Otta Seals: a sustainable and economic bituminous surface treatment*

Article image - Otta Seals: a sustainable and economic bituminous surface treatment* Photograph courtesy Krishna Shrestha, Narrandera Shire Council

Improving asset management can often come from new or improved technology not just enhanced processing and reporting of data sets.

One such instance where adopting and adapting technology has proved to be a success has been in the central New South Wales regional Shire of Narrandera.

Otta seals, relatively unknown in Australia, are a low volume, low cost road treatment.

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

The name ‘otta’ comes from Otta Valley in Norway, where the Norwegian Road Research Laboratory (NRRL) developed this bituminous surface treatment.

Otta seals are predominately used in South Africa but have become attractive to some Australian councils seeking greater sustainability, combined with a view that several current standards, if applied directly, would lead to excessive cost and overuse of scarce premium materials ultimately leading to inefficient asset management.

Narrandera Shire Council has 300 km of sealed roads and 800 km of unsealed roads, but the unsealed roads only serve about ten percent of the population.

The significance of the savings from introducing adapted technology is made clear by referencing against their annual rate revenue, where to reseal the sprayed seal network on a ten year return cycle (as recommended by Austroads and ARRB) the annual reseal budget would be in the order of 20 percent of the annual rate revenue.

If they were to re-sheet the unsealed network at a five year interval as recommended, the annual re-sheet budget would be about 90 percent of the annual rate revenue.

Council have realised considerable lifecycle cost savings with otta seals for local roads with low traffic volumes due to the use of less tightly specified materials and a longer period between maintenance interventions.

Narrandera Shire Council completed about 65 km of otta seal in last four years, 20 km of new otta seal in 2017/2018 alone.

Importantly, they have customised the process of otta seals to suit the Australian environment, including amending gradation of stone, percentage of cutter and rolling regime and signage after application.

For further information about otta seals contact the Manager of Works, Narrandera Shire Council, or Centre for Pavement Engineering Education (CPEE) Executive Manager Professional Development, Kym Neaylon, at
CPEE is dedicated to road and pavement engineering education, and because of this dedication regularly provides training in regional centres and the more remote parts of Australia.

In addition to short courses in capital cities, in 2019 CPEE is providing short courses in Broken Hill, Newcastle, Wagga Wagga, Dubbo, Bendigo, Ballarat, Launceston, Rockhampton and Maroochydore, with more courses in the works.

For further information about CPEE short course training and higher education options visit or call the CPEE office on +61 (03) 9890 5155.
*Copy supplied by CPEE