Since the Brunswick Heads Bypass in 2005, we have developed a process which has always proven effective when all of our recommendations were followed.
We have developed specialised batter chains which are used to lightly scarify the subgrade prior to the placement of topsoil, and the chains are then used again to spread the topsoil layer to average thickness of 50-100mm. the chains have 20mm ripping bars fixed at approx. 450mm centres which after several passes produce a loose topsoil bed with horizontal furrows which has been keyed into the subgrade.
Links to videos
https://youtu.be/bg4lf4BmF7k Conglomerate Subgrade
https://youtu.be/b7U6CxG15RM Spread Topsoil on Batter.
If subgrade soil test results indicate that the subsoils are significantly inferior to the site topsoil, additional ameliorants can be placed in the hydromulch applied to the surface of the spread topsoil to ensure deep penetration of all of the ameliorants.
The application also includes organic and inorganic fertilisers, microfine lime, microfine gypsum and 6mm minus AS4454 screened compost, all of which being microfine or fine varieties are readily absorbed into the topsoil surface and penetrate through the topsoil into the subgrade after rainfall. It is generally accepted but usually ignored that Soil Microbes break down organic matter, recycle nutrients, create humus, create soil structure, fix nitrogen, promote plant growth and control pests and diseases.
Lime can increase the pH of an acidic soil, but gypsum will not decrease the pH of an alkaline soil. Gypsum applied to an acidic soil will improve crop growth and gypsum, being partly soluble, moves readily down the soil profile with rainfall to reach the subsoil. Astute farmers always apply a lime and gypsum mixture rather than either of these ameliorants on their own.
When a fine organic compost is included in the mix, the microbiological action of the compost moves down through the soil profile with the gypsum, releasing the soil nutrients and improving uptake of these nutrients in the plant roots. As this process results in the rejuvenation of the soil chemistry and microbiology, it is not time-limited and so provides long-term nutrition for the developing plants.
There are no controlled, long-term release fertilisers available in a hydraulic application, as those available are usually coated with a soluble binder, which is washed off in the hydromulch process.
The soils treated with our application usually become calcic, but field trials have proven this to not be at the detriment of plant growth. This application also complies with the requirements of R178 in regard to the soil testing and amelioration to the soil scientist recommendations.
Unfortunately, there is a culture on many construction sites where the adoption of soil testing is discarded as it is assumed to be a mechanism by which costs can be increased without improving results. What in effect happens is that the money supposedly ‘saved’ by not approving topsoil investigation, is used many times over in long-term rectification and management after the works have been completed.
Over the last three years or so we completed approx. 1,000,000 sq. m of our above application on the W2B Project between Maclean and Ballina and proved that R178PC as issued for the works was inappropriate and impossible to adhere to on a construction site.
Between Aug 2017 and Dec 2017 we completed 500,000 sq m of our application on alignment clearing and grubbing between the Iluka Turnoff and Devil’s Pulpit. Our application proposed for the works was accepted by Pacific Complete as it had been proven effective, even on bare clay fill batters without topsoil, on Wave 1 Works over the previous two years when the Specification Straw Application had been a dismal failure and either washed off the batters or floated away in the drains during rainfall common to the site throughout most of the year.
The straw application on works completed by others since then continues to be ineffective and does not produce the required grass cover or long term germination and erosion control. Washouts due to lack of development of an adequate surface cover are a continuing problem along the whole of the alignment.
There is an obvious concern within RMS that all the changes to the specification over the last ten years or so have not improved results and in fact costs have risen due to the complexities and impractical methods which have been trialled based solely on some impressive advertising without any independent technical endorsement.
Although most of our work on W2B has now been removed as the earthworks proceed, we have drone footage at various stages together with extensive photographs and reports to prove our point.
We could have achieved the poor results which are apparently being accepted as conforming on the works at considerably less cost than the heavy straw application.
Early in 2018 RMS NSW adopted several of R178PC’s unworkable and impractical provisions on early Stages of the Northern Road Upgrade at Bringelly but reverted back to R178 provisions of about 10 years ago in the latest Edition of R178 issued in Aug 2018.
Soil Microbiology is an important consideration for successful revegetation.