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During the month of July, our Graduate Engineer, Nathan Raasch spent eight days in Boulia Shire supervising a bitumen double/double armour coat on the Donohue Highway, around 210km from Boulia and 25km from the Northern Territory border. The Donohue Highway is part of the Outback Highway and stretches from Laverton in Western Australia to Winton in Queensland, around 2700km – better known as Australia’s longest shortcut.

Whilst on the job, Nathan captured some great shots of the process as seen throughout.


Firstly, one of our Partners (and pilots), Stu Bourne took Nathan on a morning flight from Barcaldine to Bedourie, to Boulia, (and even to Tobermorey Station in the Northern Territory which has an unsealed dirt runway!) to introduce him to the various projects, Councils, contractors and to inspect the pavement works on the Donohue in preparation of the seal.


The Donohue Highway required a double double armour coat seal (also known as an inverted seal) which followed the below process:


The primer soaks into the pavement and helps the bitumen to stick to the pavement. We allowed three days for the primer to soak in and dry.

Initial Seal 7mm As this was an ‘armour coat’, the 7mm aggregate was spread first. This process consists of firstly, spraying bitumen on top of primer, followed by aggregate spreading on top of freshly laid bitumen.

Final Coat 14mm For the final coat, another layer of bitumen was spread on top of the 7mm seal, followed by a larger aggregate spread on top of freshly laid bitumen.


During this process, the bitumen is laid at just below 200 degrees as it comes from the spray truck

During the aggregate spreading, the whole road surface needs to be covered as quickly as possible (taking into consideration the conditions) after the bitumen is sprayed, to allow the aggregate to penetrate while the bitumen is still hot and viscous. However, applying aggregate too soon could cause it to flush and bleed. The perfect timing depends on many factors - such as the weather, ambient temperature, and temperature of the road pavement. It's a delicate balancing game but with three aggregate spreaders in a row, it was timed perfectly.

Finally, our Multi Tyre Pneumatic Rollers followed ensuring the aggregate rolls around and settle on its flattest side. This allows more penetration into the bitumen and prevents the bitumen from stripping.

This inverted seal has been trialled over a number of years and is mainly used on soft pavements (such as black soils which covers most of the outback) to reduce embedment as the large aggregate is fully supported on smaller aggregate.


This project was so close to the NT border that Nathan stayed at Tobermorey Station inside the NT rather than drive +400km a day to get to site and back if he'd stayed in Boulia.

Nathan packed his swag, chair, table and small gas cooker and was set!

"Since working further out west, I have even been able to dip my toes into the Northern Territory and South Australia! GBA Consulting Engineeers have provided me with the opportunity to see this great country of ours and experience breathtaking moments. I loved my time at the station camping…oops, I mean working. The sunrises and sets were absolutely breathtaking, and there was no need for an alarm as the big flock of Galahs would wake you up.”

Nathan also packed his film camera and captured some excellent shots of his travels.

All images captured by Nathan Raasch @natey_raasch


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