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.
A grain bunker is only as good as its seal. You can build the straightest walls, lay the flattest pad and buy the toughest tarp on the market, but if the seal along the base isn’t right, moisture, air and pests will find their way in. A poor seal undoes months of careful storage planning within weeks, turning a bulk grain receival site into a source of spoilage and lost tonnage.
The good news is that sealing a bunker properly isn’t complicated once you understand the two main setups and exactly how the top tarp and ground sheet need to come together. This guide walks through both approaches, the correct fold-and-secure technique and why one securing method consistently outperforms the alternatives growers try.
Broadly, bunkers are sealed one of two ways and which one suits your site depends on your pad design and how permanent you want the setup to be.
Earth wall bunkers use compacted soil banks, typically formed on three sides of the pad in a U-shape, to hold and support the grain stack. The ground sheet is laid up and over the earth wall before inloading, and once the bunker is filled, the top tarp is pulled down over the top of the wall to meet it. From there, both layers are folded together and secured along the join. Earth walls have been used for decades because they slope water away from the pad naturally and give the tarp a stable surface to sit against, rather than leaving it exposed at ground level.
Where earth walls aren’t practical, the “no walls” method is used instead. The ground sheet is laid straight onto the pad, the grain is stacked and the top cover is pulled over once filling is complete. The tarp is then sealed directly to the ground sheet at ground level, without any wall or bank to support it. This is a fast, cost-efficient option that suits growers who want flexibility to adjust bunker size and location from one harvest to the next.
Whichever method you choose, the same principle applies: an unbroken, weighted seal along every edge, with no gaps for wind, rain or vermin to exploit.
Most sealing problems happen at the join between the top tarp and the ground sheet underneath the grain. Get this fold wrong and moisture can wick in from underneath even if the top of the bunker looks perfectly sealed.
The correct technique is straightforward:
This same fold-and-secure method also works on the open ends of an earth wall bunker, giving you a consistent seal all the way around rather than just down the two long sides.
Once the fold is in place, it needs a securing method that holds it down continuously, in all weather, for the length of the storage period. Concrete tyres are the standout choice here, and it comes down to how the weight is distributed.
A concrete tyre sits low and hugs the contour of the folded tarp, concentrating a solid, even weight directly on the seam rather than spreading it thinly along a long, rigid object. That constant downward pressure keeps the fold pinned in place even in strong wind, and the smooth, rounded profile means there’s no sharp edge that can wear through the fabric over time. Concrete tyres are also durable enough to be picked up, moved and reused bunker after bunker, which makes them a genuinely cost-effective long-term investment rather than a one-season fix. For a full walkthrough of positioning and pulling technique, our guide on pulling a bunker tarp over a grain stack covers the process in more detail.
Two other proven methods are worth knowing, even if concrete tyres remain the most common choice for no-walls and earth wall setups:
Not every weighting method holds up once it’s tested against real weather and a full harvest. Three alternatives commonly get tried on-farm, and all three fall short for the same underlying reason, they don’t hold a continuous, concentrated weight against the fold:
If you’re currently using any of these along part of your bunker, it’s worth reviewing that section before the next fill and switching to concrete tyres, an earth trench, or wall clamps instead.
A grain bunker’s tarp and ground sheet are doing the real work of keeping your harvest dry, cool and pest-free, but they can only do that job if the seal holds from one end of the bunker to the other. Whether you’re running an earth wall bunker or a no-walls setup, the principle is the same: fold the top tarp and ground sheet together and secure that fold continuously with a method that concentrates weight rather than spreading it thin. For a full walkthrough of the entire process from pad to peak, our Grain Bunker Construction Guide covers each stage in detail. Our Grain Bunker Calculator can help you work out the right tarp and ground sheet sizes before you start.
It’s worth doing a visual inspection after any significant wind or storm event, and at least once a month during storage, checking specifically along the fold line for any gaps, wear or shifting weights.
It’s best to keep the method consistent around the full perimeter where possible. Mixing methods often creates weak points exactly where the different sections meet.
Both work well when executed correctly. Earth walls suit sites with suitable soil and a preference for a more permanent structure, while the no-walls approach offers more flexibility for growers who adjust bunker size or location season to season.
A generous overlap is essential, with both layers folded together rather than laid edge to edge. The more overlap you build in, the more forgiving the seal will be against wind and rain.
This depends on the perimeter length of your bunker. A continuous line with tyres placed close together along the entire fold is recommended, with no gaps left unweighted.
Sealing a grain bunker correctly protects every tonne of grain underneath it, and it’s not a step worth cutting corners on. If you’re planning a new bunker, upgrading an existing one or simply want advice on the right combination of tarps, ground sheets and securing methods for your site, the team at Polytex has over 50 years of experience helping Australian growers get it right.
Call 1300 059 003 or email [email protected] to talk through your setup, or explore our range of grain bunker packages built to include everything you need for a properly sealed bunker.