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ASAE Conference Proceeding

This is not a peer-reviewed article.

Bunker Silo Systems Engineered for Producer Profit

P. Miller, J. H. Marter

Pp. 186-191 in Fifth International Dairy Housing Proceedings of the 29-31 January 2003 Conference, (Fort Worth, Texas, USA), ed. K. A. Janni. ,Pub. date 29 January 2003 . ASAE Pub #701P0203


Pre-cast reinforced concrete bunker silos are cost effective as a farm bulk storage feeding system. This paper examines the costs and benefits and demonstrates how dairy producers can maximize profits. It also emphasizes the need for a stable engineering design in order to avoid common wall failures from overturning, sliding, and structural defects.

KEYWORDS . Agricultural engineering, bulk storage, dairy farming, economic evaluation, farm management, feeding, livestock farming, safety, silos, site preparation, walls

Bunker Silos

By working day to day manufacturing, selling and installing bunker silos we've been able to lead the market with design technology, storage performance and economic analysis that allows today's dairyman to make the right decisions based on facts.

Bunker silos have three (3) key elements:

  1. Site Preparation (Table #1)

  2. Walls Systems (Table #2)

  3. Floors

When producers ask the question "How much does a bunker silo cost?Ē the answer is below $30.00 per ton (as fed, wet tons) in total investment cost.

The secret to properly designing bunker silos is to first meet the technical needs (i.e. types of feed stuffs and feed off rates), then optimize the costs to make the best investment decisions.

The process of selecting a bunker silo has to do with:

  1. Need a little more storage

  2. Farm is in transition

  3. Replacing worn out uprights

  4. Not happy with current flat storage

  5. Need a primary feed storage system for a dairy expansion or new dairy

Given the answers to the above, then what is next ?

  • Look at the current system and decide whether or not expansion of the dairy is in the future.

  • Where will the feed center be located?

  • Is there good drainage at the proposed storage site?

  • Is there an all weather access road to the site?

  • Does the site allow for further growth?

The next step is to create a budget with the producer and organize the phasing of construction.

When it comes to bunker silo wall systems, the critical design issues are:

  • Slipping

  • Tipping

  • Bending

No matter if you buy your bunker walls from us (Precast Concrete Walls) or you select other systems, these three critical design issues must be resolved if you expect performance and durability. With precast concrete engineers are able to predetermine the performance and durability issues before the panels are put in service. Itís the entire reason for precast concrete.

Sidewall height is a very common question that producers ask all the time. What is the right height? The answer lies in the technical design that is tailored to each individual farmstead. It is the most important issue - size the bunker correctly - then all the other issues can be addressed.

Usually the highest walls that meet the correct feed off rate give you the best bang for your dollar. The most common wall heights available in our precast products are 6-ft, 8.5 ft, 12-ft, and 16-ft heights - all of these panels are free standing. Who knows what heights will be next?

Agronomics is the word I use often - it is a method to help select designs for maximum economy - why?

  • Best Value

  • Lowest potential for surface area exposure

  • Usually has the lowest investment/operating cost per ton of storage

You must pack and cover your bunker silo or donít build one!

Detailed Planning is a Must

Single or Multi-compartment Bunkers?

A twin cell bunker silo always costs less than a single cell of the same capacity (plus lots of other benefits). This is a site-specific decision to be made with consideration of all the factors, not just the economics.

Should the bunker have an end wall or not? Yes, if the site offers opportunity for operational advantages, no if the site is flat.

Some of the "Nuts" and "Bolts" of BUNKER SILOS

Methods of Covering Bunker Silos That Work:

The following items are needed to cover a bunker silo:

White/black plastic - 3 mil thickness and the size that will cover the bunker plus 4 feet over on each side of the walls.

Tires or tire walls that will cover the entire bunker top (pieces must touch one another)

Sand bags, you need 100 each for 100 ft. long x 30 ft. wide bunker silo

A new method is being tested, that is a ratchet strap tie-down (Table #3)

Start at the closed end of the bunker (non-feed end if both ends are open), roll out the plastic and let it drape over the end wall, place sandbags end to end across the entire end wall.

As you roll plastic over the top of the feed, place a sandbag on the top of the plastic on each sidewall and approximately every 15 feet on the top of the plastic on the feed two-foot in from the wall and perpendicular to the wall.

Plastic should always drape over the sidewall until it reaches the down slope at the feed off end.

When the plastic is not wide enough to cover the entire width of the bunker and enough extra to create a drain channel on each side, then cover across the width of the pile with several custom cut pieces and make sure the overlap is from the non-feed end to the feed end.

Then add tires or tire side walls that touch one another over the entire top surface.

Twin cell or triple cell or more cells, can be covered in the same fashion from non-feed end to feed end, the sides of this plastic must go over the common wall and past the next cell drain channel.

Instead of using tires on top of the plastic, the ratchet strap system can be used.

Bunker Silos MUST be covered to get the best value.

Floors in Bunker Silos

Reinforced concrete

Reinforced concrete floors, with a minimum of 4 inches of concrete thickness are recommended. More often 5-inch floors are placed to handle loading imposed by large equipment. Refer to ACI standards 318

The floors are always placed between the inside edge of the wall panel's integral footings. The floor will remain independent of the walls and will not be tied to the wall footings.

A concrete apron at the front or feed-end of the bunker is recommended and should be at least 30 feet in depth and as wide as the silo opening.


What about blacktop for a bunker silo floor? Blacktop has been used for bunker silo floors - however - unless it costs no more than 60% of a concrete floor, there is no advantage. Also, if using blacktop, the color must be changed (sweep lime over the entire surface) so that you do not create a heat sink, which will cause spoiled feed. One of the biggest disadvantages of blacktop, is that it requires a very expensive sub grade if it is expected to perform for the life of the storage system.

Site Preparation

The site preparation required under the precast concrete wall systems, call for ĺ inch minus crushed stone which includes the fines with a minimum thickness of 4 inches for the lowest wall systems and 10 to 12 inches for the highest wall systems. This leveling pad must exceed the footing width by at least 1 foot on each side of the panel footprint. Drainage away from the panel sidewalls is important. The fill that is required underneath the poured-in-place concrete floor could be a sand lift. Drain tile around the bunker structure is recommended. This tile will collect the leachate and transfer this to its final disposition.

Stacking Slabs

Stacking slabs are the start of a future bunker silo complex. The slab should be designed with the concept of walls being added at a later date, even if it doesn't happen, you have lost nothing and you would not waste lots of future dollars.

With stacking slabs, packing of the silage becomes a real challenge. Covering and feeding from a stacking slab can be very costly in terms of spoilage losses. The cost of the slab alone can be cost prohibitive.

Silage Bags

Bags can be a great way to store some excess feed on a temporary basis. As a permanent storage system, bags cost too much, feed too slow, and use too much valuable land area. There are many more details. For more in-depth data, we offer our Bunker Silo Manual and have a design analysis program for technical and economic review. Refer to Bunker Silo Manual by Wieser Concrete


To achieve the maximum quality of feed delivered from a bunker silo system, the technical design is the first priority. The engineering design for proper feed-off rate, gives the highest quality feed, lowest losses, and the lowest operational costs. Attention to the details of design, packing and covering will influence the desired results.


ACI Standards 318

Bunker Silo Manual by Wieser Concrete Products, Inc. Copyright 1992