Another way to see retaining walls

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Building decorative garden walls is very satisfying and rewarding, as well as being a good exercise. We define “decorative” or “landscape” walls as walls less than two feet high. Retaining walls for purposes such as holding back hills, supporting paving or buildings, and controlling runoff are more serious projects. Let’s call them “engineered walls” because when the purpose of the wall is more functional, with long term consequences, some basic engineering is essential.

Here’s a different way of thinking about a functional retaining wall: Think of the wall block as the “skin,” because a suitable wall actually consists of four other important parts. Let’s call them Footer, Drainage, Tiebacks and Backfill.

A foot of wall begins with a trench at least twice as wide as the depth of the block from front to back. The bottom of the trench should flow like a tub, to its lowest point, and not hold water anywhere. The footer trench should always be carefully compacted. The trench should be filled with layers of clean crushed stone, compacted in “lifts” no larger than four inches at a time. The taller (and heavier) the wall, the thicker that crushed stone footer should be. Since the bottom of the wall must extend underground, a good rule of thumb is to make a footer trench twice as deep as the thickness of the block and fill it halfway with crushed stone. .

Backfill is the material behind the wall, between the “skin” of the block and the slope you want to support. It should always be clean crushed stone (angular, not round) over an inch, to allow water behind the wall to seep in. Backfilling with soil is a common mistake; damp floors expand and freeze, gradually pushing back the wall from behind.

Drainage behind the wall is key; Poor water management behind and under walls is the most common cause of wall collapse. Perforated tiles at the bottom of the footer trench, draining daylight down the wall, should be installed before adding gravel. Surface runoff from behind the wall must be diverted around it; ideally, the wall should be high enough to allow a drain from one end to the other.

The wall or skin block should be “tied” into the hill behind it. Usually this is done by installing a strong geotextile fabric, called a geogrid, between the layers of blocks and extending up the hill behind the wall. This net is held in place by the weight of the block and the backfill gravel, forming a sort of sandwich. In order for the wall to fail, the net would have to break or retreat from under tons of gravel.

Understanding these basics is a good start, but there are many other tips and tricks that come with training and experience. Every wall situation is different and there are many different types of segmented wall systems, each with cost-benefit tradeoffs. It is important to know your own limits before you start.

As a certified landscaping contractor, we are often called after the fact when structural retaining walls fail for one reason or another. It is never a pleasant experience for us or the owner. It is much more expensive and time consuming to repair a poorly installed retaining wall than to install it in the first place. Usually the cause of the wall failure is built in and the only solution is to remove it and start over.

Steve Boehme is a landscaper / installer specializing in landscape “makeovers”. “Let’s Grow” is published every week; Column archives can be found on the Garden Advice page at www.goodseedfarm.com. For more information, visit www.goodseedfarm.com or call GoodSeed Farm Landscapes at (937) 587-7021.


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