Remedial works to shoddy piles
A quick blog on some activities I have been doing at Wentworth House Partnership (WHP). As part of my responsibilities for temporary works design for WHP I have been tasked with coming up with solutions to piling NCRs that have occurred at The Stage.
Prior to Keltbray taking over the piling contract a significant amount of piling works had been completed by a different contractor. This contractor had walked from/ been removed from the job because their methodology that they specified at tender wasn’t going to work. A large section of secant pile wall had been installed by these contractors prior to them leaving. This pile wall is to allow for the construction of a 2 storey basement. During the excavation of the basement a number of piles in the existing wall were noted to be raking into the basement area (out of tolerance for verticality), having large amounts of inclusions and/or evidence of mattressing of the pile rebar cages. These defects have been noted after 5m of excavation, a further 10m is still required.
3D Modelling has been used to assess how much the raking piles were intruding into the basement footprint.
As can be seen from the image the existing secant pile wall is supposed to be installed where the white circles are. The dark green circles indicate the location of the pile at a depth of 15m. This is based on examining how far they are out of tolerance at a depth of 5m and extrapolating. A number of piles are raking significantly into the basement. These piles will have to be broken out and reinstalled. The plan for this is to break the pile out in sections and pour a concrete column to replace it. Analysis is ongoing to establish the effect of removing a pile from the wall. It will more than likely be limited to the removal of one pile at a time otherwise there is a risk of the whole wall failing.
It can also be seen that a number of piles are raking away from the basement. This will result in large gaps in the wall which will allow water ingress into the basement. In this case a relatively simple formwork design has been designed in order to allow concrete to be poured into the gap. To limit the pressure on the form work I specified that the pile infill must be done in 3m high sections. This will also allow the contractors to do the infill sequentially as the excavation continues.
The above a sketch that I drew for the formwork design. This sketch was then converted into CAD by the CAD technicians so that I wasn’t giving the contractors a Freddie age 5 drawing but actually something with a whiff of respectability.
Holdfast Training Services Trial Blog 
What was the specification for the piles (contractually)?
On BPS Phase 3 pile location when exposed was identified as a risk. Since even if the piles were within specification (75mm for position and 1:100 for verticality – from memory?). If I remember correctly these specs are at piling mat level not 16m lower at formation level…
Is there any de-watering regime?
I am guessing the original design would have been relying on the secant wall to achieve an impermeable barrier.
What contingency is in place now? Assuming the water ingress could be quite large at formation level if the gaps are bad.
Rich, the specification for the piles is 1:75 for verticality. However some of them are hilariously outside of this. One of the them is projected to be 1300mm outside of tolerance.
You are correct, the original design was relying on the secant wall. Now that there are gaps in the wall this changes. The gaps are predominantly in the clay layer so I am not expecting a flood. Most likely just have a bloke with a pump to get rid of any excess water whilst the replacement concrete is being poured.
Fred
One final query – WTF were the “previous” piling contractor doing?
Name and shame?
For BPS secant pile wall a RC guide wall was constructed with pretty tight tolerances?
Actually another question too Fred
Have you considered a proprietary shutter system? Have you asked the contractor if they have anything in stock (Peri shutters etc)?
Contractors were Miller piling and i’m not sure what they were doing. At Southbank Place guide walls were put in place for the secant wall constructed there. I’m not sure if one was done at the Stage…..wouldn’t surprise me if there wasn’t.
As for shuttering,PERI is available however because each NCR has a different gap size, height, etc I’m finding it easier to design timber shuttering.
Apparently Millers were also seen using the back of an excavator bucket to force the cages in – now clear when some of the piles are being exposed. Guide walls are being used now by the current contractors but there is no evidence of them having been used when Millers were on the job. The issue Fred has now is that Keltbray (current contractors) have to be able to warranty all the work, including that done by Millers – by coming up with suitable remediation methods.
Sump pumps are on site if required but large GW inflow is not expected. There was little discovered on the ground investigations and is likely contained under the clay layers. The site is also surrounded by numerous large construction projects which are likely causing a drawdown in the local area. The excavations have only gone 5m deep at present and GW strikes were experienced at about 18m but were only experienced in a couple of boreholes. It will be interesting to see how this progresses as the excavations get deeper and I will keep you posted about any developments.
Fred, I assume you are only intending to replace the ‘male’ load bearing piles? Can you break down the others to the correct position and just leave them in situ?
TD
Tom, you’re correct. No one is all that fussed by out of place female piles. Those that are raking into the basement will be broken back in line with the rest of the wall.