sizing a drop hammer using the FHWA Modified Gates Formula 2

[foreng]

I’m looking for a rule of thumb for sizing a drop hammer using the FHWA Modified Gates Formula. One suggestion is to size the hammer so that the pile reaches the desired capacity at approximately 3 to 10 blows per inch. The project I'm working on requires a pile capacity of 142 kips, with a 4000 lb hammer and say a 5 ft drop the required blow count is 1 blow/in. Should I reduce the hammer size? or does this sound reasonable.

Due to the limited scope of the project, no PDA will be used.

 

[DRC1]

It has been a couple of years since I told this story, so it is probably okay to tell it again.

A drunk is wandering around a lamp post on his hands and knees. A cop comes up to him and asks him what is he doing. "I am looking for my wallet" he says. So the cop starts to help. After a few minutes the cop asks "are you sure you dropped here?" "No" says the drunk, "I dropped it somewhere down the block" Then why are we looking here?" asks the cop. To which the drunk replies "Because I thought the light was better here."

This story reminds me of hammer formulas. They have no good basis in science, vary widely in accuracy, were coralated to a particular set of conditions, what are used with out regard for those conditions, and really give us little useful information, yet we continue to use them because the are easy to calculate.
I sugest running a wave equation analyis to find your answers, not only will it give you bearing capacity, it will tell you if the hammer is sutiable for the pile, and at what blow count you might expect pile damage.

 

[vulcanhammer]

I'm with DRC1. The Gates formula is good to give you an idea of the first hammer to input into the wave equation.

Wave equation program (with documentation) can be downloaded at

http://www.vulcanhammer.info/drivability/weap-spile.php

http://www.pz27.net

 

[foreng]

Unfortunately in most cases the scope of the project precludes any geotechnical investigation (small temporary bridges), which makes DRC1's comment about the drunk even more pertinent.

When making any initial sizing with Gates what would be a good rule of thumb to target final set blows/in with a drop hammer? As mentioned above I’ve seen 3-10 blows/in mentioned in some literature.

 

[DRC1]

I would perfer to see at least 4 blows to the inch, just because, it is easy to get an accurate blow count at the final drive. Some times its hard to know wher the blow starts and stops and the 1 -3 blow to the inch can be difficult to be accurate. Otherwise, blow count and hammer size depends on a slew of factors - Are you looking for skin friction or point load, are you bearing in soil or rock? How much load is in the piles? are they plumb or battered? And often, the most important, what hammers does the pile driver own?
If you can do a gates formula you can do wave equation. The more information you have the better it will be, but with a few good assumptions, you should be able to do better than a hammer formula. I would talk to a local pile driving contractor. To find one in your area, contact the PDCA (pile Driving contractor Association) at

http://www.piledrivers.org/

The members want to promote pile driving and should be able to help you.

 

[foreng]

Thanks again DRC1 for the advice, I'll do that.

 

[vulcanhammer]

I wouldn't go past 10 blows/inch. That's in part the experience of a hammer manufacturer (that was our warranty for the onshore equipment) but it also reflects the simple fact that, with the higher blow counts, more and more energy goes into the hammer-pile system and less into the soil, creating a "diminishing returns" situation and increasing the likelihood of damage in same hammer-pile system.

4 blows/inch is a nice range to operate in.

http://www.pz27.net

 

[foreng]

Thanks vulcanhammer for the input. I've talked to my local geotech about wave equation without soil data and he was not interested. I'll keep looking.

 

[lawayosh]

Small bridges? In stream beds you have to consider what is visible and below the surface to determine the type of pile material necessary to be able to drive them at all. Cobble laden bottoms can be unfriendly to timber piles. If the pile lengths aren't too short, as a lot of short pieces may not be practical to salvage afterwards, steel H piles might be worth considering as they could be pulled with a vibro and not leave anything behind.

Without soils data you don't know what kind of bad stuff could be lurking below or bearing capacity that you can expect.

 

[lawayosh]

Small bridges? In stream beds you have to consider what is visible and below the surface to determine the type of pile material necessary to be able to drive them at all. Cobble laden bottoms can be unfriendly to timber piles. If the pile lengths aren't too short, as a lot of short pieces may not be practical to salvage afterwards, steel H piles would be worth considering as they could be pulled with a vibro and not leave anything behind.

Without soils data you don't know what kind of bad stuff could be lurking below or bearing capacity that you can expect.