Steel pipe piles hitting an early refusal. Any recommendations? 1

[hughits]

I am a structural engineer working on driving steel pipe piles for an oil sands treatment facilities. Based on the soils report and design drawings, the design embedment of most of the piles range from 20 m to 26 m. The pile top cut-off elev. is about 1.5 m above the final grade, so the total pile length would be 1.5 m longer than the design embedment. For additional info, each pile will support a steel column (no skid or concrete pile cap).

Most of the piles we have been driving met the design embedment, but there were several piles that hit early refusal. Some of them needed to be driven about 26 m deep but were actually driven 20-21 m. The rest of them are quite or way off of the design depth. They needed to be driven about 24 m but were actually driven only 7-8 m. Since we had more than 100 blow counts at the refusals, I wouldn't worry about the pile axial compression capacity, which was also confirmed by our geotechnical consultant. The concern is the piles tensile capacity. The design tensile capacity is way more than the actual capacities we will get from those piles.

Now we are trying to come up with ideas to fix this up. One solution we are discussing is to take them out, drill the holes down to 20 m or something, and put them back in the holes, and backfill it. I would like to perform additional borings around the area, but the client wouldn't accept it.

I am wondering if there are any better ideas out there. Any idea, opinion, and/or recommendation on this issue would be highly appreciated.

Thanks.

 

[muuddfun]

You have come accross a very difficult problem.
I have been wrestling with the same problem for more than a year on a bridge I am working on.

The first thing I would try is to relief drill within the pile to remove whatever plug has built up on the inside. Then try to redrive the pile and see how much further you get. You will learn a lot of usefull information from this. When you do the redrive make sure you have the PDA to monitor the driving. In all likelyhood you will not be able to make this work because your piles are probably damaged at the tips, and may be bent along the shafts as well.

I have done many PDA and also static tests on the piles with all kinds of results. The most difficult issue with the PDA testing is to sort our the frictional component along the outside of your shaft from everything else. The problem with PDA is that there are only non unique solutions. You have too many variables to deal with. In the end it will give you some good results especially for overall capacity, but you really have to understand its limitations to know what to make of the friction results.
The static test did show very high capacity when you have a damaged or "hooked pile".

What type of soil are you in? Is it mostly sands, or is it all cohesive? Have you looked or measured how much plugging you are getting on the piles you have driven? Have you compared the plugging on the ones driven to design depth with the ones driven short?

You should probably count on very little to zero tension capcity for piles that are as short as the ones you have stated (7-8 m). How much tension do you need to achieve?

The difficulty with open ended pipe piles is that you are driving from a condition with the only resistance being along surface of the pile, to one where as you increase the plug then you are increasingly driving a displacement pile. As the pile plugs it gets more difficult for objects such as cobbles to be pushed out of the way. The relief drilling does allow some progress in cobble zones, but it is limited.

What I have resorted to is preaugering ahead of the pile driving, and leaving all the material in the hole, then driving the pipe piles as far as I can. If they stop early I try the relief drilling, and usually can get them another 5 to 10 feet deeper (but I am in a heavy cobble zone).
If I still can't reach my tension needs then I drive some replacemnt H pile next to my CISS piles to supplement the tension capacity. This is the cheapest way to keep the job moving. The H pile drive better in the cobbly zones.

I stop all pile driving at 50 blows per foot to limit damage. I learned this from the PDA testing. During the earlier stages of work we were driving at much higher blowcounts, but found that damage started to happen as noticed by the PDA between 55 to 60 blows (on a 24"x5/8" PP with a D46-32 Hammer).

 

[PEinc]

"I stop all pile driving at 50 blows per foot to limit damage."

Only 50 bpf? FYI, PennDOT wants 20 blows per inch as refusal!

http://www.PeirceEngineering.com

 

[muuddfun]

PEinc.

Unfortunately 240 blows per foot equivalent was tearing up the pile to much, it wasn't doing any good.
Check out these pictures of some we extracted.

The main reason to stop is to be able to work inside the pile easier since we are doing CISS piles. Otherwise I might not care as much about the damage.

After a sequence of stopping driving at 50 blows, and cleaning out the interior, then redriving, the pile are getting well into any cobbly zones. I have tried a second round of relief drilling and driving but they did not advance more than a foot after that, so the extra cost was not worth it, and it gave me some confidence that I would have some end bearing.

 

[PEinc]

Everyone says 240 bpf is too much but that's what PennDOT wants - and gets. They also make you design the piles for A36 36 ksi even though the steel is A572 50 ksi. It's only money!

http://www.PeirceEngineering.com

 

[hughits]

Mostly sandy soil with a little bit of silt and clay here and there. Checked the soil plug elevations, but their pattern doesn't seem to be correlated with the driviability. The required tension capacities vary between 500 and 1500 kN.
We decided to do PDA on those piles hitting early refusal; we actually did on two of them. The results were very encouraging. The actual compresison and tension capacities are 7-8 times greater than the ones we obtained from the capacity vs. depth design chart provided by the soils consultant. Even with the short embedments, they are okay.
By the way, we got a little more than 100 blow counts per 12 cm on those piles and stopped driving as instructed by the soils consultant. I believe some or most of them must have been damaged at their tips, but we got enough capacities and moved on.
For some of them for wich we can't get enough capacities, we are planning to drive some compensation piles around them. Hammper impact weight (piston) is 4600 kg.

 

[muuddfun]

Hughits

I would be skeptical of the values for friction (in tension) being that much higher than what you get from the capacity vs depth formulas. Using very conservative delta and K values in the API formula vs using more generous ones I would only expect a difference in possible capacity ranges of about 3 for pile that are only 7-8 meters in sandy soils. [Say API using K at 0.8 for PP, and Delta of 15 deg, you would get about 60 kips (about 270kN) at 25 feet (7.6m) depending on the overburden (120pcf) vs. about 155 (about 690kN) if delta was 35 deg.]

I would expect that you had much higher that predicted capacity for the compression due to the end bearing. The hard part with the PDA is distinguishing the frictional component from the end bearing component. I have had several PhD's who do the PDA stuff doing many tests for me and lots of CAPWAP and they have come to the conclusion that it is a very hard thing to figure. It could be that high but I would not trust it from the PDA stuff only with the static tests.

You have a big hammer so you are definetly getting some good compression values if you are only going 12 cm for 100 blows.

 

[hughits]

For specifically the pile in quesiton, I wouldn't worry about it too much at this point. The design loads for tension and compression are 960 kN and 1170 kN, respectively. The capacities we obtained from the PDA test are 4100 kN and 5200 kN for tension and compression, respectively. I didn't know that it was hard to breakdown the tension and end bearing component from the total capacity. The numbers we got - 4100 kN - is about 4.3 times great than the required tension capacity though, which I thought would be okay.

 

[muuddfun]

Only needing 215 Kips for tension should give you a good bit of leeway for piles that test that high for total capacity.

You design load is very close to what I have on my project. It is very interesting for me to see what someone else is getting in a very similar situation. I am driving 24 inch PP (610 mm)about 46-58 feet (14-17.5 meters) to achieve greater than 200 kips (890 kN) tension and 400 (1780 kN) - 600 kips (2670 kN) compression. A good bit of my capacity is comming from end bearing of a plugged pile. Some of my piles are refusing earlier slighly past 30 feet (about 10 m). These ones I generally have to supplement with HP 14x117's.

We did a static test on several of the piles that were damaged and was able to pull over 500+ kips (2225+ kN). The shortest pile tested was about 35 feet long (about 11 m). We came to the conclusion that the damaged section was catching or hooked and therefore was providing some additional capacity above what one would expect from friction alone.

We also have done PDA testing on about 36 piles with lots of different results as I said before (from about 480 to 1440+ kips (2130-6400+ kN), by CAPWAP, total capacity). Results vary a bit more by Case method.

On average I would say about 50 percent is friction and the rest in end bearing. Some times friction is as low as 10 to 20 percent.

Is there any bitumen in your sand deposit? Is that contributing any to your friction values? Just curious because you said it was being done for a oil sand process facility. Just want to know how close the geology is to what I am dealing with. I am in a riverbed with about 5 feet (2 m) of sandy fill, then about 30 to 40 (9 - 12 m) feet of younger alluvium, and about 30 feet down is lots of cobbles, and boulders and heavy gravel deposits on occasion. About 45 to 50 feet (14-16 m) I get to a very dense older alluvium which is my target for reliable end bearing.

 

[Zambo]

It would be interesting (and important) to know if the toes of the piles are damaged, as already has been mentioned this could be the problem. Have the toes of the piles been reinforced with additional plate?

What about using a vibro to see if it can better deal with the soil conditions.