jargon, which if you're not familiar with, can bury you quickly.
Also, you should have a step where you load test a number of driven piles. Possibly the Geotechnical Engineer can be present for that. It's possible, although I don't specifically remember, that the testing is done pretty quick after the first group of piles go in, so the contractor can get it out of the way. So maybe the Geotech can do all of this in one trip.
Plie driving is way too expensive and hard to recover to cheap out on the engineering involvement. Do you really want to be deposed and say to save money, you limited the expert's visits?