Preboring Driven Precast Piles 2

[asixth]

Hi guys,

I am a structural engineer who is doing a structural design of precast driven piles for a bridge abutment. I have run down the loads and the geotech report makes the follow suggestions.

1. Because the piles are driven to refusal, the capacity of the pile will be governed by the structural capacity.

2. Undersized pre-boring will be required to facilitate driving through a stiff stratum of bedrock.

My questions are:

If my piles are 500 diameter, what diameter should I make the pre-bore and what should I be looking out for? All help will be greatly appreciated.

 

[DRC1]

Okay
1.) All piles driven to a blow count are driven to a termination criteria. The term refusal has many meanings to different people, but is often interpreted as the point where the pile will not advance despite repeated pounding by the hammer. Thus it is not a good criteria for termination. A better criteria is to determine what set is required to acheive required capacity times an appropriate safety factor and what set might pile damage be anticipated.
2.) I assume when you say your piles are 500 diameter, you meam 500 mm or 18 inch. These are large piles and capable of carrying high loads. Driving stresses will probably control design. Precast piles are easily damaged by driving to rock and a fair amount of pile failures should be anticipated. Precact piles are not a good choice for driving through an underreamed piolot hole in bedrock, even with point protection.
3.) Why not terminate on the top of the rock? If you used a hydraulic hammer and cast points, that would increase the potential for sucess. Not really sure why the preaugering to a greater depth in bedrock would be requuired.

Hope this helps

 

[Drumchaser]

I concur with most of what DRC1 explains.... and would switch foundation design(s) to drilled shafts...much less potential for misery.

 

[asixth]

Thanks DRC1 and Drumchaser

That is correct, the piles are 18 inch diameter, I have them stressed to 9MPa or 1300psi and I am providing 24mm diameter headbars (1" diameter).

How do I calaculate the stresses transferred through this pile section by driving so I can design the headbars for driving.

 

[hokie66]

asixth,

Why are you doing this? A more normal approach is to provide the required capacity to the piling contractor and let him decide how to accomplish it, pursuant to your review of course.

 

[asixth]

It's a government job and they are asked us to design the precst prestressed piles (PSC).

 

[hokie66]

There is no point in designing something the piling contractors are not set up to accomplish. I would consult with one of the reputable piling contractors before you finalise the design.

 

[GeoPaveTraffic]

I would never design precast concrete piles to bear on bedrock, as has been noted the potential for damage to the piles is very high. Also, it is likely that the bedrock elevation will vary across the site. How do you plan to accomodate this variation with precast piles?

Better choices would be auger cast piles, H-piles, or drilled shafts (piers). Just because "the governement" wants something does not mean that it is practial. You can bet that you will be blamed during construction if what YOU designed cannot be constructed.

 

[Drumchaser]

asixth-

We have moved away from PCC Piles due to the extra expenses/problems that can often occur. This is not to say they are not the preferred foundation option for the rest of the world.

Are your piles to be driven on a batter?

....it is my opinion
(No offense to the PCC Piling folks here) conc. piles are very problematic as compared to drilled shafts.
Requirements are usually:
1) Designed bearing achieved.
2) Pile Tip elevation met.
3) With-in specified location tolerances. (top & btm. of
pile)

As the apparent design engr. you may or may not be involved in the driving process, which according to the data provided will be tricky at best.

"Because the piles are driven to refusal"....it is my experience once refusal has been met (no more downward movement) the pile has been overstressed.
This is not good.

If you are locked-in on this design....most S.T.A.'s have specifications for pilot holes in relation to pile size, as well as other reqt's that may help you. Check your local DOT website(s).

good luck

 

[BigH]

I'd change to closed end steel tubes. It's been indicated that the government agency wants precast conc piles - but as most here seem to be "against" them, I'd say you owe it to your integrity to point out that other pile types, driven to refusal "on" bedrock would be more appropriate and less susceptible to damage due to overdriving.

 

[hokie66]

"Stiff stratum of bedrock" can mean different things to different people. If it is highly weathered, the precast piles may work just fine. I still think you need to get a piling contractor talking to the geotech before the decision is made.

Where I am, precast piles are usually either square or octagonal. Round piles may be common elsewhere, but again, check with the piling contractors.

 

[asixth]

hokie,

I practice in the same city that you do. The piles I have are actually octangonal, I was just being more general in saying they where round.

 

[hokie66]

asixth,

I thought so, but you threw me with the round piles. A lot of the rock we have here is highly weathered enough to facilitate driving precast piles. I don't work in roads/bridges, but see a lot of the octagonal piles driven with conical steel driving heads. Main Roads often uses them in bridges for the above ground column as well. They just drive the octagonal precast to the specified depth or criteria, then cut off the top and cast into a header beam.

If the rock is hard, this may not be the best solution. You might want to talk to Wagstaff Piling or Piling Contractors for advice.

 

[rowingengineer]

So which part of the Brisbane River is your project? I currently doing a small pile job in Newstead, here we are looking at composite pile, The stratum is soft enough that we can drive into the “rock bed” to refusal without pre-boring, about 10m with a 375SQ. A residential project next door, had driven pile as well, be there for about 4 years, no pre-boring.
However in saying that I believe they are going to drilled piles on the BCEC Southbank extension because weathered and highly variable nature the “rock bed”. Not involved directly with this project, so would not say my source is quotable.


When in doubt, just take the next small step.

 

[DRC1]

To answer your question about Pile stress, this is typically derived from a wave equation analysis for typical hammer systems that might be used. PCI has standard pile designs. Check with them for design details. I would as other have said advocate for H-Pile or closed pipe. It is economical and more effective than precast. Also can easily adjust to varrying heights of rock. But even with steel piles, reasonable driving criteria should be used to prevent damage.

 

[asixth]

My apologies for not replying sooner, thanks to everyone for their contribution. The bridge is in the Gulf of Carpentaria, Main Roads are improving the infrastructure because seasonal flooding often closes the highways which stops freight from servicing the Northern Territory.