Use of Preliminary Test Piles as Working Piles

[EireChch]

Can someone tell me a reason as to why we cant use preliminary test pile (so a pile that has been loaded to 2.5 times its working/allowable load, maybe to failure, maybe not) as a working pile that will support the structure.

 

[Lomarandil]

If there wasn't any large displacement event, and the pile is primarily a gravity pile, I'd allow it.

If it were a lateral pile without a lot of redundancy, or maybe a highly corrosive environment, I'd have to think about it a little.

----
just call me Lo.

 

[geotechguy1]

>If there wasn't any large displacement event, and the pile is primarily a gravity pile, I'd allow it.

Driven piles get installed by failing them into the ground with large displacements; yet we all seem quite happy to use those as working piles.

If anything test piles are better than working piles because the standard of care applied in constructing them is usually higher than a normal pile, unless you are able to conceal from the contractor that it is a test pile.

 

[EireChch]

Thanks gents, I have similar opinions.

The discussion came up in a meeting where the clients asked about re using PTPs as working piles. I said something similar that I dont see a reason not to. Your working load is obviously far less than your PTP max load so you know the pile is safe.

I was challenged with comments on skin friction is destroyed as it has failed!!! This highlighted to me that the clients engineer has limited experience. Many times we do compression tests to high loading and maybe failure followed by tension testing. Shaft capacity is still there!

The one thing that got me was " how come may specifications state that working piles should only be tested to 1.5 times their working load and not 2.5-3 times".

I told him that I didnt have a good reason as to why this is the case. I have done some googling and it does appear to be the case.

Regardless, I still dont see a reason, well at least from geotechnical performance, as to why you would limit load on working piles. I can see from a practical standpoint that if you prove your settlement criteria is ok then the capacity criteria is ok too. You need a lot less load to prove your settlement criteria.

 

[geotechguy1]

I thought that the 1 to 1.5 limit is basically about it being a 'proof load'; I think it's a crossover from structural engineering where you may want to proof load something but keep your test within the elastic zone because the plastic deformations are very undesirable for a beam or some other structural element; whereas for soil, the deformations are almost entirely plastic anyway (although we like to confuse things by using elasticity theory and calling plastic deformations elastic deformations). In many areas it's structural engineers that do the 'design' as it is and they have a habit of applying structural concepts to soil. I've had this same conversation on sites in Canada and it usually comes from structural engineers.

Edit: Or from geotechs who have been browbeat by structural engineers for to long

 

[HTURKAK]

When the static load testing of pile , Load vs Settlement of pile head and Settlement vs Time plotted , one can easily see the creep effect of the soil . That is , pile loaded upto the deflection is around 40 mm .

https://www.fhwa.dot.gov/publications/research/infrastructure/geotechnical/05159/chapter4.cfm

When the static load test is performed on working pile , the the influence of preloading on the performance of the subject pile within the structure should be considered.

That is the reason for the working piles should only be tested to 1.5 times their working load.

Moreover, if the static tests are performed on the test pile , the designer can make any modifications to the foundation design ( pile size, arrangement of piles etc..) as per the results of the tests.

IMO, the static load testing of the working pile is the pile contractor's own risk .

 

[EireChch]

Hturkak - thanks for your comment. Can you expand on what the influence of preloading is?

 

[HTURKAK]

The vertical spring stiffness may change depending on the soil conds and pile type.

 

[MTNClimber]

We’ve allowed it before. If you’re in doubt just reload the pile to prove it’s ok.

 

[Okiryu]

I think that perhaps you can provide examples...For driven piles, the soil ultimate capacity is reached when the pile is driven each blow (unless the set per blow is very small, and only in that case is there extra capacity that has not been activated). So, the capacity of the pile is still there even the soil reaches its ultimate capacity. This is also demonstrated by restrike tests which shows that the pile still have capacity (regardless if the pile is experienced relaxation or setup...there is still pile capacity).

 

[EireChch]

Hturkak - I think the preload can only increase stiffness as soils have a reload stiffness that is 3-5 times its initial loading stiffness. Can that potentially be a negative from a structural point of view, as in does it attract more load ? Geotechnically its only a positive.

Okiryu - yes I agree.

 

[HTURKAK]

Your interpretation is exactly true. The influence of the stiff pile to the displacement behaviour of the structure and the footing reinf. scheme should be checked.
Past experience, i have rerun the analysis and the pile cap with six piles reinf. affected.