06 Feb Retaining Walls Go High Tech
Twenty years ago, all you needed for a retaining wall was a chainsaw and a stack of railroad ties. While these retention systems were effective in maintaining proper drainage and controlling erosion, they deteriorated rapidly, and some leached harmful chemicals into the soil. Wooden retaining walls are still very much in use today, but rarely at Ward Design Group and only at the customer’s insistence, I am quoted as saying that “it is never a good idea to put dead wood in the ground, it will only deteriorate”. Segmented Retaining Wall systems are also known as gravity block are the state of the art in soil retention barrier construction, and one of the best suppliers is Pavestone. Here is a brief lesson in the anatomy of a gravity block retaining wall from a recent project for the Robinson family of North OKC.
The first key to a strong wall is a firm foundation and this starts with leveling the soil at the base of the wall area. In this case, we removed a failed wooden 6-by-6 timber wall that had allowed the soil to shift enough to cause the house’s foundation to sag. Once the soil to be retained is cut back and the ground leveled at the base a footing is dug and filled with either cement or crushed rock. In this case, we chose crushed rock, which was compacted and leveled to create a pad for the base course.
In the old way of doing things, the wall was built up against the soil and any space behind was filled with more soil, giving almost no shock absorption and allowing moisture to build up directly behind the wooden components accelerating decomposition. Modern construction technique for walls up to 4′ dictates a pocket of 10 to 12 inches deep be constructed between the back of the wall and the soil. The pocket is exponentially larger the taller the wall to be constructed. The block is then stacked to an appropriate height, leaving a void the full length and height of the wall. For walls over 4′ in height, geogrid is installed every 2′-3′. The geogrid is attached to the wall block and goes back from the wall into the embankment. This is a modern-day version of the dead-man supports that were used in traditional timber walls. The geogrid anchors the wall into the embankment that it is retaining.
A layer of high-density erosion control fabric is used to cover the back of the wall and the front of the earth to create a “pocket” of sorts. The fabric runs down the wall, folds in the bottom of the pocket and runs up the earth behind to keep dirt out of the area. A large corrugated, perforated drain pipe, typically 4 inches or larger in diameter, is laid along the entire length of the wall, at the bottom of the pocket to carry water from behind the wall then out of the area through discharge tubes, helping to maintain proper soil humidity. Often we attach a pipe to nearby downspouts to channel the water through the drain system and away from the house. This helps to elevate hydro-static pressure caused by heavy rains.
The area above the drain pipe, between the wall and the soil, is filled with large gravel up to six inches below the cap of the wall. The landscape cloth is folded in across the top of this gravel to keep soil out and the top six inches above the closed pocket is filled with soil. Keeping silt out of the drain rock and pipe is crucial, in our industry when this happens it’s called a blind drain or going blind. The area filled with soil is then packed in place. Sod or plant material or some type of decking material to expand the outdoor living space is placed behind the top of the wall to prevent silting and erosion.
Since the blocks themselves are interlocking, they are installed without mortar, which allows minor movement in the walls face without cracking seams or blocks. To tie the wall together, the top cap is cemented in place with a construction block adhesive, locking all of the stones below into a solid wall.