Holdfast Training Services Trial Blog

There were 3 main noteworthy occurrences when I was back in the UK for my 3 weeks of summer leave.

Pier 2
The columns on pier 2 were poured and the bracing was not installed correctly (as per the designers drawings). My site engineer had the drawings with the clear notes on them as to where the bracing should have been. However the leading hand convinced that it would be fine not to follow them and that it wouldn’t budge. This was not the case. One of the props holding the form in position failed and buckled during the pour. This was only noticed after the pour and no effort was made to try and fix this. The 2 columns needed to be poured on separate days (unlike the planned one day) as the 9m long vibrator got jammed in the reo at the start of the first column. There was no mechanical 9m hose to replace it, only an air vibrator (which has a larger diameter hose and head, meaning it is more likely to get jammed). The pour continued with this replacement and the damaged vibrator was cut off and left in place in the column. I say it was cut off, but what really happened was they hooked a crane up to the hose and tried to pull it out. That only succeeded in snapping the hose off. When I returned and saw the columns there was a noticeable slant in it. This was confirmed by the survey of the columns. The contract is very clear that the allowable tolerance for this type of element is +/- 25mm and at 57mm is well out.

I checked the formwork system and came up with a way to make it work, even though there was 65mm difference between the 2 columns. However it is up to the clients rep whether they will accept this. They are well within their rights to have the column demolished and redone correctly, however the rough estimate is that will cost around $100000 (all paid for by JHG). To make matters worse the first 3 spans of the bridge depend on span 2 being lifted in by 2 cranes sitting on either bank (one 350t and another 250t). So spans 1 and 3 can’t go in till span 2 is up. By not being able to pour pier 2 headstock the whole sequence is delayed. There is also the added pressure that the 350t crane is only available for a small window. Outside of this we will need to find another crane, which will cost much more to get on site. I made sure that the bracing on pier 1 columns was much more robust, it meant the pour was delayed by a day but both columns were well within tolerance so it was worth it in my opinion.

Broken pile
The piling rigs have now moved onto bridge 2 to start work, having crossed the Bruce highway during a night time road closure without incident. With the end of piling on bridge one this was a suitable time for a handover to another engineer who was initially only responsible for the bored piles and retaining wall. With everything handed over I was rather amused when on second pier of piles in bridge 2 they managed to snap a pile.

The clients rep blamed us, and we blamed unforeseen ground conditions. There was no-one from the clients team on site at the time, yet they are still claiming that the piling subcontractors “pulled the pile”. For those that don’t know the term, when driving a pile they can have a tendency to deviate from the vertical. In the early stages of driving some minor corrections can be made, but once a decent length of pile is in the ground any attempt to “pull the pile” into position can damage and even break the pile. To back up our case we have over 800 piles that have been installed as per our sequence without incident. I’d like to think that it was my steady hand on the tiller which stopped any piles breaking, however that is merely a happy coincidence. The next step is waiting for direction from the client. Any action prior to this could be a waste of time. Looking at things the way they are I see 3 possible options open:

1) Ignore it. Cut the pile off below the blinding level and don’t tie it into the cap. This requires the designers to go back and look at what support/resistance the pilecap as a whole needs to provide. All the other piles in the pier are well over capacity. The steelwork in the cap would likely need to be redesigned to ensure the forces are transferred away from the gap of the broken pile.

2) Use it as is. Use what capacity there is in the broken pile and tie the pile into the cap as though there was nothing wrong with it. This would also need the same checks as option 1, as the capacity of the broken pile would be unreliable over the lifespan of the bridge. In effect its just option 1 but not wanting to waste what has already been driven.

3) Drive another pile adjacent to the broken pile (towards the centre of the pier). This would be within the 3xD that the piles are currently spaced at, the steelwork would need to be redesigned in the pilecap and access to the position for the piling rig would be difficult (but not impossible). If the pile did break because of a large discontinuity then this pile may too also get damaged.

The Spaniard
Soon after my return there was a visit from “the Spaniard”. This is one of the executives from Groupo ACS who own Hochtief, Hochtief in turn own Leighton holdings, they then own John Holland. So lots of the executives from John Holland were on site to wander around. It was all to do with cashflow and why we were going over budget. His opinion/answer was that we needed to secure our revenue streams before acting on something. The example used was the large pile offcuts that we have on site. The additional cost for these offcuts should come from the client (in his opinion) as the piles are barely making it into the underdrive allowance. My simplistic understanding agrees with him. You would expect that most piles would be at the design toe depth, or that the average for the site as a whole would be close to the design toe (with some in the underdrive and some in the overdrive). However the vast majority of piles are sitting 3m above that design toe. My guess is that the designers were overly cautious in their design, and to reduce the risk of not reaching capacity, went for longer lengths of piles. By reducing the design risk they have created more work for the contractor, but at least there is plenty of capacity in the piles! The recent development that has come as a direct result of the claim for the pile offcuts is the direction from the clients rep that all piles must be driven to design toe. This is adding extra time to each pier, and could potentially damage the piles. The size of the hammer is such that were it used at full drop height the piles would likely split. The PDA gauges were connected after the split occurred to confirm it, and to give an indication of where the pile was damaged (roughly 3m below surface).

I’ve also got my hands on some aerial shots of site that were taken just before I went on leave. They’re worth waiting for the page to load to have a look at as the detail is pretty good.