Holdfast Training Services Trial Blog

This blog covers the last two weeks as it has felt like a continuous week with one aim to get 10 piles completed for the foundations to Dickson Rd overbridge. There was Fri and Mon off of course for easter and all ‘non salary’ personnel had last Thu off as well so that was a non productive day.

MGI piling were the sub-contractors who we brought in to carry out the piling and the main realisation of the last two weeks is related to the AMS and site control. MGI piling arrived the Tue before Easter and the AMS was briefed to everyone so that they can ‘sign on’ to the method, then a services brief was carried out and the permit to excavate signed (this permit is about 50 pages long with 98% of the content irrelevent to my actual site and has remained on the back seat of the Ute ever since!). It seems an AMS is similar to an “Admin Instruction’ in the Army where the Engineer/Officer spends a great deal of his time to produce a document that very little people read unless I suspect an accident/incident happened where it would be scrutinised with a fine tooth comb. Even though everyone on site had signed on to the AMS to say they agree with it etc, a sub-contractor will invariable do what he has always done regardless! I have no big specific gripe on this issue it has just been an observation throughout the piling works.

As we had ordered oversized cages in the event we had to drill further than expected the Geotech Engr and subby concluded we would just drill further anyway to the depth of the cages so that we would not have to cut and weld on site – we had already got the welder on site ready to go before this was decided to the slight annoyance of the superintenedent who had to re-employ the welder that day and is another case of the sub-contractor deciding something without consulting either the suprervisor or myself, having already agreed to the methodology. A possible concern with this was if the sub-contractor would charge us more for drilling further and pouring more concrete but the time saved would be of more benefit in the longer term.

Each pile was drilled through the road surface and was fairly straight forward, I remained on site for the majority of the works in order to check off the Inspection and Testing Plan (ITP) checklist as the works progressed for QA purposes and to control the concrete coming on to site. We encountered water at about 9m of a 9.5m depth drill and in some it was only about 50mm at the bottom once the bucket had cleaned the toe out so on most occassions I eneded up calling the concrete forward so we could pour the pile before an excessive amount of water could trickle in through the shale below.

The sub-contractor arrived on site with a different rig (T-108 rather than a Mait 130 if your interested) than what we were expecting which meant the whole works took an extra day than we thought. As the reo cages did not need cutting we did save time here and each cage weighing 1.7T were positioned to the rear of the rig with a Franna crane and easily lifted by the rigs winch and lowered into the pile hole. Concrete placement was done by a ‘lay flat’ hose as opposed to a tremie pipe due the lack of water although on one or two in my very inexpert opinion it probably should have been tremied. They would check the water level in each hole before we poured and inform either myself or the supervisor depending who was there and we would be happy to expect about 100mm (65 litres of water) with the lay flat. On one pile post pour I observed water about 100mm below the top of the reo and considering there was a 1000mm projection and 400mm overpour this meant we had a least 400mm (320litres) of water in there. Water being lighter than concrete means it probably isn’t an issue providing the ‘lay flat’ worked like a tremie as they assumed.

I learnt a great deal about the practical issue of placing concrete this past couple of weeks. I had calculated 4.7m3 per pile and thinking that this figure was not allowing for the displacement of reo an order of 9.4m3 for the first two which we poured together would be alright – it was just! There was nothing left on the last truck and we just managed to fill both piles but I didn’t want it to be that close again so ordered 5 per pile/truck for the rest. I also learnt that you can never underestimate the stupidity of a concrete truck driver. One decided to drive past the site twice after us flagging him down but he still decided to go on a personal tour of the entire alignment before realising he had legs and could get out of his cab and ask someone where he should be! This meant the concrete was an hour old before it arrived and we had 30 mins to place it which wouldn’t have be an issue if it hadn’t failed its slump test twice (tremie mix 180mm slump). We ended up ready to pour just before the 90 min mark having added the maximum amount of water we could so not to affect the w/c ratio, it was 1645 in the afternoon and the Project Verifier had just arrived to witness the pour. In the end I decided that we had been fairly quick in placing the concrete with the other piles and we wouldn’t get another truck until tomorrow so I decided to go ahead thinking if we start to pour on 90 mins it will be alright. It went alright up until the last 0.5m where the concrete was fairly unworkable and kept clogging the hopper. With hindsight I wish I had just turned that truck away and even if we poured the next day we still would have completed all works on the same day. A Non-Conformance Report (NCR) may have to be raised although the PV has not mentioned anything about it and I have got his signature on the ITP checklist already. As we are overpouring by 400mm this will help and I think the top will be where the problems may lie but we can visually check the top of the pile once we excavate and trim. Although the tremie mix is self compacting we could have also tried comapcting the top 1m or so, another lesson learnt.

 Corporate pose – inclonometer readings.

Another annoyance was the back-filling of the holes as we had at least a 1.5m drop to the top of the cages from ground level we were to backfill the holes after 24hrs for safety reasons but the sub-contractor was quite keen to do this after about 3 hrs which included covering the inclonometer tubes. Fortunately I had made sure they were capped but having to dig them out again to take readings was a little irritating. I have taken readings on abutment B (24-72 hrs after pouring follwed by a second 24-72 hrs after the first) but have been unable to take the second reading at abutment A as the civil team have arrived and on Fri morning I went on site to see abutment A piles with a little earth covering them!

 The 5 piles of Abutment A are under there somwhere.

Some interesting local Sydney news for you over the Easter weekend: “Police were called to a domestic disturbance of a retired couple over the weekend. A 64 year old male has been discharged from hospital after having superglue poured into his ears and eyes. A 62 year old female is also accused of beating him with her prosthetic leg!”

After a relaxing, if short 4 day Easter break on Rottnest Island off the coast of Perth I returned to a 4 day week that has felt like a month. Prior to leave a milestone had been missed and it was now best effort to get the South Block Upper Basement Zone 2 suspended slab (320m3) poured by Wednesday and the following Zone 3 (120m3) poured by Friday. Even to my inexperienced eye I could see this was pushing it especially because the Senior Engineer was off for the week and it was down to me to manage.

I was handed control of the pour with about 50% of the Reo on Zone 2 complete but for a first slab it was a bit of a nightmare to comprehend. It was not regularly shaped, it had 3 step ups, 6 integral beams (4 with PT), 2 Temporary Movement Joints (TMJ) (for seismic design), 3 ramped sections, 1 stairwell, and 2 penetrations (one of which was to accommodate the up ramp from Lower Basement). To deal with the more interesting details in a little more details:

The reo schedules and mark-up drawings were fairly epic in themselves and took me a good couple of days to get my head around which was unhelpful as I was being asked questions from the point of arrival on Tuesday morning. This mainly involved multiple trips to the slab deck to identify the issue, to steel fixers (NRG) office to discuss a solution, then to the site office to call the scheduler to check measurements, delivery dates and bar marks to ensure all the required steel was there to complete the task. Having been on the task for 3 weeks at this point with minimal top cover, it was inevitable that there were going to be issues. There were plenty, culminating on the night before the pour, after the structural designer had completed his structural check, that we found we were missing wall starters and were unsure of the wall dimension. After a few frantic emails and an early morning dash around site, the size of the pour became an advantage as after the 0600 pour start we had approximately 3 hours to get it rectified if we poured in the correct configuration. We didn’t pour in the correct configuration but luckily the fixers were quick and it was done in the hour.

Slab joints are a fairly new one for me, and it seems that the TMJ technique used on the NCH is for a lot of people in Perth. The TMJ’s are designed to give the structure a degree of flexibility between slab edges during construction to accommodate shrinkage due to curing and PT. The project is using a range of Ancon shear connectors that are either locked off after a specified period or allowed to accommodate movement throughout it’s life. The benefit of using the shear connectors is that it speeds up the rate of construction as it reduces the requirement for complicated or additional formwork, and only leaves a small joint and connector that required grouting to finish the connection.

Post Tensioning in the beams is quite an issue, it appears that the structural drawings when sent to the contractor to produce shop drawings get changed ever so slightly that they always clash with the top reo. There has been several issues with the lateral placement of the ducts as many are detailed to pass through columns, but as the PT shop drawings were not complete at the time of column pour the bars were cast in the standard positions, therefore requiring bars to be removed and replaced by drilling and epoxying new starters in place. The issue that I have now found to be the biggest issue is the lack of any consideration in programming for the lag that occurs in construction due to tensioning. The initial stressing can occur at 24hrs post pour (concrete approx. 9MPa), however as per the structural specification the final stress can not occur until the slab reaches 22MPa and 5 days post pour. This builds in an additional 2 days per slab to each pour date. I have looked at the sequencing of the slabs and think there may be a more efficient way of doing it, but I need to speak to the programmer to ascertain the assumptions behind the decisions.

From a RE background of pouring a maximum of 5m square flat slabs, the idea of moulding the concrete into ramped sections concerned me a little – it turns out to be pretty much by eye. Pouring 320m3 of concrete in one hit was pretty hectic considering steel was still being fixed at the other end of the slab. This was the last ‘easy’ pour as the truck mounted pumps will no longer have access to the bottom of the hole. From now on concrete will be pumped from the access slip lane into a standing boom pump, which will pour to the final destination. The complication comes in the fact that the agitator trucks will not have the space to wait in the slip lane, where all the other deliveries arrive, so coordination is vital. This will be yet another task for the site engineers but in my view is so crucial it should have a single responsible person as there is so much scope for errors and with the prospect of turning concrete/reo away due to lack of space it could get pretty costly.

One of the overarching issues for me is the complication that so many contractors on one job creates. Not just the usual issues of construction but also the issues between themselves (Rivalry, Unions). I realise the intent of employing workers and sourcing materiel separately is to cut the costs of the project, but it appears that the time taken to liaise between the various consultants (architects, structural, services), the individuals who are doing the work, and those sourcing the materiel takes the vast proportion of the engineers day leaving only a small element for actual forethought and fault finding prior to construction. I don’t think that many sub-contractors makes light work and cost effective construction, quite the opposite actually.