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 rubble trench foundations?
A rubble trench foundations is similar to a trench foundation except that instead of concrete being poured into the trench, the trench is filled with layers of well compacted stone. This is an ancient method of spreading the load from the structure to the soil below.
The rubble trench can incorporate a drain towards the bottom so that it serves a dual purpose – both as a footing and as a drain.
Frank Lloyd Wright was a great fan of the simplicity of the rubble trench footing
The trench fall into the category of “shallow foundations” . At the top of the rubble layer a reinforced concrete beam is usually placed onto which is built the structural walls.
The rubble trench has fallen out of favour with engineers, which is a shame as it can be a sound design solution that will drain a wet site, and that is also ecologically sound.
How To Build A Rubble Trench Footing
A rubble trench footing begins by having the tench dug to a depth specified by the structural engineer. Building Control will usually need to approve the soil condition at the bottom of the trench, before filling. Trench widths will 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.
A land drain will normally be placed towards the bottom of the trench to remove any water from the site. The water will normally be taken to soakaway. A geotextile membrane should be used to help prevent clogging of the drain from surrounding soils.
The stone layers need to be well compacted in 200mm layers.
The top of the stone rubble layer is made as level as reasonably possible. on top of this a “ring beam” of reinforced concrete is usually added. Once the concrete in the ground beam has sufficiently cured, then a short rising wall (sometimes called a stem wall or dwarf wall) is built from the top of the ground beam 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 Rubble Trench
- Pros – A trench footing is a cost effective solution
- It is simple to create with any digger and an appropriate sized bucket, plus stone.
- Incorporates drainage and load bearing in the same solution.
- Can use waste or recycled, reclaimed product in the rubble
- Cons – Can go deep if soil conditions are poor at the bottom of the trench
- Collapses of the soil sides can occur..
- May be treated with suspicion by those who automatically resort to a concrete filled trench.
- Difficult to make energy efficient as large thermal bridge is usually created