A variety of systems plus possible modifications results in a wide offering of retaining structures—one or even several to fit nearly any condition. Although any one of several types of retaining walls may do, specified criteria help to select the wall that is best suited for the job. These criteria may include environmental concerns, construction problems, site constraints, aesthetics, and cost.
A gabion revetment is typically built with a greater slope than a vertical gabion wall and helps protect a shoreline from erosion.
A more recent innovation involves coir fiber mats, made from biodegradable coconut fibers. The gabion is lined with a coir-fiber erosion control blanket, filled with rocks and soil infill. The coir blanket is folded across the rock and fill before the basket is permanently closed. The gabion is then sprayed with a hydroseed mix. If sufficient moisture for germination is available (either naturally or with an irrigation system), the gabions become completely vegetated in a short period of time. The benefits of the coir fiber blanket inside the gabion include high moisture retention to assist with seed germination and a means of containing the soil fill.
The simplest gabion structure is a 3 ft (1 m) -high wall using one tier of gabions. A second tier of gabions can be placed on top of the first tier and stepped back slightly. Gabions higher than 6 ft (2 m) typically require additional design constraints. Higher tiered walls require greater basal widths and/or the use of counterforts to brace the wall. Anchors are typically used with gabion walls to help ensure secure placement.
A rockery will resist lateral earth pressures to some extent and helps to prevent local slumping and slope failure provided it is well constructed, inclined into the slope sufficiently, and does not exceed design height limits. The stability of a rockery wall is governed largely by its batter angle and by its height-to-width ratio and does not contain mortar or steel reinforcement. The greater the batter (inclination off vertical toward the slope) the greater will be the permissible height/width ratio for a specified factor of safety. In most cases this ratio should not exceed three. As rocks greater than 2 ft (0.6 m) in size are seldom used, this limits the practical height of a rock breast wall to 6 ft (2 m) or less (Gray and Sotir, 1996).
Tieback anchors can be used, especially with steeper slopes, to secure the articulated blocks in place. The typical height for this type of wall is 4 to 6 feet (1.2 to 1.8 m) but can be engineered to greater heights when tieback anchors are included in the design.
Articulated block walls help to protect the environment by utilizing non-corrosive, environmentally friendly materials; since the strong concrete units need no maintenance and are typically easily handled and installed, the systems are cost-effective and economical.
For a normal base, the minimum depth should be 6 in (15 cm) of a 95% standard proctor compacted granular material. To provide additional foundation depth, for below-grade placement of block units, use a ratio of 1 in. below for each 8 in (20 cm) of wall height above grade. The area between the block units and the 12 in (30 cm) behind the units should be filled with 5 to 7.5 in (13 to 19 cm) crushed stone, or clean, well-draining granular fill. The material must be compacted around the units.
Chance helical tieback anchors can be used to add predictable load capacity to a retaining wall structure. The solid square shaft bar is screwed into stable load-bearing soil and connected to the wall facing.
To learn more about using tieback anchors for retaining wall applications, take the 90-minute Chance University course that covers this in more detail. Fill out this form to request enrollment.
Gray, D.H. and Sotir, R.B. (1996) Biotechnical Soil Bioengineering Slope Stabilization: A Practical Guide for Erosion Control. John Wiley & Sons, New York.