Rubble Trench Foundations

Posted September 24, 2008

A rubble trench foundation is a way to build the foundation that more environmentally friendly than the typical concrete footings.  It uses much less concrete than a typical concrete stem wall and also can use local materials.  It also provides both a structural foundation and drainage in the same system.  One limitation is that it can only be used with a slab on grade or with a stem wall and crawl space.  The basic process is to dig a trench to below the frost line and then line the bottom with gravel.  A perforated drainage pipe is laid on top of the gravel and then the trench is filled with stone or concrete rubble to grade. A steel reinforced concrete grade beam is then poured directly on top of the rubble.  The rubble trench foundation produces a resource efficient, high quality, low cost foundation.

The rubble trench foundation have been used for thousands of years by was popularized early in the 20th century by Frank Lloyd Wright, who used it in a number of his buildings.  Today rubble trench foundations are commonly used in straw bale buildings due to their low environmental impact.  A major advantage of the rubble trench foundation is that it is not susceptible to frost heaving. Since the foundation is made of materials that rapidly drain, there is no water to freeze, and so no frost heaving.  The disadvantage of a rubble trench foundation is that it needs to be in well drained soil where the water table is below the bottom of the rubble trench.  It also works best on a sloped site where the drainage from the bottom of the trench can go to open air.  If the water table is low enough and the soil well enough drained, drainage can be accomplished on a flat site with a dry-well.  A dry-well is a hole that is below the drainage point that is filled with loose gravel or rubble and will accept the drain-water and slowly disperse it into the ground.  The dry-well has to be above the water table in order to work properly.

When building using a rubble trench foundation, you will probably need to get an engineer’s drawing made for it, as it is not included in the building code in most places.  The soil may have to be tested to see if it support the rubble trench and the building above it.  The basic procedure to create the rubble trench foundation is to first dig the trench, usually about 16 inches wide, with straight sides to a few inches below the frost line (4 feet in Southern Ontario).  This is most easily done with a backhoe, but can be done by hand.  The trench must be dug so that there is a slope of at least 1/8 inch per foot of slope for drainage.  Once the trench has been dug, place landscape fabric on the bottom of the trench and cover it with a few inches of gravel.  Then place a 4 inch perforated pipe on the gravel and have it follow the drainage slope either to open air or to a dry-well.  The sides of the trench should then be lined with the landscape fabric in order to stop silt from migrating into the rubble and plugging up the drainage.  After the landscape fabric has been laid the trench should be filled with washed 1 1/2″ stone or crushed concrete about a foot at a time and then tamped to reduce settling.  After the trench has been filled to grade, forms must be laid in a layout according to the engineered drawings  and the re-bar installed.  The concrete grade beam can then be poured.  Once the concrete has cured, the forms can be removed and the building can begin.

Posted under Techniques

Insulated Concrete Forms

Posted September 4, 2008

You may have seen the Insulated Concrete Forms (ICFs) being used to build a house, or heard about them in discussions about green building. ICFs are Styrofoam blocks that are stacked to make walls and then they are filled with re-bar and concrete. Unlike a normal concrete wall the forms are left in place. There are different variations on this theme, with most ICFs consisting of two flat 2-3″ pieces of foam separated by a 6-8″ space and attached together by wire or plastic braces. Other ICFs have a waffle like structure on the inside of the form which reduces the amount of concrete used and increases the insulation.

If you read the literature presented by the ICF manufacturers, they come up with statements about the “effective” R value up into the 50 range. This is misleading advertising. Concrete has essentially no insulating value (.08/inch) and the foam has an insulating value between 4 and 5 per inch. An ICF with 4 inches of foam (fairly typical) would then have an insulating value of between R16 and R20, way shy of the R50 advertised. The way that an ICF make houses more energy efficient is that the shell of the house is truly air tight. If the window and door penetrations are properly sealed (spray in foam and caulked), an ICF house would be essentially air tight. As noted in an earlier post, most heat is lost through air infiltration.

There are several disadvantages to ICFs. The biggest is that they use a lot of concrete, and the manufacture of concrete is one of the larger contributors to greenhouse gases in the world. Second is that they are made of plastic, which comes from fossil fuels. Another thing I don’t like about ICFs is that they have plastic foam on the inside of the structure. I don’t like this for 2 reasons. The first is that in the case of a fire, it could possibly produce very toxic smoke. Secondly having the insulation on the inside reduced the effect of the thermal mass of the concrete.

The advantages of ICFs is that they are much more flexible in the way that concrete walls can be formed. With conventional forms, it is much more expensive to have walls taller than 8 feet, as the forms have to be stacked which is much more labour intensive, whereas the ICFs don’t have that limitation. Also, ICFs can be installed by a Do It Yourselfers with the help of a few friends, but be careful to follow the instructions and make sure the walls a thoroughly braced, as a blowout can make quite the mess. Also if you use the waffle type of ICF, you can create a very solid wall that uses less concrete than a conventionally poured wall.

For my house I decided to go with the conventional poured concrete wall for the foundation. I did this because I designed the house as a walkout with passive solar input and needed as much thermal mass exposed as possible. If the foundation was not a walkout, I would have strongly considered using waffle type ICFs for the foundation, but I would be reluctant to use it for the above ground walls due to the high greenhouse gas emissions from the manufacture of the concrete.

Posted under Techniques