Modular Retaining Wall Works on Gravity
If you ever struggled with railroad ties while building a garden retaining wall or planter, you probably wondered if there wasn’t a better way. The standard eight-foot tie, which has added a rustic Western touch to many a Southland garden, is frankly a bear to work with--heavy, dirty, hard to cut, impregnated with carcinogenic creosote and even so, prone to rot out after depressingly few years.
One alternative often seen in local gardens is a wall of pieces of broken concrete slab laid up in dry-wall fashion. With the broken edges forming the outside of the wall, it has an informal look you can enhance by planting succulents or other crack-dwelling plants in the interstices. Such rubble walls are easy to build, but unfortunately they can’t approach the strength and stability of a filled concrete block wall with a poured-in-place concrete footing.
Now a new type of retaining wall is gaining ground in Southern California. Called modular retaining walls, they are built of mortar-less, interlocking concrete blocks held together by gravity and friction, as are rubble walls and, for that matter, the pyramids of Egypt. The difference is that the new modules are shaped in such a way to resist lateral pressure that might topple a rubble wall. For example, standard KeyStone modules, one type available in the Southland, are three times as deep as they are high.
The modules or blocks that make up such a wall are shaped to fit together snugly or are connected to each other with fiberglass rods, depending on the manufacturer. Because they are unmortared, once the first course is carefully installed and leveled, the rest is mostly a stacking operation, great for the do-it-yourselfer.
Bear in mind that retaining walls are subject to building code regulations which vary considerably from one community to another. In most cases you are safe if you stay under three feet, but check with your local planning department to make sure.
Even if you need a contractor to build an important structural wall, a modular wall can save money because it is quick to build compared to a poured-in-place wall.
After building his first wall with KeyStone modules, landscape architect and contractor Giancarlo Massarotto of West Covina was enthusiastic. “The best thing is how easily you can build graceful curves, either convex or concave. You couldn’t do that with railroad ties. Also the ‘sculptured rock face’ surface of the blocks has a nice, natural look to it.”
Asked about their drawbacks, he winces. “They’re really heavy. The standard units used for a major wall weigh 94 pounds each!”
Walls three feet or under, however, can be built entirely with smaller modules that weigh 78 pounds each, and KeyStone will soon bring out an even more user-friendly 27-pound model called Garden Wall blocks that will be sold at home improvement centers.
Supervising civil engineer Edward Alexanians of the Los Angeles County Planning and Safety Department predicts that modular walls will become a common feature in Southern California construction. “CalTrans likes them very much so you’ll be seeing a lot more of them. Our department doesn’t have standards for this type of wall yet, so we are approving them on a case by case basis.”
According to Alexanians, modern gravity wall technology was developed in Europe in the early 1970s. By the mid ‘80s the idea had spread to the United States and took root in the northern plains. The severe winters of that region are tough on retaining walls as moisture soaks into the concrete and freezes, causing them to fail. Two companies, KeyStone and Versalok, were soon formed in Minnesota and began manufacturing high-density concrete modules that interlock with fiberglass rods.
The northern plains also experience heavy summer rains, and the pressure of water trapped behind a retaining wall is often cause for collapse. In modular walls water is able to escape from the cracks between the individual blocks, unlike a solid, poured-in-place wall.
An unexpected early test of a KeyStone modular wall came in July, 1987, when more than 12 inches of rain fell on Minneapolis in six hours. A 17-foot-high wall survived the epochal storm with water shooting out of it like a sieve.
Frost heave is not much of a problem in California, nor have we had to worry about torrential rains for several years. Earthquakes, however, are a constant concern. In San Bruno engineers constructed an 18-foot KeyStone wall reinforced with geogrid (a synthetic fabric that gives soil added tensile strength) to create building sites on a steep hill. Eighteen feet is a tall wall by any standards, but an unusual feature of this one was that it was built about 300 feet away from the San Andreas fault. It was completed in September, 1989, and a month later the 7.1 Loma Prieta quake, with an epicenter only 35 miles away, rattled the area but left the wall unscathed.
KeyStone modules can be ordered from Sepulveda Building Materials, 2936 W. Sepulveda Blvd., Torrance (213) 325-2173. For more information about KeyStone retaining walls, contact KeyStone, 212 Technology Dr., Suite Y, Irvine, Calif. 92718 (714) 753-1790.
