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Retaining Walls' Strength in Style

Built to withhold tons of soil and water, these barriers can also be an attractive, useful part of a home's landscape.

November 28, 1998|JOHN D. WAGNER | TODAY'S HOMEOWNER MAGAZINE

Sure, retaining walls look like simple stacked stone, block or timber. But 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 when used in terracing sloped properties (something not uncommon in Southern California and increasingly important as flat home sites become ever more scarce in many regions).

In addition to sloped landscapes, where water runoff causes hillside erosion, ideal locations for a retaining wall include spots downhill from soil fault lines and wherever 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.

A casual check around the neighborhood is likely to 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 feet high, 15 feet 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 such as these in play, you should limit your retaining wall efforts to walls under 4 feet tall (3 feet for mortar-less 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 to design the work. Have the architect or engineer inspect the site thoroughly. Experienced builders should do the installation.

To have a 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% 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.

Building It Right

Poor drainage resulting in saturated soil is the main cause of failure. That's why all good walls begin with three features:

* landscape fabric.

* backfill.

* 4-inch perforated drainpipe.

The depth to excavate depends on the wall and soil type. If you live where your soil drains well, scrape away topsoil to form a base for non-mortared 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 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 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.

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 feet long, T-shaped tiebacks buried in the hillside) attached to the wall every 8 feet, extending 6 feet back to a 2 feet wide T-bar.

Deadmen are not included in some interlocking-block systems if the design allows backfill to secure the blocks individually. 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.

Timber

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. Allow about three days to build a wall 4 feet tall by 15 feet long.

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