Pile load test-elastic or plastic settlement??? 2

[22222222]

I am looking for an information how to distinguish between plastic and elastic settlement after unloading the pile. Pile settlement was 52 mm on 1900 ton lading.Pile diameter 2 meters and installed in mudstone, sandy mudstone and sandstone stratas to depth 30m. After unloading there was residual settlement 44 mm. People say it was plastic deformation, but I am not sure. What is the percentage of residual settlement to say is plastic or elastic? Mudstone UCS is between 500 Kpa to 5 Mpa. SPT values taken were more then 100 all the way. Before this test we loaded one pile that settled 64 mm and rebounded to 58mm.There was a time gap between first and second test 4 month.Any influence on time delay.later you test less settlement we may get?

 

[wiktor]

Any textbook on the subject would be helpfull - "Pile Foundation in Engineerig Practice" by Prakash, or "Foundations Analysis and Design" by Bowles have some information on the procedure and interpretation of the results.
The difference in between maximum measured and permanent settlement (less elastic shortening of the pile) is a plastic deformation. This should be clearly visible when you prepare diagrams presenting relation in between load and settlement.
The time lapse in between the tests have some bearing on the results. The skin friction usually will increase with time, which could be noted on the load test results.

 

[22222222]

Hi Wiktor.
Thanks for your answer. I got already diagram of loading/settlemen and rebound line is almost straight, but still not sure about plastic elastic settlement.We have loaded now only 900 tone(working load) and settlement was 2mm and after unloading the rebound line is ower the settlement/loading line. This I would understand like a elastic deformation.I did not expect this would happen in 1900 ton load but some small rebound is there. I guess that plastic deformation would be with no rebound or some small rebound, but how small compare to settlement, do not have any idea so far.I try to check more even in books you have mentioned.

 

[longisland]

Sounds to me it's plastic settlement. I suggest doing a few more points with PDA test. I presumed there were several rigs during piling; check your piling record to look for any irregularity. I hope it helps & not telling you anything you've known. wiktor's right on the time lapse especially in clay.

 

[22222222]

Hi Longisland,

Thanks for your answer.
What is PDA test?

 

[Focht3]

Pile Driving Analyzer (I think) - it measures hammer force, hammer/cushion/pile/soil system response. It is becoming common on larger pile driving jobs in the U.S. It's a great tool in experienced hands - and (in my view) pretty useless otherwise. Unless the project ends up in litigation, of course.

Did you measure pile tip deflection during the load tests?

I have assumed that you have tested two piles that are nearly identical with very similar driving records (measurements of the blows per foot taken during their installation.) If your first load-settlement curve is almost linear and elastic (it rebounded almost immediately and completely), and the second has significant "set", one of two things may be going on:

1. Your two test piles are significantly different for some unknown reason, and/or

2. The four month delay between tests allowed the skin friction profile along the pile to change, with some of the residual forces in the pile transferred downward and/or dissipation of the negative skin friction in the upper portion of the pile.

These are my first impressions of possible causes for the difference in behavior. What were the weather conditions during the four month interval? Go back and look at your second test pile - do you still see a 44 mm set?

Finally, let me remind you that you are not dealing with a manufactured product - you are primarily dealing with inherently variable natural deposits of soil/rock. I fully expect that the ultimate pile capacities - were you to test some of them until they settled 6 cm or more - would vary by 30 percent or more. You should expect to be puzzled by some of your full-scale load tests. After all, if every foundation behaved exactly as predicted by some formula, then there would be no need for geotechnical engineers!

 

[22222222]

Hi Foch,3

Thanks for your answer. Our piles were 2 m in diameter, bored piles using large oscilator and casing for excavation.Not driven piles.
The two piles tested(Static loading test)were in the same group but in second test we have lowered down the load about 100 Tone.Now we are going to performe another test but quick cycling to avoid the creep effect and hoping to reduce settlement.Test will be on the neighbour pier about 50 m apart.Geology is mudstone sandstone with SPT more then 100. After first test we had to excavate the ground about 6 meter deeper to extend the pile and then I found vertical joints that could not be seen from the surface outcrops.I think that this could be a cause of our bigger settlement.After test I will tell you the result.
I will try to check the old pile if the settlement value is still the same. Weather in those 4 months was also rainy and sunny, so hard to say.Thanks.

 

[Peter888]

so so...............

 

[BigH]

Just saw your query and replies; only perused and have not looked into it to any depth so will not comment as such.

However, I bring up a point that might be of interest to geotechnicals in separating shaft and end-bearing proportions of a pile load test. An experienced geotech professor that I've recently met has suggested to plot a normalized settlement vs load. Take the settlement and divide by the pile diamter. This appears to be a good way to separate the two support mechanisms. The break (suggested to be in the range of 1 to 2%) is the change over from shaft to end-bearing.

Secondly, I strongly suggest geotechs to obtain M.J. Tomlinson's book on Pile Design and Construction (Viewpoint Press). It is a most awesome reference.

Best regards to all.

 

[Focht3]

BigH has a good suggestion; but I'm not sure it will help in your case. You don't really need to divide the settlement by the pier diameter; just look for a distinct change in your load-settlement curve. Mudstone may offer too much bearing capacity to allow for a distinct transition between skin friction and end bearing.

Drilled piers do not have the same kinds of residual stresses as driven piles; weather and the time delay may or may not be factors. The vertical cracks are of interest - but may be a function of severe drying of the ground rather than the load test itself. If the four month period between tests was "dry", then the soils could have experienced shrinkage and moved away (laterally) from the pier. (We have shrinkage cracks that have exceeded 7 meters in our area.) This could significantly reduce the skin friction contribution to pier capacity.

By the way, did you measure pile tip deflection during the load tests? Did you apply the same test load to both piers?

 

[22222222]

Hi All,

Thanks for your remarks. We have finished the cyclic test on neighbour pier.6 cycles on 150 and 2 cycles on 200% of working load. Total settlement for all cycles was 24 mm and after unloading was 17 mm final residual settlement. For 6 cycles we had about 1 mm additional setllement between each load unload.Unloading was always back to the working load and waiting time was 6 hours before each cycles.I think the test was Ok, but some people think that there should not be any settlement on the working load(800 Tons in our case, saying that all shaft friction was fully mobilized).
We had 2mm settlement up to working load and after we changed loading to 1200 Tons we had additional 4 mm.
At 800T, we held load for 12 hours(less then 0.25mm setlement rate per hour).Loading was quick for all stages and it took less then 4 minutes to apply the load and unload.Settlement between 2 loading for 200% of working load was 0.5 mm and may give some hint that during quick loading ground gets stiffer and no more settlement should take place.Cracks I mentioned before were joints and not shrinking craks and we also measured movement on anchore piles.Up lift was about 4 mm per anchor.Anyway, does anybody have any idea why somebody thinks that there should not be any settlement on working load? Thanks.I am going to get this M.J.Tomlinson's book you receommended.

 

[Focht3]

22222222 -

I've encountered "those people" before (usually in an expert witness context.) I qoute Terzaghi in these circumstances - and those that argue that the foundation "cannot move" have a hard time disputing the opinion of such a prominent engineer in a 66 year old reference!

The next paragraph is mine; Terzaghi's follows (in bold text.) You may paraphrase my work, but do not use it directly (unless you cite me as a source, of course!)

Careful, detailed observations are crucial to understanding foundation movements. A quote from Dr. Karl Terzaghi, Ph.D., P.E. in his presidential address to the First International Conference on Soil Mechanics and Foundation Engineering (ICSMFE) is presented below. Dr. Terzaghi is widely recognized as the “father” of modern soil mechanics (now known as geotechnical engineering). His address to the engineers assembled at Harvard University, Cambridge, Massachusetts in June 1936 contains a number of observations that are still valid today.

Second, the observation methods must be reliable; otherwise there is too wide a margin for interpretation. If an observer claims that a building did not show any signs of settlement, the structure may have settled through a distance of one-tenth of an inch to four inches, provided the settlement was uniform and the distance to the neighboring structure was appreciable. Finally, the report on the observation must be accompanied by a statement of all the vital factors which were likely to have influenced the object of the observation. Otherwise the observation cannot be used as a basis for a valid conclusion. In order to satisfy this third requirement, the observer must be familiar with the physics and mechanics of the observed phenomenon.

Have fun!

 

[22222222]

Hi Focht3,

Thanks for your view on the problem.
I am going to have fun next week at the meeting, hoping finall to close this long lasting issue.You konw, 100 people is sometimes 100 opinion(different usually).

22222222

 

[TCL]

I am trying to understand the evaluation results of a pile test.

The pile is a 1200 mm dia bored pile, designed for skin friction and end bearing in sandy/clay soil, for a high rise tower, which is sensitive to differential deflection. The required test loads are the working load for the first cycle and twice the working load for the second cycle. The maximum settlement registered for these two cycles are well within the permissible values.

The residual settlement at the end of the first cycle is within the permissible value. However the residual settlement is in excess of the permissible of 6mm residual settlement.

My question is: How and why will the test be seen as a failure if it breaches the permissible residual settlement after the first cycle (1 x working load) and similarly after the second cycle (2 x working laod), i.e. the implication on the working piles.

Any reference to authors such as Bowles, Tomlinson,or Terzaghi will be most welcomed.

 

[Focht3]

TCL:

Why not start a new message thread?

Hmmm, you won't likely find any "standard" reference for your problem. You're in the "thinking man's" territory!

What constitutes a "high rise" to you? A 1.2 meter drilled pier isn't all that large. Where is the site? Soil conditions? Groundwater? Special concerns?

Why such a small residual settlement tolerance? A know of quite a few high rises (40+ stories) with expected elastic settlements of 150 mm and total settlements of 400 mm - or more. Total settlement isn't the issue - the concern is differential movement.

Let us hear from you!

 

[TCL]

FOCHT3,

Thank you for your reply. It does not seem to be a problem after all. The source of the problem was the manner in which the specification was written. But after checking the specification again, the 6 mm permissible residual settlement is for the first cycle (i.e. unloading from the 1 x working load) and not for the second cycle as mentioned earlier.

Thank you once again.