The ARC Rock Wall Construction Guideline

Rock Quality and Density

• Every piece of rock used in a rockery must be angular ledge rock that is in sound shape and has a resistance to weathering by outside elements. In addition, the longest dimension of any individual rock is not to exceed three times its shortest dimension. The density of each rock should be greater than or equal to 155 lbs. per cubic foot.

Slopes

• Any slope above the rock wall should be kept as flat as possible and should never exceed an inclination greater than 2 horizontal units for every 1 vertical unit unless specifically engineered to handle the slope’s surcharge load, which is the amount of lateral stress applied to the back side of the wall. Slopes should contain significant deep-rooted vegetation and covered in mulch to protect against erosion induced by surface water flow.

Fill Compaction and Construction Reinforcement

• For any rock walls built in front of a fill, it is important to ensure the fill consists only of organic, debris-free granular material that is no bigger than 4 inches apiece. All fill should be placed in thin lifts no greater than 10 inches in loose thickness and compacted to at least 95 percent of its maximum dry density. The primary method of constructing a fill for a wall less than 8 feet high is to overbuild it and then cut it back, while walls higher than 8 feet require a geogrid or geotextile reinforcement.

Rock Wall Keyway and Drainage

• The keyway is the first step in the rock wall construction following general excavation. It must compose a trench not less than 12 inches deep where the base row of rock will be set and must extend for the entire length of the wall. Additionally, install a slotted or perforated plastic drain pipe with a minimum diameter of 4 inches along the rear of the keyway behind the base rock for drainage, ensuring a sufficient gradient for flow to a low point.

Wall Thickness

• The thickness of the rock wall, determined on a case-by-case basis, is typically equal to the width of the individual rock plus at least 12 more inches for the drain rock layer. The higher the wall is, the greater the width each rock must have. In addition, if the wall is to have any retaining function at all, the thickness must be adequate to resist the intended load weight. In select instances, more than one row of rock may be required.

Rock Shape and Stability

• All rocks integral to the foundational stability of the wall should have a generally rectangular, cubical or tabular shape, while those with a circular or tetrahedral form should be used only for filling large voids or be rejected altogether. Only use rocks with no major cracks, seams or foliation joints as well, unless firmly confined by surrounding rocks.

Rock Placement

• The base row of rock must be set on firm, unyielding soil and should reach at least 12 inches below the lowest adjacent site grade. As subsequent rows are built, no continuous joint planes can exist in either a lateral or vertical direction. And when possible, all rocks should have a minimum of two rocks beneath it. Also, the top layer of rock on the wall should slope backwards toward the protected soil face since stacked rocks tend to topple outward when weakened. Similarly, no rocks should have top surfaces that slope downward from the wall face.

Other Considerations

• The gradient of a rock wall's face should never be flatter than 1H:4V (1 horizontal unit for each 4 vertical units), with a recommended incline of 1H:6V back toward the face that's being protected. In order to provide drainage behind the wall and prevent significant soil erosion, install a 12-inch drain rock layer (18 inches for walls higher than 8 feet). Also, voids in the wall are not necessarily problematic and do not need to be chinked with smaller rock as long as firm contact exists between all rocks surrounding the void. The larger the rocks used, the larger the acceptable void may be.