Load Testing of Large Diameter Piles and effect on superstructure

[Hariharan]

I wish to discuss the various acceptability criteria for safe working
loads of piles from Pile load tests. The most commonly quoted
criterion is: "Safe working load is 50% of the load corresponding to
a settlement of 10% of pile diameter".

When we consider large diameter piles, say 1500mm, the load
corresponding to 150mm (nearly 6&quot would need to be obtained.
Testing for such large settlements is possible only if we have
separate 'test piles' not used in the project. What is the practice?
Some bridge engineers may be able to clarify.

Further, the settlement would be about 25-30% (30-50mm)
under operating loads. Is this information given to and used
by structural engineers in their analyses? I was trying a test
example, and the design bending moments could be drastically
different if the settlement is considered in a struture with
continuous beam elements supported on piles. This kind of
situation is likely in bridges and marine structures.

Any feedbacks?

Thanks in advance,

Hariharan

 

[daveross]

You are absolutely correct. Unless your structure can tolerate large settlements (which is unlikely), you have to limit the settlement criteria in a different way for large diameter piles. That is, the allowable settlement at working load should be based on the settlement which the structure can tolerate. If you have a bridge superstructure which is sensitive to foundation settlement I think you would need to consider the settlement as a load case in the analysis, and limit the allowable settlement according to the limits which the superstructure can reasonably and economically sustain.

 

[KAM_imported]

There are many methods of determining the "failure load" of a pile or drilled shaft. It is important to differentiate between a defined "failure load" and the "ultimate load". The latter refers to the applied top load at which a tested pile plunges at constant load. Because the latter often results in very large deflections, most people use a defined failure load and then divide this load by a factor of safety (usually 2.0). The FHWA failure criterion for piles having a diameter greater than 610 mm. is:

Failure Deflection = (elastic deflection) + b/30, where b equals the pile diameter in mm. The failure load of the pile is that load which causes a movement of the pile head equal to the failure deflection.
For piles less than 610 mm. in diameter, the FHWA criterion is:
Failure Deflection= (elastic deflection) + (4.0 + 0.008b)

Another method, called the Davisson Method, is used for driven piles and can be found in most geotechnical textbooks. If your structure is very settlement sensitive, you may have to base the allowable load on an allowable deflection criteria, in which case none of the commonly accepted failure methods may apply. Hope this helps.

 

[Hariharan]

Thanks, gentlemen. Some more queries:
1. The FHWA criteria for the two cases do not
seem to result in similar or converging values for b=610mm.
From larger pile criterion, failure deflection = elastic + 20.3 mm
from smaller pile criterion, failure deflection = elastic + 8.8 mm.
One cannot justify such a large jump! Or have I misunderstood
something?

2. Is load testing performed to 'failure deflection'?

3. In such cases, is the 'ultimate load' of any significance? Is it
computed or used anywhere? Seismic analysis perhaps?

4.In offshore piling, for example, we use only pile
ultimate capacity and use a minimum factor of safety as per code.
Deflections are included in the analysis model in the form of soil
load-deformation curves.

hariharan

 

[Hariharan]

Still looking for some response to my queries! Thanks
in advance

M. Hariharan

 

[StrucKh]

Just a suggestion.
You can use Osterberg cell testing otherwise known as O-cell testing.This test is useful when you have to test pile for loads in MN range.
StrucKH

 

[Focht3]

Hmmm,

There is a difference between the "safe working load" and the amount of deflection a particular structure can tolerate. Let's not confuse the two!

I admit that I'm a bit confused by the blending of bridge foundation criteria and offshore design methods in Hariharan's questions/discussion. You can't "mix and match" - stick with one or the other, at least until you have completed the design. Both methods check for a "safe working load" and maximum allowable movement, but by very different methods.

If you are familiar with offshore design methods and understand t-z curves, then you can design almost any pile or pier by this method. Just be aware that the local transportation officials may require you to prove acceptability by the FHWA approach...