Creating Safe Rock Anchors

The many sport routes being developed in Muir Valley required rock anchors... lots and lots of rock anchors.

In the beginning, Rick and Liz, being outdoor rock climbers, were very familiar with the rock anchors that were installed on sport climbing routes. But, they were not knowledgeable about details of how they were installed. The Webers, of course, wanted to be sure that high quality hardware was being safely installed in the Valley. And, as they were both engineers, they decided to research all the different types of rock anchors used in setting up rock climbing routes and test them when they were installed in the Corbin Sandstone walls of the Valley.

To set up an accurate, realistic testing program necessitated the purchase of laboratory-quality testing apparatus and measuring instruments. Rick Weber, with the help of some of Muir's volunteer route developers, used this equipment to conduct a number of tests on rock anchors installed in Muir's rock.

It is important to note here that a rock anchor system consists of both the hardware and the rock in which it is installed. One particular type of hardware installed in granite will be stronger than its installation in Corbin sandstone. This is because, of course, granite is much stronger than sandstone. Engineers measure the strength of rock as "compressive strength". Granite, as found in Yosemite National Park, has a compressive strength of 19,000 pounds per square inch (psi). Here in the Red River Gorge, our sandstone varies from only 2000 to 9000 psi. This means that it is extremely important to install the best quality hardware using the best tools and techniques. The people developing the rock climbing routes in Muir Valley enthusiastically subscribed to and supported this quality commitment.

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Rick Weber drilling a hole for a rock anchor on one of Muir Valley's sandstone walls.

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Hydraulic cylinder to provide the pull-out force for a rock anchor

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Different types of rock anchors being tested for their pull-out strength

The procedure for creating this type of rock anchor is as follows:

  1. Using a special rotary hammer drill, fitted with a masonry bit, drill a ½-inch diameter into and perpendicular to a rock face to a depth of about 5 inches.

  2. Clean the hole of rock dust created by the drilling.

  3. Insert a ½-inch diameter x 5-inch long expansion bolt through the hole in a hanger bracket (specifically made for rock climbing anchors) and into the hole drilled into the rock until the hanger bracket is flush with the wall.

  4. Using a foot-pound torque wrench, tighten the expansion bolt to 45 foot-pounds of torque.

 

In about 2007, a new type of rock anchor system was developed by an Englishman, Jim Titt, that showed promise of being even superior in several respects to the expansion bolt/hanger bracket system we had been successfully using. This new bolt was made of 1/4-inch diameter stainless steel rod, twisted into a shape as shown in the accompanying photo. Instead of screwing it into a hole in the rock, a super strong epoxy resin was first injected into the hole, and the bolt was simply inserted and kept in place until the epoxy cured. This type of bolt is referred to as a "twist" or "glue-in".  And the testing Weber did on them showed that, generally, they comprised an even stronger hold to the wall than did the expansion bolt anchors. They also had the advantage of resisting loosening, which the expansion bolts did on rare occasions.

In the beginning, the rock anchor of choice was an expansion bolt/hanger bracket assembly. And, these worked very well, many of which are still solidly in place today after almost 20 years from when they were first placed.

 

In 2004, we discovered that the International Climbing and Mountaineering Federation (UIAA) had established a formal standard for installing rock anchors along with specified forces that an anchor should be able to withstand without failure. Naturally, we were eager to find out if the anchoring system that the route developers were using in Muir Valley could meet this standard. So, Rick Weber set about testing these anchors as installed in Muir Valley's Corbin Sandstone.

 

Rick:  "And, it was both a satisfaction and relief that, indeed, the UIAA-approved hanger brackets we had been affixed to the rock walls with ½-inch diameter expansion bolts in Muir Valley met or exceeded the UIAA Standard 123 for rock anchors."  So, the developers of rock climbing routes in Muir – and throughout the Red River Gorge region, for that matter – continued using this anchoring system. These anchors worked very well, many of which are still solidly in place today after almost 20 years from when they were first placed.

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Expansion bolt anchor after being pulled out of its hole in a rock face with a force in excess of that required by the UIAA Rock Anchor Standard.

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The twist bolt, developed by Jim Titt that is inserted into a hole in the rock that has been filled with a high-strength, long-life epoxy.

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Instrumentation and force-generating rig for testing rock anchor bolts set up on a vertical section of one of Muir's Walls.