From the foothills to the high mountains, our terrain provides us with some beautiful vistas — and some practical dilemmas as well. How do you deal with all the grades and inclines in your backyard when you need an area for the kids to play or a place to plant your petunias?
“Often, home construction is thought out with limited parameters,” observes Marc Archambault, stonemason and project manager for The Unturned Stone in Asheville. “The initial priority is ‘we want the greatest view around,’ but then it turns out that there isn’t anywhere on the property, aside from the porch, where you can stand on flat ground.”
Stone retaining walls offer a time-honored solution. Entirely sympathetic with their surroundings, they allow the homeowner to create level, usable areas in a sloping landscape and establish the framework of gardens and pathways without jarring the eye or intruding on the serenity of the scene.
1. HARVEST OR GATHER?
WET OR DRY?
The primary material options for retaining walls are rough-quarried stone or fieldstone. Quarried stone is removed from large veins in the earth by employing dynamite or drills, and is often used in formal wall designs. Fieldstones are found loose on the ground or embedded in the soil. Because they have been exposed to the elements, fieldstones possess a more weathered, organic quality that lends rustic appeal to the project.
Stonemasons employ two fundamental techniques in building with stone – dry laid (or dry stack), in which gravity alone provides the structural stability, and mortared work where the stone is held in place by a sludgy mixture of sand, water, Portland cement and lime.
2. STICK TO IT
Mortar work can allow a great deal of creative license. Since the stones don’t have to fit together perfectly, decorative puzzle piece compositions are possible. This is particularly useful in veneers, where stones are applied to a concrete base and held in place by the mortar.
There are several grades of mortar, with varying rates at which water passes through them. The mason will choose the appropriate mortar based on the selected stone, the composition of the surrounding soil, and other environmental conditions.
3. DRY LAID STACKS UP
Dry stacked walls require considerable skill, but the results are both beautiful and durable (witness the ancient stonewalls in Ireland). This is due in no small part to an area where dry laid stonework excels — drainage.
“Water always wins,” observes Archambault. “Mortared walls in the ground don’t last as well as dry stacked because of hydrostatic pressure.” The shifting of soil behind the wall creates an opposing force and most of that movement is created by — you guessed it — water.
Add this to the corrosive effect of absorbed water and, over the long term, the mortar and the structure will weaken. “Even solid concrete walls are not impervious,” Archambault continues. “You can put in weep holes to drain water, but very often they get silted up. Then the walls will bend, or they will bubble out in the middle and burst.”
4. IN A FIX
Despite the finest craftsmanship, accidents and wear happen. “Another great thing about a dry stack wall is that if you do need to repair it, within a year you won’t be able to tell that it was fixed,” says Archambault.
“With mortar, you’ll always be able to see the difference, because it’s so difficult to get the mortar color to match exactly.” If you do need to do restoration on a mortared area, you can avoid the most obvious differences by adding soot or some other coloring agent to blend the new application with the aged mortar.
5. BEHIND THE SCENES
“A structural wall has a complex set of principles that guide its construction and create a unified whole so that weather and traffic won’t alter its stability,” says Archambault. Soil will naturally shift over time due to the effects of freezing and thawing — an expanding and contracting known as heaving — as well as gravity and water run-off from the surrounding landscape.
With a dry stacked wall, rather than placing stones directly against the soil, a trench is created behind the structure that is then backfilled with gravel — washed crushed stone and rubble stones — to act as a buffer between the earth and the face of the wall. This helps to compensate for the inevitable settling and fluctuations in the soil bank.
6. PROTECTIVE LAYERING
Lining the bank with geotextile fabric, which resembles those flat polymer scouring pads, can add 50 years to the life of your retaining wall by allowing water to pass through, but filtering out soil. “We do it for two reasons,” Archambault explains. “There’s so much red clay in the soil here, the textile filters it out so it doesn’t stain the stone. It also helps to maintain the air spaces in the backfill and the joints of the wall.”
7. WHAT’S THE ANGLE?
In a retaining wall, the force of gravity is going to push straight down. Ideally the top of your wall should be sitting pretty directly over the bottom of the wall, but you also need to compensate for the pressure of the soil behind the structure. This is accomplished with a batter, a surface angle that slopes gently backward from the bottom to the top, usually at a five to ten degree angle, depending on the height.
If you put too much of a slope on it, however, the top will push the bottom of your wall out. “The batter is important,” says Archambault, “but don’t be fooled into thinking that a steep batter will make for a sturdier wall. The surface should appear to be relatively vertical to the eye.”
8. A SHORT COURSE IN JOINTS
Each layer of stacked stone is a course, and the spaces between the stones within a course are joints. In a more formal wall, the joints are arranged somewhat uniformly, while a rustic wall will generally feature a more random pattern.
In either case, care must be taken to avoid running joints, where the joints between the stones in a series of courses fall along a vertical line. This creates a weak and vulnerable area that can threaten the structural integrity of the wall.
9. BUILDING ON THE FOUNDATIONS
Once you’ve established the bones of your landscape with retaining walls, the possibilities are endless. Patios, planting beds, seating areas, steps and decorative features such as niches can be incorporated into the overall design to add additional function and charm.
10. THINKING IN FOUR DIMENSIONS
Designing stonework goes beyond the three dimensional process of considering line, surface and structure and takes into account that fourth dimension — time. And time isn’t strictly chronological. “Time includes weather, children playing, delivery vans turning in the driveway and, of course, water.”
“Time as a design piece is also important in that you want to create something that will last aesthetically,” Archambault notes. “One of the beauties of stone is how permanent it can be, so you don’t want to get too cute or trendy.”
With carefully planning and execution, your stone retaining walls will create enduring outdoor architecture and grace these hills for many generations to come.
For more information about The Unturned Stone, visit their website at www.unturned.net. You may also want to peruse two excellent reference books by local authors – Garden Stone by Barbara Pleasant (Storey Publishing, 2004) and The Art and Craft of Stonescaping by David Reed (Sterling Publishing, 1999).
