Do you need to build a shipping container foundation? It’s fair to expect that foundations for container-based houses should be simple and inexpensive to match the characteristics and nature of such houses (short construction time, low cost, flexibility, and in many cases temporary character). Such assumption doesn’t mean however that foundation can be ignored as unimportant. In fact, the proper type, good quality foundation is the starting point and basis (as the name suggests) for the success of your project. It must support the weight and guarantee the stability of the structure in all predictable weather, climate, and soil conditions over the long years of its lifespan.
Factors determining the type of shipping container foundation
1. Total weight of containers’ structure
The foundation must provide stability to the overall structure over its lifetime. Of importance is not only the total weight but also its distribution. Note that the small-footprint multi-level structure needs a different foundation than the same weight but large footprint single-level one.
2. Local building regulations
Obtaining pertinent permissions may be a long process because not all states (as of the year 2019.) have valid guidelines regulating the construction of container-based houses. When working on it, you will find that different regulations may apply to temporary structures compared to permanent ones, to locations in residential areas or industrial zones, to city or rural environments, to commercial or residential structures, etc.. But the complexity of this issue does not change the picture – local regulations are some of the most important factors determining the type of foundation for containers-based houses and the starting point of the overall construction effort!
3. Type of soil on the construction site
Setting the structure on the soft, sandy area will require deep foundations to reach stable, compacted layers of soil or rock. Such rules also apply to moist, rain-saturated lands typical for tropical zones.
In contrast, on firm soils or rocky terrains, you may get away with any shallow foundation. Low cost, easy and practical solutions (for example pavement blocks, treated timber logs, slabs…) most likely will do the job.
4. Climate zone
Geographical regions with temperatures dropping below the water’s freezing point set another limitation. Once apparently stable, firm soil freezes, it will change its volume due to the content of water (moist, rainwater…). As a result, frozen soil will push upwards and either destabilize or crack foundations. The only way to prevent such effects is to put the foundation’s footing below the frost line. In such zones, the minimum depth of foundation is usually specified in local building regulations.
5. Cost & Time of shipping container foundation
We are all wallet sensitive. Part of the overall construction cost is material (cement, stones, iron rods, gravel, etc…), labor (including also the need for specialized equipment (if any), and time necessary to accomplish the task. This prompts the conclusion that what you really need is just a “Good Enough” solution for the task you try to accomplish because the cost and time necessary to build a foundation for the container-based house should match the characteristics and nature of that house.
6. Risk zone (flooding, tornados, earthquakes…)
Under normal circumstances, single module structures (1-TEU module) may not even need any special foundation (paving blocks will perfectly do the job). However, exposure to natural disasters completely changes the situation. For example, in flooding zones, the practical solution may be concrete footings (piles) not only deeply buried in the soil but also elevating the structure well above the expected water level. In hurricanes and tornadoes zones, the foundation should be at the soil level and strong enough to anchor the container’s structure the way it can withstand the forces of nature. Similarly, seismic zones have their own set of regulations that must be met by the construction industry (when it comes to containers, they are robust enough to survive even powerful earthquakes, so most likely foundations will be the first victims).
7. Nature of the container’s structure
Temporary structures (usually one to a few 1-TEU modules) may not need any foundation, while permanent multi-level and multi-module structures will certainly need robust foundations guaranteeing their stability.
In many cases, especially when it comes to small, one-level TEUs structures in a “friendly” environment, the foundation may be a sort of DIY project. In general, however, especially for larger residential or public, multi-level container-based structures, it will be necessary to hire specialists (civil engineer(s)) and contractors to make sure that the foundation guarantees stability and safety.
Foundations for Container-Based Houses
Note that the floor of Shipping Container(s) does NOT need to rest on the ground. It is supported by cross-member beams making the bottom part of the frame. The whole SC structure is fully supported by the frame and needs a firm foundation only at its corners, right under the load-bearing beams. The extra benefit is that lower parts of the shipping container structure do not have to be in permanent contact with soil.
Typical structure of the ISO Cargo Shipping Container (Source: Foshan Tianpuan Building Materials Co. Ltd). Structures of Customized Container houses are very similar, only their maximum strength ratings are much lower as they are not designed for the transport of cargo.
As mentioned above, the basic 20ft-equivalent module requires strong and stable support only at its 4 corners (40ft modules usually need extra support along its long sides). Practically, it means that if the terrain is stable and level, you may use just four concrete pavement blocks (any precast concrete blocks) to do the job. This is the typical solution when SCs are deployed on paved parking lots, sidewalks, plazas serving as kiosks, info-centers, fast-food stores, etc. It will be also acceptable to use 4 concrete blocks on the grass or bare soil especially when SC is deployed temporarily for the duration of the project (typically in construction zones, areas of infrastructural projects …). In such cases, small gravel beds under each pavement block will help to drain the water.
The goal is to provide stable, leveled support for the container’s frame and at the same time to keep its floor slightly above the ground. The main conditions are stable, firm soil (ground), and “blessing” from local authorities.
Note that pavement blocks also offer a reasonable solutions for permanent, small, often single module countryside (weekend) houses.
Temporary Shipping Container House on pavement blocks (Source: “Shipping Container Village in Bristol” ).
When it comes to multi-module/multi-level container-based structures, more robust foundations will be necessary, since such structures (especially multi-level ones) are heavier and will not be considered temporary ones. In warm and moderate climate zones with dry, compacted soil, piers may offer the optimum solution. They are easy to make in the DIY approach, inexpensive, and good enough to offer a long-term solution even for larger residential structures. Just to give an idea – the popular pier foundation applicable to shipping containers will consist of four 20” x 20” x 20” (about 50cm x 50cm x 50cm) steel-reinforced concrete blocks, strategically located at the corners of the 20ft module (40ft modules will need extra two pier blocks – each one in the middle of the container’s long side).
Example of Piers foundation for multi-TEU Customized Container House (Source: Shenzhen Utwo Technology Co. Ltd).
The trench (often also called Strip) foundation provides continuous support for the walls. While trench makes sense for traditional structures, for container-based houses it seems to bring too little of the necessary effort. Note that container-based houses by their nature do not really need continuous support all along the walls because the whole load is transferred on the foundation by the frame’s corner studs.
On the negative side – continuous trench blocks access to underneath utility lines (when needed) and conceal any potential degradation of the bottom part of the structure. Let’s also mention that trench (strip) foundations should not be used in seismic zones as they have a low resistance to earthquakes.
The bottom line is – Trench (Strip) foundations are less expensive than slabs but for container-based houses, they do not bring substantial advantages that can be weighed against their inherent drawbacks.
Two-storeys Container house on Trench-Slab type foundation (Source: Liaoning Tianying Steel Structure Engineering Co. Ltd)
The slab is a few inches thick, monolithic layer of concrete (usually steel-reinforced). It requires more material and labor than mentioned above shallow foundations as all the area under the structure must be excavated several inches deep to have extra room for a gravel bed on which the concrete will be poured. Its stronger version known as “Slab-on-Grade” is significantly thicker at its edges to better handle higher loads at its edges. Thanks to the footings along its perimeter, the slab-on-grade is better protected against sinking (settling) so it can be also used on softer soils.
Left: Concrete slab (Source: FamilyHandyman), Right: Slab-on-grade (Source: ConcreteNetwork)
Slab foundations are more expensive than other shallow foundations but, in some specific circumstances, they have noticeable merits.
The slab foundation allows the load to be more uniformly distributed across the house’s footprint. This makes the foundation (and house) more stable because the load per square foot is now much lower than the weight-bearing limits of the soil under the slab. For small container structures, slab foundations can be used even on “less compacted soils”, especially its slab-on-grade version.
Another (pragmatic rather than technical) reason for slab foundation may be a location in warm, tropical zones with a lot of rain. In such cases, the slab foundation (especially when extended beyond the house’s footprint) helps to create a clean, mud and weeds-free area under and around the house. It also prevents pests, rodents, and other creatures (moles, raccoons, etc) to build nests underneath the house.
Another advantage comes from the fact that the slab foundation eliminates the need for entrance steps. For many, it may be a crucial element of the decision process.
While the “extended” concrete slab foundation may be an elegant solution for residential container structures, slabs also have their drawbacks.
a) As mentioned earlier, it’s more expensive and labor-intensive than all other shallow foundations;
b) Unless the design includes special provisions for utility lines, the typical slab foundation blocks access to them when needed (and as Murphy’s law states – “if something can happen, it certainly will!”.
c) It’s prone to cracks due to the movements of soil caused by shifting (on weak soils) or changing temperatures (freeze/thaw cycles) in northern climate zones). Note that the bed of gravel under the concrete slab is intended for water drainage, so temperatures in the range of single digits below the freezing point should not cause problems.
The real problem starts when the depth of frost lines goes well below the footings of the slab (in fact of any shallow foundation).
Things may be different in the countryside and in general in colder climate zones. Even apparently “stable” ground will move with time. Such an effect will be accelerated by winter freezing and summer hot temperatures. The moist soil increases its volume when freezing, causing the rise of the ground level. The magnitude of movement will be proportional to the level of water contained in the soil and the depth of frost lines (more water and deeper reaching freezing temperatures will magnify the effect). The movement of soil maybe not be noticeable to the human eye, but most likely, its effects will be visible. For example, you may find that doors will not open or close easily because the container’s structure slightly shifted from its ideal horizontal position or got twisted. Also, the foundation may sink or show cracks…
In such cases, a deep foundation is necessary with concrete posts/piers buried below the maximum frost line estimated for that climate zone (most likely determined by local construction regulations). While layers of ground following freeze/thaw cycles will be moving up and down along the concrete foundation (typically “smooth” tube piers), such movements will not affect the stability of the pier itself, provided it is sitting on the stable layer of the ground below the frost line.
Weak (soft) surface soil unable to bear the load transferred by piers will have a similar impact on the stability and level of the container house as freezing temperatures. In such cases, the traditional piers (supporting posts) should be sitting on footings installed deep enough to reach stable, compacted layers of soil. Footings, by spreading the load over a larger surface of soil help to bear the load by weaker soils eliminating the need for costly pipes.
Customized Container House on Deep concrete footings (Source: Liaoning Tianying Steel Structure Engineering Co. Ltd)
Deep concrete footings can be still a sort of DIY job (just deeper digging or drilling but usually in the firm soil). For example, in northern climate zones, all above-the-ground decks are built on buried (4-to-5 feet deep) concrete tube posts. In most cases, with appropriate depth, diameter, and steel reinforcement, such a foundation may be good enough for small container structures.
Bigger, family-size, multi-story container houses will certainly need robust foundations reflecting not only the weight but also the permanent character of these structures. Under normal circumstances (favorable soil and climate) it can be either pier or trench-type foundation, but certainly deeper and more robust. If preferred, you may also borrow from the traditional residential industry (full or partial basement). Please note, however, that these two construction technologies: “low cost, fast set-up time, portability…” characteristic for containers and “high cost, long construction time and permanency” characterizing traditional “in-ground” basement – are worlds apart.
In any case, for safety and compliance with local regulations, technical details of such foundations should be proposed by authorized persons (civil engineer(s)).
Multi-directional Steel Pipe Footings
SureFoot is a new foundation technology particularly popular in Australia and New Zealand. Answering the question of what exactly is SureFoot, we will take excerpts from the manufacturer’s description:
… Surefoot is an “all-in-one” hybrid concrete-free footing that can support residential homes, decks and commercial housing developments worldwide…. As a cost-effective alternative to traditional footings, it is suitable for a varied selection of soil types as determined by both geographical and climatic range…”
In fact – this new foundation technology is particularly attractive for container-based structures. Eliminating a huge amount of labor and necessary time characterizing traditional concrete-based foundations, SureFoot closely matches the short set-up time of container-based structures. Even more, these footings (similarly to container structures) can be removed and reused at the new location so they share the characteristic of portability!
SureFoot footing consists of a pile cap (baseplate) made from 4-to-10 mm (160 -to-400 mils) thick steel with custom-made driving guides for micro piles (steel pipes). Pipes length depends on the characteristics of the soil and starts from about 1m (40 inches) but may reach 2-to-3 meters when required.
The micro piles (steel pipes with a hollow diameter of 32mm (1.26”) and minimum gauge of 2.6mm (0.1”)) are driven into the ground at angles customized for a given soil. The simplest, low-demanding footings use 3 light micro piles, the largest HD footings may use up to 16 heavy micro piles. Once installed (with the help of a jackhammer), thanks to the strength of tree-roots like configuration of multi-directional piles the SureFoot can provide stable, low-cost footing for very heavy loads (heaviest model up to 36,000 kg (almost 80 thousand pounds).
All components are made from high-strength, hot-dip galvanized steel and are rated for 50 years of service. The installation of one footing takes about 10-20 minutes depending on the complexity of the terrain. The extra beauty – they can be easily installed on terrains where the access to heavy machinery is limited (for example on slopes). According to the manufacturer, they can withstand not only compressing loads but also uplift and lateral forces (think about tornadoes/hurricanes zones). They are suitable for a variety of soils including sand, silt, clay, gravel and sedimentary rock.
Left: The concept of SureFoot footings, Right: Container Café-Bar on SureFoot footings
Left: SureFoot footings with piles allow for construction on slopes
Right: Almost finished installation of the SureFoot footing (sticking-out micropiles must be either fully hammered into ground or trimmed)(Source: SureFoot)
Concept of SureFoot footing (here with pile extension)
Rarely (only in very specific cases) you may need a pile shipping container foundation. Such cases may include – large multi-story public or residential buildings located on weak grounds where traditional deep-foundations may not meet relevant requirements. Another such case may be a container house located partially (or fully) on the water. Knowing how appealing is the “Waterfront Location”, the idea may be tempting ?.
Piles (typically steel tubes and reinforced cement) need specialized equipment because they must be hammered through the soft soil or water until the firm layers are reached.
Piles (Source: Designing Buildings Wiki)
Elevated versus Ground-Level foundations
Piers, Piles (including multi-directional Steel Pipe SureFoot Footings), and to some extent Pavement blocks keep container’s structures above the ground. This arrangement has several advantages:
a) Prevents floor’s contact with soil, rainwater, vegetation, improves the ventilation… which has a positive impact on the durability of the structure;
b) Allows access to utility lines (water, sewer, electricity) when needed (let’s call it practicality);
Deep foundations make also possible to significantly raise the container’s structure above the ground level. While high elevation is beneficial in flooding zones, it also creates an opportunity to use the space for a garage, sheltered patio, or simply utility space…
But nothing is free – structures elevated above the ground level are more exposed to the weather, which may cause problems in extreme climate zones. For example, cold winds freely moving under the structure will make it more difficult (and costly) to keep the interior warm. By the same token, hot air accumulated under the container’s structure will reduce effectiveness and increase the cost of cooling. Note, that for structures sitting at or below the ground level, the soil, thanks to its thermal inertia effectively “shelters” their bottom parts from the exposure to extreme temperatures.
Good news – similarly to in the case of 4-season RVs, skirting the structure can reduce the gravity of the problem.
Shipping container foundation in High-Risk Zones
While under normal circumstances the pavement-block foundation may be the perfect solution for small container-based structures, it is easy to predict that such structures will freely fly when touched by tornado-strength winds. The pavement blocks cannot secure containers to the ground, they would rather join the flying container in the air. The bottom line is that in windy, tornado or hurricane-affected areas, container-based structures must be anchored to the foundation.
It’s easy to guess, that such foundations must meet not the only static weight of the container’s structure, but also much higher uplifting and lateral forces imposed by strong winds. Unfortunately, due to the containers’ “box-like” shape, their aerodynamic profile is very bad. The good news is, however, that the containers themselves have very good wind tolerance (or resistance if you wish). If not properly anchored, they may become “airborne”, but it is difficult to tear them apart. For these reasons, they are often deployed as temporary housing in sinister zones.
Special cases require special means, so when planning to build a container-based house in high-risk zones (earthquakes, flooding, tornadoes, hurricanes…, it will be in your best interest to check with local authorities for guidelines and regulations before starting your project.
Hopefully, now you have a clear idea about what kind of shipping container foundation you need for your home or commercial structure.
Learn more about the shipping container foundation here