Engineering a Retaining Wall
A retaining wall can hold back a hillside and turn steep slopes into living space—if you pay attention to the basics
A retaining wall can hold back a hillside and turn steep slopes into living space—if you pay attention to the basics
Sure, retaining walls look like simple stacked stone, block, or timber. But in fact, they're carefully engineered systems that wage an ongoing battle with gravity. They restrain tons of saturated soil that would otherwise slump and slide away from a foundation or damage the surrounding landscape. These handsome barriers also make inviting spots to sit, and can increase usable yard space by terracing sloped properties, something that is increasingly important as flat home sites become ever more scarce in many regions.
Along with sloped landscapes where water runoff causes hillside erosion, ideal locations for a retaining wall include spots downhill from soil fault lines and where the downhill side of a foundation is losing supporting soil or its uphill side is under pressure from sliding soil.
If your property needs a retaining wall, or if the one you have is failing, review these descriptions of the four most common types: timber; interlocking blocks; stacked stone, brick or block; and concrete.
Although retaining walls are simple structures, a casual check around your neighborhood will reveal lots of existing walls that are bulging, cracked, or leaning. That's because most residential retaining walls have poor drainage, and many aren't built to handle the hillside they're supposed to hold back.
Even small retaining walls have to contain enormous loads. A 4-foot-high, 15-foot-long wall could be holding back as much as 20 tons of saturated soil. Double the wall height to 8 feet, and you would need a wall that's eight times stronger to do the same job. With forces like these in play, you should limit your retaining wall efforts to walls under 4 feet tall (3 feet for mortarless stone). If you need a taller wall, consider step-terracing the lot with two walls half as big, or call in a landscape architect or structural engineer for the design work (have the architect or engineer inspect the site thoroughly) and experienced builders for the installation.
If you have your retaining wall built, figure about $15 per square face foot for a timber wall, $20 for an interlocking-block system or poured concrete, and $25 for a natural-stone wall. Preparing a troublesome site—one that includes clay soil or a natural spring, for example—can raise costs substantially. Add 10 percent or so if you hire a landscape architect or engineer. But shop around; some landscape firms do the design work for free if they do the installation.
Timber walls are only moderately challenging to build by yourself up to 4 feet high
Building It Right
Poor drainage resulting in saturated soil and frost heaving is the main cause of failure. That's why all good retaining walls begin with landscape fabric, backfill, and 4-inch perforated drainpipe.
The depth you need to excavate depends on frost depth as well as the wall and soil type. Mortared or concrete walls in heavy-frost areas require footings dug below the frost line. Nonmortared walls should be built on a gravel-filled trench dug below frost line. If you live where it doesn't freeze and your soil drains well, you may be able to just scrape away topsoil to form a base for nonmortared walls.
Before adding gravel, lay down enough landscape fabric to contain the new gravel. Form the fabric into a large C shape, with the open mouth of the C facing downhill. The fabric should wrap around and create a border between the gravel and topsoil to keep sediment from clogging the gravel and drainpipe.
Replace native soil with 3/4-minus gravel (no stones under 3/4 inch in diameter) or "bank-run" gravel (washed stones 1/4 inch to 6 inches in diameter). Shovel at least a 4-inch layer of gravel onto the landscape fabric. Grade this layer so it slopes 1 inch for every 4 feet, allowing water to drain away. Then lay in 4-inch perforated PVC drainpipe at the base of the wall and cover it with gravel.
Shovel in backfill as you build the wall, one tier at a time. Don't add all the backfill at the end—it won't compact thoroughly. Tamp down the gravel as you go with a heavy hand tamper. Behind the top tier of the wall add 6 inches of topsoil and lightly compact it.
Battering and tiebacks
All retaining walls should lean into the hill 1 inch for every 12 inches of height. Timber walls 4 feet or higher should be tied to the hillside with "deadmen" anchors (6-foot-long, T-shaped tiebacks buried in the hillside) attached to the wall every 8 feet, extending 6 feet back to a 2-foot-wide T-bar. Deadmen are not included in some interlocking-block systems if the design allows backfill to secure the blocks individually in place. Still others require geo-grid, weblike tiebacks that get buried in the backfill. Check the manufacturer's literature.
A final heads-up on masonry walls—concrete blocks chip and crack easily. Carefully inspect the blocks upon delivery, and don't be shy about returning damaged blocks for credit.
A timber wall made of 8-foot-long 6x6-inch pressure-treated beams needs tiebacks and deadmen spiked in place every 4 feet. Landscape fabric keeps the gravel from clogging with silt, while 4-inch PVC pipe drains most water.
Types of Walls
Upside: Only moderately challenging to build by yourself up to 4 feet high. If an engineer has designed the wall, located the deadmen, and specified the backfill and drainage, you can install an even taller wall yourself.
Downside: Not as long-lived as masonry. Making square cuts is challenging. Also, components are heavy and hard to manage alone. Plan on about three days to build a wall 4 feet tall by 15 feet long.
Cost: $10 to $15 per square face foot installed, depending on your region—higher if extensive excavation, soil prep, and backfilling are needed.
Use 8-foot-long, 6x6-inch pressure-treated wood designated "For Ground Contact," and have all materials delivered. Follow all rules for landscape fabric, drainage and backfill. All timber walls require deadmen every 4 feet at midwall height or higher. Pin the first tier of timbers to the ground with #4 rebar.
Interlocking Concrete Block
Upside: Also called segmented retaining walls, interlocking-block systems from Keystone, Risi, Rockwood, Tensar, Versa-Lok, and others are mortar-free and easy to assemble. Units are small and modular, so walls can taper, turn, wrap, and curve. Available in many textures, shapes, and colors, these engineered systems, which can be used for walls up to 20 feet high, rely on several techniques including:
•Keyed, battered design (block shapes key into one another and are stacked so they lean into the hillside)
•Backfill trap (block shapes allow backfill to be shoveled into the block webbing, trapping each block individually)
•Geo-grid webs (block maker supplies geo-grid plastic-net tiebacks that attach to the block and are buried 5 feet in the hillside at specific heights).
Downside: You can't mix and match manufacturer's systems. Block systems that use metal pins to tie blocks together can be a challenge to line up exactly.
Cost: About $12 to $20 per square face foot installed, depending on block configuration and site. More expensive systems tend to be stronger and stack higher.
Arrange before delivery from the masonry yard where materials will be stockpiled in your yard and if the forklift used to off-load the truck will fit through backyard gate, etc. Follow all rules for landscape fabric, drainage, and backfill. Use manufacturer's calculators to determine how many blocks, pins and tiebacks you'll need. When stacking blocks, sweep off each layer; small pebbles can disrupt the pattern. Cap walls with flat units or stone held down with silicone caulk.
Stone, Brick, or Cinder Block
Upside: For stone, a handsome rustic appeal. Collecting stones on site and doing the work yourself can also save money. Brick provides a more formal look. Cinder block is inexpensive and can be reinforced with steel and concrete.
Downside: Stone-wall masonry is harder than it appears. Fitting the stone is exacting work and making mortar joints look natural requires experience (nonmortared stone walls don't offer much holding power). Brick masonry also requires skill to hit the visual standard all of us are used to. Cinder block has to be faced with stucco, brick, or stone or overgrown with plantings to make it attractive.
Cost: About $10 to $12 for cinder block; for brick and stone, around $20 to $25 per square face foot (double that figure for a two-sided wall).
Follow all rules for landscape fabric, drainage, and backfill. A mortared wall needs a footing and a drainage system that will defeat frost heaving. A dry, nonmortared wall allows water to seep through, relieving pressure behind the wall naturally.
Upside: Strong. Well-designed and properly drained and backfilled, concrete walls rarely fail.
Downside: Bare concrete isn't particularly attractive. It can be veneered with masonry, or special forms can be used that embed decorative designs in the finished wall. Also, if a wall fails, patching may not be possible and removal is costly. Walls over a few feet high should be formed and poured by a pro unless you've had experience with vertical pours.
Cost: Around $16 to $20 per square face foot installed.
Follow all rules for landscape fabric, drainage, and backfill. The footing should be below frost depth or on well-drained gravel that reaches this level. Use 3/4-inch ply and 2-by-4 bracing to form the wall. And install #4 rebar wired in 12-inch grids for added strength. Use mechanical vibration or strike the forms with a rubber mallet every 6 inches when concrete is wet for a smooth finished face.
Safeguarding Against Three Common Failures
Retaining walls usually fail slowly. Common problems can often be fixed if you act quickly. You can also protect a new wall in the building process by safeguarding it against the three most common failures:
What happens: A load is added within 3 feet of the top of the wall. The wall leans out at the top and eventually tips over
What to do: Tell your landscape architect or engineer if a car or shed will be placed near the wall. The pro should then beef up the footer and increase the number of tiebacks or deadmen to add strength. Adding retrofit tiebacks is expensive and requires excavation, partial dismantling, and reengineering the wall
What happens: Soil behind the wall gets saturated, causing hydrostatic water pressure and weight to topple the wall.
What to do: Replace native soil behind the wall with 3/4-minus or bank-run gravel for 2 feet. Line the inside base of the wall with 4-inch perforated tile drain on a gravel bed that slopes 1 inch for every 4 feet of run to carry water to daylight or a dry well. Topsoil should take up only the top 6 inches behind the wall.
What happens: Retaining wall lacks proper drainage or a footer. Soil becomes saturated and freezes, heaving upward and breaking the wall apart.
What to do: Walls should rest on 3/4-minus or bank-run gravel, with the footer or wall base buried beneath the frost line (6 to 48 inches, depending on region). For deep frost, use concrete block rather than retaining wall to ground level, then build the retaining wall on that. Well-drained gravel behind and beneath the wall can substantially diminish frost heaving.
Where To Find It:
1955 Lake Park Dr., Suite 250
Smyrna, GA 30080
Keystone Retaining Wall Systems
4444 West 78th Street
Bloomington, MN 55435
Osmose Wood Preserving
1016 Everee Inn Rd., Box O
Griffin GA 30224-0249
Risi Stone Systems
8500 Leslie St., Suite 390
Thornhill, ON L3T 7P1 Canada
Rockwood Retaining Walls, Inc.
7200 N. Highway 63
Rochester, MN 55906
Tensar Earth Technologies
5775-B Glenridge Dr., Lakeside Center, Suite 450
Atlanta, GA 30328
UltraWood ACQ Preserve
Chemical Specialties Inc.
200 East Woodlawn Rd.
Charlotte, NC, 28217
Versa-Lok Retaining Wall Systems
6348 Highway 36, Suite 1
Oakdale, MN 55128