Open End Pipe Pile Plug Length

[PEinc]

Does anyone have any information on or a reference for the minimum length of soil plug in order for the entire pile tip area (including the plug) to be used in calculating the point bearing capacity of the pipe pile? The open end pipe pile will be driven through soft organic silt, through a sand layer, to a very dense sand, gravel, rock fragment stratum where it will achieve its capacity almost immediately (but not before).

 

[Focht3]

The best references were published in the Proceedings of the Offshore Technology Conference - held in Houston every spring. I don't have copies, so you'll have to look around for a list of published articles.

In essence, you want to make lower- and upper-bound estimates of the soil friction acting on the inside pile diameter during pile installation, and compare that to the ultimate bearing force on the soil in the end of the pile. The plug won't form until the total soil friction force exceeds the bearing force on the soil plug. Be warned: you should get a range of answers, which will probably vary quite widely.

This is becoming kind of a lost art since PDAs have come into commonplace use. But it is still an important factor to estimate -

As a side note: why not use close-end piles?

 

[PEinc]

Closed end piles? It's a LOOOOOOOOONG story! The engineer is very worried about displacement and vibrations, among a few other concerns. He's very nervous and seems to be trying to dump everything on the contractor in the form of a VE proposal.

FHWA has some guidelines for the plug. Basically, they're saying that in sands plugging generally begins at a pile penetration of 20 diameters but can be as high as 35 diameters. For soft to stiff clays, it's about 10 to 20 diameters.

FHWA's Driven pile design program assumes that during driving of the open end pile in granular soil there is no plugging. However, for the static (restrike) or long term (ultimate) conditions, 30 diameters of penetration is required for assuming full tip bearing.

The engineer thinks the pile has to be driven 20 diameters into a very, very, hard (N >> 50 to 100/1&quot granular layer in order to develop a plug. He is ignoring the sand layer(s) above the bearing layer. WEAP analyses using conservative soil values for the bearing layer show the pile can't be driven more than a foot or two into the layer without damaging the pile with a larger hammer. It'll never go 20 feet.

Most references I read about open end pipe piles (and H-Piles) say that the plug forms - period, no penetration requirements. Other references talk about concreted open end piles where the plug keeps being removed as the pile is driven. This is not what the involved parties are leaning toward.

The more I read, the less clear the issue becomes.

 

[BigH]

I believe you are right in that there will be very little penetration into the very dense granular layer. If these values are true, this is almost akin to weathered rock. An effective plug should develop within a very short distance into the very dense stratum. Overdriving is to be avoided.

I know that most engineer's are always of the opinion that driving equations are unreliable, but I remember a paper by Flaate and Olsen (1967 or so) with their or someone else a few years later doing a follow up that did statistical study - you might find these (ASCE SMFE Journal) and check to see if there is open ended pipe piles. This statistical study did some "revisions" of formulas. When you get a pile into the very dense stratum, make an estimate of the penetration likely for your projected hammer size (Gates Formula is rather an easy one) and see what you capacity is. Use a SF of 3 or 4 and use this as your allowable capacity.

Also suggest that you refer to Chellis' book. While "old", it is extremely practical with "real" problems and you might find a discussion on this very topic. Also, see M.J. Tomlinson (Viewpoint Press).

Also to consider - if your sand stratum is sufficiently thick and reasonably compact, you will develop considerable skin friction in this stratum. Using conventional analyses, I usually found that the allowable bearing pressures were in the same order of the frictional component with the safety factor being held in the base resistence. Given that base resistance needs much more movement to be mobilized, you might consider to use the calculated frictional resistance as your allowable load and whatever end bearing as your SF - driven into the very dense stratum until practical set refusal should ensure plug. Compare to estimates from likely pile driving formulii taking estimated sets (say 5mm/blow) with adequate safety factors, and structural capacity of the pile and make a judgement. The problem is, many times, that analyses just don't give you what your "gut" says (which, with experience is many times "right&quot.

Best regards.

(note: I am new to this site and just want to say that I enjoy Focht3's most insightful comments on many of the queries posted.)

 

[Focht3]

Thanks, BigH - I try to "think twice before speaking once."

Driving 20 diameters into a material with N = 100/1" is absolutely nuts - it won't happen. The piles will hang, and repeated driving will only damage the pile, hammer, top cap, leads, etc. This is a formula for a lawsuit.

PEinc: you do have a problem - strictly speaking, a "plug" needs to have more friction inside the pile than bearing force applied to the plug "face." But BigH is right - the pile will undoubtedly "hang" within a short penetration of the very dense layer. (I saw this happen on a load test for a project in Pensacola. I gave the owner a recommended length based on geology and SPT blow counts, with the tip one foot into the dense sand - he extended the test pile by 10 feet to "be sure." Those 10.5 feet of instrumented precast concrete pile were hell to remove after the pile met refusal!) But your piles won't hang because a plug formed and the hammer can't overcome the tip resistance from the plug...

I don't know your geology, but if the materials are silica sands and gravels, then grain crushing begins at about 100 ksf on the pile edge. That is likely to control the pile penetration.

I don't know your problem, but if you don't have significant lateral loads you may still want to consider close-ended piles. Skeptics of the 100 ksf "rim" capacity approach are likely to accept "refusal" on a closed-end pile tip. You'll just have to convice them where refusal will be met!

You'll get about the same vibrations with either end (open or closed) since most of your driving will be through relatively soft materials - most of the energy will go to overcoming side friction; and if vibrations are an issue then driven piles are out anyway. You should specify drilled piers - this will also address the displacement issue. And you can do a much better job of controlling the foundation's geometry and integrity.

 

[PEinc]

Thanks guys, for your input. I already had to submit wave equation analyses for the piles. The piles do no get anywhere near their ultimate capacity until the piles hit the dense layer. The piles have only 5 to 10% skin priction. The engineer is requiring 2 feet penetration into the hard layer so friction is totally out of the question.

I checked Chellis - found nothing there on open end piles. Closed end piles were originally specified and, I believe, that's what will ultimately be installed.

The piles are in cofferdams out in the water. Access is not very easy. Due to vibration concerns, the engineer originally called for pre-drilling the piles to the dense bearing layer using drillers mud. However, DEP got involved, saying the drilled piles were essentially wells which could cause cross contamination of the aquifers. DEP wants the piles grouted if they are pre-drilled.

My job in all of this is to try to keep everyone else from doing something stupid which costs my client money and liability or is detrimental to the bridge.

 

[Focht3]

PEinc -

The new information is helpful. I think that I understand why the designer is so intent on significant penetration into the hard layer. I will assume that the piles you are discussing represent the cofferdam itself. (While this does not seem likely, I'll run with this assumption for now.) Lateral loads are a concern since you are constructing a cofferdam; the materials above the very dense layer don't sound like they will offer enough lateral (passive) resistance inside the cofferdam. Failure to "fix" the bottoms of the piles/piers will make construction of the cofferdam slower and significantly more expensive. And the extra internal bracing that will be required will substantially reduce the working room inside the cofferdam.

Why not use drilled piers that are cased rather than mudded? The casing can be vibrated into place, and the presence of the casing should take care of the aquifer cross-contamination issue. It also reduces the barge space and other handling issues. The bottom of the casing won't be water tight, but you can fill the casing with river/sea water to reverse the piezometric head. A drill rig can advance the excavation the desired 10 or 20 diameters; and the casing can be forced down into the very dense layer a bit after the auger has cut into the very dense layer. This won't work as described, of course, if the very dense layer you are trying to penetrate is one of your aquifers and the river/sea water has poorer quality that the aquifer's water. You still have a contamination issue.

You can drill the piers dry, but this is more problematic. Another solution to satify DEP would be to add potable water, rather than river/sea water, into the casing to reverse the piezometric head. Expensive, but it can be done.

Now, let's assume that the cofferdam exists and we are only discussing the bridge piers. Significant penetration into the very dense layer only appears necessary to "fix" the pile tips because of the expected large lateral loads. I would expect that you could avoid this problem by spreading out the piles and using frame action to pick up the lateral forces. This comes at a price, though: your cofferdam will likely be larger to accommodate the increased pile cap size. You may also run into minimum lane spacing requirements for barge and/or ship traffic. And larger pile caps will likely increase river scour during floods. It's a trade-off between design constraints, time, and cost.

Anyway, let us know the details of what your client decides to do.

 

[Mikeki]

Have a look in Pile Design and Construction Practice by M J Tomlinson. I find the book very useful.