Holdfast Training Services Trial Blog

It has been a long Christmas break. Partly because BG&E extended their close-down period to 3 weeks, but mainly because we had both sets of parents over. It has been like trying to herd deaf, incontinent cats for 2 weeks. I have been at my wits end, and I have actually been glad to get back to work. 

The start of the new year has brought with it a few new projects for BG&E, not many but the year is still young. It is still quite evident that the company is scratching around for work in order to fill time until something more substantial comes along. At present I have continued to work on culvert sizing reports for the Main Roads WA panel contract, I am still waiting on a response to a proposal to upgrade a culvert crossing in the Perth Hills (though this may be shelved by the Department of Parks and Wildlife whilst the deal with the fallout of the recent bushfires), and I have recently been handed management of a bridge replacement proposal in the middle of nowhere. 

The Shire of Murchison strangely prides itself on being the only Shire in Australia to have no Township. With a land mass of 50,000 square km and a population of 117. The Ballinyoo bridge replacement project was initiated by MRWA in 2006 but now in 2012 on it’s 4th iteration it looks like the planets have aligned and it’s a goer! 

Only 9 hours from mine

The main reason for now replacing the bridge (aside from it’s amazing state of disrepair) is ironically due to the Shires isolation. The government will be starting the Square Kilometre Array (SKA) project to place loads of satellite telescopes in the area where there is limited ambient light interference. They expect that the construction traffic alone will increase the roads daily vehicle use to a whoping 15 from a mere 3. Couple to that the fact that it is only serviceable for 8 months of the year when not inundated by the massive 700m3/s flows it regularly expects in the wet season, and you wonder why they are bothering. The bridge was the first precast concrete bridge in WA, and though it is 85years old, the initial quality was pretty poor standard and it has required significant maintenance just to keep it open over the years. 

My main effort this week has been baselining where the project sits with BG&E. One of the main issues has been what the client actually wants. A 20year dry serviceable structure has been requested, though not formally. It would be easy to provide a simple design to achieve a 20 year dry bridge but due to a total of a 2400m floodway along the sides of the structure with a dry serviceable return period of 7 years, it hardly seems worth it. I organised a kick start meeting with anybody who had corporate knowledge of the project which was useful to keep all informed and for me to establish a route forward. Following the meeting I have been in comms with the client to backbrief them my recommended scope and to confirm the findings of my previous work review, in the hope that once confirmed, I can produce an accurate proposal for completing the work. 

The project comprises 3 BG&E elements:

• Waterways – Design flows and stage heights to set bridge soffit height and culvert sizes
• Civil – Approaches, road alignment, culvert
• Structural – Substructure and Superstructure

Waterways

The catchment area is pretty huge at approx 82,300 km2. It’s flood flow at 20 year return period is approx. 750m3/s at a height of 246.7m AOD. When the deck height is currently 245.9m AOD this is a problem as the 5 year period is actually higher. The issue with raising the bridge to meet the 20 year period is that to raise the bridge means to raise the road, which will increase the headwater upstream of the bridge and therefore mean that the deck will need to be higher still. An option to reduce this is to install culverts into the approach rises to lower the stage height. By placing a culvert through it attenuates the storage somewhat and allows a lower road/bridge. It is an iterative process that can only really be obtained accurately using computer software. That said, design is based on historical flood measurements, estimations of flow, surveyed levels and guessed manning’s numbers so ‘accurate’ is a pretty vague word! The question remains whether it is financially viable to raise the road height along the whole 2.4km alignment.

  Civil 

Once the bridge height is set and the intent for the alignment is established (floodway/culvert/nothing), the design of the approach embankments and road can be completed. Nothing much has been done on this yet, as they originally worked up 2 alternative alignments in the 3^rd iteration when the existing alignment was off the table. The client now wants it on the original alignment. This is difficult for BG&E as the project is currently in the proposal phase and therefore not billed. This deficit will be made up in the fee if won. 

Structural

The design at present will be a 72m proprietary Rocla bridge system (so not much in the way of superstructure design) with a piled/rock socketed foundation that will be connected to the newly raised road by 2 earth bund approaches with an additional culvert to maximise the dry serviceability period.

Rockla Bridge Deck System

  The original plan was to use the Rocla precast spun concrete piles, but due to them cracking on a previous build, standard circular concrete piles or H piles will be driven. These piles form the piers that join with the headstock – this is something I need to look into this week.

Still a long way to go on this one. Not entirely looking forward to the site visit, but at least I will get a bit of red dust on my boots. Finding it particularly hard to pin anyone down on anything, so I have started a decision register that I can roll out at meetings to track past and current decisions made.