Rock Retaining Wall
DEAR TIM: I want to build a rock retaining wall, but have no clue where to start. Rock retaining walls look great and seem to be a very green method as Mother Nature created the material herself. What are some of the most-important things I need to know to build a rock retaining wall that will last for generations? What kind of tools will I need? Should I install the rock in mortar, or can I just dry stack the stones? Greg S., Irving, NY
DEAR GREG: Building a rock retaining wall is not for the faint of heart. It takes lots of muscle and the three D’s: Diligence, Determination and Discipline. After one day of moving rock around, you’ll possibly be muttering to yourself that this was a very ambitious project.
Retaining wall rocks are plentiful in many parts of the USA, but in some areas they are as scarce as rainfall in the Atacama Desert. Before you go too much farther into this project, make sure you calculate the correct quantity of rock you’ll need. You can do this by estimating the square footage of exposed wall you intend to have. Once you have a quantity, calculate the cost of the rock. If you have to buy it, you may be in for a surprise as to how much it costs. The farther you are away from the source of the stone, the more it will cost. Freight costs to transport the rock can be very high.
Rock for a retaining wall must be durable if you want it to last generations. Not all rock is the same. Some like granite and limestone can easily last for hundreds or thousands of years. Cold weather is rock’s enemy as water that seeps into tiny cracks can freeze and blast apart the rock over time. I can see that happen with the rock in my yard here in New Hampshire. It’s a very durable granite, but this spring I saw some small quartz crystals that were broken away from the stone during the winter.
Rock retaining wall construction can be very challenging. The rock themselves are very dense. It’s not unusual for a cubic foot of rock to weigh between 150 - 200 pounds. Large boulders will either require you to have a piece of mechanical equipment or you have to master using gravity to help you lift the stones in place.
Gravity can help you lift stones as crazy as that sounds. A retired carpenter, W. T. Wallington of Lapeer, Michigan, discovered just a few years ago how to lift and move huge 10-ton blocks of stone and concrete by simply taking advantage of the center of gravity of the object. You can see how he did this at his website: www.theforgottentechnology.com
Retaining-wall science can be complicated, especially as the height of the wall increases. Walls over 3 or 4-feet tall can begin to have significant loads against them from the soil that you’re trying to hold back. If you’re planning a tall retaining wall, I highly recommend that you have it engineered. You don’t want the wall falling down injuring or killing someone or damaging your property.
Your rock retaining wall design will require you to decide between a dry stacked wall versus one made where you mortar the stones together. I’m a fan of dry-stacked walls because they can be long lasting, easier to construct and resist the freeze-thaw cycles that create frost heave and soil expansion that tips over retaining walls. Come to New England to see a dry-stacked moss rock retaining wall and you’ll realize the wall has been standing for generations to have accumulated the natural moss patina.
If you decide to set your rock in mortar, I suggest you take the time and build a small test wall until you master the technique. Make the mortar very weak using a minimum amount of hydrated lime and Portland cement. This will allow you to take apart the wall and salvage the rock. I would mix ten parts sand to one part each of lime and cement for this lean mortar mix.
Most rock, because it’s so dense, doesn’t have the suction of brick and concrete block, so make the mortar mix fairly stiff. Without suction, the mortar mix stays plastic for some time. If the mortar doesn’t get stiff from water loss fairly quickly, you can’t stack more rock on the wall as the mortar moves in the lower rows or courses.
Soil can be very heavy, especially if it’s wet. A cubic foot of soil often weighs nearly one-hundred pounds. Keep this in mind as you start to build a rock retaining wall. Your wall needs to resist the push of this soil against the wall. If the ground above your wall is sloped, you have tremendous pressure pushing against the wall as the soil succumbs to the pull of gravity.
When dry stacking a rock wall, you need to be concerned with the friction between the individual pieces of rock. This friction needs to be greater than the push of the soil behind the wall, otherwise the soil will push a course of stone from a lower one.
If you intend to create a flat area above a retaining wall that will act as a parking area for cars or trucks, you need to account for this extra surcharge load. Some of the weight of the vehicles will push against the wall, especially if the tires are close to the edge of the top of the wall.