The basis for the foundation for the Madoc Performing Arts Centre is a rubble trench. This type of foundation uses compacted stone in an excavated trench to provide bearing capacity for the building above. We use a grade of crushed limestone called "3 inch minus" which includes any aggregate that would fall through a 3 inch screen. This type of material can be well and evenly compacted. The compaction is done with a mechanical plate compactor, and is done after adding a depth of 12 inches of stone at a time.
The sides of the trench are lined with a material that will keep the soil from migrating into the stone. In our case, we used recycled carpeting for this task.
The bottom of the trench has a drainage tile running in it, which slopes to carry water away from the building.
This type of foundation is strong (the same type of compacted stone foundations support roads, railway tracks and other high-load situations), is self-draining, and is not prone to failure due to cracking like solid (concrete) foundations.
Our main reasons for choosing a rubble trench foundation are:
On top of the rubble trench, we have employed two different styles of grade beam foundations.
The earthbag grade beam that supports the main octagonal space are composed of woven polypropylene sack material that we obtained as a continuous tube. These tubes are filled with a site soil mixture of gravel, sand, silt and clay and then compacted firmly. Barbed wire is run between each course of earthbag to prevent slippage. We used two parallel runs of narrow bag with insulation (hempcrete) between the runs. Earthbag foundations are very low cost, and are strong enough for a large building like MPAC. They are quite easy to build as they don't require formwork.
If you'd like to learn more about earthbag building, check out the book Earthbag Building by Doni Kiffmeyer and Kaki Hunter. They helped us with this foundation, and their web site www.okokok.org is a good place to start your research.
The hempcrete grade beams are being used for the first time ever on this building. Hempcrete is a mixture of chopped hemp, hydrated lime and a small amount of either portland cement or quick-set gysum. A reaction between the lime and the hemp results in a very lightweight material that still has reasonable compressive strength. The advantage of hempcrete over regular cement is that the hempcrete is both structural and insulative, so both ends are achieved in the same pour. It is also lower in embodied energy. The disadvantages are a longer set time (2-4 weeks) and lower strengths. However, where the high ultimate strength of concrete is not necessary, this option works well.
Our chopped hemp came from Moorcroft Hemp Farms. You can find out about Grant Moorcroft's hemp operation at www.hemphomes.ca .