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.

 

[aj_k]

Thank you for all your reactions.

The layer in which the necking was occurred has a qc value of approximately 0.35MPa and a low friction ratio.
We are not yet sure about if the vacuum was created below the tip while extracting.
There was no casing used. the mandrel/ flighting and grout were used to stabilize the bores.
There was no heaving of the soil found while casting and after casting.
But the centre to centre distance between the piles was not satisfied based on the undrained shear strength of the weak layer. Can this be the only reason for necking in all piles?

 

[msucog]

Withdrew too fast. Or throttled the pump back at that level and grout flow stopped. QA should have strokes and rigs should have AME/PIR to verify. It's possible they sucked down the pile on the next one and topped off later...QA should be catching that and reporting. Wrote this before reading all the replies, so that's my take and think many of you are of same opinion now that I just read a few replies. In the end, the contractor should be responsible for their work.