Consideration of Weight of Soil Plug in tubular Steel pile

[Angre]

Hello All,
In the case of open ended, tubular steel, friction piles, should we consider the weight of soil plug, inside the pile, as demand on pile ?
Can it not be argued that the soil has done necessary readjustment to itself,whilst driving and thus the weight of plug has the same status as soil outside the pile
and demand on the pile is purely from superstructure?
Rgds

 

[MotorCity]

The weight of soil is the same before and after the pile is driven, so there is no net increase in load due to the soil.

If the soil is clay, I suppose you could make a case that some portion of the clay "sticks" to the pile wall, and therefore transfers the soil load from the bottom of the soil plug to the wall of pile. I would probably assume this effect to be negligible on pile design.

I would be interested to hear from any geotechs out there if this does in fact occur and if so, is there any significant impact to design?

 

[SlideRuleEra]

Soil (clay) "stuck" between the flanges of a driven H-pile could be a reasonable analogy to the soil plug in a pipe pile. The book "Bethlehem Steel H-Piles" talks about soil between the flanges:

When an H-pile is driven... into clay, quite often the soil trapped between the flanges and the web becomes so hard due to compression that it is carried down with it. The pile therefore becomes in effect a displacement pile, and the core of soil trapped on each side of the web performs the same function of displacement that timber lagging serves in softer silts and clays. In such soils H-pile will develop very satisfactory load carrying values.

From that statement, IMHO, the soil plug is important for developing load capacity - in that type soil. Not a demand on the pile but a contributor toward improved performance.

You can download the Bethlehem book at this page of my website, see page 13: Link

http://www.SlideRuleEra.net

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[Angre]

Hi SlideRuleEra and Motorcity,
Thank you guys for the suggestions and SlideruleEra for kindly sharing the resources on Pile Design.

The effect of self weight is not negligible, as I have a large dia pile with a lengthy penetration.Let me illustrate my case with numbers.

I have a pile that is 1.8m in dia and 90 m deep pen in soil.
The weight of the plug is pi/4 * 1.8m^2 * 90m * (2.00-1)t/m^3*2.00(F.O.S) = 457 MT.
Which is quite substantial increase in demand on pile.

I have not come across anything in literature and geotech books which deals with this topic in some detail.

I have an argument developed to justify NOT taking the weight of plug.

My argument is that self weight of soil and plug gets adequately supported by rearranging the soil matrix during driving.
It is as if I have manufactured a big piece of rock at foundation point.
If indeed there was real rock at foundation I dont take the weight of rock in determining the demand on the foundation.
In short Weight of plug is supported by soil matrix rearrangement by driving, superstructure loads are supported by "pile soil composite structure".

Rgds
Angre

 

[SlideRuleEra]

Angre - If the soil plug is 90m long, and the pile is driven to a depth of 90m, that sounds like the plug remained stationary during (impact?) driving. The pile would have just moved through the soil without altering the plug. If that is the case, during driving, the plug would have increased friction and slowed the driving process.

After driving, the weight of plug may neither create a the load on the pile nor contribute to support of the load.
Am I misunderstanding something?

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[Angre]

Hi SlideRuleEra,

You are right that if plug is formed at the toe, soil will not enter the pile.
This though is not easy to ascertain(and convince others).We cannot justify as to when and when not a plug gets formed.
So to be on safe side, it is assumed that all of the pile is filled with soil.

Rgds
Angre

 

[Retrograde]

A 1.8m diameter pile is unlikely to plug, although the soil in the tube will get dragged down to a certain extent.

 

[SlideRuleEra]

Angre - Wow! A 90m long plug is very conservative... and potentially a costly assumption if weight of the plug is considered demand on the pile. If a large number of these piles are required, I suggest driving an identical, expendable index pile. Open the top of the pile and measure the distance down to the top of the plug. IMHO, that would be money well spent.

Edit: Another option could be to ask Dr. B.H. Fellenius for his opinion. I see on his website that he does respond to emails: Link

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[BigH]

Have you actually driven a pile and measured the "free" depth to the top of the plug?

 

[Ron]

Driving a pile of this size is similar to a vibratory deep compaction method known as TerraProbe. The effect is the same. The plug within the pile becomes more compacted (as was hinted by BigH... ). This increases the lateral stress on the pile from the soil, thus increasing friction on the inside face of the pile. Now you have friction resistance on the inside and outside; however, I would neglect the friction on the inside for design purposes.

So to answer your question, the plug weight may be neglected.

If you would like to test this, do a cone penetrometer in the center of the pile before driving and after driving to determine the difference. Cheap compared to driving a 90m pile!

 

[Angre]

Hi Retrograde,
The formation of plug cannot be ruled out.
Such "plugs" can form even in diameters as large as 20m.Iam thinking of cement silos and design of silo walls.
Rgds
Angre.