Drystone Walls

for reference: https://www.hammerheadstoneworks.com/2018/10/09/montreat-big-wall/

Montreat Big Wall

I helped build this wall when I worked with Fred Lashley at the Unturned Stone. We built it 18 years ago and it remains the tallest drystone wall I have ever worked on. At its tallest, it is 7 feet high. Made mostly of fieldstone from Maggie Valley, we supplemented with Hoopers Creek, which shows up as the rusty colored faces in the pictures.
The wall is in Montreat, and I like to visit when I make my way out to Black Mountain. It is an impressive structure. It looks largely the same now as it did then, except for the glorious patina of lichen. I don’t think I will ever build another wall so tall.

Codes? What Codes?!

These days there are building codes that limit the height of a retaining wall. In a residential setting, like this, the max is 48 inches tall. Anything taller, must be signed and sealed by a licensed engineer. In general, engineers do not love stone walls. They like things that are consistent and predictable and more easily quantified than the crazy ecology of a stacked wall. The human elements, care, intention, and experience that the builders put into the structure cannot be factored into mathematical formulae.
Drystone walls have fallen out of favor in preference for segmented block, prefabricated panels, and poured concrete structures. They are all the same and more easily analyzed. It’s an apt metaphor for the state of modern society, our blind devotion to conformity and standardization.
When building with prefabricated predictable materials, it matters much less if the people building care at all about what they are doing. And if you can subtract care from the process, you can subtract cost. It’s always cheaper if you don’t give a shit. Until later.

Whether or not the same codes were in effect 18 years ago, I don’t know; I just worked on the wall and was not much involved in the process that brought the project to be. I know it was subcontract work, for a landscape guy in Black Mountain with a stellar reputation and very organized and disciplined approach to his work. If anyone was likely to get the necessary approval, it would have been him. To my eye, the wall has not moved at all and I see no evidence of it having needed repairs for the last 18 years.

What Makes it Stay Up?

I get a version of this question quite frequently, especially when people discover that we usually work without mortar or concrete. The answer is amazingly complex, but I generally begin with a very simple answer: gravity and skill.
I’ve already mentioned how important care is in the creation of a structure like this. Care plus practice equals skill.
The rules that govern the construction of a drystone wall are simple and robust: two over one and one over two, end them in, maximum contact, minimum movement, batter back, the top must sit over the bottom, weather always wins. From the simple rules emerge an infinite number of possibilities. Bird flocking is often used as the epitome of the principles of emergence, simple rules lead to complex (and beautiful) results. A drystone wall is another stunning example.

Two Over One: One Over Two.

This is a very simple principle that you must cross your joints. Tall vertical lines in masonry construction “often called running joints” weaken the structure significantly. Crossing joints imagine the traditional running bond of the most common brick wall and creates a network. That web of dynamic forces is fundamental to what holds a wall together.

End Them In

Another simple rule: stones should run deep into the wall. Sometimes, in an effort to conserve their rock pile, people will use the long side of a stone as the face. This is called tracing and it makes for a shallow structure and therefore a weak wall. If you have a stone that is shaped like a french fry, run it long into the wall.

Maximum Contact & Minimum Movement

The next stone you put in should make as much contact as possible with the stones that are already in the wall. And so on and so on.
One of the great attributes of a dry stone wall is its flexibility. It has the capacity to move and therefore respond to environmental factors. The day-to-day expansion and contraction that comes with being heated by sunlight and cooled by frost may cause minuscule movements in a wall but not crack it apart. It is not supposed to be a rigid structure. Inflexibility fails in walls, as in life.
Minimal movement is a great attribute. On the other hand, stones that pivot from side to side on a single point cause trouble. Maximum contact, minimum movement is where practice becomes most important to a wall builder.

Batter Back

A good dry stone wall has a slight backwards slope to its face, called the batter. Imagine the wall leaning into the hillside which it is intended to retain. This strengthens the wall and gives it leverage against the slope behind it.

The Top Must Sit Over The Bottom

Lean back, but not too far. The top of the wall, where your caps rest, must be sitting squarely over the bottom of the wall. It is a common beginners mistake to lean the wall back too steeply. A very steep batter on the face will have the top of the wall sitting over soft soil backfill instead of the hearty structure of the wall. The weight of the cap will compress the soil. The path of least resistance for that soil will be forward; it will blow out the bottom of the wall, collapsing the whole structure. This is where the concept of gravity really comes into play with the construction of the wall. With practice and care, gravity is a great ally.

Weather Always Wins

Fundamentally, all construction is an attempt to resist the forces of weather. And while weather is not a singular force, in my experience, water is the prime mover.
Almost all of the structures we build live their lives in the weather. We have to constantly be conscious of what happens when it rains. Where does the water go? Where does it collect? What happens when it freezes? What happens when it builds up or when it moves the earth?

With this particular wall, so tall and at the bottom of such a steep bank, managing water was a primary concern. We lined the cut bank behind the wall with a filter fabric that allows water to pass through, but holds back the soil. In addition to keeping the backfill of the wall from getting silted up, it prevents red clay from washing through the wall and staining the face.
This wall is also very deep or thick, depending on how you want to look at it. Behind the face there is an enormous amount of ugly stone, chips, and gravel. Rain water runs through it and does not linger long enough to build up hydrostatic pressure, the force that pushes over so many rigid, impermeable walls. There are no drain pipes behind the wall, because the whole wall functions as a drain.

Eighteen years and counting.