The rubble trench foundation, an ancient construction approach popularized by architect

Frank Lloyd Wright Frank Lloyd Wright (June 8, 1867 – April 9, 1959) was an American architect, designer, writer, and educator. He designed more than 1,000 structures over a creative period of 70 years. Wright played a key role in the architectural movements o ...

, is a type of

foundation Foundation may refer to: * Foundation (nonprofit), a type of charitable organization ** Foundation (United States law), a type of charitable organization in the U.S. ** Private foundation, a charitable organization that, while serving a good cause ...

that uses loose stone or rubble to minimize the use of

concrete Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most wi ...

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. However, some soil environments are not suitable for this kind of foundation, particularly expansive or poor load-bearing (< 1 ton/sf) soils. A rubble trench foundation with a concrete grade beam is not recommended for earthquake prone areas.Chiras, Daniel D. (2004). ''The new ecological home: the complete guide to green building options''. White River Junction, Vt.: Chelsea Green Pub. Co. p. 75. . 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 ensure good soil drainage, the rubble trench foundation serves both foundation functions at once. To construct a rubble trench foundation a narrow trench is dug down below the

frost line The frost line—also known as frost depth or freezing depth—is most commonly the depth to which the groundwater in soil is expected to freeze. The frost depth depends on the climatic conditions of an area, the heat transfer properties of the s ...

. The bottom of the trench would ideally be gently sloped to an outlet. Drainage tile, graded 1":8' to daylight, is then placed at the bottom of the trench in a bed of washed stone protected by filter fabric. The trench is then filled with either screened stone (typically 1-1/2") or recycled rubble. A steel-reinforced concrete grade beam may be poured at the surface to provide ground clearance for the structure. If an insulated slab is to be poured inside the grade beam, then the outer surface of the grade beam and the rubble trench should be insulated with rigid XPS foam board, which must be protected above grade from mechanical and UV degradation. The rubble-trench foundation is a relatively simple, inexpensive, and environment-friendly alternative to a conventional foundation, but may require an engineer's approval if building officials are not familiar with it. Frank Lloyd Wright used them successfully for more than 50 years in the first half of the 20th century, and there is a revival of this style of foundation with the increased interest in green building.

References

{{DEFAULTSORT:Rubble Trench Foundation Shallow foundations Sustainable building Civil engineering