The role of ground sheeting in bunker storages is to exclude ground water and contaminants from gaining access to the grain stockpile, and also to prevent any escape of the fumigation gas which is of critical importance protecting the stored grain from insect attack.
The evolution of ground sheeting materials and systems has been a central element of the success of the grain bunker storage system. Without this, it is doubtful that the bunker system would have become as widely adopted as has occurred throughout Australia and increasingly in other parts of the world.
The bunker (bulkhead) grain storage system had its origins as a temporary ‘overflow’ solution in the event fixed storages had reached full-capacity during ‘bin-buster’ harvests. Often this Australian invention involved holding overflow grain on the local cricket ground. In Western Australia bunker storages are still sometimes referred to as TS’s (temporary storages) despite little grain now being stored any other way.
In the early years of the evolution of grain storage bunkers very few of growers co-operatives (one per state) had the necessary funds to invest in asphalt hard-stands on which bunkers could then be constructed. Although CBH in Western Australia invested heavily over many decades in asphalt hard-stands for its bunkers, for most grain handlers, bunkers were built on compacted gravel sites. These sites therefore needed plastic sheeting underneath the grain to protect it from ground water, contaminants and the risk of escaping gas. At the time, the most suitable ground sheeting material available was extrusion-blown black or clear polyethelyne (PE) film having a thickness of 200 microns and a maximum available width just 4.0 metres.
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Cargill operations manager, Richard Shields, recently recalled that at SACBH (now Bunge) and at AWB GrainFlow (now Cargill) … “we were forced to use 50 metre length rolls of black builder’s film taped together … it was a nightmare in windy conditions”.
The situation changed dramatically about 25 years ago when a Queensland AWB GrainFlow site manager, Jim Moran, suggested that the rolls of narrow width PE ground sheeting film should be replaced with large dimension ground sheets fabricated from woven/coated PE fabric. A typical ground sheet was 42.0m x 30.0m, and had a folding pattern which enabled progressive and quick deployment even in windy conditions. A fabric weight of 120 gsm was adopted, although in more recent years there has been a move to a more study 180 gsm. A major West Australian fabricator specialising in supplying the Grain Industry materials took-up the suggestion and this ‘new’ approach to the ground sheeting system was adopted for AWB. It was quickly also adopted Australia-wide. The only exceptions were the sites lucky enough to have asphalt hand-stands, and also a major operator in South Australia which maintained the use of rolls of PE ground sheeting film.
Grain stored on a polyethylene ground sheet prior to covering.
The woven/coated ground sheets overcame the contamination of grain in bunkers during out-loading activities – when fragments of cheap PE film found their way in the grain.
The use of large-dimension prefabricated woven/coated ground sheeting accelerated the acceptance of grain bunkers by replacing almost entirely the use of unmanageable narrow-width rolls of black and clear PE films. This is turn added motivation for grain handlers, traders and growers to convert from fixed storages to bunkers which are now the dominant grain storage system in Australia due to their flexibility and low cost.
Ironically, over the past 30 years progress has been made in the development of wider-width blown PE films, first to 6 metres, than 8 metres and then 10 metres. Today widths of up to 20 metres are available. The other important development was in the film itself, by adding metallocine to the PE formulation. This ‘new’ film mPE is very much tougher than the standard PE’s. Wide-width and more sturdy mPE films has led to ground sheeting film making something of a comeback in some quarters as a ‘back-to-the-future’ option for ground sheeting at a very similar square metre price.
One unwanted feature of woven/coated ground sheets is the risk of ‘pilling’ during out-loading activities. Pilling is the formation and de-lamination of tiny balls of coating material caused by the pushing of grain back and forth by the buckets on front-end loaders. Although good qualtity ground sheeting material minimises the risk of pilling through better coating adhesion strength and the chemical formulation of the coating itself. However, pilling can still be encountered if cheap ground sheets are used.
Another key aspect of ground sheeting, whether film or fabric, is ensuring generous over-laps between sheets when deploying. Overlaps should be 1.5 metres minimum and should run with down-hill lapping to minimise the risk of ingress of ground water. Stitching of laps is preferable over lapping alone. The fins created when stitching should preferably face down rather than up into the grain pile. Being water tight fabric, if these procedures are closely followed, very rarely can the root cause of water ingress into bunkers be blamed on ground sheeting.
As can be appreciated, ground sheeting is a core element of the grain bunker storage system. The progressive improvements in ground sheeting films and fabrics has been central in enabling the adoption of the bunker system as Australia’s preferred system for low-cost flexible grain storage.
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