EBC: Rubble Trench Foundation

I want to keep the three cedar elms (circled in red) so their locations determined the dimensions of the earth berm cottage (18 feet x 12 feet)

Site Preparation

While waiting for a septic inspection I spent a couple of days using the mini-excavator to remove a large portion of earth from a slope on the back section of our property. I’ve had this vision of a little cottage tucked into the hillside under a canopy of trees. There are three large cedar elms I used as my size constraints for the cottage dimensions of 18 feet by 12 feet.

After two days of excavation I had made a 5 foot cut into the hillside. I had videoed and taken pictures of the septic and pond excavations extensively and realized it just wasn’t that exciting. So, if you want to watch the excavator in action I recommend my pool and septic tank videos. Here are some before and after pictures of the site.

The Rubble Trench

For the rubble trench I dug a trench 18 inches wide and a minimum of 12” deep. The frost line where I am is six inches so that’s plenty. I’ve read that rubble trench foundations are resistant to frost heaving so even if the frost line were deeper it wouldn’t be an issue. Rubble trench are also better for expansive clay because they won’t crack like a concrete footer would. To move water that ends up in the trench I sloped it from the back corner to the front corner. From the front corner I ran 4 inch corrugated pipe downhill to a dry well.

I put landscape fabric down in the trench to keep the rubble from becoming infilled. After that I put down 4 inch corrugated drain pipe and then back filled the trench with 3” river rock tamping as I went. The river rock is rounded so it provides plenty of space for water to infiltrate. It took seven yards of river rock to back fill the trench. I brought the rock up an inch or so above ground level and used the laser level to get it fairly level.

The Grade Beam

The concrete grade beam is 8 inches wide by approximately 8 inches high. The forms are 2x8s fastened together with screws. I have two identical 100 foot tape measures I used to check the diagonal lengths to square up the forms.

To secure the forms I used these outriggers made with 2×4 scraps staked down with concrete form stakes. I like the outriggers because it makes it easy to adjust the level of the forms. It also makes it easier to remove the stakes after the concrete has been poured.

I’m installed two runs of ½ inch rebar to reinforce the concrete. Where two pieces meet there is a 20 inch overlap. I used two steel pipes to make the 90° bends for the corners.

To maintain the 8” grade beam width I fastened 8” sections of pressure treated 2x4s between the inner and outer forms boards. I also drilled holes in each spacer board for the 10” anchor bolts. I could remove the spacer boards after the concrete cures and mortar the opening but I’m planning to leave the them in the foundation.

I elevated the rebar sections by tying them off to the spacer boards with bailing wire. The rebar is at least 2 inches away from each surface. Just after pouring the concrete we’ll unwrap the bailing wire and push it into concrete while its still wet.

The Pour

Our weekends in May have been pretty busy so fortunately Jean and my Dad were able to help me with a mid-week pour. This May has also been very warm so “we” decided on getting started at 7am.

I calculated that we would just under 50 80lb bags of ready mix concrete. Check out the video for how I made that calculation. On pour day I did the mixing and wheel barrowing while Jean and my Dad worked the concrete into the forms.

We used a reciprocating saw without the blade to vibrate the forms to get the air bubbles to rise out of the concrete and various sticks and shovels and hoes to poke prod and generally agitate the concrete to really fill out the forms and embrace the rebar.

I could only carry 25 bags of concrete on our trailer so mid-pour I had to run over to the hardware store to get the rest which took about an hour. Even with that delay we finished before noon.

A few days later I removed the forms. The outriggers with form stakes came out easily as previously claimed. The only stake I couldn’t get out was one I attached directly to the forms. It’ still there so I might just grind it off eventually. I had watched videos advising rubbing the forms down with oil before pouring concrete but I had forgotten to do that. Even so, the forms board came off fairly easily. The inside ones were a little tight so I ended up cutting of the end of one.


Corrugated Pipe & Connectors$130
7 Yards of 3″ River Rock$460
Form Wood & Rebar$216
Anchor Bolts & Bailing Wire$32
46 80# Bags of Concrete$214

There were some materials that I already had on hand like the landscape fabric, form stakes, screws and scrap wood that would have increase the cost by $120-$150 or so. I had accounted for the excavator rental for the septic install but if I had just rented it for a week for this project it would have been around $1250.

Another advantage of a rubble trench foundation besides being resistant to frost heaving and expansive clay is that its less expensive then a concrete foundation. 46 bags of concrete is approximately 1 cubic yard. If I had used 7 yards of concrete instead of 7 yards of river rock for the foundation it would have been ~$1500 for just the concrete not including the rebar. I would also need to install a french drain along the outside of the foundation. Then there’s also no way a septuagenarian, a statistician and yours truly could ever hope to mix and pour 300+ bags of concrete in a day and if we called on a concrete truck it would have to traverse across our septic drain field. Alright, I convinced myself.

The next step is framing the walls and roof. I have materials coming in this week so stay posted.