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





RE: The Use of Pin Pile (or Mini Pile) Below Shallow Foundation
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?
RE: The Use of Pin Pile (or Mini Pile) Below Shallow Foundation
Dik
RE: The Use of Pin Pile (or Mini Pile) Below Shallow Foundation
www.PeirceEngineering.com
RE: The Use of Pin Pile (or Mini Pile) Below Shallow Foundation
Dik
RE: The Use of Pin Pile (or Mini Pile) Below Shallow Foundation
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.
RE: The Use of Pin Pile (or Mini Pile) Below Shallow Foundation
Dik
RE: The Use of Pin Pile (or Mini Pile) Below Shallow Foundation
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.
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RE: The Use of Pin Pile (or Mini Pile) Below Shallow Foundation
RE: The Use of Pin Pile (or Mini Pile) Below Shallow Foundation
More info OP please....
RE: The Use of Pin Pile (or Mini Pile) Below Shallow Foundation
www.PeirceEngineering.com
RE: The Use of Pin Pile (or Mini Pile) Below Shallow Foundation
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.
RE: The Use of Pin Pile (or Mini Pile) Below Shallow Foundation
www.PeirceEngineering.com
RE: The Use of Pin Pile (or Mini Pile) Below Shallow Foundation
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.
RE: The Use of Pin Pile (or Mini Pile) Below Shallow Foundation
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!).