6. Materials: Foundations

6. Materials: Foundations

(a) Concrete slab

Despite the fact that this is far and away the cheapest solution, I am reluctant to build on a slab on the ground. I don’t want to contribute to the already considerable problems Cumnor Hill has with flooding, and neither do I want Cumnor Hill draining into my house. The section is already continually damp, owing the clay substrate, and I would ideally like my building to be raised from the ground so that the air can circulate underneath it.  Despite my reservations, the plain concrete slab is the solution urged on me by all builders and architects I have consulted. Given my budget, and having given the other possibilities dues consideration, I may come back to it. However, it is estimated that production of 1 ton of cement will produce 0.8 tons of CO2, so I am investigating the possibility of using either of the following: Hempcrete[1] and Limecrete[2]

(b) Raised floor construction on concrete piers[3]

A – Floor system

B – Wood or steel beam

C – pier/post connector

D – Concrete pier

E – Foundation bolt

F – Shear panel

G – Insulation and vapour barrier

H – Foundation Vent

Admittedly, though, there is a lot of engineering involved, and it would be expensive. We still have concrete in the form of piers, but in addition, we have steel beams and quite a lot of ironmongery in the form of vents, foundation bolts, shear panels etc. How green would it be?


(c) Recycled tyres[4]

Recycled tyres are often used for the foundations of straw bale buildings. A good example can be found in a community health building, the Beehive in Manzil Way, off Oxford’s Cowley Road. Tyres filled with rubble are strong and stable, but opinion varies as to how long they will last if not exposed to the air. Estimates range from 300 years to thousands.  So, as long as you bury them, they could last forever. If you leave them exposed to the air, they will degrade. At Manzil Way, they have plastered over the tyre foundations with a natural clay plaster,  but it is cracking already, after just a couple of years, and it looks as though it is not going to last very long.

(d) Timber piles

I like the idea of timber piles, but in this case using them could be problematic. They would have to be pile-driven, which would mean bringing heavy machinery on site. Or they could be dug in, but in this case would still need to be cemented into place.  Also, once I have the piles in, I would still have to fix the base to them (see b. raised floor construction above).

(e) Micro-piles

I thought I liked the idea of micropiles, until I read more about them: “Micropiles (typically 6” – 12” diameter) are excellent foundation elements constructed using geotechnical drilling techniques and high strength materials. Micropiles are preferred supports to stabilize buildings, bridges, highways and other man-made structures that have restricted access, karst geology or are vibration sensitive. Micropiles can penetrate to hundreds of feet in depth and support high compression and tension loads. . .” at which point I stopped, feeling really stressed. I think micropiles, although they sound small and friendly, are not required for a small Ecobuild in Cumnor. Another case, I think of an over-engineered solution

(f) Helical foundations

Browsing through construction sites on the web, I came across a company called Screwfast Foundations and was really excited by their product. This simple foundation system is based on the principal that a screw can do the work of a nail, only with longer lasting effect and with less ground disturbance. The idea of driving piles into my plot is a pretty scary one: pile drivers are noisy and brutal, and the pile would have to be concreted in anyway. If you could screw a pile into the earth, how elegant a solution is that! This is a system used by the Victorians, for the first fixed shipping lights in the Thames Estuary and for the construction of the big promenade piers along the South Coast. These are still in daily use after 150 years, so for this product, my build would be small beer. This may, of course, be my problem. Would this be using a sledgehammer to crack a walnut?  And, elegant and exciting as it is, how green is it? Interestingly, though, my approach to the company elicited the response that they would be too expensive for my purposes, and the suggestion that micropiles would be better.

(g) Steel spanning concrete slabs

Another exciting innovation by major building company Roger Bullivant is “System First” a modular foundation method comprising galvanised steel beams spanning concrete pile caps. On top of the beams sit pre-cut high-density polystyrene slabs in metal supports; and on top of this is poured a screed of concrete, thus creating a relatively lightweight and highly insulated floor slab.

Roger Bullivant “System First” foundation

Roger Bullivant claims dramatic savings in carbon emissions, water consumption and raw materials: “Compared with a traditional trench-fill foundation for a house with an 80m2 footprint, it says that CO2 emissions are reduced from a typical 73 tonnes to just 13 tonnes. Water use is reduced by almost 90% from a staggering 36,000 litres to just under 4,300 litres.” The company also cites a 75% reduction in construction time on a traditional house foundation, with comparable cost savings.[5] This is a very attractive option, but, again, it could be a bit over-engineered for my purposes.

(i)Tyres and gabions finished with Limecrete

Matt Muldoon, of the Natural Building Company, advises that I should assume a dead load of 300kg/m2 for an intensive green roof, which could be a problem for a load-bearing straw wall, but would be OK for timber frame. The foundations could be of gabions or car tyres, even without crete, although, if I used tyres, I would, of course, still have to shelter them from the elements, so limecrete could be used for that. He writes:  If I was building it, though, I would put some foundation in beyond car tires/gabions . . .  I’d do limecrete trench fill, or better still you could save on limecrete by making a rubble trench and capping it with limecrete. This should be fine – your building is lightweight, and if you’re building on clay it’s better to have something flexible (limecrete) than inflexible (concrete). Just a note though – if the ground is very clayey at the bottom of your foundation, so clayey that it can’t drain, then it’d be better to do trench fill limecrete than rubble trench. If your foundation can’t drain and it’s full of rubble then it’ll just fill up with water like a pond – and nobody wants their house on a pond.[6]

(j) Filcris Ecogrid

Just when I thought I had covered all the bases, I heard back from a company I had contacted with a view to using recycled plastic as a foundation. They suggested two products from their range: one was recycled plastic posts, and the other a kind of recycled plastic mesh that I could fill with compacted earth, sand, or shingle. It is a product called Ecogrid[7], and is used for car parks and as a guard against erosion and as a foundation for lightweight garden buildings. Could this be the answer?


I think I will let Malcolm, my architect brother-in-law from New Zealand, have the last word on this one. He writes to me about a 3-day course on Eco-design he attended in Wellington, New Zealand: “One of the workshop sessions discussed the comparison of building materials to compare the energy input needed to produce the material.   The scientist projected an ordinary drinking glass on the screen and then proceeded to add the energy inputs back to the beginning of sourcing the sand to make the glass.  The steps covered the entire screen in a network flowchart.  Because of the complexity of inputs to create a product it was impossible to compare one product with another in a quantifiable way.   What you are doing is all that is possible where you are looking for materials that are as close to the source with the least work required to make them usable.  You are then listing all the merits and disadvantages of each and leaving it up to the subconscious to weigh it all up and tell you which one you prefer.”

It’s not a quick job, planning a building. If I thought that ten weeks would be enough time to get it sorted, I was mistaken. It takes a lot of time to research techniques and materials . . . or, at least, just to learn which techniques and materials I need to research. And then, of course, there is the mathematics: learning about loading and stress, ratios, volumes, bearings etc. But an Ecobuild isn’t an easy option. As the Einstein Principle has it: Things should be as simple as possible, but no simpler.

First, I have to understand the complexity of my building. Then I have to simplify it. There is much left to do.

[1]See Appendix 1

[2] See Appendix 2

Illustration: Nucentury Home Inspections

[5] https://www.building.co.uk/sustain_story.asp?storycode=3126869 consulted 18 Feb 2009

[6] Email from Matt Muldoon (Date)

[7] See Appendix 3