We have created this Foundation Help & Advice centre to answer your questions about GreenRaft and its insulated foundation solutions. We are also providing information on various foundation types to explain the reason why a particular foundation might be used or has been selected by your structural engineer.
It is not an exhaustive list, so if you have a question that needs an answer, please let us know. Hopefully, we will add your question to the section.
There will be some linking between the topics where subjects overlap, This should give a better understanding of the various foundations and how they are used.
It is a huge topic, so you will also find links to other authoritative sites where more in-depth articles are available.
The main factors that influence what type of foundation might be used are the load of the structure, the bearing capacity of the soil, the width of the foundation upon that soil and other site conditions. These site conditions may be soil and strata mixtures, water tables, nearby trees or removed trees.
Help Centre Main Topics
Across the top are five topics related to foundations, that make up this Foundation Help & Advice Centre. The topics cover foundation types, foundation design, thermal performance, costings, and installation.
Pick which one seems most appropriate for your question and see if the answer is in there.
What are trench foundations?
Trench foundations are the most common type of foundation used in UK housebuilding today. They are popular because they are relatively cheap and can be formed simply without too much fuss.
The trench fall into the category of “shallow foundations” and are similar to a “Strip Foundation“. These two terms, “trench” and “strip” are sometimes used to describe either type. The main difference is that trench foundations are usually deeper and are formed of mass concrete without reinforcement. A strip foundation contains much less concrete but has steel reinforcement to provide it with its strength.
Strip footings were mostly replaced by trench footings when the cost of labour increased beyond the cost of materials.
How To Build A Trench Foundation
A trench footing is exactly what it says. The digger digs a trench under where the main load bearing walls will be. The depth of the trench has to be a minimum of 450mm below ground level, as that depth is the frost line in the UK. Trench widths usually relate to the standard bucket attachment widths of 16″, 20″ , 24″ etc.. The depth of the trench can be unknown at the outset, as it has to go down to a solid bearing soil.
Building Control will usually inspect the trench after the dig, and they need to approve the condition of the soil or more digging may be required.
The top of the concrete is made as level as reasonably possible. The top of the concrete is usually just below ground level. Once the concrete has cured sufficiently a short rising wall (sometimes called a stem wall or dwarf wall) is built from the top of the trench to above ground level. This wall can be brick, block, trench block or can be reinforced concrete. A suspended beam and block floor can sit on top of this wall if a cavity is required under the floor. Alternatively a ground bearing concrete slab can be poured to create the floor at ground level.
Pros & Cons of Trench Foundations
- Pros – A trench footing is a cost effective solution
- It is simple to create with any digger and an appropriate sized buket
- Well known and common solution
- Cons – Can go deep if soil conditions are poor at the bottom of the trench
- If deeply dug then collapses can occur requiring time and cost consuming shoring for the concrete pour.
- Uses a high volume of concrete, so poor for carbon footprint
- Difficult to amke energy efficient as large thermal bridge is uusally created