pile test

[geotechnicalnep]

Hi ! I have recently designed pile foundations for a reinforced road embankment. This section is 80 m long and 22 m wide. I am using precast concrete piles. Now I have to carry out pile test.

Could any of you please advise me whether I go for Static or Dynamic test? How many test do I need to carry out?

 

[dgillette]

"...whether I go for Static or Dynamic test?" Yes. ;-)

The static test gives you a more definitive measure of bearing capacity and the load-displacement curve, but the dynamic test (Case Method pile-driving analyzer) gives you information the static test can't, like whether the hammer was putting out full energy when it reached your criterion for ending driving. It can also give you an indication of damage in the pile. I think it would also be quicker and cheaper because it just requires mobilizing a guy with a few suitcases full of equipment for a few days. The slowest part of the test itself is installing a few instruments on the top of the pile. Depending on what level of precision you need, the bearing-capacity results may be available immediately straight out of the PDA, or the data may need to be run through a program called CAPWAP. (Hammer energy comes out right away.) To test setup of the soil around the pile, come back the next day and hit the pile again with the PDA installed.

How many? Depends on variability of soil properties and depth to bearing layer across the site. You can have the dynamic test run on several per day during production driving, until you are sure you have all the bugs worked out of the hammer, refusal criterion, etc.

vulcanhammer - You have more experience with this?

 

[Ron]

The static test tells you after the fact if you have an issue. PDA lets you do ongoing evaluation and modification as the driving takes place.

You can obviously do both, but if I had to choose only one, I'd look at doing PDA.

 

[DRC1]

If you only do one, not knowing the site, I would go with the static. Dynamic tests can sometimes be misleading, and work very well if there is a static test with which to correlate.However the static will be significantly more time and money. Generally in a static test, a few piles are driven and one is or two are selected for testing. If the pile passes, the balance of the piles are driven in the same manner. A PDA can be a lot cheaper and give you other information. This is done durring driving. It should be noted that the results from the on site PDA or approximate. The final results are generally run off site using CAPWAP and follow a few days later.
To really get a good idea of what will work, call GRL. They developed and market dynamic pile analysis.You can reach them at

http://www.pile.com.

I would not worry about them pushing their application for the wrong site. Their business is built on reputation and they are very concerned about the imporoper application of their products. Talk to one of their engineers and you will probably learn more than you thought possible about pile testing.

 

[UcfSE]

The type of testing may affect your assumed resistance factor if you are doing an LRFD design. Does that apply to you?

 

[dgillette]

"...pile foundations for a reinforced road embankment."

This suggests that you could tolerate a bit more settlement than for a building or bridge foundation, which might affect what you need in the way of testing.

 

[geotechnicalnep]

Thanks dgillette, Ron, DRC1,UcfSE.
Yours advices and information have been valuable for my work. As you adviced I contacted GRL too and recieved valuable advices. I talked to other colleagues who are around me here in Ireland. Now we are going with a static test and dynamic tests on 10% of the piles.
I have no idea about LRFD.
However these piles are being driven in clay and reach limestone bedrock - eventually it will be practically end bearing.
Thanks a lot.

 

[BigH]

Given the fact that you have to "pile" your highway embankment, and are driving the piles to end-bearing in rock, downdrag forces can be significant on a unforgiving tip condition (see Scandanavian experience - many articles published in the 1965-1975 era. If I remember correctly many piles 'broke' due to the fill placement - then the load bearing would be reduced. It might be advantageous to drive piles solely within the clay and not to end-bearing. This can give you a mat effect and still maintain global stability. I believe that Zeevaert has discussed this in his book of Soil Mechanics for Difficult Soils.
Is this embankment a MSE style wall embankment on soft clay? If so, you might consider putting in wick drains and permit quick consolidation of the clay under stage loading in order to build your embankment (settlement implies increased undrained shear strength - so you improve your shear strength and hence global stability while you build the structure). We did RE Walls that settled up to 1000mm during stage loading construction. I would think that wick drains would be much quicker and cheaper than pile driving. Just a thought.