Trench fill foundations are routinely used in the construction of low-rise buildings such as houses, garages and small commercial buildings. This type of foundation is constructed by digging a trench around the footprint of the building, which is then filled with concrete.
Trench fill foundations are a popular choice for many types of construction project, because they are quick and easy to construct, require less excavation than other types of foundations, and are suitable for a range of soil types. They are also effective at distributing the weight of the building evenly across the ground, which helps to prevent subsidence or other types of movement.
While trench foundations often do not require reinforcement, it can be a sensible addition to add rebar mesh, particularly if the depth or proximity of trees requires it. Ultimately if you’re not sure, or the foundations will be supporting more than a very lightweight construction, adding mesh is a pragmatic approach. To be clear, we would advise consultation with a civil or structural engineer in any and all circumstances.
In this article, we will evaluate the pros and cons of trench fill foundations, and look at how they stack up when compared with strip foundations, before finally considering what factors need to be taken into account when calculating the depth of trench fill foundations for a building project.
What Is A Trench Fill Foundation?
In a trench fill foundation, a trench is usually dug to a specified depth that is at or just below ground level, and the bottom of the trench is typically lined with a layer of hardcore to provide a stable base for the concrete. Concrete is poured into the trench, over the hardcore base, to create a foundation that is both level and stable.
Of course, as with all parts of the construction process, trench fill foundations require careful design and execution to ensure that they are strong enough to support the weight of the building and the loads it will be subjected to over time.
Trench Fill Foundations - Pros and Cons
Trench fill foundations offer several benefits, as well as some disadvantages. Some of the benefits include:
- Speed of construction: >Trench fill foundations are quick and easy to construct, making them a popular choice for projects that need to be completed in a short timeframe.
- Minimal site excavation: Trench fill foundations require less excavation than other foundation types, offering savings in both time and money.
- Suitable for different soil types: Trench fill foundations are suitable for a range of soil types and can be designed to accommodate various loading requirements.
- No complicated formwork: One of the main advantages of trench fill foundations is that they do not require costly or complicated formwork.
There are also some downsides to trench fill foundations, which may mean other types of foundation are more suitable. These disadvantages include:
- Cost: Without formwork, trench foundations require more concrete than some other types of foundations, so they can work out to be more expensive.
- Limited suitability for larger projects: Trench fill foundations may not be suitable for large buildings or structures that require significant load-bearing capacity.
- Risk of cracking: Trench fill foundations may be prone to cracking if not designed and constructed correctly, which can cause structural problems over time.
Ultimately, the suitability of trench fill foundations will depend on a range of factors, including the size and weight of the building, the anticipated forces that the building needs to withstand, soil conditions, and budget constraints.
Trench fill foundations versus Strip Foundations
Another common type of foundation used in a wide range of construction projects is the strip foundation. Whilst strip foundations and trench fill foundations are similar in some ways, there are also some significant differences between them.
Trench fill foundations are constructed by digging a trench around the entire perimeter of the building’s footprint and filling it with concrete, whereas strip foundations are long, narrow concrete footings that are placed under load-bearing walls.
One of the main advantages of trench fill foundations is that they require less excavation than strip foundations. This is particularly beneficial in areas where excavation is difficult or expensive, such as in urban areas, or sites with very limited access. However, trench fill foundations require more concrete than strip foundations, which can work out to be much more expensive.
Strip foundations are generally more suitable for larger buildings, as they can support much heavier loads. They are also more versatile than trench fill foundations, and can be used on uneven ground or designed to support different types of loads.
One of the main disadvantages of strip foundations is that they require more formwork than trench fill foundations, which is both time-consuming and expensive. Strip foundations also require rigorous planning and design to ensure that they are strong enough to support the weight of the building and the forces acting on it.
Whether you decide on a trench fill foundation or a strip foundation will depend on factors such as the size and weight of the building, the soil conditions, the site access and the budget.
Depth of Trench Fill Foundations
Working out the depth required for trench fill foundations is an important part of the design process, as it ensures that the foundation is strong enough to support the weight of the building and that it can resist any external forces acting on it. Some of the considerations to take into account when calculating the required depth of trench fill foundations are:
The load-bearing capacity of the soil: A geotechnical engineer can carry out soil testing to determine the strength and composition of the soil.
The weight of the building: The weight of the building will also influence the depth of the foundation. The building’s architect or structural engineer can provide this information.
The width of the foundation: The width of the foundation should be at least equal to the width of the wall it is supporting.
The frost line depth: The frost line depth is the depth to which the ground freezes during winter. The foundation should be below the frost line to prevent damage from freezing and thawing.
In the UK, there are a number of regulations governing the depth of trench fill foundations, and guidance on these regulations can be found in the NHBC’s Technical Requirements documentation. For example, in the UK, in frost-susceptible ground conditions, the depth to the bottom of the foundation should be at least 450mm.
Once all of the above information has been finalised, the required depth for the foundation can be calculated. Obviously, working out the depth required for trench fill foundations requires careful consideration of all relevant factors, and it is important to work with a qualified engineer or architect to ensure that any foundation is designed and constructed to meet the specific requirements of the building project.
As can be seen from this article, trench fill foundations offer a cost-effective, practical solution for many smaller construction projects. Whilst they are used predominantly in the domestic residential sector, trench fill foundations may also be suitable for smaller commercial projects, so a careful evaluation of various foundation types should always be made before reaching a final decision for your project, in order to deliver a solution that meets both budgetary and structural integrity requirements for the build.
And finally, if you are advised to use rebar reinforcement in your trench foundation, or you’d like to operate a belt-and-braces approach, contact us for a quick, no fuss quote.