The rubble trench foundation (RTF)



Drainage & Foundation Combined

The rubble trench is a favourite type of foundation among many natural builders due to the fact that it substantially reduces cost, time, labor and the need for cement and rebar. It allows you to even eliminate the need for cement and rebar completely if needed, which we will illustrate here. It also combines two key features that any successful foundation needs to achieve drainage and an even load distribution.



The Frozen Landscape

In temperate climates where winter frost should be expected a foundation must be designed to keep water from accumulating underneath the foundation walls in order to avoid frost heaving. Frost heaving is when water in the ground expands as it freezes and subsequently pushes the ground upward. And in spring the water thaws and the ground sinks back down again. This constant pushing up and down will force the house to twist and skew which might not only create problems with opening and closing of doors and windows but even structurally compromise the house.

The RTF deals with these challenges by consisting mainly of a drainage trench, much like a French drain, filled with crushed stone that lets any water drain down and away from the foundation. The basic principle of this has been used in many shapes and forms for thousands of years but it was American architect Frank Lloyd Wright who brought this technique (which he called ”dry wall footing”) into the 20th century.


1. The Trench

The trench should run underneath all the external load bearing points, continuously around the full perimeter of the building. Any internal load bearing points (underneath the house) can simply rest on undisturbed earth (stable soil cleared of top soil) so the amount of digging needed is minimal compared to your standard concrete slab.

The minimum depth of the trench is determined by the local frost-free depth. This is the depth where the temperature never descends to zero C° so any water ending up in the trench will therefore not freeze. The bottom of the trench then needs to slope with an even descent of at least 3 cm for every 1 meter of trench, diverting the water towards one point. From then on the water can be diverted away from the foundation through an outlet to either daylight or to a dry well.

2. Filter Fabric Geotextile

The trench must then be lined with a geotextile to prevent any of the surrounding soil from clogging up the trench and outlet. This geotextile is really the only non-natural material that you need for this foundation to work properly. The trench would probably work quite well without it for at least a while but any silting would gradually reduce the trench's ability to drain away water, eventually rendering the rubble trench completely useless, with frost heaving just waiting to happen.

3. Crushed Stone

After the trench is lined with Geotextile it is gradually filled with angular and washed stones (see picture bottom right) that have an average size of between 2,5-5cm, compacted at every 30cm layer using either a hand powered tamper or a pneumatic tamper. The crushed stone needs to be washed before filling the trench otherwise it will contain sand and smaller particles that could silt up the trench and/or outlet over time. You continue filling the trench with crushed stone and tamping it every 30 cm layer until you reach about 20-30cm below grade (ground surface). This is where the stonework begins.




4. Stonework

At this point you could of course construct a formwork, setup with rebar inside it and pour a concrete grade beam, in which case the rubble trench would greatly reduce the need for cement. However, the RTF can be constructed without a single drop of cement, using the oldest building material known to man; stone. It needs to be a continuous wall made of either really large blocks of stone, large enough for one row to sufficiently lift the house above ground or smaller, more easily managed stones arranged in a beautiful dry stacked stone wall (see below left). Both choices would create a look that definitely matches the natural materials of your house a lot better than cement ever could, and lets the building truly marry the landscape. The foundation should extend at least 40cm above grade to keep the house away from any possible splashing and normal masses of snow.

5. First Line of Defence

The rubble trench should now be able to deal with most of the percipitation that one can expect but as an extra precaution you should add a small slope all around your entire foundation. This works as a first line of defence by keeping much of the precipitation from ending up in the trench in the first place. Simply let the Geotextile continue a bit up along the side of the foundation stones and fill up with earth against the stones with the Geotextile in between. This way you keep any of the added earth from ending up in the trench, avoiding any potential clogging. Then shape and compact the earth into a slope that leads water at least 1 meter away from the foundation. When done correctly this should leave you with a very dry and strong footing that will do its job for a very long time.