Driven Pile vs Vibrated Pile 8

[pelelo]

Engineers,

I am working on a small project that involves driving few H piles, 40 ft embedded to support a work platform.

One of the contractors asked is there is any problem if instead of driving the piles, they are vibrated. Even though they are different construction procedures I think they will provide the same result: install the pile on the ground, at the required depth.

I recommended piles being driven because that is what I have seen in the past. At the same time I can test the piles during the driving procedures (using the PDA).

But that question made me think what would be advantage or disadvantage of vibrating the piles instead?. To my knowledge a vibrated pile can not be tested (PDA), am I right?.

FYI, the stratigraphy is 5 feet of muck (N<5), then 5 ft of loose sand(N between 6 and 10), then for the next 40 ft, medium dense sand (N between 20 and 30).

Please let me know your thoughts. Thanks

 

[oldestguy]

Without full information, such as loads expected, and seeing it is not a fussy job, go for it either way.

 

[pelelo]

Axial load ~ 25 Kips
LAteral Load ~ 4 Kips
Uplift 10 kips

Do you think you can test a vibrated pile?

The only advantage I see if is driven, I can test it while is being driven. If vibrated, I don't think I can.

 

[oldestguy]

Testing load capability by standard methods usually involves a few more piles for support or a reaction to the load imposed. So how the pile got there does not change that. We are not here to interpret your boring log for estimating pile capacity however. I don 't think the type of job justifies a load test. You can estimate all you want, but it still is an estimate.

 

[PEinc]

Maybe you can try vibrating the piles in with a backhoe-mounted vibro and then, when they are almost fully installed, finish them off by driving, to required capacity per a dynamic formula, with a small air or diesel impact hammer hung from the backhoe.

http://www.PeirceEngineering.com

 

[SlideRuleEra]

One of the contractors asked is there is any problem if instead of driving the piles, they are vibrated.

Vibratory hammers are quick and quiet (compared to impact hammers). Are either of those properties an advantage on the project?
If speed is an advantage, is the Contractor offering to share labor/equipment cost savings with the Owner?
If reduced noise is an advantage, a vibratory hammer may be worthy of consideration.

I've allowed a vibratory hammer for permanent piling on one project. Speed was important, and we (electric utility) were the Owner, Engineer of Record, and Construction Manager. Also, the piling were point bearing and design loads were light plus pile driving characteristics for the site were well established.

Vibratory hammers are great for temporary piling... if the pile can be vibrated "down", the same pile can almost always be vibrated "up" (extracted) when no longer needed.

IMHO, don't even consider risking your reputation (just to help a Contractor) unless there is benefit (speed, reduced noise, or lower cost) for the project.

http://www.SlideRuleEra.net

 

[HENRYZAU]

Vibratory hammer tends to cause less shaft resistance than impact hammer. Brown method from my memory recommends about 30% reduction in shaft resistance if vibratory hammer is used.

 

[Okiryu]

I think if the contractor is proposing this change, they should make the effort to proof the design capacity and find a hammer to do a PDA. We have forced this when a contractor wanted to change from driven piles to grouted piles (predrilling and fix the pile with grout). If the change benefits the contractor, they will find the hammer for the dynamic test.

 

[BigHarvey]

Only drawback is that there is no way to check bearing capacty with driving formula as for driven pile. We generally drive the pile fast with vibratory hammer and then use an impact hammer for the final set. Make a load test if you use only vibratory hammer.

 

[MTNClimber]

I've seen this done on a lot of jobs. Like others have stated, finishing off the piles with an impact hammer and PDA testing at least one is pretty standard in my book for permanent piles. I've seen temporary piles vibrated in without proving capacity with an impact or PDA test, but the contractor was accepting the risk in those situations.

Another benefit of vibrating is the ability to extract the piles if there are obstructions.

 

[BridgeSmith]

With a 40ft embedment depth on an H-pile, even with a very conservative estimate for side friction, it should be adequate for the axial and uplift loads. Only the resistance to lateral load, or more specifically, the lateral deflection, would concern me. I'm not really familiar with the effects of the vibratory installation, but I think I remember reading that the material around the pile, especially near the top, ends up being fairly loose. If that is true (which hopefully some of the geotech guys can confirm or deny), the deflection under that 4 kip lateral load could be significant.

 

[BridgeSmith]

For a small project, I can understand the contractor wanting to avoid using a standard hammer. The mobilization costs are high compared to the vibratory. As PEinc noted with the picture, a vibratory hammer can be easily attached to a trackhoe. All the diesel hammers I've seen for driving H-piles required a lead cage suspended from a crane. We usually see a $5-10k fixed mobilization cost for driving piles for our bridge foundations.

 

[SlideRuleEra]

HotRod10 - See "Comparison of Impact vs. Vibratory Driven Piles", published by the Deep Foundations Institute. Per the report and as you have assumed, acceptable lateral loading for piles installed with a vibratory hammer is the "wild card". Using an impact hammer for driving the last few inches will not help with lateral loading.

With that said, a pile with 40' embedment, 30' of that is in "medium dense sand", certainly sounds like a very conservative design. The 1930's rule of thumb for embedment of cantilever sheet pile (with very high lateral loading, of course) is embedment length equals cantilever. No matter what equipment (impact or vibratory) is used for pile driving, I would expect an H-pile with 30' of meaningful embedment to handle 4 kips of lateral loading (applied at any reasonable elevation) with no problem.

Also, we don't know how much of the H-pile is cantilevered above the 40' embedment. Total pile length is most likely >40' and possibly >>40'. Assuming the piles are on more or less level ground, I would want a crawler crane to accurately position, then stand up a pile with total length >40'. Then put a hammer (impact or vibratory) on top of the pile... that's going to take a fair amount of boom. The crane may (or may not) have to boom out a fair distance from the crane's location to reach the pile... more boom.

Bottom Line: Highly recommend the OP revisit the 40' embedment requirement. If that embedment depth is just for CYA, it's going to cost the Owner a bundled of (wasted) money for larger than necessary equipment and excess pile driving labor.

http://www.SlideRuleEra.net

 

[BridgeSmith]

Thanks for the confirmation, SRE. As I said, I would be more concerned with the displacement of the top of the pile than the strength, assuming, as you said, that the 4 kips is applied anywhere near ground level. I agree with the 40' seeming excessive.