How To Build A Stackable Rock Retaining Wall
DEAR TIM: I'm lucky and have lots of medium to larger rock on my lot that can be easily harvested. I want to build a stackable rock retaining wall that will be about 3-feet tall and hold back that amount of dirt. Is there magic involved in this process, or do you just stack the rock and hope for the best? What would cause the wall to fail? I don't want to have to redo the job in a few years. I've seen photos of stone walls in New England farm fields and woods that seem to have withstood centuries of Mother Nature's weather and abuse. McKayla S., Pittsburgh, PA
DEAR MCKAYLA: Believe it or not, there's not too much magic involved in building a stacked rock retaining wall that will last generations. It does help if you paid attention in your high school physics class, but if not, I'll provide the crash course below.
First, let's make sure we're talking about the same type of wall. You mentioned the photos of the rock walls you've seen. I live here in New Hampshire and am surrounded by these walls. In fact, I own a piece of property that has stone walls on it that are probably 170 years old. Many of these walls are in excellent shape.
However, the stacked rock walls on my land are not retaining walls. They were primarily built as pasture borders when New Hampshire had millions and millions of grazing sheep. The wool from these domesticated animals satisfied the enormous raw-material appetite of the woolen mills both before and after the War of Northern Aggression, most commonly known as the Civil War.
These rock walls are not retaining walls. They're just free-standing walls that used to be in the middle of fields that acted as crude, but effective, fences. They had no dirt pushing against one side of the wall. The rock walls you see in the photos still look good because gravity is just pulling them straight down. Retaining walls have an added sideways thrust component in addition to the downward pull of gravity. In cold climates, this thrust is multiplied as the soil freezes behind the wall and expands sideways as well as up.
In other words, when you decide to use rock or stone as a retaining wall, and you plan to just stack them instead of interlocking them with mortar or concrete, you need to make sure the weight of the stones and the friction between them are greater than the sideways thrusting force that's constantly trying to tear down the wall. That's pretty much all the magic you need to know, unless you have great skills and can levitate the heavy stones into position!
The first thing, in my opinion, that you need to know is that weight is everything. Heavier stones, ones that weigh in excess of 150 pounds or more, are highly recommended. Just as it takes great effort for you to move these small boulders, it will take more effort for Mother Nature to do it too.
You say the wall will only be 3-feet tall, but is this wall cut into a hillside where the ground slopes up and away from the wall? If so, there's a tremendous amount of weight and soil upslope that's working to push over your wall.
A 3-foot-high retaining wall that has just level land at the top doesn't push that hard against the retaining wall. However, if the small retaining wall is immediately adjacent to a parking area where heavy trucks can load the soil just behind the wall, this added weight can cause a poorly designed wall to collapse, even a short wall just 3-feet tall.
I recommend that you dig a 6-inch deep trench that the first row or rocks rests in. This small keyway helps hold them in place. Deeper is better, but you don't gain much if you bury more than one third of the height of the first row of rock.
If possible you want to have a slight backwards lean to the wall. You want the face of the wall to be out of plumb with a slight tilt back towards the dirt that's being retained. Four inches of backwards tilt in three feet of height is plenty.
Do your best to tightly interlock the rock. Jagged or misshapen rocks work much better than rounded river rock that are nothing more than giant balls. The more surface area of each rock that touches adjacent rock the better. This friction allows the stones to work together instead of each rock just counting on its own weight to hold back the soil.
If the soil at the top of the wall is level, you can gradually decrease the size of the stones or rocks as you get to the top of the wall. Try not to use rock that weigh less than 25 pounds each unless they're used to fill voids in the back of the wall.
Stand back and view the wall as you build it. Look to make sure you like the look of it and look down the wall to make sure it's running in a somewhat straight line. You can use a string to help you keep it straight, but often stacked walls look best if they have a little wiggle or two in them. It creates the illusion that the wall has been there waging a battle against Mother Nature for some time and it's a tie!