Otta seals investigation to help dust problem on Valley's dirt roads

Otta seal works being carried out in Africa. Photo sourced from Alchetron
Otta seal works being carried out in Africa. Photo sourced from Alchetron

The engineering services department at the Nambucca Shire Council has been delving deep into the world of innovation, of late.

A few weeks ago Guardian News reported that Mann St is being pegged as the first possible North Coast road to trial plastiphalt – a recycled waste product road surfacing technology.

But there’s another revolutionary trial that is currently being investigated by our Council, and one that could help a very real issue for rural residents – dust from unsealed roads.

Currently, Bellingen Shire is the first local government area in Australia to be trialing Norwegian Otta seals – a type of road surface treatment formed by adding graded aggregate to a soft bituminous binding agent, typically emulsified asphalt. 

“Otta Seals have not been used in Australia as yet, they have been used with good results in New Zealand for the past 12 years and most recently adapted to steep gradient/curves road pavement,” assistant manager Nambucca Shire Council engineering services Paul Gallagher said.

They have also been tested extensively throughout Norway and developing countries across Asia and Africa.

Otta seals were developed to be used as a temporary surfacing on new roads, but outstanding performance results relating to strength and endurance have proved them useful as a permanent road sealant too.

Otta seals are typically used in low-traffic areas (around 500 vehicles per day).

The stone used in an Otta seal is much more variable in size than the aggregate used in a standard spray seal.

The stone used in an Otta seal is much more variable in size than the aggregate used in a standard spray seal.

And if the Bellingen trial goes well, Nambucca will consider implementing it into their current gravel resheeting program.

Currently, Council’s unsealed road network – a vast 345km expanse – is maintained through a 10-year program of regular grading with the addition of fresh gravel sheeting as necessary. 

But Council’s Asset Management Plan states that this procedure is under scrutiny, largely because of cost of maintenance, but also because “the condition of unsealed road surfaces is very dynamic and subject to sudden change under circumstances of differing weather conditions and changed traffic usage”.

The issue with ‘just sealing all the roads’ is that a standard 250 metre stretch of gravel road will cost around $60,000 to coat with a regular chip seal, plus costs for upkeep on top of that.

Just to lay a seal down on all of our currently unsealed roads would cost around $83 million to achieve.

And chip seals last around seven years, on average.

But varying reports suggest an Otta seal will last anywhere between 12 and 25 years (reported in South Africa), and perform 50 per cent better than a regular chip seal in maintenance terms.

The other huge benefit is the cost: “Bellingen Shire Council has estimated that the cost of sealing 250 metres of road with an Otta seal is $32,000,” the council report said.

So, this means an almost 50 per cent saving on your average chip seal.

“And the Otta seal is more waterproof that a conventional bitumen seal due to the mastic formation [the range of differently sized aggregate particles used fill most voids in the surface, making it less porous],” Mr Gallagher said.

He said during a trial in New Zealand, one road did experience a ‘washout’, but said this was a result of poor drainage. And pot holing and stripping was minimal in that trial when considering the length of road which was trialled.

Possible failures of the seals could include “ravelling and cracking due to hardening of binder (this can occur in a conventional bitumen seal), failed drainage due to lack of maintenance, base failure due to water incursion or high stresses breaking up the seal”.

But Mr Gallagher believes being on the precipice of trialing the Otta seals “is an exciting space to be in”.

This may offer a real option to some of the issues we have with dust and corrugation.

Mayor Rhonda Hoban agreed that a trial would be beneficial to help mitigate problems in areas where it “would realistically be 100 years before we could afford to put down asphalt”.

Where we’re at

Last week, representatives from Fulton Hogan (the New Zealand company spruiking the seals) visited the Shire to inspect the roads earmarked for the trial, if Council chooses to proceed.

“The advice I provided to Council was to try and dovetail the Otta Seal with our extensive gravel resheeting program as we have committed the plant and equipment to these projects,” Mr Gallagher said.

The roads inspected included South Arm Rd, Missabotti Rd, Newee Creek Rd, North Bank Rd, Taylors Arm Rd, Tamban Rd, Searles Rd, Spokes Rd, and the 2.5 km of Congarinni Rd South that has just been resheeted.

“We have asked the company whether they are willing to fund a full trial pavement and they have indicated they would, we haven’t entered into any formal agreement at his stage but, if this is successful, Council would enter into a partnership for further trials,” Mr Gallagher said.

Also in the news