Pile load test, ASTM vs BS

[mbilal89]

When we compare the requirements of compressive axial pile load test of BS standard and ASTM standard, it is observed that the requirements of ASTM are very strict when compared to the BS standard. For instance, ASTM requires that the clear distance between reaction pile and test pile should be a more than 5 times the pile diameter (test pile or reaction, whichever is greater), whereas the BS standard requires only 3 times pile diameter, that too centre to centre. So the difference is too much. Also, the ASTM required calibration of jacks and provision of load cells, and there is no such requirement in BS standard.

I think that the ASTM standard is too conservative. I also see that in US, mostly driven piles are constructed. So there are greater number of piles but less pile capacity. So it is easier to make a smaller platform for the test. Dynamic test is also carried out more frequently so the requirement of static is also reduced.

Please share your opinion.

 

[SlideRuleEra]

I think that the ASTM standard is too conservative.

You can do something about that. Here is the procedure: "How ASTM Standards Are Developed - Modify Existing Documents"

BTY, I don't say this to be "funny" or sarcastic. I have submitted a detailed, documented proposal for a change to an AISC document. Nothing may happen, but I've tried.

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[oldestguy]

I've run many a pile load test. The circumstances you have to work with sometimes don't permit exactly followi9ng all details,such as spacing. Assuming the test is run by a geotech PE, I really doubt that anyone will second guess the procedures or check the book. Never had any test questioned.

 

[mbilal89]

That is a good idea slideRuleEra. I shall try it.

 

[mbilal89]

Dear oldestguy,

I'd say that you are lucky that no one checks the books. I have also conducted a number of load tests, i have even failed some of them and learned quite a few things from these experiences. Astonishingly, I did not omit anything from the book in these tests and still the load tests failed; actually the setup failed.

It is true that in the presence of a geotech expert, noone really questions validity of the setup.

We have to conduct a load test of 1500tons on a project but the consultant is very stubborn and is not willing to compromise the code. It took us around 3 months to actually make the setup as per the book. We will be starting soon.

 

[ATSE]

If the reaction piles are less than 5 diameters horizontally to the test pile, the tensions and compression stress overlap ("fans") becomes non-negligible.

 

[jjsim]

I think it is to the benefit of the contractor to keep the reaction piles further from the test pile so that the stress bulb of the test pile and reaction piles will not overlap. In this way, the geotechnical capacity of the test pile will not be compromised by the reaction piles.

 

[ATSE]

To be clear, the interaction between the reaction tension piles in close proximity to the center test pile produces an increased test load and decreased vertical displacement. Both theoretically and in practice. That is, reaction piles too close to the test pile gives unconservative results.

 

[mbilal89]

I surely agree with the concept of safe distance between the test pile and the reaction pile. My question is on the rationale in deciding the safe distance. The BS standard defines it to be three time max dia c/c distance whereas ASTM is tad more conservation and asks for five times max dia CLEAR distance. SO, what's the rationale? In sandy soils the influence of reaction piles will be high whereas is case of rock, the influence will be minimal. so applying the same principle for all types of soils is unfair. It has to be standardised ofcourse, but is should be done based on the soil properties,in my opinion.

Also, in case of a Kentledge system, it is very expensive to arrange long beams to built a platform for large diameter piles. And the influence of pad footings is not like that of reaction piles. The requirements should also be segregated based on the method used for the testing of the piles.

 

[mbilal89]

And it is not just about the requirement of safe distance. The requirement in ASTM of jack calibration and load cells is also an additional requirement when compared with the BS standard. Not every country has the facility to calibrate jacks.

 

[SlideRuleEra]

mbilal89 - I believe you can make a convincing proposal that the 5x diameter ASTM spacing can be considered for revision.

For friction piling, 3x spacing is often considered a reasonable minimum for a specified design load (P). Concept pressure distribution shown below:

Then, what spacing is reasonable for a friction pile load test at 200% of P? Concept pressure distribution:

A point-bearing load test concept pressure distribution looks reasonable, too:

A compromise answer may be > 3x, but could easily be < 5x.

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