Hammer size and Bearing capacity of pile 1

[Shakta]

I am debating which hammer size should be more practical in this scenario. A concrete square pile need to be driven to 50' below ground. Mostly silty-sand material. It is reported that shaft resistance of the pile 450 kips and end bearing 600 kips at 50' depth, so total 1050 kips. If we consider the total 1050 kips for hammer selection it should work. However the question I have that if 600 kips or something over 600 kips (not total 1050 kips) were consider for hammer selection, will that work? As the skin friction will be disturbed during driving. Do we really need to consider 1050 kips? Your thought or any reference. Thanks!

 

[SlideRuleEra]

You are right that skin friction is much reduced during the driving process.

Hammer selection is normally made by the Contractor as part of his means and methods. The Engineer typically has the right to review and accept / reject the Contractor's hammer selection... but the Engineer's review is often (but not always) just a formality. A qualified pile driving contractor should have far more insight than the Engineer concerning the capabilities and limitations of various available pile hammers.

 

[Shakta]

Thank you so much SlideRuleEra. Agreed with you.
Also would like to get some idea from contractor's perspective. Is there any rule of thumb or some tips on it.

 

[SlideRuleEra]

Shakta - It's been many years since I drove piling as part of the work constructing a bridge, but will try to revisit that time, simplify and focus on the basics.

Pile hammers are sized by their energy rating (foot-pounds). There are three basic operating principles for large pile hammers:

Single Acting (Pile impacted with a free-falling weight).
Double Acting (Pile impacted with a weigh that is accelerated "down"... instead of just free-falling).
Vibratory (Pile is "shaken" by the hammer, causing it to penetrate soil). Vibratory hammers have unique characteristics that may not be suitable for permanent piling. They are great for temporary piling that a contractor may use as part of his means and methods.

There are several potential power sources (steam / compressed air, diesel, hydraulics, etc.).

Each type hammer has advantages and disadvantages. For general purpose, heavy duty pile driving, the single acting principle is preferred by most contractors... this is what we used. The following comments apply to single acting hammers:

The goal is to use the most powerful hammer practical that will not damage the piling. A concrete pile, like you are considering, withstands a powerful hammer very nicely. A contractor has to compromise on this goal often. Problems with hammer availability, hammer weight, cost, etc.

Another goal is to have the weight of pile hammer "ram" (the striking part) equal or exceed weight of the pile... hard to do for a large concrete pile. This ties into the following goal:

When comparing hammers with the same energy rating, pick the one with the heaviest weight falling the shortest distance. The goal is to have the impact of the ram and pile to be at as low velocity is possible. (Actually, the ram strikes a "follow block" that is positioned on top of the pile... but we will ignore that for this discussion). A low velocity impact allows more complete energy transfer (moving ram to stationary pile) during the time (milliseconds) that they are in contact. Many engineers are of the mistaken belief that a high-speed, elastic impact is preferable... this is just plain wrong and if fairly easy to disprove by looking at examples of actual pile hammer specs.

Another goal is to have the hammer operate at a high blow count (blows per minute). Ideally, a pile would be "pushed" continuously into the soil... not possible with an impact hammer.

As you can see, there are a lot of compromises and the contractor has to balance all of these (and few more) to select the best available hammer for a project.

Fifty feet deep is a lot to ask of a pile driver. Depending on the design and use of the pile, you may want to consider allowing the contractor to earth auger a slightly undersized hole for much of that depth. The last several feet should be driven without earth augering.

 

[Settingsun]

I also like a good rule of thumb but hammer selection is a highly variable area. FHWA document FHWA-NHI-16-009 Volume 2 gives a minimum ram weight of 1-2% of the nominal pile resistance as a starting point and a minimum hammer energy guideline that depends again on the pile resistance. These are in section 15.19 of the document. Probably not worth getting fancier given ask the variables involved.

Your local bridge branch may also include 'local' guidance in their specs.

Whether 50 feet is a big ask depends on the ground conditions. I worked on a highway job where several bridges had concrete piles to 100 feet. They were for about 350 tonne ultimate design load, so around 700 tonne ultimate geotechnical capacity. Same order of magnitude as yours.

Contractors will mobilise a hammer to prove the pile capacity at end of drive IME. They don't want to come back to restrike.