Not sure what size Grain Bunker Size you need for your desired capacity? Try our Grain Bunker Calculator.
Not sure what size Grain Bunker Size you need for your desired capacity? Try our Grain Bunker Calculator.
Bunkers are one of the most cost-effective ways to hold large volumes of grain on farm, but their performance is set long before the first load arrives. It is set by the foundation, the pad the whole bunker sits on.
A correctly designed pad does not manage water. It removes the opportunity for water to accumulate anywhere. Rain moves off the cover, off the surface, away from the toe of the stack and stays clear of the storage area. Once that profile is built and compacted, wet-season grain quality stops being a variable you monitor, but an outcome you have already decided.
Six decisions determine whether a pad performs that way: where it sits, how far it is raised above surrounding ground, what camber runs across it, what gradient runs along it, how well it is compacted and where the runoff goes once it leaves the tarps. All six are made before any grain arrives, and all six are expensive to change afterwards.
When grain spoils along the base of a stack or a soft patch appears mid-bunker, the cause is almost never the tarp. It traces back to one of five decisions made during construction:
Each of these is a design fault, not a weather event. The rest of this guide addresses them in the order you would build the pad.
Site selection is the cheapest and most-effective drainage work available, because good country does most of the job before any earth is moved.
Look for:
Orientation is part of the same decision. Positioning the closed end of the bunker into the prevailing wind reduces wind loading and lift on the covers, which protects the tensioned profile that sheds water in the first place.
Growers who get this right tend to describe it the same way. Avondale Farms at Terrappee in northern Victoria now runs three bunkers on a site chosen for its natural fall away from the grain face and rates drainage as the precondition for the whole system working.
This is non-negotiable. The pad must sit above the surrounding ground so natural drainage moves water away from the grain rather than toward it.
On sloping land, a well-chosen site may already provide most of the required elevation. On flat regions, the pad has to be built up, either by importing material or by cutting and filling on site. Taking that material from immediately around the pad achieves both objectives at once: the pad rises and the ground beside it drops, establishing a fall away from the storage footprint before any drainage channels are formed.
Prepare the pad with a slight camber to either side and a small rise through the centre. This crown moves any moisture that reaches the surface out to the edges instead of lettingit track lengthways beneath the stack, where it has no way out and stays in contact with the grain the longest.
The volume of earthworks involved is small. It repays itself the first time the site takes an unexpected 40mm of rain.
Along the length of the bunker, maintain a gradient of at least 0.8% toward the open end, so rainfall drains away from the grain face rather than pooling against it.
Consistency matters as much as the number. Inconsistent grades create intermediate low spots that trap water in the middle of the pad, which is the hardest place to detect a problem and the most expensive place to have one. Set the grade with a machine that can hold it and check it across the full length before the ground sheet goes down.
The pad must be compacted well enough to sustain loaded trucks and machinery without rutting.
Compaction is what protects everything built in the previous three steps. A pad that holds its shape keeps shedding water for years. A soft one loses its camber in the first wet week of harvest, and the ruts that replace it channel water directly under the stack. Compaction also determines whether the site remains trafficable when you most need to move grain out.
The final element is a drainage area around the base of the pad to receive and divert runoff coming off the tarps.
Position drainage lines and any pits away from high-traffic areas so trucks and augers are not running across them and progressively destroying the profile.
A well-built pad still needs a barrier between the ground and the grain. Ground sheets laid across the entire pad before filling are standard practice for two reasons: they stop residual ground moisture migrating up into the base of the stack, and they prevent contamination and physical grain loss into the soil.
Treat the ground sheet as a specified component of the pad rather than an optional extra. It is a small investment against losing the bottom layer of a 1,000-tonne stack.
Wall options belong in the design stage, not after the pad is built. Earth, concrete or modular poly walls increase the capacity you can store on the same footprint, and they change the pad dimensions you need.
Rounding the ends to match the auger’s natural filling shape is a common way growers gain capacity without moving to larger tarps. You can model capacity, footprint and cover requirements before you turn a sod with the grain bunker calculator.
The pad manages water from below. Filling technique and a correctly tensioned bunker cover manage water from above, and a smooth fill profile is the reason water never sits on the tarp in the first place.
For the full step-by-step process, the grain bunker construction guide collects proven practice from growers and bulk handlers around the country, and the construction and operation video walks through the setup visually. Ahead of each season, work through the checks that get a bunker ready before the first truck arrives.
A properly designed pad carries an up-front cost, and it should be assessed against the alternative rather than against zero.
Bouts of heavy rain can spoil enough grain to exceed the entire earthworks bill, and a poorly drained pad continues to cost money every wet season it remains in service. Bunker storage typically runs at a fraction of the cost of vertical storage per tonne per year, but those economics only hold if grain comes out in the condition it went in. We break the numbers down in our comparison of grain storage cost efficiency.
Growers who have moved from single-use grain bags to reusable bunkers, including a 25,000-tonne wheat operation at Esperance in WA, consistently identify the pad as the component worth doing properly the first time.
A well-designed pad and a quality cover are what separate grain that stores predictably from grain that spoils. For over three decades, Polytex has manufactured high-strength bunker tarps and ground sheets for Australian growers and bulk handlers, and we work with sites at the planning stage rather than after the first wet season.
Browse the on-farm grain bunker packages or contact the team on 1300 059 003 for a custom quote and free advice on planning your storage.