When constructing a shipping container building, a foundation must be laid for the containers.
Decisions about the foundation type will vary depending on your budget, structural requirements, local soil type, and local building conventions.
Getting this decision wrong can be devastating and it can bring your entire project to an abrupt halt. This is one of the biggest mistakes people make when constructing a container building.
Today we are going to explore the various types of foundations that can be used address the pros and cons of each foundation type.
What Type of Foundation Should I Use?
The two major factors that need to be considered when designing your foundation are cost and what’s required structurally. Let’s address them each point at a time. First, what’s required structurally?
To calculate what is structurally required, consult with a qualified builder or engineer. Just be aware that both your soil type and the number of containers you are planning on using have a large impact here.
If you are constructing on soft soil, then you will need a deep foundation type such as pile foundations.
Whereas, if you’re building on hard soil, you can use minimal foundations. This is because the majority of the weight of the container will be distributed onto the existing hard ground.
Sometimes people prefer to ‘over-spec’ their foundation, which is where you make your foundation stronger than necessary, either for peace of mind or because you prefer that particular style of foundation.
Ultimately though, the decision will revolve around cost, design, and what’s structurally required.
Types of Foundation
Typically, any foundation which is used in traditional home construction can be used for shipping container foundations.
These can be classified into either shallow or deep foundations.
Shallow foundations are laid very near to the ground level, as opposed to deep foundations which can be laid at depths of up to 10 meters.
We are going to focus on the three most commonly used foundations: concrete piers, slab foundation, and piles.
Concrete piers are a type of shallow foundation and are one of the simplest and cheapest routes to go.
In their basic form, they are concrete cubes which have reinforced steel within them. Reinforced steel can be either steel bars or a mesh of steel wires used to strengthen the concrete.
Image From Larry
This foundation is definitely the most DIY friendly. It is also the least expensive type of foundation. A great advantage of using this type of foundation is that because the containers are up off the ground it allows for ventilation and prevents condensation forming underneath the container.
Six concrete piers are laid for each container. One pier is laid for each corner of the container and two piers are used in the center to support the middle of the container.
Image From Sea Cabin
This is by far the most popular shipping container foundation and is the one we recommend in 8 out of 10 cases.
A slab foundation is more time-consuming and more expensive than a pier foundation. It is an exceptionally good foundation to use on softer soil types. If you are going to use a slab foundation, be prepared to dig a lot!
Slab on Grade Foundation
We often see slab foundations used in warmer climates where freezing isn’t a concern. The advantages are that it’s quick and easy to build, and because there are no hollow spaces in the foundation these foundations are less vulnerable to termite infestation.
The disadvantages of a slab foundation are the lack of access to utility lines once the concrete has hardened and the potential for heat loss when ground temperatures drop below the interior temperature.
Pile foundations are used when the soil type is too weak to support a concrete base. This type is the most expensive type of foundation covered here.
If you remember, pile foundations were used in the Graceville Container Home Case Study.
Piles (cylindrical solid steel tubes) are hammered into the ground through the soft soil until the piles reach more suitable load bearing ground.
Example of Pile Foundations
Once the piles are secured in place they are traditionally capped with a block of concrete. So once you have secured all your piles you end up with a grid system of concrete caps which above ground look visually similar to concrete piers.
Pile foundations are not recommended for a DIY builder. A contractor would be needed to install pile foundations due to the specialist equipment needed, such as the pile driver.
Strength of Concrete to Use for Your Foundation
If you chose to use either a concrete pier or slab foundation, this section is extremely relevant for you.
Once people have decided to use a concrete foundation, their next question is normally about what strength of concrete to use.
The strength of concrete you need to use for your foundation will be primarily decided by the geo-technical engineer’s report.
The concrete strength will be referred to as a C value. C15 concrete, a general all-purpose concrete, is made by using 1 part cement, 2 parts sand, and 5 parts gravel. The higher proportion of cement used, the stronger the concrete. For instance, C30 is very strong concrete made up of 1 part cement, 2 parts sand and 3 parts gravel.
Example of Concrete Pier Foundation
If you are mixing small quantities, then you can either do this by hand or by using a cement mixer. For anything more than 1 cubic meter, consider having the concrete delivered directly to your site, ready to use.
Note that, if you are mixing the concrete yourself, make sure you thoroughly mix all the elements together properly, otherwise the strength of the concrete is greatly reduced.
To determine how much concrete you need, just calculate the cubic meters of your foundation. Multiply the width by the height by the depth.
For example, to calculate how much concrete is needed for a 10-foot wide, 22-foot long, 2-foot deep slab foundation, multiply 10 x 22 x 2. The amount of concrete to be ordered would be 440 cubic feet.
Once the cement is mixed with water, it will start to cure. Ensure the concrete cures properly, since this improves its strength and durability. The concrete only cures properly if the concrete’s temperature is kept within a suitable range (refer to the manufacturer’s packaging).
It typically takes from 5-7 days for the concrete to cure. During this time it needs to be kept moist.
Pouring Concrete in Hot Weather
If you are laying concrete in hot weather, it’s crucial you prepare your site correctly before pouring the concrete. Place temporary sun shades up to block any direct sunlight onto the concrete. Also, before you lay the concrete you should spray the ground with cold water. While you are mixing the concrete, make sure you use cold water.
Another good idea is to pour the concrete either later in the evening or first thing in the morning to avoid peak temperatures.
Pouring Concrete in Cold Weather
Like pouring concrete in hot weather, special measures need to be taken when pouring concrete during cold weather.
Cold weather is classed as the average temperature being below freezing for more than three consecutive days.
Before you pour the concrete, make sure any snow or ice has been cleaned from the base and forms. Remove any standing water. Once you have laid your concrete, cover it with insulating blankets right away. Use the blankets for 3-7 days while the concrete is curing. After the concrete has cured, remove the blankets gradually, so the concrete doesn’t crack due to quick temperature change.
Make Your Choice
So hopefully by now you will be able to select the type of foundation you require and also know exactly how to mix the cement for this type of foundation.
Make sure you pay special attention to the advice regarding how to lay in extremely hot and cold climates as this can make or break your container’s foundation.
The next step now is getting your containers delivered and installed on your foundation.
Let us know below it went when laying your foundations!