This project features limestone block. Limestone blocks can be stacked two or three high to create a low, stable landscaping wall. Once the height of the wall exceeds those limits, however, stability becomes an important issue. For a wall to be stable, the weight of the limestone blocks must exceed the force of the soil behind the wall. As moisture in the soil behind the wall increases, the force attempting to push the wall forward increases. Important considerations in constructing a limestone retaining wall are foundation, backfill, block size and anchoring system.
Prior to placing the first layer of limestone block, remove all topsoil and soft subgrade material. A 6- to 12-inch layer of crushed limestone should be placed and compacted for the wall’s foundation. The width of this foundation must exceed the width of the limestone blocks by at least 6 to 12 inches. The top of the crushed limestone should be 12 inches below the finished grade at the face of the wall.
Since accumulation of moisture in soil behind a wall is the cause of most wall failures, a perforated drain pipe must be placed on top of the foundation at the back of the wall. Large sized crushed limestone is placed around the pipe to act as a filter. Two different backfill materials are often required. Large-sized crushed limestone is placed against the wall creating a wick drawing the accumulated moisture down to the perforated pipe. Low swell potential material (small-sized crushed limestone or sandy clay material) is placed between the wick and the undisturbed earth. All material is compacted in layers to reduce settlement potential. Additional drain pipes are installed in 4- to 5-foot increments of wall height.
In a typical limestone wall, the weight of the limestone blocks is the only force holding the soil back. Taller walls of larger limestone blocks will be more stable than those using smaller limestone blocks. As the wall increases in height, however, it becomes more difficult to place large limestone blocks without special equipment. Reducing the size of limestone blocks in the top of the wall reduces its weight and structural integrity. It also has a detrimental effect on the wall’s appearance. It is important in building limestone walls to make sure an ample supply of limestone blocks of similar size and color is available to complete the wall.
As the wall’s height increases and block size decreases, other methods of countering the force of the soil behind the wall must be evaluated. Geogrids are a type of plastic fabric used as a wall anchoring system. Geogrids are sandwiched between rock layers and extended into the backfill material. The weight of the backfill material on the geogrid holds the wall in place.
A second option is the use of geotextiles to create soil envelopes. Geotextiles are synthetic fabrics designed for structural integrity, durability and resistance to deterioration. As shown in the cross section above (click to enlarge), sheets of geotextiles are used to create envelopes of soil behind the limestone wall. The weight of each soil envelope holds the preceding soil envelope in place. The limestone wall does not retain the soil. With geotextiles holding the soil back, a designer can be more selective about size, color, availability and visual appearance of the limestone blocks.
The natural appearance and beauty of Iowa limestone should be emphasized by building tall, limestone retaining walls. Local limestone quarry operators can provide advice as to availability of quality limestone blocks and the height at which additional consulting and construction requirements are needed.