Originally Published as: Why Do Pole Barn Posts Settle?
Mike Momb has been Technical Director for Hansen Pole Buildings, LLC of Browns Valley, Minnesota for more than 20 years. His daily post-frame blog, as well as his weekly “The Pole Barn Guru” column can be followed at the company website, www.hansenpolebuildings.com/blog/.
A pole barn’s structural integrity depends heavily on its foundation. When posts settle, an entire building can shift or lean. This movement often indicates that the soil’s load-bearing capacity beneath the structure has been compromised or was never properly prepared, leading to uneven weight distribution causing frame distortion over time.
Pole barns are often built using agricultural exemptions to building code requirements, or in areas that don’t require structural plan reviews or inspections. Fully engineered post-frame buildings, conversely, involve highly trained design professionals who follow sound engineering practices to prevent columns from settling.
How can one tell if pole barn posts are sinking?
When a pole barn structure begins to settle, signs are often subtle at first, appearing as small, manageable issues. However, these can escalate if left unaddressed. A noticeable slope at doorway thresholds or doors suddenly requiring a shoulder check to close suggests the frame itself is twisting due to uneven support. Exterior splash plank gaps are particularly telling, as they often reveal independent movement between building and surrounding grade.
You can confirm active settlement by marking cracks and monitoring them over time to see whether movement is ongoing. Visual indicator checks include walking the perimeter after rain to see whether steel siding is separating from the splash plank, a major red flag for foundation failure.
Every pole barn project starts with dirt. To achieve a quality outcome, it takes a quality beginning. It’s hard to believe, but there is a lot of “stuff” to cover about dirt.
Building site preparation begins with proper drainage. Plan to keep the building grade higher than the surrounding site. On an ideal site, water drains naturally away from the building. Since few sites are ideal, grade work is often required to keep surface water away from the building. Keeping the finished building floor higher than the surrounding site reduces the chance of flooding during heavy rainfall or rapid snowmelt.
In floodplains, consult your building department first to determine their requirements. A typical recommendation is to set the finished floor level higher than the flood level. This may require importing fill to raise grade. You can hire a surveyor to determine these heights accurately. In some cases, vents may be installed below flood level to equalize interior and exterior pressures,
Many sites can be graded with a skid steer (a.k.a. Bobcat) or backhoe. Some cases require heavy equipment to properly grade the site so water drains away from the building.
At a minimum, site preparation includes removing all sod and vegetation. For ideal site preparation, remove topsoil and stockpile for later use in finish grading. In frost-prone areas, remove any clays or silty soil from within the future building “footprint”. Replace subsoil removed around the building with granulated fill to help drain subsurface water from the building. Distribute all fill, free of large debris (no pit run), uniformly around the site in layers no deeper than six inches.
Compact each layer to a minimum of 90% Modified Proctor Density before adding the next layer. A soil (geotechnical) engineer can be engaged to perform this test and verify that compaction is adequate. Usually, adequate compaction takes more than driving over fill with a dump truck or earth moving equipment. When any building portion sits on fill, rest columns, as well as any concrete encasement, on or in undisturbed soil. In many cases, building inspectors will require engineering to confirm compaction adequacy on filled sites. Geotechnical engineers can be expensive; however, they are even more costly when called in to do analysis “after the fact”.
Soil compaction is defined as the mechanical process of increasing soil density. If performed improperly, soil settlement can occur, leading to unnecessary maintenance costs or structural failure.
So, what is soil actually? Soil is formed in place or deposited by various forces of nature – such as glaciers, wind, lakes, and rivers – residually or organically. Key factors in soil compaction are soil type, soil moisture content, and the compaction effort required.
There are five principal reasons to compact soil:
Increase load-bearing capacity,
Prevent soil settlement and frost damage,
Provide stability,
Reduce water seepage, and
Reduce swelling and contraction.
Soil can be compacted by vibration, impact, kneading, or pressure. These compaction efforts can be accomplished using two main force types: static or vibratory.
Static force is simply machine dead weight, applying downward force on the soil surface and compressing soil particles. The only way to change effective compaction force is by adding or subtracting machine weight. Static compaction is confined to upper soil layers and is limited in depth. Kneading and pressure are two examples of static compaction.
Vibratory force uses a mechanism, usually engine-driven, to create a downward force in addition to the machine’s static weight. The vibrating mechanism is usually a rotating eccentric weight or a piston/spring combination (in rammers). Compactors deliver a rapid sequence of blows (impacts) to the soil surface, affecting top and deeper layers. Vibration moves through soil, setting particles in motion and moving them closer together for the highest density possible. Based on the materials being compacted, a certain amount of force must be used to overcome a particular particle’s cohesive nature.
Poor, improper, or no compaction can result in concrete slab cracks or frost heaves, foundation erosion, and/or building settling. Proper compaction can extend structural life.
Every soil type behaves differently with respect to maximum density and optimum moisture. Therefore, each soil type has unique requirements and controls for both the field and testing purposes. Soil types are commonly classified by grain size, determined by passing soil through a series of sieves to screen or separate different grain sizes. Soils found in nature are almost always a combination of soil types. Awell-gradedsoil consists of a wide size range, with smaller particles filling voids between larger particles. This results in a dense structure that compacts well. Soil makeup determines the best compaction method. Three basic soil groups exist: cohesive, granular, and organic. Organic soils are not suitable for compaction.
Cohesive soils, such as clays or silts, have the smallest particles. Cohesive soils are dense and tightly bound together by molecular attraction. They are plastic when wet and can be molded but become very hard when dry. Cohesive soils feel smooth and greasy when rubbed between fingers. Clay soil is less than ideal for constructing a pole barn and should be removed and replaced.
Granular soils range in particle size from .003” to .08” (sand) and .08” to 1.0” (fine to medium gravel). Granular soils are known for their water-draining properties. Sand and gravel obtain maximum density in either a fully dry or saturated state. Granular soils feel gritty when you rub them between your fingers. When you shake water and granular soils in your palm, they mix; when you stop shaking, they separate. When dry, a soil sample will crumble.
Gravel and sand can be compacted either by vibration (using a vibrating plate compactor, vibrating roller, or vibrating sheepsfoot) or kneading with pressure (using a scraper, rubber-tired roller, loader, or grid roller). Both are good to excellent in foundation support terms and as a subgrade. They compact easily and are not expansive (expansive soils tend to be prone to frost heave issues).
Soil’s response to moisture is very important, as soil must carry loads year-round. Rain, for example, may transform soil into a plastic state or even into a liquid. In this state, soil has very little or no load-bearing ability.
Soil moisture content is vital to proper compaction. Moisture acts as a lubricant within soil, allowing particles to slide past each other. Too little moisture means inadequate compaction – particles cannot move past each other to achieve density. Too much moisture leaves water-filled voids and subsequently weakens load-bearing ability. Most soils reach highest density at a certain water content for a given compaction effort. As soil dries, it becomes more resistant to compaction. In a water-saturated state, voids between particles are partially filled with water, creating apparent cohesion that binds them together. This cohesion increases as particle size decreases (as in clay-type soils).
To determine whether proper soil compaction is achieved for a specific construction application, several methods have been developed. The most prominent is soil density.
Soil testing measures soil density to compare the degree of compaction against specifications for the structure to be built; measures the effect of moisture content on soil density vs. specifications; and provides a moisture-density curve identifying optimum moisture content.
Tests to determine optimum soil moisture content are done in a laboratory. The most common is the Proctor or Modified Proctor Test. A particular soil needs to have an ideal (or optimum) moisture amount to achieve maximum density. This matters not only for durability but also for cost, because less compaction effort is needed to achieve the desired results.
A quick moisture-determining method is known as a “Hand Test”. I love these test kinds – they are quick, simple, and I don’t have to get out my checkbook or credit card to pay for them.
Pick up a handful of soil and squeeze it. Open your hand. If the soil is powdery and won’t hold the shape your hand makes, it is too dry. If it shatters when you drop it, it is too dry. If soil is moldable and breaks into only a couple of pieces when dropped, it has the right amount of moisture for proper compaction. If soil is plastic in your hand, leaves moisture traces on your fingers, and stays in one piece when dropped, it has too much moisture for compaction.
Backfilling pole holes with soil rather than concrete can significantly increase settlement risk, as earth-only foundations often lack the bearing capacity to distribute structural loads. Backfilling with soil offers no mechanism to prevent posts from being pushed deeper into the ground under the building’s own weight.
Over time, that loosened or disturbed soil will settle and compact naturally. Soil particles that are moved around and aerated will eventually be pulled down by gravity and water, densifying into a compact mass rather than loose soil. This happens when loose or disturbed earth densifies under gravity and moisture, causing structures on top to sink. When builders place a post in a hole and refill it only with displaced earth, this material remains susceptible to this natural compaction process. That is why many pole barns move significantly without a proper foundation.
Concrete cookie footings placed at the bottom of holes are a common but often inadequate way to save costs. Usually, they fail to meet the required load-bearing surface area. A footing should distribute building dead weight PLUS all imposed snow and live loads to the soils beneath the building.
Examine a typical pole barn example – a 40-foot-wide building with columns spaced every eight feet. Actual building weight (dead load) varies greatly depending on the materials used. Steel roofing and siding will be lighter than shingles and wood siding. For this example, we will use a light building weight of 10 psf (pounds per square foot). The building code specifies a minimum 20 psf roof live load. This means each footing must carry the weight of one-half of the width (20 feet) times column spacing (eight feet) times 30 psf, or 4800 pounds.
In many areas, soil bearing pressures are as little as 1500 or even 1000 psf. Basically, the easier it is to dig, the lower the soil’s capacity to support a vertical load.
For every foot of depth below grade, soil capacity may be increased by 20%. Other than with 1000 psf soil, for every foot of width over one foot, this capacity also increases by 20%.
With 1500 psf soil, and footing bottom four feet below grade, a 12-inch diameter footing will support 2400 pounds per square foot.
A 12-inch diameter footing covers 0.785 square feet; a 16-inch diameter covers 1.4, 18-inch 1.77, 24-inch 3.14.
An 18-inch diameter footing would support 4513 pounds from our example. Close, but no cigar. However – lots of places have snow loads (footings must support them) and many buildings are wider than 40 feet, or have columns placed over eight feet apart.
Increasing to a 40 psf roof snow load, with the same eight-foot column spacing, would mean resisting an 8,000-pound load! With 1,500 psf soil, even a two-foot-diameter footing would be inadequate.
How about Sakcrete® ready-mix concrete for footings?
The most familiar and most available option is Sakrete® general-purpose High Strength Concrete Mix. When mixed per manufacturer’s instructions, this mix affords a compressive strength of 4,000 psi (pounds per square inch) at 28 days.
Instructions: Empty contents into a mortar box, wheelbarrow, or mechanical mixer. When mixing by hand, form a crater for adding water. Add water slowly. Avoid a soupy mix. Excess water reduces strength and durability and can cause cracking. Mix a 60-pound bag with three quarts of water, and an 80-pound bag with four quarts.
Now, realities of using bagged concrete for pole barn footings….
It is not unusual to have concrete encasements 24 inches or larger in diameter and 18 inches or more in depth, to prevent building settling and uplift issues. One hole this size would take 4.71 cubic feet, or about 700 pounds of concrete! Even a very small building with an 18-inch diameter and depth takes 2.65 cubic feet, or about 400 pounds of concrete.
With either 60- or 80 lb. bags, it is going to take a lot of bags! An average building could easily have 20 posts, and if looking at 700 pounds per post, we are talking about seven TONS of concrete (3-1/2 yards).
Setting aside the huge bag numbers, other realities remain. Some builders insist on throwing an entire bag (usually including the bag) into a hole. They believe groundwater will cause ready-mix to harden. Why does this not resemble the manufacturer’s instructions?
Engineering a reliable foundation requires addressing both a structure’s downward pressure and potential for vertical uplift caused by wind. Professional industry guidelines emphasize that a properly treated post with a concrete base provides essential vertical load distribution necessary to prevent long-term shifting. Relying solely on original soil leaves a building’s stability to chance, particularly in areas where soil bearing capacity is inconsistent or prone to high moisture. Concrete bottom collar pours extending up post sides serve a dual purpose: preventing the building from sinking while also resisting wind-driven uplift forces.
