necking in piles

[aj_k]

what can be the possible causes of necking in piles (displacement piles) in clay layer? the clay layer is about 5 metres depth, but the necking was found to be in the top 1.5m depth.

 

[EireChch]

You will have to explain what type of pile you are using and what the ground conditions are (i.e. layering and strength)?

Necking is typically a structural flaw in cast in place pile. But this isnt a displacement pile..its a re-placement pile.

Poor concrete placement? Did you use a tremie pipe?

 

[aj_k]

hi.. thank you for your reaction.
we have used atlas piles( full displacement piles).

the soil condition: sand layer- for first 2 m depth
clay -for the next 3.2 m depth. (undrained shear strength of average 35kpa.)
silty sand (with few clayey silt layers)- for next 6 m depth (undrained shear strength of 40kpa)

there were around 260 piles constructed (completely fully displacement piles) and almost all the piles had necking.
there was also overconsumption of concrete for almost all piles by 5% and more approximately.

but the centre to centre distance was not properly followed.

the concrete was placed with a tremie pipe.

 

[EireChch]

An Atlas pile seems similar to a CFA, and for some reason I forgot about those. Too long working with bored piles now, but you are right it is a replacement pile.

Its hard to give a reason tbh. How was your QA/QC procedures, did you have a site engineer observing, taking samples, preparing a pile construction record sheet (which shows actual toe level, cut off level, concreted length, casing length etc.)

Not much use to you know, but always a good way to approach pile construction.

Regarding your necking, does necking occur at approx 1.5m in all the piles or does it vary?

Its close to the interface between sand and clay layer. Is the auger pulling some clay material into the concrete (at the interface between the two) on extraction of the drill bit? Im thinking out loud here....

Wouldnt be concerned with 5% over consumption...

 

[aj_k]

The site engineer observed everything and all the data were recorded.
The necking was observed at a depth of 1.75 m depth to 3.5 m depth from the ground level and it was the same for all the piles.
The necking is in the clay layer which is close to the interface of the sand-clay layer.
For all the piles, there was the overconsumption of concrete which varied from 11% to 33%.

 

[oldestguy]

Is that an expansive type clay? Relieve overburden pressure and will it expand?

 

[aj_k]

No, it is not an expansive clay.
it should be noted that the groundwater level is 1 m below the ground level.
Does the reduced centre to centre spacing of piles cause necking?

 

[jdonville]

@aj_k,

For those unaware, 'necking' refers to a contraction of the as-constructed diameter of a bored pile, usually to a dimension below the specified nominal or minimum required diameter for the element.

Please confirm that the type of pile you are referring to is similar to this:

https://franki.de/en/services/displacement-piles/atlas-pile/

.

If so, then the piles (according to the graphic at the link) are constructed by screwing a displacement mandrel into the soil to the target depth. The displacement head is larger than the mandrel shaft diameter. Reinforcement is installed through the mandrel. Concrete is then placed through the mandrel and the tool withdrawn, creating a ribbed profile at the pile surface to promote enhanced skin friction.

In this case, you variously describe the grout take as being 5% or 11 to 33% above expected. First, there is a great deal of difference between these two ranges. Second, if there was a necking problem, I would expect the grout consumption to have been generally lower than expected rather than higher.

How was the extent and location of the necking determined? PIT testing? Exhumation/excavation of the pile top?

My guess is that the proximate cause of the necking could be sloughing of the near surface sandy soils into the concrete/grout. Proximity of piles C-C seems not to be an issue, but hydraulic communication could have drawn down the concrete/grout in an adjacent fluid pile and exacerbated the sloughing whilst hiding the fact that such sloughing might have occurred.

 

[emmgjld]

I am having a hard time getting all the given info to 'jive'. Please describe this 'necking', as I would think that less concrete/grout would be required, as questioned by jdonville.

 

[aj_k]

Thank you for your reactions.
The type of pile I am referring to is atlas piles, which are full displacement piles.
There were around 300 piles constructed and 44 piles were excavated which showed necking. And the rest of all the piles showed anomalies when a pile integrity test was done.
The concrete consumption data provides overconsumption of 5% to 33%.
The necking is determined by excavation.
First layer: sand (2 m thick)
The necking was seen in the second clay (which has a thickness of 3.2m thickness)layer.
And the necking was found in the first 1.75m thickness in this clay layer. The necking range of all piles was in this range of depth.
Sloughing: there was no heaving of the sand observed at the time of construction.
Can there be some other reason which caused necking?

 

[jdonville]

Please describe in detail the installation methodology and/or provide a 'storyboard' that illustrates the construction sequence for the type of elements that you are referring to. We would like to help, but are obviously confused by the nomenclature that you are using. The naming you are using may not be as common as you assume. Please also indicate where this project is in the world, as many of us are based in North America and may not be familiar with practice in other regions.

The only other type of foundation that I am aware of that uses the trade name 'Atlas' are hydraulically installed steel displacement piles with a circular pipe section and closed end.

Again, the grout takes indicate that in addition to the necking there was likely some bulging at a lower elevation that overshadowed the necking issue.

The reason for why the necking occurred is important to understand, but there does not appear to be any "smoking gun" in your summary. Since it was observed at nearly all the piles (according to your summary), I would suggest that the Contractor's means and methods to provide adequate support of the surrounding soils during all phases of the construction should be looked at carefully.

 

[MTNClimber]

Atlast piles are not used where I practice so I don't have experience trouble shooting them. Maybe they retracted the mandrel too soon, before there was a proper head of concrete over the layers which are showing necking. Although there aren't dimensions posted on Franki's website, I would assume concrete take would be highly variable since the graphics show the mandrel head has large diameter flights. Not sure if concrete take will actually tell you much.

How bad was the necking (design vs actual diameter)? What's the contractor's opinion on it?

 

[aj_k]

Thank you for your reactions.
The piles are atlas piles, full displacement piles.
The project is in the Netherlands.
There was no casing used, not even a temporary casing.
The overconsumption shows there should be bulging at lower depths, as we assume.
the diameter of piles is 560mm/460mm. and the necking diameter was found to be 240mm, some piles are crushed.

Manufacture.
A steel pipe onto which a replaceable auger head is fastened at the bottom is pressed into the ground in a screwing motion. Due to the auger head, the soil is laterally displaced or compacted. The auger head is sealed water-tight with a lost closing nut tip.
The auger head and the pipe are screwed vibration-free into the ground using a rotary drive with simultaneous pressure. The hydraulic drive pressure is measured when it is screwed in and compared with the foundation soil information, such as the drilling profiles, sounding diagrams, etc. After achieving the required drive pressure, the reinforcing cage is inserted. The pipe and the reservoir are filled with concrete. The pipe and the auger head are withdrawn by screwing them backward. During this process, the base tip is released and the auger head forms the thread-like pile. The concrete column in the pipe and reservoir with its large static overpressure immediately fills the released cavity with concrete. Therefore, a spiral concrete bulge approximately 5 cm in thickness is formed in the ground around the pile shaft.

Soil profile:
first layer: sand (2m thickness)
second layer: clay (3.2m thickness) - necking occurred in this layer.
the lowest Cu value(undrained shear strength) found in this second clay layer is 25kpa.
According to the site contractor, the concrete head was maintained properly.

 

[MTNClimber]

What’s the pile spacing and how long did the contractor wait until drilling the adjacent pile?

 

[aj_k]

Thank you for your reactions.
Half of the piles were not cast according to the c/c distance, which requires 9D (according to the Cu value =25). Also, these piles were cast adjacent to each other within a time limit of fewer than 4 hours.
Half of the piles were casted with satisfactory c/c distance from the adjacent piles.
But when we excavated, the piles which cast with c/c distance satisfied and not satisfied were found to have necking.
Still, we could not find what could be the cause for the necking in the piles with c/c distance satisfied.

 

[aeoliantexan]

I am not experienced with that type of pile, so this is s guess: Perhaps the concrete or grout pump was shut off too soon so that a vacuum was created below the tip as the extraction was completed. This "sucked" the sides inward in the weakest layer.

I can't explain the excess concrete consumption, unless the weak soil was pushed outward by excessive concrete pressure during the removal process. The two problems may be related only by the weak clay soil.

 

[r13]

I think the lateral pressure due to fluid-like concrete had exert a pressure that was well in exceeding the compressive strength of the silty sand, resulting in bulging to occur in that layer. If you have data on the silty sand, you should be able to do a quick check on that. The budged segment will draw more fluid-like concrete from above, resulting in drag and/or negative pressure (similar to pipe flow) on the clay layer, and cause the layer to collapse. The speed of tremie tube withdraw/lifting could be an influence factor too.

I am not a trained geotechnical engineer, please kindly point out any mistakes in comment above.

 

[Ron]

The most common reason for necking in wet placed concrete piles is sloughing of soils or lateral soil pressure. The static head of the concrete must always be greater than the lateral pressure so that this does not occur. Sometimes during placement, the concrete in the tremie does not have enough head (depth of concrete in the tremie) to overcome the sloughing or lateral pressure and the result is necking.

 

[r13]

Given the high water table and presence of the sand and sandy layers, in absent of casing, what was used to stabilize the bores?

 

[jdonville]

@retired13
The way I understand from the initial description, the mandrel/flighting and grout was used to stabilize the bores. This is done with augercast technique also.