Piling case pulling out reinforcement – Request for advice.
I am working on Phase 2 of the Battersea Power Station redevelopment, inside the power station itself. Within this I am managing the piling package inside the Boiler House, which in way of orientation is the main, central building of the power station, with the towers and chimney in the corners.
I have recently encountered my first issue, and although we have found a solution to allow construction to continue, we have not found the cause. I was wondering if anyone has any previous experience of this and knows the cause, so that hopefully I can prevent it happening again.
The design for this pile had the toe depth located in London clay, and as such the casing was used to seal into the clay, but no further, with the undrained strength of the clay allowing the rest of the bore to be drilled without the need for bentonite support. However, after over drilling beyond the London clay layer and into the aquifer below, and with the bentonite plant not yet operational, further casings had to be used to prevent the Thanet sands from collapsing into the bore. With this new depth a pile redesign was required, resulting in little more than a longer length of reinforcement cage needed, which was spliced on, and the concrete pour went without incident.
A total of 30m of casing was installed on this pile, far more than had been designed for, and it is when the casing extraction began that the issue arose. As the second section (3m) was extracted, the reinforcement cage rose with the casing. The cage had somehow become stuck in the casing and was being pulled out with the casings. Bauer (the piling contractor) were unable to push the casing back down, and after numerous attempts to rectify the situation, Bauer made the decision to remove the entire cage for the pile, and core through the existing pile to form the new one.
Unfortunately the first (top) splice joint in the reinforcement cage failed (it was not designed for tension with a load a wet concrete on it) during the casing extraction, and only the top cage (approximately 12m long) could be removed. After several hours of failing to dig out the cage (I believe the concrete was still too wet) it was left as is. Currently the dip to concrete is 7.3m from ground level and there is 8m of casing on, but lifted above the concrete.
It has been decided to backfill the void and to place a new pile either side, with a capping beam on top, rather than try to rectify this pile. Space allows for this and schedule is the driving factor, with this being put forward as the quickest solution.
Where I am after advice is what caused this to happen. Currently there are 2 possible options being discussed, but I am having difficulty accepting how either of them could have caused the issue. I have outlined the options and my thoughts on each below:
- The cage was simply caught on the teeth of the casing. The spacers should have kept the cage away from the teeth, and if this was the cause then I can struggle to see why only the top cage was caught. It would make more sense to me that the entire cage would be pulled up if it was caught on the teeth. Or am I missing something?
- Grout loss during the pour. If there was a large amount of grout loss then could the denser concrete and increased aggregate have simply jammed the cage into the casing? If this grout loss was towards the top then I can see how that could explain why it was only the top section which was jammed, but I fail to see how there could be grout loss inside a casing?
If anyone has any further information on either of these options, or has other theories to throw into the mix, then please get in touch. I have about 200 piles to go and would like it if the rest went more smoothly!
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Laurie,
I never had an issue like this, but could theorise (probably incorrectly) on the causes for days – there are many factors how a pile cage can become caught – but I would hazard a few reasons, poor execution or rushing to fix the issue.
During piling on my site I caught guys numerous times batter the bejesus out of the cage (especially the spacers) to make them fit. This was often required if the cages have been left on uneven ground and have deformed under their own weight. You only have to make them pull a cage out once (and make their concrete stand) and fix the cage properly to stop them doing it again. Wrecking bars and some elbow grease works.
In your case, it may be the cage is banana shaped and caught on one of the intermediate casings during extraction. Although I don’t know why this wouldn’t catch on the initial cage extraction, you would think the mass of concrete would hold the cage down, even if it was bent.
More likely as to how you would extract the whole cage with the casing is that there is a mechanical interlock somewhere – maybe a rock/clay plug in the bottom casing. But again you would pull the whole cage out – unless after it got stuck, Bauer extracted the top section and broke the splice connection.
What length was the cage, shorter than the casing – this would explain how you got the first case out but not subsequent ones?
Did Bauer use the piling rig to extract the top cage?
I have uploaded a PDF on to the media section if you want to have a look – it might help explain what I am saying.
Good luck.
30m of casing!
How are the cases being extracted? Presumably you are not able to vibro them and are therefore using the rig to “unscrew” them?
In my experience (on Phase 3) this was a pretty blunt tool and it would not surprise me if this caught the cage somewhere?
We routinely had cages stuck which required a jacking rig attachment to remove them and that was only 12.5m of casing.
Are the casings removed in sections just ahead of the concrete as it is poured?
Laurie,
It definitely sounds like a mechanical lock between the casing and the reinforcement which I would not in the least bit find surprising. I found that spacers were utterly rubbish if contractors used the cheap ones, which they almost always do. We were forever finding spacers floating on the surface of our polymer which indicates they had been broken during the insertion of the cage due the forces pushing the cage toward the pile face. How tidy were the splice joints? If the cages were not perfectly aligned vertically it is likely the cages will be at an angle to another vertically therefore might explain why only one was caught and not another. I don’t think you will ever really know how they caught but better quality, stronger spacers may assist to retain your cover and prevent the cages moving towards the pile faces. Maybe also put them on the cages at more frequent intervals.
We also had an issue with a voided pile (rig snapped due to weathered basalt which we then couldn’t extract). We did exactly as you described by constructing two new piles either side with a beam linking them. One important thing to bear in mind is how deep does this beam need to be and how are you are going to construct the beam – completely dependent on site conditions and access. The distance between the piles will influence how deep the beam needs to be and this will need to be excavated. For us, it meant a near 3m excavation for a 1.8m deep beam – no-one realised it would be this big until the the structural drawings arrived the day before construction was about to commence. Needless to say, construction did not commence as planned as a whole new methodology to retain the ground had to be implemented first resulting in the best part of a weeks delay.
Also. a diagram of what you were describing would be really useful. I followed it but a diagram would have made it quicker and a lot easier.
Hope you don’t have many more issues with piling, although I suspect you probably will and for fear of self-advertising, I posted a blog about piling risk and management mid last year as we had many many significant problems. Could be worth a read.
Jo
Thank you for all of the replies, I’ll try to answer the questions below:
Chris – After checking the storage of the cages they are stored on level ground and they aren’t big cages yet, so I do not think that they are deforming before use, but you are quite right about them being ‘persuaded’ into the case with quite some vigour. I have had a word with the guys and I’ve not seen them do it since, so hopefully that will put a stop to it. The cage and case were the same length, both 30m, and in terms of extraction of the cage, they used the piling rig to remove the casing which lifted the cage with it, but once it was decided to extract the top cage they used the crane. Bauer now seem pretty adamant that it was, as you suggest, a mechanical interlock between the casing and the cage at approximately 12m (the length of the top cage) and once this section removed and the splice broke, the remainder sank back into the concrete.
Dickie – 30m of casing was not the plan, in fact it meant that the other rig had to stop work as all of the casing was required. It just became necessary when the clay boundary was a long way from anticipated. Hopefully the rest of our piles will only involve half of that at most. Yes, they are only being removed with the rig itself, and there have already been 2 incidents of cases getting stuck (albeit in obstructions where the old power station foundations are not as per the designs we have) so they are indeed looking at bringing a jacking rig of some sort to site. The casings are being removed after the concrete is poured and not ahead of it – did you have issues doing this and had to switch to removing the casing ahead concreting?
Jo – The splices on the designed cages were tidy, however I did not see the splice which actually failed, so I can’t comment on that. There is every chance that it was rushed and not as good as the others. I have raised your scheduling concern with the redesign with the senior engineer and he seemed impressed, so if nothing else it has made me look good! I will read your old blog to see if I can find any other gems to keep the illusion alive. Sorry for not including a diagram, I will do on future blogs but the idea didn’t even come to mind for this one, but I can see the use.
There are all sorts of things that I don’t quite get here:
Firstly the toe was never intended to go to the Thanet Sand- but it was over drilled and due to the confined pore pressure it was considered that the toe had to be dealt with right?
The decision, in the absence of drilling fluid, was to extend the casing – I’ve never heard of casing this deep- but I guess they’ve done it before
They then had to drill and hope the bore standing water was keeping the base stabilsed? Again I suppose they know what they are doing but it was presumably a buck auger and as it withdraws it will really destabilize the base if not under fluid pressure
OK so let’s say they were happy with this- then what is a reinforcing cage for?
I find this time and time again- it is never for compressive resistance. The only reason for is is for potential tension in the piles piles. I presume any tension resistance was there through the clay so the extension of the cage was…err…we.. was barking