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Home & Garden

Terraced gardens require more than stacking blocks

| Sunday, July 3, 2011

Terraced gardens present a simple and attractive solution to dealing with a fact of life in Western Pennsylvania: a property built on a hill.

But Safei-Eldin Hamed, a professor and director of landscape architecture programs at Chatham University in Shadyside, has a warning to homeowners considering one.

"Don't do it without considering all of the factors or because it is a fad or it would be cool," he says.

He and others involved in landscaping offer warnings about paying attention to the type of soil, the size of the terraces, the height of the walls holding them and sometimes the need for government approval.

It is not simply a question of moving some dirt. But that work presents more problems than it seems, says Bill Reese from Creekside Landscape Supply in Greensburg.

"It seems simple, but you have to consider you are moving dirt from point A to point B, and then back again," he says. "It can be a bigger job than it seems."

Joel Schroeder, president of Lincoln Way Landscape Supply in White Oak, agrees.

"Typically, it is a matter of putting up a retaining wall to hold back some dirt," he says. "But you can easily get into a situation where you need to start thinking about other matters."

One of them, he says, is the use of geotextiles -- woven, synthetic sheets, that are placed horizontally to reduce pressure from the soil.

Often, he says, the work is being done as a way of dealing with hilly property and does not involve creating a garden.

Facing all of the issues

Louis Viens, a landscape engineer from a Canadian firm that makes blocks for retaining walls and other uses, says the size of the site and the nature of the soil must be considered, too. Clay-ridden soil, so dominant in this area, is prone to lateral movement, so could require more support.

"All of these present issues you hadn't though of," says the engineer from Unilock in Quebec province.

One of the most serious elements of terraced gardens is legality.

Robert Welling from the R.I. Lampus Co. says how many municipalities require walls four feet or higher to have a plan from an engineer. The Springdale firm is a dealer in block, stone and concrete products, such as segmented retaining block from Versa-Lok.

While those restrictions may differ from place to place, one thing does not: Any wall that high demands a building permit and that could entail an engineer's work, says Bob Vita, building inspector and zoning officer for Shaler.

The whole issue there, Welling says, is not building a wall that is too weak to hold back the dirt behind it. Generally speaking, he and Schroeder say, the size of the earthen area behind a wall should be no more than 1 1/2 times the height of a wall. For instance, if you have 2-foot wall, then there should only be a three-foot terrace behind it. Subsequent layers are measured the same way, not from the total height of the area.

When that becomes impractical, it can call for the use of geotextiles to reduce the pressure of the soil.

"Some people forget the basic engineering principals," Welling says, or don't know them to begin with.

Reese and Hamed also cite the importance for effective drainage. If water is not allowed to run from this area of accumulated earth, it will put even more pressure on the wall meant to hold it, they say.

Hamed also says the use of the terraces comes greatly into play. Many times, he says, the layers are simply for gardening, so deal with no "load-bearing" issues. If there is any kind of traffic, however, the strength of the walls becomes even more important.

Do it yourself, with wisdom

Reese says building a terraced garden is "definitely a do-it-yourselfers' job," but is easier to "do it in phases."

Digging the dirt from its original spot is one job; building the wall another; replacing the earth a third. It can be tough, but it you find a supplier who will deliver your supplies as you need them, it can really help," he says.

Viens says the job is not that difficult, but its success is dependent on the knowledge of the homeowner involved.

While Reese points out boulders can be used for the retaining wall, he Schroeder and Welling all say segmented walls are used most often. They come in different construction types, making them appeal to a variety of workers. Some lock with pins while others do that in a tongue-in-groove fashion, Schroeder says.

Hamed says understanding all of the construction issues is important so you don't get involved in a project beyond your ability.

"Terraced gardens go back to the Italian Renaissance and to Islamic Spain, so it is easy to be inspired by history," he says.

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