Pipe Pile Plugged and Cored Conditions questions 1

[irishengdave]

Hi There,

I am researching how to determine the axial capacity of open ended tubular piles in primarily cohesive material. The methods I have read about say you should check the plugged and unplugged conditions.

1. The plugged capacity = Outer skin resistance + Base resistance of a closed ended pile - weight of soil plug.

In order to calculate the weight of the soil plug I have to determine its length. How do I do this?

2. The unplugged or 'cored' capacity = the outer + inner skin resistance + the base resistance of the annulus of the steel pile.

Question is do you need to check the soil stress at the bottom of the internal soil column to ensure the soils compressive capacity at the pile tip is not exceeded.

I cannot find and good design exapmples online but am trying to get my hands on a couple of technical papers such as the API method, ICP method and NGI methods that A-pile software uses to do the calculation.

Any guidance or clues appreciated!

 

[SlideRuleEra]

At least on land... not meaning to be flippant, couldn't you lower the weighted end of a tape measure inside the driven pipe pile until it contacts the top of the plug? Knowing the pile length and using subtraction, the difference would be the length of the plug.

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[irishengdave]

Sliderule, not at all its a legitimate question..

I need a way of calculating the target depth of the pile before the job goes to site. We are talking about a very large contract with many piles.

To do it in the field you would need to know whether a true plug has fully mobilized, which would be impossible to do by visual observation. There is a formula by Tomlinson to do this in the field.

 

[SlideRuleEra]

Google on "steel pipe pile soil plug", there is quite a bit on the web on the subject. Likely some research there that will help. Here is one paper that came up:

Effect of Soil Plugging on Axial Capacity of Open-Ended Pipe Piles in Sands

Edit: No matter how the calculations are made, for a large project it may be a good idea to have a stand-alone test pile program before pile design is finalized. On our green-field electric generating stations we did exactly this. A contract for driving (or drilling) instrumented test piles was used to verify preliminary pile design. This contract required the pile driver to work in conjunction with a qualified testing lab. The test results were used to finalize the design. The savings realized by having a firm basis for the design of several thousand piling required for each project, easily justified the up-front cost of this contract.

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[LRJ]

Driven pile plugging is a dynamic problem rather than a static problem, so predictions of plug length are quite difficult. Add to that phenomena like arching and compaction of the soil plug and you've got yourself a very complicated problem. Nevertheless it still seems to be standard in industry to calculate a fully-plugged pile capacity and fully-unplugged pile capacity and then use the lowest capacity given (i.e. path of least resistance). However, that standard practice won't typically include the weight of the soil plug in the pile capacity calculation - the weight of the plug usually gets included in the design axial load from the structural engineer.

What I would say is that if you have a circular pile with an internal diameter greater than approximately 1.4 m then you can count on the pile being unplugged. I've known PDI (creators of GRLWEAP and engineers with extensive pile driving and monitoring experience) provide this recommendation though I can't remember the formulation of their calculation (which involved inertia force amongst other things - maybe it's published somewhere?).

Regarding the capacity methods you state, note that the ICP method doesn't have an internal friction component in the calculations: it is just external friction plus the end bearing component.

If you're looking for a method which includes dynamic properties in the calculation then the method presented by Dean and Deokiesingh (2013) titled 'Plugging criterion for offshore pipe pile drivability' looks quite good. This method includes dynamic properties from pile driving to determine whether plugging occurs. I've never actually used this method, however, but I recall it looked quite reasonable.

With respect to your question, well your calculation of capacity is essentially performing that check. Or do you mean to ask whether you should perform an in-place analysis? If so: yes. Normally you'll find that the Q-z (end bearing) spring is mobilised less than the t-z (friction) springs.

Hope the above helps!