Friday, February 25, 2011
Retaining walls serve to retain the lateral pressure of soil. Basement walls, for example, are retaining walls, but the term is most commonly used to describe the decorative or structural build-up of material to hold back soil on a sloped surface. Most commonly, these are seen on shore lines of creeks, or on hilly slopes.
As a landscape feature, retaining walls provide an eye catching focal point in a yard layout. They must be designed to resist the pressure of soil, but also often are required to resist hydraulic pressure. Because these pressures are the least at the top of the wall, most designs allow for a tapered construction, thicker at the base than at the top.
Gravity walls are constructed in such a manner that they rely on their mass to hold back the soil, while cantilevered walls often have a footing that helps to distribute lateral pressure into vertical pressure on the ground below. They generally are reinforced internally with steel. Anchored walls are more commonly used in rocky terrain, with the wall anchored into the solid rock behind the wall.
For the typical homeowner, though, most retaining walls are designed with both practical and aesthetic uses in mind. While a piled wall, consisting of precast heets of concrete, slabs of steel or thick timbers pounded into the ground may serve the practical purpose well, they may be quite unsightly. Sometimes such designs are augmented by dry-stacking stone or brick in front of the sheet, to provide the illusion of a retaining wall constructed solely of that decorative material.
The most common landscaped decorative retaining wall is the cantilevered structure, using poured concrete, decorative imitation stone, or real rock and bricks. These allow for a continuation of the lawn or garden surface right to the lip of the wall, without unattractive gaps.
A design option that is gaining in popularity is the use of interlocking bricks, that work much like a Leggo block system. These bricks come in a plethora of styles, sizes, shapes and colours, and are easily installed by most dedicated handymen, in one or two days.
When installing a retaining wall that will also act to resist hydraulic pressure, it is important to install proper drainage systems, as well. This is where the home handyman most often fails, and where consulting with a professional fence installer or landscaper becomes critical.
Professional installers also are able to provide the guidance that will turn a functional retaining wall into a work of art, deserving of attention and praise. While the cost may be substantial, the benefits, in the log term, will readily exceed the investment.
Contrary to myth, solid fencing does not provide the best wind break. The force of the wind on the solid flat surface creates both pressure on the windward side and pressure differences on the leeward side, somewhat similar to the way an airfoil works.
To illustrate this, note that snow fences are designed with porosity, allowing some of the wind to work its way through, yet decreasing and deflecting the air flow. As a consequence of the design, snow will tend to accumulate a few feet in front of the fence, as the vortices of wind redirect.
When designing a wood fence as a windbreak, stagger the boards 2 inches apart for every six inches of width. The ideal porosity for windbreak fences is 25-33%, and such a gap will give a 25% porosity. This provides a protection that will extend eight to ten times the height of the fence. Another option is to slope the fencing material, if you are intending to use the fence as a shelter for livestock.
For home and farmyard applications, consider building the fence in staggered sections, with each panel offset from adjacent ones by a foot or so. This gapping increases airflow redirection, while also reducing wind load on the panels. Varying the height of the panels also will aide in this redirection. The intent, of course, is not to stop the wind, but to reduce and redirect.
When installing a wind barrier that is intended to act as a snow fence, do not install the fence right at the point where the snow is to be stopped. Rather, set the fence back several yards, at least, so that the snow that is impeded will drop and accumulate prior to the road way or clear area that you require. Remember that, since the wind break (if properly installed) will create a protection area that is eight to ten times the height of the fence, a four-foot fence will provide a dead zone that is up to 40 feet in front of the fence.
Some permanent wind breaks are nothing more than a good tree or brush line. However, solid rows of trees provide less protection than porous rows, so do not plant so that an impenetrable barrier is created. Instead, use staggered plantings, offset against each other.
Although it may seem counterintuitive, mesh fences can provide good wind barriers, as well. Materials such as the mesh seen along the perimeter of golf driving ranges, plastic and resin meshes and even chain link fencing provide a nominal measure of protection. To illustrate the effectiveness of a simple screen, open your house window on a cold, relatively calm winter day, and feel the coolness of the air coming in against the screen. Then stick your hand outside and feel the difference. That screen has partially blocked the transfer of heat due to convection. It works similarly for wind.
Wind breaks, then, are not so much wind barriers as they are wind speed bumps. Holding back the wind, to paraphrase Jim Croce’s song about spitting in the wind or tearing the mask off the old Lone Ranger, should not be foolishly attempted!