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Retaining Walls Revisited

February 16, 2009

MJ (my wife) reads this blog, and she made a good point about my last post (“Retaining Walls- Always Needed?”). Her point was that this blog is for a general audience, and since I excerpted a conversation with another designer, the language was a little contractor-y. Re-reading the post, I’m inclined to agree, so I wanted to clarify some of my points in laypersons’ terms.

The first is the issue of “cutting and rolling” a grade. Essentially, if I have 12″ of grade change or less, I can often grade that into a gentle slope. This slope will grow grass just fine, and it’s gentle enough that it can be mowed pretty easily. This sketch illustrates that (the dashed line indicates the original shape of the soil):12in-for-web

The next scenario I discussed is a 12-24 inch grade change. A wall could certainly help things out here, but you can also grade it down without a wall. You’re left with more of an embankment than a slope, however, and how you plant it changes. It’ll be tough to get grass seed to take before it washes away. Sod will work, but you’re left with two concerns: because of the steep slope, the grass is in a perpetual “drought” condition, and cutting the grass with a mower will be pretty difficult. You’re more likely to need to trim it with a string trimmer. In a situation like this, I’d rather see the slope planted with a groundcover- be it a shrub or perennial- that will stabilize the slope. See the sketch below (again, dashed line indicates original grade):24-in-for-web

From 24 inches on up, retaining walls are a good solution for enhancing function and ease of maintenance. It’s always important to consider the visual impact of a wall. Unless there’s something you’re actively trying to screen, I’m a firm believer that the smallest wall possible is the best. A seat wall is typically in the 18-22 inch range. This is a comfortable sitting height for most adults, and it doesn’t block the views out of the space. The wall defines the boundaries of the space, but even from a seated position, you can see to other parts of the yard. It’s also a great way to get multiple functions from one investment: not only does it hold back the grade, but it provides overflow seating for parties. I’ve got one job in progress right now where the homeowners are maximizing space by using a seat wall as built-in seating for an outdoor dining table. To give you a sense of scale, here’s a sketch of a seat wall:seatwallweb

The only downside to a seat wall is that without a backrest, it doesn’t provide comfortable seating for extended periods of time. If you were to build an additional wall behind the seat wall, to act as a backrest, you’d ideally have something in the 30-36 inch tall range. Whether it serves as a backrest or it’s an independent wall, a 36 inch wall is beginning to visually impact your space. That’s the height of your kitchen counters. From a standing position, you still have unobstructed views to other portions of the yard, but from a seated position, there’s more of a sense of enclosure:36wallforweb1

In extreme situations, a wall even taller than this is required to get the function we need from the space. The biggest detriment to such a tall wall is the impact it has on the space:60wall4web

The aesthetics aren’t the only concern, of course. If you’re building a retaining wall in Virginia, you (or your contractor) are required to get a permit for any wall that will retain over 24 inches of material (local codes may vary, ALWAYS check with your local building office before beginning a project). Most municipalities offer a standard retaining wall detail, which makes it a simple process: if you build a wall from the materials they specify, in the manner they outline in the wall details, they have done all the engineering work for you. Typically, these packets cover construction of timber, block, and solid concrete retaining walls. Fairfax County’s wall detail is a good place to get an understanding of what goes into a properly constructed wall. If you’re looking to build a segmental retaining wall (products such as Belgard, EP Henry, Techo-Bloc, etc.) retaining more than 24 inches of material, that will also require a permit. In many cases, the supplier can provide you with the documentation required to get a permit.

While these standard details are excellent, they only cover walls up to 60 inches (5 feet) tall. Anything above that must be designed and stamped by a professional engineer. Not only is it the only legal route, having such a large wall engineered is critical for the safety of anyone who will set foot on the property. When a big wall fails, it can be pretty spectacular:wall-fail011

The bottom line is that there are always multiple solutions to any situation. Instead of one large wall, perhaps we can use several smaller walls and adjust the grading a little bit. Maybe a small wall can be used to reduce the slope a bit, and we can plant a groundcover for erosion control as well as beauty and habitat- turn a negative into a positive with a lush butterfly garden, for example. If there’s no getting around the fact that you need a big wall to make it work, starting the process with a good landscape designer can reduce your overall expenditures by minimizing the amount of walls needed and designing them in such a way that they unify the space. It also reduces your engineering fees if the wall is already conceptually designed. I can make it pretty; the engineer makes it work.

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