It’s undoubtedly true that everyone who sets out to build a retaining wall believes their efforts will last. Trouble is, too few understand the basics behind success. That’s why so many installations fail prematurely.
Retaining Wall Construction is not a Recreational Sport
Success here begins with an appreciation of what you’re up against.
- It’s hot, heavy, dirty and expensive enough to make an understanding of the basics essential.
- Think of sloped earth like a slow-flowing river. It wants to move downhill and it never stops trying.
- The pressure it applies to anything in its way is strong and steady; each gain it makes against an unworthy retaining wall is gain that can never be won back. It’s like a big ratchet wrench.
- Add to this the dam-busting power of frost-heaved soil and you’ll see why building successful retaining walls requires beefy techniques, especially in Canada.
Drainage is Key to Retaining Wall Life
- If the soil behind a wall becomes saturated with water during late fall, it’ll expand with unstoppable force as it freezes. And it would take a nuclear submarine to survive that kind of pressure.
- That’s why the consistency of the soil behind the wall is crucial.
- Light, sandy soils and gravels drain well naturally, and are an excellent choice for backfilling.
- At the other extreme, heavy clay soils can hold water for a long time, even where drainage systems are properly installed. That’s why it’s dangerous to use heavy soil as backfill. Never do it.
Geogrid to the Rescue
- In the early 1980s a synthetic, flexible sheet material called geogrid made its way to Canada from England.
- Geogrid is installed within layers of backfill held behind a retaining wall, giving the soil enough tensile strength to resist the forces of gravity pulling it downhill.
- This takes much of the weight off the wall by thwarting the tendency of soil to flow downhill, boosting retaining wall reliability in a big way.
- This material comes as large pieces of flexible synthetic mesh laid down in layers within back-filled soil.
- Geogrid is easy to use, invisible in the finished installation, and an inexpensive way to ensure long-lasting, trouble-free retaining wall performance.
- Cost is less than a dollar per square foot for residential-grade versions.
Built-in Back Slope
- Heavy timbers, blocks or stone help to create a strong wall, but even these can topple under pressure.
- You can effectively increase the strength of any retaining wall by sloping it back into the earth.
- Five or even ten degrees of tilt is fine.
- This is why manufactured retaining wall systems include features that assure built-in back slope by the way parts interlock.
While slope and weight make retaining walls strong, sometimes even these aren’t enough. Why take a chance? To be sure that timber walls higher than 3 feet stand the test of time, consider running anchor cables back into the slope before back filling.
- The best approach is to twist some spiral earth anchors into undisturbed soil uphill from the wall.
- Then run galvanized cable back to the wall for support before burying everything.
- Earth anchors look like big corkscrews with a loop on top.
- You can get them at farm supply stores that carry fencing supplies.
- If there’s only bedrock uphill from your wall, that’s even better. Drill holes for 5/8-inch diameter expanding metal anchors to hold the cables before backfilling.
With the main secrets of retaining wall success laid out in the open, the job becomes less of a gamble and much more fun. And even if you hire a professional to do the work, it’s in your best interest to know about all the latest techniques and materials that lead to reliable results. Nothing is complicated when you’ve got all the facts. And in this case, facts translate into peace of mind. Rest assured, the only thing that’s going to defeat your well-drained, stoutly built, geogrided retaining wall is the next glaciation.