Pendle Hill Greenhouse Project - Rubble Trench Foundation
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In November, we began digging the foundation. The idea was to dig a fairly wide trench corresponding to the wall, put some gravel at the bottom of the trench, then lay a stone wall in the trench which extended well above ground level. For drainage, we put gravel around the wall. This style, called a rubble trench foundation, was popularized by Frank Lloyd Wright to reduce concrete use and improve drainage.
We also excavated the floor of the greenhouse about eight inches lower than ground level. We will set reused bricks and flagstone into a dirt and lime mixture to make the floor. The piles of excavated dirt will be used for making cob.
Laying a Wall
It takes a long, long time to build a stone wall by hand.
We first had to haul stones from a ruin on Pendle Hill's campus to the greenhouse site. We soon learned to select big, box-like stones because they are worlds easier to dry-lay. Dry laying is the next step, in which you fit rocks together so snugly that, without mortar, you could walk on them. It's like a giant jigsaw puzzle with 50 pound pieces that may not actually fit together. Once you've dry-layed a section, you mix up a batch of mortar and set the rocks.
We recycled concrete rubble into the below-ground foundation, but used stone above ground for aesthetics.
We were blessed with cheerful volunteers, and the walls slowly grew.
By May, the foundation was finished. The north wall was 2 feet above ground level, the south wall 3 feet.
There are 3 tubes running through the base of the north wall. After stacking straw bales on this wall, we will run cable over the bales and through these tubes to secure the straw bales to the foundation.