What You'll Learn
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.
Digging details. 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.
Backfilling basics. 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.
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.
Digging details. 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.
Backfilling basics. 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.
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