DryStoneGarden

crack width ruler

Clay is the dominant soil type of the Bay Area, and it has two main properties that affect how you build on it:

The first is that clay swells when it absorbs moisture, and contracts when it dries, causing anything built on or against it to move. A lot of buildings in the Bay Area have cracks that open and close as the seasons changes. The crack width ruler in the photo is measuring a crack in an adobe wall at the Presidio. It’s probably monitoring the gradual opening of the crack over time, but it might also be measuring the crack’s seasonal opening and closing.

The second is that clay has a very different shear strength when it is wet compared to when it is dry. Shear strength is the amount of weight a material can support. Think about walking on clay soil when it is dry, then walking on it after a rainstorm; your foot sinks into the wet clay. If you put something heavy, like a house or a stone wall, directly onto clay soil, the house or stone wall will sink when the clay gets wet in the winter. Cracks will start to appear. 

Gravel, in comparison, is the anti-clay. It has a consistent volume and a consistent shear strength. It doesn’t care if it gets wet. Also, the gravel helps keep the clay dry around it, and disperses the weight over it and resists pressure from clay beside it. Put enough of it under your adobe wall, and you won’t get cracks or need a crack width ruler. Put enough of it under your dry stone wall, and the wall can last forever.

A friend of mine who used to consult on big construction messes (like what to do when a parking lot fails and the heavy machines needed to fix the parking lot keep making bigger and bigger failures) says his job was to walk in and say, “Add gravel.” Gravel is essential. The easiest way for a homeowner to judge prospective wallers is to ask if they backfill with gravel (or stone rubble is okay too, same thing essentially). If they backfill with dirt, they aren’t serious about building long-lasting walls.