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RUBBLE TRENCH FOUNDATIONS
an edited version of this article was published in the May 2003 issue of Building Safety Journal.

historical context
the basics
why it works
step-by-step process
benefits & challenges
getting a building permit
additional resources
related articles

HISTORICAL CONTEXT

Various forms of the rubble trench foundation have been used for thousands of years in construction.  Earthen walls in the Middle East and Africa, for example, are built on top of shallow ditches filled with loose rock.  Frank Lloyd Wright came across the rubble trench foundation system around the turn of the 20th Century.  He observed the structures to be "perfectly static" with no signs of heaving, and thereafter built consistently with what he termed the "dry wall footing".  Many time-tested structures stand as testimony to the longevity of the rubble trench.

THE BASICS

Notice how little site disturbance is required for this foundation type.

A rubble trench, as it's name reveals, is comprised of a continuous trench filled with crushed stone and topped with a grade beam. This type of foundation is unique in that it provides structural bearing as well as water drainage in one system. The result is a resource-efficient, high quality, low cost foundation system.

The trench is typically dug with a backhoe bucket several inches below frost depth, and sloped to daylight (or a dry well) for drainage. The trench should have straight sides and a minimum width of 16 inches. A wider trench provides additional bearing area if soils do not have adequate bearing capacity. The bottom of the trench is tamped flat and lined with several inches of gravel, on which a standard 4-inch perforated drainpipe is laid. The trench is then filled to grade with gravel, tamping every vertical foot to ensure compaction. The steel-reinforced grade beam is cast directly on the stone fill, and can either support a stem wall and crawl space (see illustration) or become the turned-down edge of a slab-on-grade. The "rubble" fill may be stone or crushed concrete, but in either case, it must be washed and should provide a variety of sizes with an average of 1- inches. Where silting-in is of concern, the trench may be lined with a geotextile filter fabric prior to being filled.

WHY IT WORKS

The compacted gravel acts both as a "French drain" system as well as a spread footer that provides bearing capacity for the grade beam. The required width of the trench is determined in the same way that a standard footing is: according to the building loads and the bearing capacity of the soil. The reinforced concrete grade beam distributes the building load evenly across the gravel footer. The size of the grade beam and placement of rebar depends on building loads and should be designed by an engineer. (For context, a typical single-story residential structure requires approximately a 16-inch wide by 8-inch high grade beam with three continuous lengths of -inch rebar.) Because the footer itself is literally a drainage way, water cannot settle in or around the structure of the foundation. Without water there is no opportunity for freeze/thaw cycles to cause detrimental heaving of the grade beam.

STEP-BY-STEP PROCESS

 

1.  Dig 16-inch wide (minimum) trench to frost depth plus 4 inches and slope to daylight or dry well (1/8 inch per foot minimum).  Note: centerline of trench aligns with centerline of grade beam.

2.  Tamp any disturbed earth in the bottom and line trench with filter fabric geotextile (highly recommended, but optional).  Filter fabric helps prevent footer from silting-in over time.

3.  Layer in 4 inches of stone and tamp. Ensure that surface of gravel fill maintains drainage slope and is at or below frost line.

4.  Lay continuous 4-inch perforated drainage pipe and slope to daylight (as for a standard foundation footer).

5.  Fill remainder of trench flush to grade with 1-inch gravel, tamping after every vertical foot of fill.  (Hand tampers work well for this.)

6. Coat formwork with biodegradable oil to ease release of wood for reuse.  Any vegetable oil works well.

Note: formwork can be reused for structural framing once the grade beam has cured.

7. Set formwork for grade beam, adding steel reinforcing as required.

Note the polypropylene sheeting under the slab to prevent moisture from creeping up.

8.  Pour grade beam and integral slab.

Note:  a soil-cement slab is shown here; a 12-inch x 12-inch column of pure concrete with rebar is placed only under points where posts will be set above.  The clay in the soil-cement is from the site excavation and minimizes the total cement used for the foundation.

BENEFITS & CHALLENGES

BENEFITS OF RUBBLE TRENCH FOUNDATIONS CHALLENGES OF RUBBLE TRENCH FOUNDATIONS

  • Lower cost than a concrete footing

  • Uses much less concrete (production of concrete requires a great deal of energy and generates greenhouse gases)

  • Can use recycled crushed concrete fill

  • Provides excellent drainage, and thus a "static" foundation system

  • Soils with low bearing capacity may require an extremely wide trench (or some other footing alternative) to achieve adequate bearing area
  • Not specifically addressed in building codes; requires additional dialog with permitting officials

GETTING A BUILDING PERMIT

A rubble trench foundation meets the requirements and the intent of U.S. building codes.  However, since this system is not specifically outlined in current codes, acceptance is provided on a case-by-case basis.  Since this puts permit approval at the discretion of individual building officials, it is recommended to initiate a dialog prior to submitting for a building permit.  This provides an opportunity to inform and educate permitting staff and provide adequate information to satisfy their desire to ensure a safe structure.  The article written by Elias Velonis for Fine Homebuilding (see "Additional Resources") provides excellent technical information to this end.  It is recommended that stamped structural drawings be provided so the burden of proof is not purely conceptual.

My experiences with rubble trench foundations have been rather positive. I interact with the permitting office well ahead of time, and have not encountered rejection or delays. In one case the building inspector required that the structural engineer be present to verify tamping. I have needed to increase the trench width to 24 inches when a 16-inch backhoe bucket proved too difficult to find. (The only impact was the need for additional gravel mix to fill the trench; even with additional gravel, the cost of the foundation was lower than a standard concrete footer would have been.)

ADDITIONAL RESOURCES

WEBSITES

DCAT
www.dcat.net
Development Center for Appropriate Technology with lots of information on strawbale

The Last Straw Journal
www.thelaststraw.org
Quarterly Newsletter - a must for anyone serious about strawbale

DOE Demonstration Project
www.eere.energy.gov
U.S. Department of Energy article on strawbale construction

Straw Bale Central
www.strawbalecentral.com
general info, books, and additional resources

ARTICLES

Koko, Sigi. "Rubble Trench Foundations - A Brief Overview", Building Safety Journal, Volume 1, number 3, May 2003.

Rob, Tom. "Rocks In Your Shoes", The Last Straw Journal, Issue #16, Fall 1996.

Velonis, Elias. "Rubble Trench Foundations: A Simple Effective Foundation System for Residential Structures", The Best of Fine Homebuilding. Taunton Press: Newtown, CT, 1997.

RELATED ARTICLES

Overview of Strawbale Construction

 

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