Shipping Container Foundations Guide

A solid foundation is crucial for any construction project, including shipping container homes. Without a sturdy base, your container home might suffer from structural issues over time. Moreover, proper foundations ensure that your home remains level and secure. According to the search results, "The foundation of your shipping container building must be sturdy, level, and provide adequate support for the container home." Ensuring that your container is properly supported is the first step towards building a safe and enduring structure.

A foundation is often the first significant site work undertaken for a shipping container project. It is, literally, the element upon which the rest of your container home (or other container building type) is constructed.

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But do you really even need a foundation for a shipping container? And if so, what kind of foundation is required? Is it something special compared to other types of construction?

Below, we'll answer all your questions about foundations for containers to ensure your project is safe, secure, and will stand the test of time.

What is a Container Foundation and Why Do You Need One?

Whether you're building a container pool, a container house, or just placing an empty container on your property for storage, you need to safely and securely support it. The ground beneath us may seem simple and static, but it can be uneven, unstable, and difficult to predict.

The movement of moisture, changing temperatures, the decay of organic matter, and the growth of nearby vegetation all can cause the ground to rise, sink or slide. These changes are typically very slow, and it often takes months or even years to visually see their cumulative effects.

But don't let the slow speed fool you. Unless you build on solid rock, odds are high that the ground will eventually affect the position and level of your container home. And even if you have a foundation, don't assume that it is simple to build the right type of foundation in the right way. Just look at the number of foundation repair companies in your area...clearly, foundations are tricky!

A well-built foundation provides a solid, stable platform for your building, ensuring that its weight is evenly distributed across an area of ground that can adequately support it. A foundation can prevent costly aesthetic, functional, and even structural issues in the future.

And, in a worst-case scenario like a flood or tornado, a good foundation ensures your container will stay right where you put it. It really one of those areas where “it pays to do it right the first time”. So let's get started on understanding the theory behind foundations so you can make one that works!

Foundation Theory: How Foundations Work

The basic function of a foundation is pretty simple: hold the building up straight. But understanding the way a foundation is able to do this across different geographies and project types is important to ensuring project success.

There are a few key principles that determine how well a foundation will work. Most of them tie back to soil mechanics, the study of how the ground behaves. Intuitively, you know that soils made of sand, mud, or gravel will all support you differently. One of the main purposes of soil mechanics, and more broadly, geotechnical engineering, is applying some math and science to those intuitive feelings to ensure you have adequate ground supporting your foundation.

Bearing Capacity in Foundations

Bearing capacity is the ability for a particular soil to support the force of a load above it. It's typically measured as a pressure: exceed that pressure and the soil can no longer support the load.

If you've ever stepped on fresh powdered snow, you've seen a material that has almost no bearing capacity. The weight of your foot will sink right through the snow, stopping only at the next layer of material (ice, compacted snow, or soil) that has a higher bearing capacity.

An important thing to understand is how a bearing load is supported by soil. An easy way to think of this is by visualizing a pile of dirt. You'll notice the dirt pile isn't vertical like a tower; it's shaped more like a pyramid.

As you get lower on the pyramid, the size of the horizontal layer of soil gets larger. This means that the pressure on any individual grain of soil is highest right at the foundation, then gets smaller as you descend. In the same way, when you put bearing pressure on the ground, the force spreads through the ground in a cone or pyramid shape that's typically assumed to be about 60 degrees wide.

Factors that affect bearing capacity include soil density, cohesion, organic matter content, moisture content, and friction angle. If you end up needing a soil analysis from a geotechnical report, a company will measure many of these factors via soil boring or in situ testing on site.

Luckily, while a geotechnical engineer needs to be familiar with all of these factors and the complicated equations that connect them, you do not. Even better, for most projects, you likely don't need a geotechnical investigation anyway.

Instead, scientists and engineers have broadly categorized types of soil with different methodologies, such as the Unified Soil Classification System (UCS). And if you know the type of soil you have at your project site (such as its UCS classification), in most cases you should be able to find out its bearing capacity without having to do any on-site measurements.

To start, you can use the United States Department of Agriculture (USDA) Web Soil Survey. This system enables you to find the type of soil in the specific area where you're building. If you want to know how to use this tool, keep reading to the end of the article where we’ll give an example with screenshots.

After you have a soil type from the Web Soil Survey, you still need to know the bearing capacity. That's where the ICC Building Codes come in.

Both the International Residential Code (IRC) and International Building Code (IBC) give allowable bearing capacities for different types of soil. Table R401.4.1 from the IRC gives the presumptive load-bearing values of foundation materials.

Skin Friction in Foundations

With bearing capacity fully covered, we now need to talk about a related concept known as skin friction. The easiest way to understand skin friction is to picture a stake, like the kind you use to hold up a camping tent.

A stake is long and slender, and when you drive it into the ground, hardly any support is coming from the very tip of the stake pushing against the ground below it. In other words, very little of the stake's support comes from bearing capacity.

Instead, it's the soil pushing on the side of the stake that primarily holds it in place. As you drive a stake into the ground, you're displacing and compressing the soil around the stake to make room for it in the ground. This pressure, correspondingly, pushes against the side of the stake too.

The pressure increases the shear friction that we call skin friction, which makes the stake resist both pulling out and being driven in deeper. It's the same frictional force that holds a nail in a piece of wood, even though the sides of the nail are smooth.

Compared to bearing pressure, skin friction is a much more complex topic. Fortunately, pile foundations (discussed later in the article) are essentially the only foundation type that gets the majority of its load capacity from skin friction.

For foundations that primarily use skin friction for load support, you really need the assistance of geotechnical and structural engineers. As you go deeper into the soil, you often encounter multiple layers called soil strata, each with its own specifications and strength.

Due to these complexities, we don't recommend trying to design your own foundation based on calculating the skin friction. Instead, we just want you to be aware that skin friction exists, and know who to talk to in case you need to know more about it for certain types of foundations.

Foundation Settlement

Our next topic has a more indirect effect on the load capacity of your foundation. As you know by now, your soil is not a solid, monolithic material, but rather an aggregated collection of particles with different sizes, shapes, materials, and properties. And under certain conditions it can settle, meaning it occupies a smaller volume and has a lower elevation than it did before.

If your soil settles, it can be localized to a particular side of your project (differential settlement), or the same magnitude across the entire project site (uniform settlement). Uniform settlement is much more desirable, as it doesn't put bending or shear stresses on your foundation and container structures. Rather the whole system just sinks slightly into the ground evenly, ideally without causing problems (though utilities can become an issue even with uniform settlement).

Three of the most influential materials that may be part of your soil are organic matter, clay, and air. Why? Because more than other materials, these three soil constituents make the soil more likely to move in certain cases (and that's usually not good!)

Let's start with organic matter, which typically means things like decaying leaves, animal waste, etc. The problem with organic matter is that it slowly decomposes via biological processes. As this soil decomposition happens, the properties of organic matter change. And any time soil properties are changing, its ability to support a load is changing as well.

Clay is a type of soil based on a particular set of minerals with especially fine particle sizes. Clay can be particularly problematic because it tends to swell and shrink based on the amount of moisture it contains.

Finally, there is air. Due to the fact that soil is made of particles, there are tiny voids between particles where air (and as we'll discuss in a later section, water) can reside. With enough pressure or vibration, that air can be forced out of the soil, increasing the soil density and compacting it in a process called soil consolidation.

Failing to account for these factors can result in soil that contracts and a foundation that may crack or shift. Perhaps the most famous example of this is the Leaning Tower of Pisa in Italy, which was built on unstable soil with a large percentage of clay. This building site experienced significant differential settlement on one side of the tower, causing it to lean.

Soil Expansion

Just as problematic as soil settlement is soil expansion. Soil expansion primarily occurs due to water acting in two ways.

First is the clay soil that we mentioned before. When clay dries, it contracts and shrinks. But when it is saturated, it expands.

Expansion due to clay and other fine particulates can cause a lot of problems for foundations. Section 1808.6 Design for Expansive Soils in the IBC actually addresses this problem, but the summary is: get professional help.

Water also plays a role in soil that is below the freezing temperature. Prolonged exposure to freezing temperatures can literally freeze the water in the soil's voids, leading to frost heave. This condition can lead to significant soil expansion that can severely damage foundations that weren't designed correctly. Once again, we'll share more about how to deal with this in a later section of this article.

Types of Shipping Container Home Foundations

There are several different types of shipping container foundations you can use, varying in their functionality and material. We categorize them by their expected service life: temporary, semi-permanent, and permanent:

• Temporary Foundations: These are the easiest and cheapest method but the riskiest. They get your container level and up off the soil to help with corrosion, but the container is still able to move freely.
• Semi-Permanent Foundations: These will fix your container in place securely and provide all the benefits of a permanent foundation while being removable.
• Permanent Foundations: The type of foundation you normally think of. Once built, they aren't moving without heavy equipment and demolition.

Below, we'll go over the common types of foundations for a shipping container, noting the service life categorization for each.

Wood Beam Foundation (Temporary)

A wood beam foundation is literally just placing the shipping container on top of some large pieces of wood. Most commonly, you put the container on railroad ties, though other types of timbers and lumber can be used. Railroad ties have the chemical treatment to endure prolonged ground contact, and the size to distribute the weight over a large area.

Gravel Foundation (Temporary)

A compacted gravel bed may seem like it is not much different than placing your container on the ground. But by setting a container on gravel, you allow water to drain through so the bottom frame rails aren't in contact with moist earth. This helps prevent rust and corrosion.

Plus, gravel will settle less than added fill dirt, so the container will stay level until it is moved to a more permanent location. Although, with proper container anchoring, you could consider a gravel foundation for a more permanent placement.

One important note about the type of gravel you use for a gravel bed foundation. What you really want is crushed stone with jagged edges that interlock together for better strength. Common gravel, especially river gravel, is usually smooth and not nearly as strong of foundation material.

Concrete Block Foundation (Temporary)

While at first glance this foundation type may appear similar to some of the later options, it's actually a different category. Here, we're talking about placing a container on concrete blocks that are set right on the soil. Regardless of whether you purchase prefabricated blocks and stack them, or build your own in forms on-site, the results are the same.

Since the blocks aren't attached to or embedded in the soil in any way, the only thing holding them in place is weight. So even though the presence of concrete may make this type of foundation appear permanent, it is clearly not. Furthermore, stacking the concrete blocks as you see in the picture makes them even less sturdy.

Helical Pier Foundation (Semi-Permanent)

This particular type of container foundation goes by several names: soil screw, screw pile, helical pile, helical pier, screw anchor, helical anchor, and others! Whatever you call it, the result is the same. A large metal screw (typically several feet long and several inches thick) is twisted into the soil with hydraulic machinery and can immediately support loading. No waiting on concrete to set and no dealing with forms or excavated dirt.

Depending on the size of the helical steel plate that forms the threads and the length of the screw, a screw pile can support a surprising amount of weight. They work through a combination of both the bearing capacity of the screw's helixes and skin friction of the screw's shaft.

Screw piles also provide a huge amount of uplift resistance compared to many other foundation types, which is helpful for securing a container during a storm. And when it's called an 'anchor' versus a screw, pile, or pier, it typically means the intended use is for pull-out resistance.

The best part of helical piers is that they can be removed and reused. Just unscrew them from the ground, move them to a new location, and screw them back in. They leave behind only a narrow hole the size of the screw pile shaft, so with a few minutes of work you could make it appear that they were never even there. A few examples of screw pile manufacturers are Heli-pile, Helical Anchors Inc., and Techno Metal Post.

Specialty Pin Pile Foundations (Semi-Permanent)

Pin piles are a form of micropile, essentially a pile that is much narrower in diameter than typical. Pin piles are usually so small that you need several of them to give the capacity of a single regularly sized pile or pier.

But now, there are a few manufacturers that make a system of pin piles and pile caps that work together to form an integrated foundation solution. Three or more steel pins are driven with a pneumatic hammer through the head/pile cap, angled slightly off vertical. On top of the pile cap, you can attach whatever structural member you need. Examples of these specialty pin pile systems include Diamond Piers and Sure Foot.

Pile Foundations (Permanent)

Pile foundations, sometimes called friction piles or driven piles, are used when the soil near ground level has a low bearing capacity. They are long, slender foundation members that work primarily through skin friction from the surrounding soil. However, piles may get a portion of their strength from end bearing as well if they reach a lower soil stratum with greater bearing capacity.

There are a number of materials used to make piles, including wood, steel, and concrete. Regardless of the materials, piles are typically pushed or hammered into the soil using heavy equipment like a pile driver.

Alternatively, piles can be concrete cast in situ, meaning the concrete is poured inside an excavated hole in the ground. This typically involves boring a hole, temporarily lining it with a steel casing, then adding concrete as the casing is removed, as in a Franki pile.

Given the requirements for expensive, specialized equipment, piles are typically used for larger commercial projects and aren't DIY friendly. An exception to this is the much smaller pin piles shared in the section above.

One of the most common places you'll see piles is near the coastline. If you've noticed a beach pier or even a structure (such as the lifeguard station in the picture above) built above the water on poles, you've seen piles.

While the piles in the picture above extend above the surface of the ground, often piles are terminated below the soil surface. Then, a concrete cap is poured on top of the pile to give a uniform bearing surface for the structure above.

For this reason, it can sometimes be challenging to recognize a pile foundation after construction has moved forward. Often all you'll see are the concrete caps, while the piles are buried beneath the soil.

Pier Foundation (Permanent)

Pier foundations are actually quite similar to pile foundations in appearance and the two are commonly confused for each other. The differences between the two are in shape, function, and installation.

First, a pier is both thicker in diameter and shorter in height than a pile. Second, a pier works primarily via end bearing, not skin friction. Finally, piers are almost always cast in place by placing wet concrete in an excavated hole.

To elaborate, most pier foundations are constructed by boring a cylindrical hole in the soil with an auger, then placing concrete in