Holdfast Training Services Trial Blog

The last two weeks since returning from Tasmania have been rather hectic. It was nice of my site engineers to leave everything untouched so that it took the first week just to catch back up.

The Form, Reo, Pour process is now in full swing for the pilecap and column section of the substructure. There have been a few teething issues with the formwork, particularly where the pilecaps interact with the columns. This is now resolved and we’re pouring about 2 pilecaps and 2 sets of columns every week, with 2 headstocks soon to be added to that weekly tally. The 280t crane has also arrived on site and been commissioned, ready to lift the girders…if we ever get any.

The changes to the reinforcement in the headstock were finally approved after a meeting with the us (main contractor) the steelfixers (our subby), the clients rep (on site inspectors) and the designers (AECOM). We came to a workable option that satisfied our need for prefabrication and the designers need for torsional reinforcement. Though I still can’t understand their concern when we only needed to change a small section of bars. I did quiz them but got nothing back. I’ve also seen the cage successfully lowered into the bathtub (formwork) and the pour is planned for tomorrow.

There have been issues up at the precast yard recently that I will probably cover in another blog, as the safety investigations are on going. As a bit of background john Holland have built their own precast yard for this project to make the 700 or so girders for the 2 bridges. These are 35m long prestressed pretentioned girders with a lot of strands in them (I’ll get more info in the following blog). The first incident was a prefabricated section of reinforcement for the girders toppling over. While no one was injured, due to the size of the cage and potential to harm this is being classed as a 1P incident (notifiable to the Aussie equivalent of the HSE). The second was during a girder pour 4 strand couplers failed while in tension. With 200KN on each strand this was really dangerous. I will get some of the photos of them smashing through some wooden boards and into the stressing trench wall. Again no one was injured as the controls were in place to stop this. I’ll get more details when I can. Without wanting to second guess the investigation the control appear to have been in place, and as they have let us continue work they must be pretty happy with the process and controls.

The piling crew has now kicked things up a gear and one rig managed to install 8 piles on its own yesterday, when they are only planned for installing 4. This might have a major impact on the supply of piles to keep the rigs busy. At this rate they will catch the casting program in 4 weeks time. To mitigate this the plan is to split the rigs and send one over to start bridge 2 early. All the piles on bridge 2 are spliced and this will slow the rig down considerably. It will also buy more time for me to prepare everything for the cofferdams that are needed for pier 1 and 2 in scrubby creek.

There have been a number of minor dramas and modifications to the headstock formwork to get it to the stage shown in the photos. The first drama was the project manager not approving the purchase order for the stressing jacks in time to get them on site! I’ve now had to go to VSL at short notice and get them here to fill the gap till I could get some kit here (as it happens I got a bargain on some kit from another JHG job that is coming to a close). Trying to rush through a short term subcontract has been a nightmare, mainly due to the terms and conditions that VSL work under. They only want to be liable for 5% of the contract value, but as the contract may only be a few thousand dollars this just isn’t acceptable – if their stressing fails the damage could be in the tens or hundreds of thousands of dollars. However, I’ve managed to get someone onto site though a bit of a loophole (but with the commercial managers blessing).

In other news I would thoroughly recommend Tasmania for a holiday – it was great, and they have even named a lake after me. Went and did the Gold Coast Half marathon with Ben and Sharna (though Ben managed to pick up an ITB injury that meant he had to withdraw, so he consoled himself with a few drinks).

The final pour of the HQ’s Area F for slab on grade happened today…actually it was supposed to be tomorrow! The concrete subcontractors took it upon themselves to pour the slab – in my view so that they could rush off site finally and start another project down south. I’m yet to get to the bottom of what exactly happened, but as I was about to start to look over the rebar I found the concrete pump vehicle parking up next to me and the concrete crew moving to my area from their last slab pour. Having not seen anyone, I presumed all must have been in place with regards to checks and that maybe I had just missed a beat.  Nevertheless, I continued to check over the steel as the pour started and started to find problem after problem. What then ensued was one of those awkward situtions where the USACE QA guy with the funny accent had to step in and stop 10 grizzly concretors and cause a back up of several concrete trucks. Clearly the commotion and grumbling resulted in the QC manager and various Walsh engineers rushing out to see what was going on. It then turned into a mess of fresh concrete being dug out, formwork being replaced, and concretors swearing at each other trying to rectify errors I kept raising – Joe, I can sympathise! Problems, to name but a few, were as simple as:

1. Top rebar not being tied down.
2. Top and bottom rebar having clearance from formwork as little as 1cm (should be 2” = 5cm)
3. Laps not being tied together.
4. Rebar seats having been crushed causing a spacing of, at worse, 2cm between top and bottom rebar (slab thickness – 25cm)

Thoughts….

– Only one external QC guy is employed to QC all concrete, cementitious material and rebar across the whole site. He believed the pour was happening tomorrow and intended to check the rebar this morning with me; this meant that when the concrete trucks turned up, he couldn’t check over the steel and was pulled away to do slump checks on the incoming concrete. To add to his confusion he was trying to QC the grout being used for masonry. It is clear to see that QC is starting to slip off the agenda as contractors rush to make up time, and QC is not a deciding factor when scheduling works that overlap – IT SHOULD BE!  QC employment and scheduling needs to be thought through cleverly – many a day he has nothing to do, and is there purely because contractually he has to be.  This is crazy and I feel his presence is starting to become a box ticking exercise!

– I read an interesting article about ‘QC and continuous education: providing tools for contractors to make ethical decisions’ – BLUF how HR depts need to employ QC workers by personality type, by using tests such as Myers-Briggs in the interview phase, to ensure that QC workers are capable of making ethical decisions.  Today was case in point –  Walsh’s QC manager told me he had walked the rebar this morning (he was clearly aware that the pour was happening); that was before I then showed him the errors.  Hoping he would then rectify the problemsand relieve me by directing the concrete sub-contrators, he limply tried to stop the concretors pouring concrete but then just shrugged his shoulders when they couldn’t hear him…I then had to wade in again.  In addition, the external QC checker should have had the balls to raise the issue straight away to the QC manager; if I hadn’t have turned up, the pour would have just carried on!

Never a dull day…

A combination of Imran leaving and some of the other SPAs being on holiday have contrived to make my life considerably busier, but in a good way.  I have now taken over all of Imran’s tasks, the HP Cooler Replacement, the Minox Blower, Flowline Supports and the closeout of LP and MP Cooler Replacement from 2013.  On top of this, I am still responsible for the Retrofit of Cabins and I’m intrinsically involved in the Lifeboat Modifications.  I’m not going to touch on all of these areas, just the closest crocodile, which is a problem with a flushing skid for the lifeboats.  I would appreciate any good ideas that may be out there.

The Detail

This job has not gone smoothly, it has had problem after problem and nearly all of them could have been avoided.  The latest issue involves a flushing skid, which is a piece of equipment used on hydraulic equipment to clean the components and test the system.  For the lifeboats job we are recertifying the davit A frame, which is used to recover the free fall lifeboat from the water if necessary but on Clair the davit and A frame have not been used in 9 years.  Anyway, the flushing skid required has to meet certain criteria in order to adequately flush the system, in this case it needs to produce a Reynolds number in excess of 4000 and NAS of 6/7 but not to exceed 7.  The job is due to start on 7 Jul, with the skid required in early Aug and a skid has been procured and been shipped offshore.  The flushing skid that has been procured has a motor with the following ratings; 37kW/400V/50Hz and 43kW/460V/60Hz. 

The platform conditions are 440V and 60Hz so a fair assumption is that the power output is closer to 43kW than 37kW.  Depending on power factor and efficiency of the motor, it could draw up to 78A.  I do not have a data sheet for the motor so I have assumed 0.85 for both.  I have approached the manufacturer and vendor and requested the data sheet.

The flushing skid starts using star delta and as such it requires a 3 phase and neutral supply.  The problem is that the platform only has a maximum of a 63A, 3 phase welding point from which to power the skid.  Since it is the SPA in charge of this job in on holiday I have been left to try and resolve the issue and I am trying the following three options.  If anyone has any better ideas then I’m all ears:

• Install a temporary JB and use a single cable from this to hook up the flushing skid once it is in place.  Then, detach the cable, move the flushing skid to its second location and hook the downstream end of the cable back up to the flushing skid again.  There is a switchboard that appears to have a spare cubicle, it looks like it has 3 phase and a neutral and is capable of taking 200A.  However, there is conflicting information because the platform as built shows no neutral but the ABB as built shows a neutral. I have requested that the REP offshore checks what’s inside the switchboard and get accurate information.  I sense this will take a lot of time and effort because it will require workpacks, isolations, method statements, a temporary electronic management of change, cable calculations and the procurement of materials.

• Get a new skid with a motor of a suitable rating to be able to be powered from the 63A welding socket and yet supply enough turbulence.  This is my preferred option.  It sounds the best on paper but I have no idea whether a suitable unit even exists.

• Source a suitable generator to power the flushing skid.  It sounds a bit like overkill to get a generator for one flushing skid, and there may be problems with exhaust fumes and footprint depending on size of generator.  That said, I know I’ve got a flushing skid that works already offshore so this is just the other half of the equation.

Since time is tight I have decided to pursue all options with equal measure and try and get a solution asap.  I wanted to avoid holding out for option 2 and then find out it’s not viable in a week’s time.

What is frustrating is that this machine was released by OIS, an inspection vendor, as safe to use.  This is the second time on this project that an electrical engineering problem has slipped through the net and caused problems further down the line.

In other news…

Brendan and I had to help out at Banchory Beavers Summer Camp.  All I can say is that Brendan is bottom of my prospective babysitters list.  He was beasting 6 year olds and at what point said “Listen in means you stop talking and listen to me or Nick.  Oi chap, do you have a problem?”

Imran’s leaving do was a success.  He got so drunk he lost his keys and slept outside.

One of the constituent parts of the plethora of concrete mixes used on the Crossrail project, is Pulverised Fuel Ash, or PFA. A waste product from coal fired power stations, the material is ground down to a fine powder, mixed with heated air and burned. This forms a proportion of the material (about 20%) into fine glass particles which are separated from the remaining material.

This material has a number of uses in the construction industry;predominantly as a construction fill; as a raw material in cement manufacture, and most pertinent to me as a partial replacement to cement in concrete. This has a number of benefits:

Economic -reduction in the overall quanitity of cement purchased, and consequently a reduction in cost by using a waste product (relatively very cheap to transport from the power stations)

Environmental – green credentials in recycling waste product without the need to dispose of the stuff.

Technical. A number of technical benefits occur from the use of PFA as a constituent of the concrete mix, including:

– improved long term strength performance 

– improved durability through reduction in mixing water

• – improved cohesion and workability for a given water content

  – improved surface finish

  – improved resistance to sulphate attack(ground water levels are on the rise in London, and in my little corner of the Lambeth Beds, there is an increased risk of oxidised pyrite leading to corrosive acidic groundwater)

  – reduced heat of hydration (important in our specification as the concrete musnt remain below 60degrees during curing…difficult in summer…our tunnel reached 42degrees yesterday!)

  – reduced shrinkage and cracking (see above!)

  – reduced bleeding

  – resistance to alkali-silica reaction

All in all, its good stuff to have on board…

So why do I tell you this? Keen to hear from all you concreters out there about reduced stocks of PFA in the UK. Our supplier CEMEX has withdrawn all supplies without notice. Without all the facts, it seems that it is a direct result of many of the coal fired power stations switching to gas. Hansons, the concrete supplier, run out of PFA as at today, and having explored all other options with no joy, have advised that we revise our mix designs sharpish! This affects all their customers, not just Crossrail, and I would imagine other suppliers are feeling the pinch too. Anyone else had this buzz? (When I asked Steve whether he’d had the email about the PFA, he point blank refused to return to Chatham to run a mile and a half! Thats not what I mean….)

So what?

Hansons are contracted to supply us with concrete to specification. They ultimatley are not achieving that, so contractually it seems very simple. However, the pressure of the program mean that the blame game is slightly moot. This apportionment will no doubt will be thrashed out later, but right now it’s in our interests to review these mixes and return to production

In terms of where the blame does sit and who ultimately wil pay for this…does this count as a ‘force majeure’, being as it was, unforeseen?

Hi all!

Good luck to all those preparing for CEng and E +M interviews…

I’ve posted a few photos of Bahrain.
WWW.ROSELLIOTT.WORDPRESS.COM

I’m being replaced in November – one of you may find yourself here…no…really!

RE

As part of the Liverpool street station the northern wall of the Blomfield box which will create the interface between the Blomfield superstructure and the London Underground (LU) Circle and District line was due to be an RC pour insitu wall with aluminium rain screen cladding. Well not any more. Following extensive campaigning by Laing O’Rourke since 2012 to produce this wall in precast and countless rejections Crossrail finally agreed to completing the wall in precast concrete, however this has left Laing O’Rourke only 4 weeks in which to turn the designs around in time for the next design review which is critical to the 3D modelling.

The Issue.

Build-ability. The original wall and cladding were due to form the interface between the LU and the permanent structure of the Blomfield Vent Relief Shaft. Between the LU and the construction site is a temporary heavy duty hording. The space between the hording and the back of the RC wall was only 350-400mm, which given the size of the form work sections required to pour it would prove difficult to fix in place and even harder to then remove.

H&S. The only means to remove the formwork would be to remove the heavy duty hording. This presented a considerable H&S concern due to working next to live track.

Commercial. Working track side could only be achieved during track closure (engineering hours, 0200-0500). Working during engineering hours would present considerable time constraints and incur additional cost.

The Proposal.

Replace the RC wall with a precast twin wall. Precast twin wall could be constructed off site and the lowered into place reducing construction time and also negating the need to work track side (when constructing the wall). The overall cost for the wall could be reduced with reduced working hours, reduced transport cost of only transporting the twin wall rather than rebar deliveries and concrete pouring deliveries.

The Actual Project managers instruction

The RC wall and cladding are both to be replaced by architectural grade single skin precast wall. Of which LOR are to take design responsibility (actual instruction a little more indetail)

Having been given the responsibility to deliver the precast facade as it is now known as it is doing the job and the cladding but is not structural I have become embroiled in the dirty world of commercial and design responsibility. While i have very little engineering input with respect to design the facade for loading but I have all the responsibility of coordinating the effort of the architects and engineers from Crossrail, LOR and Arup. Lessons learnt to date.

Open to Interpretation. The PMI was written in such a way that Crossrail believe that it allows us the scope to think outside of the box and come up with our own design. As LOR do not have in house architects or detailed design teams we have had to employee Arup to complete architect designs while LOR would use its subsidiary company ‘Expanded’ to complete structural designs of the precast facade panels. The vagueness of the PMI has resulted in each stakeholders understanding of what is required being different and therefore what the facade is suppose to achieve what loads if any it is to carry and how it will look.

Design responsibility. The vagueness of the PMI has also resulted in endless wrangling over who owns what design responsibility. The architects claiming that they cant design the facade overall appearance unless they know how it is to be constructed fixed and loads transferred to the superstructure and the engineers from Explore claim that until they know how the facade is to interface with the superstructure they can’t design the internal rebar and how the loads are to be carried. At present there is no clause in any contract apportioning design responsibility either in the Crossrail contract, the PMI or the LOR to Arup subcontract. Until this is assigned then designs will not progress.

Ground Truth. The discussions were further stalled by the realisation that the as built drawings that were being worked to by the architects from Arup and engineers from Explore were in fact wrong. This was only picked up during a meeting when I noticed that the drawings that were being used and assumed did not match with my knowledge of ground truth. Lesson here always check that as built actually reflect what has been built and not what people want others to believe has been built to avoid penalties.

Commercial Knowledge. As John has often stated do not assume that others know what they are taking about. This goes for contracts as much as engineering principles. Members from the Crossrail team had been stating a claim that I found out later to be false; Cross rail claimed that Laing O’Rourke were responsible for removing the heavy duty hording. This turned out to not be in the scope of works and is important as it removes the problem of removing a concrete plinth that would once expose a section of the superstructure not covered by cladding. Not the end of the world structurally but for Crossrail and the architects a disaster. As we do not need to remove it the problem of extending the facade beyond the concrete plinth goes away.

Relationships RA and foresight. There appears to be an approach in the industry that its not my problem i don’t need to consider it. The original proposal state a twin wall to replace only the RC wall on H&S grounds and build-ability grounds. When the PMI was issued complaints were made that it was not inline with the proposal, however the engineers and construction managers (who are the guardians of RAs) had failed to note that simply replacing the RC wall did not remove the risk posed to a further sub contractor who would then be required to install the aluminium rains screen cladding. This should have been picked up at the design stage but then again by LOR. If the risk is too great for your own work force why pass on the risk to a further organisation, why not eliminate the risk for all. The replacement of the RC wall and aluminium rain screen with the single precast facade does just this.

Conclusion

The whole contractor, designer and client relationship is crucial and I am not convinced that many in the industry understand that. A client and their designers must make use of ground truths and experience from the coal face as much as the coalface must understand the designers and clients intent. When designs change and there is an opportunity to pass the buck to someone else people will take it, and architect and engineers just aren’t capable of playing nice.

I am very proud to say that I am following in the footsteps of Mike Burton. I don’t mean drinking too many ‘Quad Vods’ at Jester’s nightclub, telling war stories to impressionable students…I mean in making my mark CrossRail’s Fisher St shaft. Good to finally be on site full time…

Overview

The Fisher St project consists of two distinct sections of works;

Phase 1 was the excavation of a large access and maintenance shaft, 25m in depth running from ground level to running tunnel. Adits on the northern and southern sections of the shaft adjoin the east bound and west bound running tunnels. Mike saw this section of works completed throughout his time here. Its function during concstruction is to act as access and egress for equipment (including the removal of 2x1000tonnes tunnel boring machines), material, spoil and personnel aswell as a mean of escape. In operation it will act as a maintenance shaft and proivide ventilation to the running tunnels

Phase 2 is the construction of the cross over section. The only point throughout the 42km of line that the two tunnels interact, the section will be used for train maintenance, in the event of a breakdown, in order to clear the line. Work is underway to excavate the ‘binocular sections’…essentially the section on each running tunnel where the tunnel divides in two to form the crossover section. My part of ship from here on in…see the overarching plan for the picture to paint my 1000 words.

Plan and Geological Section

Geo Legend

Ground

I remember the Great Orator mentioning that the ground is actually quite fundamental….he mentioned it once or twice. The nature and engineering behaviour of the ground in tunneling is obviously critical to the design considerations of the underground structures. Crossrail tunnels have been bored at an average depth of 25m along their length, with the odd fluctuation to avoid existing underground structures. The tunnel boring machines and station tunnels and caverns have been bored/dug with relative ease due to the excavations being conducted largely in the London Clay Group. Having looked  at some of the ground investigation data, it seems that this consists of stiff or silty clay ranging right through to mudstone. These Palaeogene marine deposits, I’ve been assured, make for an excellent tunnelling medium, due to their relative homogeneity offering some predictability, and their short term stiffness, which allows brief unloading without collapse, ahead of stabilisation with SCL.

However, in the Fisher St/Farringdon area, the base of the London Clay sits at approximately 30m below Ordanance Datum. This area coincides with a ‘busy’ underground infrastructure network (proximity of Central and Piccadilly Lines, aswell as numerous underground services) which constrains the allignment depth. You will see from the Geological section above that just at the point of constructing arguably the most complex section of works in the allignment, we pitch head first in to the Lambeth Group.

At this stage, its probably worth thinking back to the days of the McGuirk Bluffing Face…a slight nod of recognition toward said lecturer as if I was following, when internally I was in a flat spin. Well, the same thing happened when I was told this; as if I understood immediately the implications of it…I didn’t. So I did some reading!! Yea alright, stop laughing….

This is what I learnt….we’re predominantly in the red/brown or green/grey ‘Upper Mottled Beds’, but there is quite a bit of other stuff (technical term); from fine silty sands, shell beds and sandy flint. Bottom line, its a bit of a mess; in contrast to the relative homogeneity of the London Clay belt, this material is vertically and horizontally inconsistent. For tunneling, this presents risks; potentially water bearing, the excavations could cause a flow of material along failure planes as they are uncovered, major face instability prior to concrete spraying, and greater than expected surface settlement with associated damage to surface structures in prime central London real estate.

The Tunnel Boring Machines have passed through the allignment, without reported incident, and as such I have had a few raised eyebrows for asking what the perceived risks are in relation to our excavation. This would be ok…but I since discovered that the TBM has no means of recording the nature of the ground that it passes through, for its properties, contamination or indeed the ground water regime. So essentially, we’re operating solely on a triangulation of two boreholes located 30m and 20m distant from our crossover section, and a baseline statement in the Geotechnical Baseline Report that the lower aquifer on our location is approximately 30m below the excavtion and is therefore of no significance.

Groundwater

I think that the proximity of the borehole data gives us a fair idea of thewhat the enlargement excavation will uncover, so the primary risk will be in the groundwater regime. We are aware that the Lambeth Group is likely to bear isolated pockets of water; so the water is anticipated but the quantity is unknown. So this leads to a number of risks, (based entirely on ripping off Johns notes…)

1. Potential for inundation into the tunnel as a pocket of water is uncovered.
2. The Lambeth Group is non-cohesive, so this may lead to an increased risk of heave in the tunnel invert if the resistance to this uplift is less than the head created by this water
3. Instability in the face due to ground movements caused by pore water pressure.

I must admit, I’m not yet sure what we’re doing about this, if anything, and so I will revisit in future blogs….

What next?

A part of the Phase 2 works, the binocular section on the westbound tunnel (WBX) has been excavated and the primary lining sprayed.

The construction of the cast in situ secondary lining works will now go ahead, with me firmly installed as the section engineer! Blog to follow….

I’ve spent the last couple of weeks bouncing between office based negotiations to continue to try and get the Danville flood protection project off the ground and the HQ site.  The former continues to be frustrating; I have spent many hours checking through submittals for USACE approval from the contractor for lift plans, labour training, concrete mix designs, schedules etc – the majority of which I have rejected as they just don’t meet the USACE construction specifications or our Health and Safety requirements manual…frustrating when they have the same documentation that I do to work through and cross-check!  They are either trying to cut corners at every opportunity or are still trying to get their heads around defence contractual requirements and rigidity (this is their 1st) – I am optimistic and think it’s the latter, but having seen some of their method errors I am starting to worry about their ability.  Work plans to start 7 July, and end Dec.

The HQ site is starting to look remarkably different now that pre-cast curtains are being installed.  In a non-official manner I have been assisting the QC team as the QA inspections tend to be purely the final product which quite rightly contain few issues so I am learning far more hitting issues immediately after or during construction alongside the QC team – this has actually made the QA inspections even smoother, plus having a uniformed guy, with a British accent and a USACE hard-hat emblazoned with safety control logos tends to create confusion in the ranks of hard-nosed tobacco-spitting sub-contractors resulting in less-arguments.  Despite being ‘qualified’ inspectors, it is interesting how little structural knowledge they have, and I impress myself at explaining to them why rebar is placed where it is as they ever keen to learn as well.  I have noted that the QC has a non-purposeful habit of swaying towards areas that the QA inspects the most – for example QC checks on rebar prior to a concrete pour tends to focus on cleanliness of the deck surface and rebar ties, rather than what I’ve continually brought out – ensuring drawings are abided by (and rectified if needed!), spacings are correct and dealt with correctly around vertical conduit, and rebar has the correct clearance vertically and horizontally.

Conscious the blog is a means of sharing in order to educate, rather than just a site-diary I thought some may be interested in some of the testing methods and concrete I am involved in.  Deck concrete has to be pumped to three floors high so we’ve used air-entrained, lightweight concrete.  This has used pumice as the aggregate – a featherweight volcanic porous rock.  Slump tests have been done, as standard, on every 2nd truck, aiming for a slump of 180mm- 230mm, making it very easy to work with.  28 day tests have given a strength of 52000kN/m2, when all that’s desired is 27600kN.m2 – this has consistently been the case! The second on-site test we’ve been doing is to measure the amount of air-voids as it’s air entrained – the specs require 5-7%.  Air entrainment tests take one of three forms:

1. Pressure testing: using a pressure device, you measure the air content of fresh concrete based on the pressure-to-volume relationship of Boyles Law. Pressure is applied to the sample to compress the entrained air in the pores, then measures the change in air volume to detremine the air content.  We couldn’t use this due to having porous aggregate.

2. The volumetric method or roll method (pictured below).  Relying on an instrument with an in-built gauge, it is filled with fresh concrete and agitated with an excess of prescribed water and alcohol.

3. Air Indicator Kit: This kit provides a quick and easy method to check air content. A sample is placed in a vial and alcohol is added to free the air. The change in level of the alcohol in the vial stem indicates the air content.

I have also been conducting grout testing for the stairwell masonry – this has involved the rather non-tech grout block testing pictured below.  The paper enables a cube to be moulded whilst also allowing a transfer of water out of the grout to simulate the grout in-situ where the bricks absorb some of the water with cementitious materials in solution.  We are still awaiting test results but we require 20500kN/m2; slumps 200-280mm.

30 days post steel beam fireproofing we’ve jumped onto testing them for density and adhesion. Again, a very crude method but clearly laid out in the ASTM (EC equiv).  The density test is done with a 12×9 inch removed area and lab-tested, while the adhesion test is done as follows: a mason jar-like cap with a hook is mastick’d onto random vertical and horizontal areas.  After 24hrs of setting, the hook is pulled.  In our case with a 1,75 inch thick material it must withstand at least 12psi of pull, unfortunately the material itself broke apart with a mere 5psi….oh dear! This has quickly shot up the pay-grades and we await a plan forward and response from the fireproofing manufacturers!

On a side, I made the most of being able to jump on the back of the District Commander’s diary so tagged along for some hobnobbing with a US Congressman who wanted to see the USACE island restoration project in the middle of Chesapeake Bay – Poplar Island.  I was blown away…sustainability and PPP’s at its best…reconstituting an eroded island with rock armour and dredged material from Baltimore and Virginia ports and Chesapeake Bay that enables deep water harbours to remain open, prevents open water dumping, all whilst making a carefully balanced ecosystem haven that harbours 1000’s of terrapins and over 180 species of birds acros 7km squared.  2.4million m3 are dredged each year in the areas mentioned; the island at its present perimeter can house 30 million m3 of which 19million has been used; an expansion of the island is being applied for which will enable an aditional 23million to be used.

So I mentioned previously that I’m now running with the construction of a 619.310m cantilever dwarf retaining wall stretching the length of the site separating the coal storage area from the biomass handling facilities. In many ways it’s a breeze compared to the previous thrashing of the C620 foundations (hence my upsurge in blogs!) but I don’t want to tempt fate too much at this early stage.

The wall ranges in height from around 1m to over 3m with 2 distinct gradients and a varying footprint profile. The wall itself is 250mm thick, off set 500m from the edge of a 900mm long base forming a cantilever retaining wall. There are ‘Rainbirds’ positioned at regular intervals that are effectively large plinths supporting a water suppression system (to limit the dust) cast into the wall itself and contraction joints every 15m.

As it seems the assist in future discussions (thinking back to numerous piling methodology blogs) this is the construction process:

I mark out the base centre-line with the TS and off set 600mm either side allowing 100mm for a little extra space for the steel fixers to work in. Then set the height on the laser level and get the excavator in to dig to the correct level – usually about 500mm below the prepared ground surface. Compact some type 2 aggregate into the excavation and order some ST1-S3 concrete for blinding and get that in to a depth of 50mm.

I then get out the TS again and set nails in the blinding for edge of concrete at the base and the centreline of the off-set wall for the starter bars.

I’ll check the steel alignment and then order C32/40 S3 concrete and pour the 225mm base. I have been instructed not to place shuttering parallel to the direction of the wall on either side for speed and as the excavation is only 100mm extra either side the additional concrete wasted has been accepted.

As a side issue – the excavation is usually a touch bigger than 100mm either side (operator) so assume 620m of 300mm additional excavation at 225mm depth. A quick time/cost analysis of 620m of 300mm additional excavation at 225mm deep is 42m3 of wasted structural concrete at £65 a m3 = £2730. That’s 68 hours-worth of time for 2 labourers at £20 an hour to fit some reusable shuttering either side. It would take them significantly less time as it would only require single planks and minimal bracing and thus could be a small saving but I suppose it’s a drop in the ocean in the grand scheme of things. The counter argument is the time saved but I have the base slab poured at least 60m+ in advance of the much more time consuming wall construction.

Anyway, I then get out the TS again and set nails in the structural concrete base for the lines of the wall (250mm apart) and the expansion joint positions and hope that the starter bars have not moved during the base pour and appear in the correct position inside the wall makers!

The shuttering is then placed on the rear side of the wall and I use the dumpy level to put nails into the rear shuttering dictating the correct gradient for the top of the wall (1/143). This allows the joiners to place the chamfer filet. Once in place the steel fixers move in and erect the internal wall steel (just mesh) using the filet as there TOC measure to allow 50mm cover.

I then check the steel and horizontal cover prior to the front wall face shuttering being placed and second filet line positioned. Once the shuttering is complete I have been using the TS to check the top alignment of the shuttering which as long as they have used my original nails in the base and kept it vertical it has generally been within 5mm, therefore acceptable. The wall section is then poured using a 2m3 skip and crane.

The interesting issue the wall has raised is in the contraction joint detail and there has been much discussion as to what a ‘formed day joint’ (as can be seen on the picture below) entails.

My understanding is that this is a contraction and expansion joint justified by the use of the 600mm half debonded dowels placed at 300mm centres, connecting each wall section. These are a common feature of expansion joints as the sections are tied together by these dowels but the sleeving in one of the sections allows expansion to take place without generating additional stresses within the wall. Expansion joints also feature a definitive gap between the sections to accommodate this potential movement and reduce the risk of potentially damaging forces with the structure. Clearly this gap and dowel also serve to accommodate contraction of the concrete.

To facilitate the gap of the expansion joint I have been placing 10mm sheets of flexi cell inbetween the wall sections to create the gap. These will then have 15-20mm cut away from the exposed edge and a rubber sealant will be used to waterproof the joint. The picture below shows the joint with the flexi cell prior to the sealant. This flexi cell allows contraction and expansion and increases the protection against corrosion of the steel dowels if the gap was not filled. This is the method that the joiners/fixers have used across the site for similar joints.

Questions have been asked (by other engineers) as to why I’m doing this? Their understanding of a formed day joint is simply a construction joint ie you pour up to the end of the section, allow the concrete to cure and simply pour the next section on another day from that surface. Even if a very thin divider was used to ensure clean edges this joint would surely allow only contraction and almost negligible expansion.

The GA does detail a ‘construction / contraction joint’ . A contraction joint only accommodates shrinkage so the correct procedure, by the title, is likely to not involve flexi-cell. However the use of dowel bars to accommodate contraction alone is surely a bit ‘belt and braces’ and indicates the requirement for an expansion capability?

Incidentally the concrete has contracted during the curing process, as can be seen in the flexi cell photo above. This would subsequently leave a more significant gap for future expansion if a simple construction joint was used but surely the dowel bars would be too exposed with a clear gap?

I must add that I have not been flying solo with this and checked the procedure with the MS (which mentions flexi-cell) and my line manager prior to cracking on.

Thoughts?