New gravel road sealer put to test

Published on 16 April 2021

Horsham Rural City Council is hoping a new gravel sealing product being trialled at Lower Norton could lead to significant improvements in the condition of rural roads.

Krishna Shrestha, the Manager Strategic Asset Management at Horsham Rural City Council, is a civil engineer who has piloted successful trials of ‘Otta seals’ at Narrandera Shire Council in New South Wales.

Relatively unknown in Australia, Otta seals are a cost-efficient road treatment best suited for rural roads where traffic volumes are less than 200 vehicles per day.

Put simply, graded gravel/crushed rock is placed on a relatively thick film of soft bitumen, and then rolled.

Initially, the surface does not look dissimilar to a typical gravel road, but over time, the bitumen works its way upwards through the aggregate and turns into a smooth, interlocked bituminous premix.

The name ‘otta’ comes from Otta Valley in Norway, where the Norwegian Road Research Laboratory developed the treatment.

Now Otta seals, re-adapted for Australian conditions, are being put to the test south of Horsham at Plush Hannans Road.

Mr Shrestha is anticipating the same positive results as in Narranderra, where the road surface has outlasted traditional and more expensive pavement methods.

“We learnt that the varying sizes of rock protected the bitumen from direct solar radiation and associated oxidation and embrittlement. Because of graded aggregate used on sealing, there was also greater flexibility and therefore longevity,” he said.

HRCC is responsible for inspecting, maintaining and repairing 1,059 kilometres of sealed roads.

But it is on the 1,830 kilometres of unsealed roads throughout the municipality where major cost efficiencies could be made. 

That’s because constantly regravelling unsealed roads is becoming an unsustainable and increasingly costly exercise.

“Recent financial analysis suggests that Otta Sealing an existing unsealed road will reduce the lifecycle cost by half as the need for continuous resheeting will be curtailed.”

Contrary to casual assertions, Council invests heavily in local roads. In this year’s budget, $3.845 million is being spent on rural roads, and that doesn’t include funding sourced from state and federal governments.

“If we can upgrade many of our gravel roads to sealed roads using this new method, it will help us minimise the maintenance costs over the longer term,” Mr Shrestha said.

“We are using locally available graded gravel which meets the required plasticity index, gradation and hardness standards. This reduces cost of material and material haulage cost due to reduced travel distance. There will be additional cost reduction due to simplified installation and ability by local contractors to install this type of sealing.”

Because of its lower specification, this type of aggregate is not used on roads that carry thousands of vehicles per day.

“But that concern is irrelevant for a high number of our local roads. We have many which see as few as 10 vehicles per day and yet have to be maintained at the cost of our ratepayers,” Mr Shrestha said.

Otta Sealing at Plush Hannans Road will be monitored for one year to determine its performance under Wimmera weather and traffic conditions.

Following the seal, a 40 kilometre per hour speed limit will be in place for about six weeks while the pavement surface strengthens.

Residents are asked to adhere to the road signage and take care when traveling on the new pavement as there will be loose aggregate on the road.