The rubble trench foundation, an ancient construction approach popularized by architect Frank Lloyd Wright, is a type of foundation that uses loose stone or rubble to minimize the use of concrete and improve drainage. It is considered more environmentally friendly than other types of foundation because cement manufacturing requires the use of enormous amounts of energy.
A foundation must bear the structural loads imposed upon it and allow proper drainage of ground water to prevent expansion or weakening of soils and frost heaving.
While the far more common concrete foundation requires separate measures to insure good soil drainage, the rubble trench foundation serves both foundation functions at once.
To construct a rubble trench foundation a narrow (approx. 400 mm.) trench is dug down to about 100 mm. below the frost line. The bottom of the trench would ideally be gently sloped to an outlet.
The bottom of the trench is flattened and lined with several inches of clean gravel, on which a standard 100 mm. perforated drainpipe is laid.
The trench is then filled to grade with either screened stone (typically 20 to 50 mm) or a clean recycled rubble. Note: where silting-in is of concern, the trench may be lined with a geo-textile filter fabric prior to being filled to eliminate the trench being silted up over time and losing its drainage capacity.
A steel-reinforced concrete grade beam may be poured at the surface to provide ground clearance for the structure.
Advantages of a Rubble Trench Foundation
- 7% of all greenhouse gasses come from the manufacturing of portland cement. Reducing the amount of concrete reduces the amount of air pollution.
- Concrete is expensive. Reducing concrete can reduce cost of building.
- Less mass below ground reduces the loss of heat from the floor
Disadvantages of a Rubble Trench Foundation
- Can not be used for structures with basement.
- Some soil environments are not suitable for this kind of foundation; particularly expansive or poor load-bearing soils.
- Not specifically addressed in building codes; requires additional dialog with permitting officials