Holdfast Training Services Trial Blog

1.     Project Update.  In order to raise the Earth and Rock-filled Embankment dam here at Chaffey Reservoir, a reinforced earth wall (RE Wall) is currently being installed along the existing crest of the original dam (broadly speaking). As the engineer responsible for raising the dam crest, the RE Wall installation falls firmly within my remit.  The works have been subcontracted to Fusion Civil Pty, an established earthworks company specialising in such structures.  However, aside from the considerable size (500m x 7.5m x7.22m), it is the fact that the earth core is composed of three very differently graded aggregates, vertically aligned into three specific zones that makes this particular RE Wall quite unusual.  Such a design has been adopted in order to control the rate of seepage whist preventing any water being visible on the downstream face.  The zoned fill technique has also been adopted within the existing embankment dam below, however this also has rock armour installed on both the upstream and downstream external gradients.  The primary impact of having zoned fill material within the RE Wall is the slower rate of construction; a fact that Fusion Civil are currently discovering to their cost (quite literally).

Chaffey Dam & RE Wall Upstream Face

2.     Quarry.  As well as being the engineer responsible for raising Chaffey Dam I also manage the on-site quarry which includes the sourcing, screening and crushing, load/ haul and all associated QA. A great benefit of the location here at Chaffey Reservoir is the local geology, perfectly lending itself to an on-site quarry for the both RE Wall Fill and road base material.  Whilst it may sound to be a fulfilling and interesting role, the quarry has been the source of great frustration and angst since day one.  Primarily, this is due the ‘cheggars’ subcontractor that John Holland selected to conduct all crushing and screening of the raw material.  More to follow…

Chaffey Dam On-site Quarry

3.    At present the RE Wall has had six 0.3m layers of fill and three staggered layers of 2m high precast concrete wall panels.  Each 0.3m layer of fill material is hauled up to the crest from the quarry (≈1.1km), placed by a single CAT 730 articulated dump truck (due to with restrictions) before being spread and finally compacted by Fusion Civil.  Once 50% of a layer has been placed and compacted I request that compaction is checked.  A local Geotechnical firm provided the material technician who then determines the densities of the in-situ material using a nuclear densitometer.  For QA purposes I chose to divide each layer of fill (≈ 950m3 aggregate) into two work lots (chainages 0-250m and 250 – 481m) based upon work lot sizing guidance within Australian standards (Roads & Maritime Services Q6).  Finally the layer is strapped with the reinforcing galvanised steel straps as shown in Fig 2.  Remarkably, it is the steel strapping that provides the structural integrity of the RE Wall.  The straps provide the tensile strength that holds the precast concrete panels in position simply from the vertical load imposed by the compacted earth layers.

4.   Volcanic Resources Pty are the subcontractor who are responsible for all crushing and screening of raw materials.  Their primary function is to produce the correctly graded material as specified within the principle contract (CDASU GC21).   The three materials are categorised as zone 4, 4a and 4b and increase in aggregate size respectively.  Zone 4 material is the initial material located in the zone closest to the upstream internal face of the RE Wall in a 2m wide strip running the length of the wall.  It has the lowest permeability of the three fill materials and is a reddish/ brown, well graded, angular, silty SAND.  Zone 4a is the next material located in the centre 2m strip of the RE Wall.  It is also reddish brown in colour and well graded but is a sandy GRAVEL that is designed to allow water through at a greater rate whilst providing structural integrity within the earth core.  Finally, is the zone 4B material.  It is placed in a 3.2m wide strip between the centre and downstream internal face of the RE Wall.  It is a grey, poorly graded, angular, gravelly COBBLE that is designed to be highly permeable whilst providing structural integrity.  Due to the high voids ratio water is able to pass freely and is a requirement as it is intended to prevent water ever reaching the very large rock armour on the downstream face of the Embankment dam.  The idea being that if the public were ever to see signs of water penetration on the downstream face of the dam, panic would ensue.

5.  Material Conformance.  The latest issue has been one of material conformance.  Until recently, Volcanic Resources produced all fill material simply using an industrial rock crusher and a mechanical screen.  The machinery was set up to produce all three required products concurrently

( 4, 4a and 4B.)  However, approximately five weeks ago the large zone 4B material was no longer required as I determined that the stockpiled volumes met the estimated demands for the remainder of the project.  Despite this, the 4B material continued to be produced as an unwanted bi-product at a considerable rate.  At this time, half way through the project, the subcontractor demanded that material prices were renegotiated due to the sudden reduction in his earnings.  JH politely refused yet were unsurprised by the request due to previous grievances raised by the subcontractor.  The subcontractor subsequently request that JH purchased a cone crusher, in addition to their own crushing machine.  The idea was sold to JH under the guise that all the oversize waste material could then be re-processed, saving JH the need to excavate and haul further raw material.  An agreement was reached whereby by JH agreed to hire a cone crusher up to a limit of $45 K (AUD) after which time Volcanic Resources would assume the financial hire costs of the machine.

So What?

7.    Upon receiving the shiny cone crusher, material production rocketed and waste material stockpiles quickly diminished; a relief for me as the zone 4 stockpile was shrinking faster that it was being replenished.  However, all was not well.  Upon inspecting the material produced, it was apparent that there were distinct differences between the “pre” and “post” cone crusher zone 4 and 4A materials.  The zone 4 material appeared overly coarse and failed to bind when moulded.  Equally, the previously well graded zone 4A material was now very clean, uniformly graded gravel (perfect for a drive way NOT an RE Wall..).   I immediately interrogated  the Volcanic production staff on how they verified that they were producing conforming material (As the owner had already reassured me all was well).  They simply said there were given a maximum aggregate size for each material and that was it…

8.   48 Hours later and the results confirmed the my concerns.  Overly coarse zone 4 material and a uniform 4A material lacking sufficient fines to meet conformance.  Fortunately the material had not entered the wall.  Unfortunately, the material had contaminated existing stockpiles (but was fairly straight forward to identify and separate).

Lessons Learnt.

9.   Do not trust a subcontractors’ word without checking, even if they are contractually obliged to make the product.  Do not assume the obvious has been done; Common sense would say that as a material production specialist (i.e Volcanic Resources), if you change anything in a system you would re-test the new material to check for conformance.  Clearly this was not done and they simply did not care.  When I confronted the owner of Volcanic on the issue I was met with the following response:

“If you want your material to pass, why not test a stockpile that you know will pass?”….

Fill Material Production

10.   Since this time, they have struggled to produce conforming material.  The cone crusher failed to deliver what was promised and despite clear direction, they have attempted to load failing belt material directly in supply of the RE Wall.  Much of my time is now spent in the quarry..

Solution.

11.   I directed that we reverted to using predominantly raw feed (high % fines) with a blend of the oversize waste product.  Whilst better, the material was still struggling to pass due to a lack of fines.  As the ambient temperature has recently risen considerably, I directed that the raw feed stockpile was saturated in an attempt to bind the fines with the larger rock.  This had an immediate positive impact and produced conforming material.  However, the SPE was not overly happy as JH pay per tonnage and do not wish to pay for hauling water.  My dilemma was, do I delay Fusion Civil in the production of the RE Wall due to lack of fill material or pay for the haulage of water?…  The saga continues…

Reinforced Earth Wall – Left Abutment