My last post (February 1st, in case you missed it! Riveting stuff!) was an introduction to wall-building theory. Now let’s start getting into different types of walls commonly constructed in the Bay Area. First, one of the most commonly built walls in residential landscape construction:
‘Drystack’ Stone Walls
When walls are referred to as ‘Drystack’ or ‘Dry Set’, it means they are constructed without mortar (cement) to hold them together. This is one of the oldest wall construction techniques on earth, and uses stone, or in some cases broken pieces of concrete or other blocky materials, to retain soil. Even though this is a simple and thus relatively inexpensive wall building practice, there are some principles that must be followed to ensure successful, long-lasting retaining walls.
As I explained in my last post, properly built walls include a footing. This is a portion of the wall which lies below the top of the adjacent surface (grade). With most drystack walls, at least 1 course (horizontal layer) of stone is buried to create the footing. The ultimate height of your wall will determine the depth of the footing- a taller wall requires a deeper footing to support it. Even curbs or edgings will have a few inches of the stone buried below grade.
Constructing a Drystack Wall
A drystack wall is constructed using the principles of gravity and friction. Larger stones, or a mix of sizes, are preferable to small stones in order to maximize both principles. Vertical seams will weaken a wall, so proper wall construction overlaps joints as much as possible. The flattest side of each stone is what you see, so the ‘face’ of the wall has a uniform look. Angular types of stone will fit together tighter, with smaller joints, and will actually become stronger as the wall settles. Rounded stones, like the ones found in river beds, are obviously tougher to stack- they don’t have as much surface contact within the wall, so there is less friction. One great advantage to drystack walls is that they drain naturally. Water weeps through the small openings (joints) between stones, relieving pressure behind the wall. Other types of walls require additional components to ensure drainage, because water pressure can severely compromise wall strength.
Unlike most mortared, poured, or pier-supported walls, which are constructed to be upright (plumb) drystack walls are battered. They are built with a slight tilt against the soil being retained, which reduces force on the wall. So,if you were looking at the wall from the side, (in section, see below) it would appear to slant just a bit towards the retained area. This allows the wall to settle over time without compromising stability. As we know in the Bay Area, the earth moves. Even without tremors, water and time will compact the soil above the wall. If a drystack wall is properly built, patios, paths, even driveways can be constructed near the retaining wall and the wall will hold. However, a more structural retaining wall may be a better choice in some landscapes, depending on the conditions and to ensure longevity. If the earth moves enough, even a perfect wall could fail.
Cut and Fill with Walls
‘Cut and Fill’ is a term you have probably heard in relation to retaining wall construction or grading. With so many sloped landscapes in the Bay Area, creating flat spaces often means moving soil. Removing soil from a site (especially with limited access, like 3’ wide side yards with old, treacherous stairs, which most Bay Area homes seem to have) can be extremely time-consuming, and thus, costly. If you can keep soil on site and utilize it to enhance your landscape instead of removing it, you can streamline your construction project. ‘Cut and Fill’ means cutting (removing) soil from part of your landscape, and using it to level depressions or infill low areas elsewhere. Here’s where retaining walls come in. We can take sloped areas and use retaining walls along with cut and fill to create level spaces. See the section detail below. There are a couple of important things to remember with cut and fill. First, the cut soil expands as you move it, which needs to be taken into account when you’re calculating how much soil to is being relocated. The material gets broken up into pieces when it is cut, and gets fluffy (highly technical terminology here). Second, to prevent the fill soil from settling too much and disturbing the retaining wall holding it up, it must be compacted. Even with careful compaction, fill soil is less stable than native soil, and less stable than it was before cutting. For this reason, we don’t use drystack walls as often to retain fill. Reinforced or mortared stone walls are likely better suited to holding filled soil.
Because there are a lot of types of stone available to use in wall construction these days, drystack walls can vary significantly in appearance. Some stones are very monochromatic, while others are quite colorful. Some have an angular, blocky appearance, while others have more rounded edges or are longer and flatter. When improperly built, any drystack retaining wall will appear sloppy. In some landscapes, a sleeker look is preferable, so even a perfectly constructed drystack wall may not fit the bill. But drystack is simpler to build and less expensive than constructing other types of walls, so the technique in itself shouldn’t be written off. There is probably a material out there that would suit your design aesthetic, even if you lean towards a contemporary style. And stone isn’t the only option– brick can also be used in drystack wall construction. Broken concrete woudn’t be my first choice in most cases, but recycling a broken-up patio or driveway to create a wall that can be obscured is environmentally considerate and can help extend a budget.
I had intended for this post to be about different types of retaining walls, but I got on a drystack bender (this happens) and here we are. I’ll get into other types of walls next time. For now, I hope these posts are informative about the landscape elements you see in your neighborhood. But I also hope they help to explain why we, as designers, recommend certain materials or construction techniques over others. We want to give you the garden that looks best with your home and suits your personal style, but we’re also considering your budget and trying to help you prioritize your project goals. Charming drystack stone walls are budget friendly, naturally draining, and can serve multiple purposes to level spaces and create an architectural focal point in your garden at the same time.