First and foremost, when installing expanding rock anchors for foundations, it’s important to choose your anchor site carefully.
Being mindful of expanding rock anchors for foundations
- Rock anchors will only perform effectively in solid competent rock (class 0 soil).
- Drill the hole into the competent rock a minimum depth of 1 foot. Be sure to drill so that the anchor rod will be in line with the guy.
- Holes should be drilled so the diameter is optimally 1/8 inch larger than the diameter of the unexpanded anchor. (For example, the hole drilled for a R315 is 1-7/8 inches in diameter. However, drill bits are usually available in 1/4-inch increments. In practice, a 2-inch diameter hole is drilled for the R3 series anchors and a 2-1/2-inch diameter hole is drilled for the R1 series anchors.) Be sure to thoroughly clean the drilled hole of rock dust and debris.
- Slide the anchor down in the hole. Use a bar through the eye to turn the rod until the anchor is fully expanded against the sides of the hole. Removing the wedge tie will occasionally help facilitate wedge expansion.
- Rock anchors set in holes that are drilled 1-foot deep in competent class 0 soil (hard rock) will develop the full strength of the anchor rod. This can be either a 3/4-inch rod (a minimum ultimate strength of 23,000 pounds) or a 1-inch rod (a minimum ultimate strength of 36,000 pounds).
- It’s good practice to grout the rock anchor after it’s installed. This is an added measure to help protect the rock from degrading. Grouting may be performed using a funnel and PVC grout tube, which extends down the hole to the top of the wedges. A flowable, neat cement grout (no aggregate) with a .4 to .45 water-to-cement ratio is suitable for grouting rock anchors. The grout will cover the drilled rock and prevent it from weathering over time.
- The guy strands can then be attached to the anchor eye and fully loaded. The optional grouting may be done before or after the guy strands are attached.
This topic was initially presented by Hubbell Power Systems. Check out Hubbell’s original article, “7 steps to install expanding rock anchors.”
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