DryStoneGarden

I’m hoping this photo from last fall will sell my Mom on some gravel. She’s been resistant to the idea of using gravel in her yard, but I think she’s picturing gray construction gravel. I probably need to start calling it Tuscany Gold, the way they do at the stoneyard, make it sound more stylish. The Tuscany Gold looks a bit mediterranean/southwestern for her taste, but gravel has some advantages over flagstone, mainly that it drains well and it’s inexpensive and easy to install and maintain. It’s not so good for patio furniture, it’s not as nice to walk on, and it can get stuck in the soles of your shoes and scratch the hardwood floors, but none of those issues would be a problem for that section of my parents’ yard where they want to take out some lawn. And I think the ease of installation should trump everything in this instance. A mother shouldn’t want her only son carrying a ton of flagstone into the backyard.

ryan 3/4

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.