26 January 2011

What is a Rubble Trench Foundation?

The function of a foundation for any structure is to take all of the weight from the building above, and distribute that load evenly to the ground below.  The type of foundation that makes sense for a particular structure depends on the size and shape of the building, the type of structural system for the building, the slope of the land, the capacity of the soil to support weight, and how deeply the ground freezes in your area in winter.  A rubble trench describes one low-impact option for the footer, which is typically the part of a foundation that is below ground.

WHAT IS A RUBBLE TRENCH FOUNDATION?
A rubble trench is simply a continuous trench footer around the structural perimeter, dug as deeply as the ground freezing point in winter, and filled with stone.  A structural (usually concrete) grade beam (a beam that rests on the ground) is poured on top of the stone-filled trench, and distributes the structural loads of the building across the surface area of the trench below.  This type of foundation uniquely provides both structural bearing as well as water drainage in a single foundation system.  The width of the trench determines the bearing capacity for loads above (as with a standard concrete footer).  A filter fabric liner between the soil and the stone provides insurance against silt filling-in the cavities between the stones, which would impede the flow of water over time.  Drainage is important with most foundation systems, since water is the single largest culprit for foundation failure.  Liquid water can erode the ground bearing around a foundation footer.  Frozen water expands when it freezes, which causes the ground around your foundation to also expand, which subsequently causes the foundation to heave upward in Winter and drop it back down when the ground thaws in Spring.  That heaving movement can crack the structure or cause uneven settling of the building.  When installed correctly, a rubble trench results in a resource-efficient, high-performing, eco-friendly, and low-cost foundation footer.

All sizing & structural details must be provided by a structural engineer

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.  The ditches are shallow because the ground does not freeze.  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", meaning they showed no signs of heaving, because of the complete elimination of water around the foundation.  From then forward, he built consistently with what he termed the "dry wall footing".  Many time-tested structures stand as testimony to the durability of the rubble trench. 

STEP-BY-STEP PROCESS
1. Dig a trench to frost depth plus 4 inches and slope to daylight or dry well.  (Slope should be 1/8" per linear foot, minimum.)  I usually do a minimum of 16" wide trench, but the width required for structural support depends on the specific soil bearing capacity and building loads.

2. Line trench with filter fabric to prevent silting-in of the footer over time.

3. Add 4 inches of stone and tamp it once around, by hand is fine.  Ensure that surface of the gravel fill maintains the drainage slope and is at or below the frost line.

4. Lay 4-inch diameter perforated pipe continuous on top of the sloped stone.  Slope the pipe to daylight, as for a standard foundation footer.

Note: technically the drain pipe is optional, since the entire rubble trench footer provides drainage.  In some jurisdictions, I have found that including the drain pipe, even though it is redundant in function, facilitates getting a building permit.


5. Fill the remainder of the trench flush to grade, or just below, using 1-1/2 inch gravel, tamping after every vertical foot of fill.  (See note below on stone size.)  Hand tampers work just fine for this application...no need to use a pneumatic tamper.  Just walk along the entire trench and drop the tamper over all areas.  Tamping locks the stones together to provide strong bearing that won't shift over time.

Note that the rubble fill may be stone or crushed concrete, but in either case, it must be washed free of fines and should provide a variety of sizes with an average of 1 to 1-1/2 inches and a minimum of 1/4-inch.  Fine dusty particles or sand-sized particles can clog you rubble trench and then it will not act properly as a drain.  You can test the drainage in your trench with a hose before continuing.

6. Coat your formwork (for the grade beam) with biodegradable oil.  This ensures easy release of your form for potential reuse.  Any vegetable oil works well.  I use 2x12's for the grade beam formwork and then reuse the same wood for structural framing once the grade beam has cured.



7. Set formwork for grade beam or slab-on-grade thickened perimeter beam.  You will likely need  steel reinforcing bar (rebar) inside your concrete.  Have a structural engineer designate the structural requirements for your grade beam.  If you are pouring a slab-on-grade with a thickened perimeter beam over the rubble trench and will install in a single concrete pour, the slab preparation is the same as what you would install with a conventional footer.

8. Pour concrete grade beam.  The grade beam can be a discrete structural element around the entire perimeter of the structure, or can be integrated into the thickened perimeter of a slab-on-grade foundation (as this photo shows).  Any structural elements above this point are completely standard, whether installing a stem wall and crawl space, a full basement, stud walls, a post-and-beam structure, or whatever.  An engineer should size all structural elements.

Note the rebar still sticking out of the ground to support the wood form until the concrete has fully cured.

BENEFITS
Lower Cost.  A rubble trench foundation requires less labor, uses less material, and reduces material cost compared to a standard concrete footing.  There is no over-digging, no footer forming, and no backfill.
Minimal Site Impact.  Digging is limited to only the outline of the building, so site disruption is minimized.
Lower Greenhouse Gas Emissions.  Rubble trench footers reduce concrete use by an average of 80%, compared to a standard footer (depending on frost depth and the type of foundation installed).  Production of concrete requires a great deal of energy and generates 1.25 pounds of greenhouse gas for every pound of cement in the mix.  Reducing total concrete use translates to direct reductions in greenhouse gas emissions.
Can Contain Recycled Content.  The rubble fill can use recycled crushed concrete instead of gravel, as long as fine particles are washed out.
Improved Drainage & Foundation Performance.  A rubble trench provides full water drainage under every structural bearing element of the foundation, ensuring that the footer remains dry at all times.  This type of static foundation system ensures that water cannot freeze under the foundation.  When water freezes in the ground, the water expands, which can heave a building foundation.

CHALLENGES
  • Soils with low bearing capacity may require an extremely wide trench or some other footing alternative to achieve adequate bearing area.
  • Rubble trench foundations are not specifically identified in building codes, so may require additional dialog with permitting officials.  It helps to provide drawings stamped by a licensed engineer.

BUILDING PERMITS
Rubble trench foundations meet the requirements and the intent of U.S. building codes, however, since this system is not specifically identified 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 everyone's mutual desire to ensure a safe structure.  The article written by Elias Velonis for Fine Homebuilding magazine provides excellent technical information.  Stamped structural drawings are also highly recommended.

For additional information on rubble trench foundations, see also my online article: Rubble Trench Foundations.

3 comments:

  1. What is a rubble trench foundation ? I don't Know but read this post now i am clear. Thanks for the post.
    rebar reinforcement

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  2. I'm curious about this ... is the textile liner water permeable? If so, what keeps the flow of water from changing the soil around the foundation? If not, why have the perforated pipe 4" up from the bottom of the trench, why not just lay the pipe on the bottom of the trench and lead away the collecting water out somewhere else?

    If you have a rubble trench foundation, can you also have a basement provided the basement is far enough away from the perimeter of the foundation that the ground can support the foundation without caving in or subsiding eventually?

    This is very interesting.

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    Replies
    1. Yes, the liner is a filter fabric...lets water flow through but keeps silt from building up inside the stone trench. There shouldn't really be a "flow" of water at the depth of the foundation. Dampness yes, but you shouldn't really be building where the water table is high. The perf. pipe is technically not needed. I only include it when I have a permit official that is unfamiliar with this type of foundation and wants to see conventional water drainage included with the footer. And the rubble trench simply replaces whatever your foundation would have been down into the ground, so you can building anything up from here...basement, crawlspace, slab-on-grade...whatever. Hope that helps.

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