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.

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