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.

20 comments:

  1. 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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    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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  2. I am wondering about the continuous cement beam. Is it possible to use cement pavers/mortar and get the same or similar outcome? I was considering flat cement pavers with pre drilled holes for rebar to fit in vertically for strength and to give vertical wood plank- footers(?) something to attach to the foundation with in order to create a wood frame. The wood frame would be filled with straw/light clay. Is this possible?

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    1. I would run your specific structure past a engineer....there are too many variables to give you specific structural advice. But in short, you need something that will distribute any point loads from the building, across a larger area of earth. Otherwise you have sinking, settling, shifting, etc.

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  3. Thank you for the interesting post. I have a question concerning the materials used as 'rubble'. I have plenty of bricks from old houses (full size, half and smaller ones). Can they be used in the rubble trench?

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    1. So the answer is maybe. What depends is the strength of the brick and the loads of the house. If the brick is very soft, easy to crush AND the weight that it will support is very high, then it could crush the brick and cause sinking. However, you could overcome even soft brick with a wider grade beam (ie, the concrete part at the top). So I would say if your brick is hard and strong, and you are building something small and not crazy heavy (like no bigger than a modest house), then you are probably fine with the brick. But if your brick is soft, or what you are building is big/heavy, then I would talk with an engineer about it. (Might be a good idea to talk with an engineer regardless, as I can't really give you structural advice....especially without seeing the specifics of your situation.) I hope that helps.

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  4. Thank you for your quick answer!
    Actually I was worrying about the size of the brick because the most of them are bigger than the usually mentioned gravel or stone.

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    1. What you need to be sure of is:
      1) you can tamp the material so it is well seated and won't move over time
      2) that the material you use is not at risk of crushing (and thus creating a void)
      3) that water can flow easily through (since you are really making a drain)

      And of course that the bearing area is sufficient for whatever you are building on top. Does that help?

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  5. Thanks for your blog, it's been really usefull..

    I have a question about foundation/grade beam choice, which alternative you think it's best for a strawbale house: rubble trench and concrete, earthbag, tires with rammed earth?
    Considering we live in an earthquake area (really strong, in Chile) and also cost/time/work because we are building the house ourselves.
    We have worked before with rammed earth tires and it's really hard work and slow, earthbags seem easier, but rubble trench seeams much easier. I only doubt if it is strong enough.
    What would be your advice?
    thanks!

    ReplyDelete
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    1. I don't design in an earthquake region, so my knowledge of earthquake-safe structures is purely theoretical. That said, I would do a rubble trench foundation with a grade beam, but place the beam so that it is partially below the finish level of the ground. That way it is locked into the earth, but not in a way that it can't move with the earth (if/when it moves due to an earthquake). Kindof like a raft foundation. But I would seek advice from a structural engineer just to verify what the recommendation is. I would not use the tires due to toxicity. And I would not use earthbags as your foundation because think about what happens when the bags disintegrate...then you just have compacted fill as your structural bearing.

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  6. Hi Sigi, in attaching the sill plate to the concrete bond beam can I shoot the pressure treated 2x4s with a ramset since I didn't set J bolts into the concrete?

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    1. typically, yes. I would check with a local structural engineer to confirm and to sort out size & spacing. Also don't forget the sill seal between the wood & the concrete.

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  7. Yes, I had planned to get a seal for that purpose. Thank you!

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    1. Hi Sigi, I am using 4x4's for my toe ups. My bond beam is 12 inches wide but my bales are 21" wide. Should I place the interior toe up (4x4) on the inside edge of the beam and then use the knee wall to support the inside balance of the bale or just use the interior knee wall to support the whole bale? Clear as mud? Thank you so much!

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    2. Yes, you need some support for the whole bale. If you are framing a floor, then the bales can be on flat 2x's right on the framed floor. But if you are creating a curb for under the bales, then support the whole thing. And think about your plaster as well. Thick plaster needs something to sit on...if it's floats and is just "glued" to the wall, it will eventually sag and crack at the top. In terms of what solution you have to supporting the whole width of your bale wall, either solution is fine as long as what is below the wood is appropriate. So you can either move your 4x4 to the face of the wall or build a little knee wall. (Don't forget to insulate the cavity)

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  8. Thank you Sigi, I am planning a earthen floor and the interior plaster will rest on the compacted gravel underlayment.
    So I will place both 4x4's on the edges of the 12x12 bond beam and build a knee wall up to the level of the 4x4 toes ups to support the full bale.But what will the exterior lime plaster rest on?

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    1. can be the ground (with a capillary break) or a plaster stop (like a ledge for the plaster to sit on)

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