The Use of Pin Pile (or Mini Pile) Below Shallow Foundation 6

[GGGGeo]

Guessing contractors finally tired of remove and re-compaction below footing and due to design-build approach, contractors propose using steel posts driven by pneumatic hammer below the shallow foundation. I read through the threads that the steel posts (or mini piles) are to be driven to "refusal". Then perform pull or compression test to obtain performance data then adjust quantity accordingly.

Could anyone familiar with mini pile (and helical anchor) kindly share opinions on:

(1) What parameters are typical needed for structural engineer to design mini pile? I can only think of friction.
(2) Since the mini pile is driven by pneumatic hammer and pneumatic hammer is not as heavy as the diesel hammer, the soil around the pile is likely disturbed since more "pounding" by pneumatic hammer are needed. Therefore, it may make more sense to provide reduced friction resistance.
(3) Field proof test will play more important rules for mini pile. What frequency in field testing is reasonable?

Thanks in advance!

 

[EireChch]

Ground conditions? What depth are they proposing to drive to? Often skin friction is ignored in the upper 2m.

Screw piles with flanges are also an option. Less vibrations (though this may not be an issue for you).

1) Friction is normally based on shear strength so i would think you would need some sort of strength testing (SPTs or Vanes).

2) There can be different factors applied to friction values based on driven or bored. I am not aware of a reduction between driving with pneumatic or diesel. Maybe others can advise. I would just consult your local design standards.

3) It depends on the size of the project. How many piles will you have. Some codes recommend testing at least 10% or working piles to justify reducing reduction factors. Again, your local design standards should have this information. Where in the world are you?

 

[dik]

Are you talking about micropiles that are inserted through a cored hole in the existing spread footing? If so, they are occasionally installed in this locale... a little pricey, but, they work.

Dik

 

[PEinc]

Sounds like a regular, but smaller, driven pile to me. Shouldn't the design be the same as for a driven pile rather than a micropile?

http://www.PeirceEngineering.com

 

[dik]

Micropiles have limited load capacity but are very effective for some applications. In these enviorons, they are normally drilled and may only have an OD of 6". The existing concrete foundation is cored to accept the pile. The existing foundation is secured to the micropile after installation.

Dik

 

[Okiryu]

OP said: "contractors finally tired of remove and re-compaction below footing and due to design-build approach, contractors propose using steel posts " So, I guess these proposed minipiles are an alternative method for new foundations. Looks like the original foundation design was based on shallow foundations with some subgrade preparation consisting on over excavating and replace with good material or re-compact the in-situ soils. So, perhaps the structure is small and/or soils are not too bad?

I agree with PEinc, perhaps design them as end-bearing driven piles. Because of the original design which considered over excavations and re-compaction, I assume that the minipiles may not need to be too long which will limit the friction contribution. Also, they have small diameters so friction may be small comparing to end-bearing capacity contribution.

 

[dik]

I'm aware of the shortcomings, but, as noted, "...are very effective for some applications."

Dik

 

[SlideRuleEra]

...pneumatic hammer is not as heavy as the diesel hammer...

Compare the specifications for the two hammers. Weight of the hammer is not the only factor. Energy developed by the hammers should be compared, not the weight. For single-acting hammers (the most common type - both pneumatic & diesel), energy is the product of the weight of the ram and ram's stroke length.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[Okiryu]

How much do you need to overexcavate and re-compact? If it is shallow and have large areas, you may look also at soil improvements with cement slurry: mix in-situ soils with cement slurry with an excavator. The cement slurry gets pumped to the excavator (which has some nozzles) from a small cement plant.

 

[EireChch]

It seems I misinterpreted the original problem.... But coring holes through an existing footing to install piles sounds like a bad idea. What would the minimum hole dia be? 100-150mm? I am thinking that it must be a relatively wide footing? 2m maybe? Coring a 100mm hole say every 2m in a smaller footing say 1m or less would just provide a path for cracking? Saying that I'm no structural so what do I know!

More info OP please....

 

[PEinc]

GGGGeo never clarified if the footings are existing or not. If existing and small, possibly, helical piers attached to the 4 edges of the footings may work. If the footings are not existing, install driven, small diameter piles or helical piers and then construct the footings as usually done for piled foundations. If rock, cobbles, boulders, very dense, or very soft soils are encountered, it might not be a job for helicals.

http://www.PeirceEngineering.com

 

[GGGGeo]

I am so thrilled to receive so many responses from all different aspects and I would like to thank you all! I am at northetn California and it has almost been a standard practice now (for jobs need soil report) to require remove and recompact soil minimum 2 feet below new footing. Last time I read a report to allow footing to be directly sat on bottom of excavation was many years ago.

So this particular job contractor wants to support new pad footing on driven posts (he/she calls it pile) W6X9 with peumamic hammer both are typically used for PV solar panel.

You can see PV solar panel on posts driven with nice rigs on YouTube. Hardly any information of vertical and lateral loads can be found. I am guessing for 7-10 ft spacing, total weight needs to support is about 1-2 kips. So the post may be really lightly loaded. Therefore driving them to 5-7 feet may be "good enough?!" since they are posts!

For building foundation support, calculations and more frequent pile proof testing may be needed. Since these piles will not be driven by production hammer but by tapping, will pneumatic hammer (impact and high frequency) comparable to conventil drop hammer? There may be reduction to friction and end bearing for post driven by pneumatic hammer?

I read article that the select of pile driving hammer selection is bigger the better but "the weight of hammer shall be such that no more than 10 blow/inch when reaching ultimate capacity". What will be minimum weight requirement? Or need to convert in between ft-lb to J.

 

[PEinc]

You can drive the piles with any type of "hammer" that will get them down as long as you don't damage the piles and you do at least one pile load test to confirm the pile capacity.

http://www.PeirceEngineering.com

 

[EireChch]

Agree with PEinc - doesn't matter what Hammer as long you test and know your capacity.

Whether 5-7ft is good enough is anyone's guess. Depends on the soil conditions at that depth (expansive soils an issue?)..dia of the post etc.

If it's clays then you could use an auger attachement for a typical excavator and use a bored pile with rebar cage tied into footing foundations.

 

[Ron]

Most micro or mini piles are not driven....they are either hydraulically pushed or hydraulically screwed in place (helical anchors, for instance). The "capacity" desired is measured by the hydraulic effort of each process. This process is done for a variety of reasons, most notably limited access to existing conditions and minimizing vibration effects that you would have with a driven pile.

Yes...they have limited capacity...but that's a given. You set the spacing of the piles to accommodate the needed load. I've used them to restore foundation settlement issues in both sands and clays. Spacings varied from about 3 feet apart to 6 feet. If done properly and with adequate knowledge and forethought of the existing conditions, they are effective. Keep in mind that these systems are used most likely because you are out of other options, so use appropriate safety factors and don't promise that they will solve the world's problems (or even the one you have!).