Bolton Valley Notes

The problem

The fixed protection used in the Bolton area cliffs have gone through several iterations of based upon many different factors. Conventional small diameter mechanical bolts have been found to loosen over time in the medium-soft schist that is prevalant in the Green Mountain range. The one exception to this observation being the double-sleeve style wedge bolt available from Fixe (and other harder to source European manufacturers). Additionally, there are installed pieces of fixed protection that diverge from contemporary best practices in some of the following ways.

  • Use of an unvetted concrete anchors that most likely fail to meet industrial construction/manufacturing or UIAA/CE standards for climbing protection.
  • Improper material selection/combination (i.e. using plated steel or mixing plated and stainless steels)
  • Poor placement selection. A surprising number of bolt placements on some routes are in disturbingly hollow rock or do not meet manufacturer specs for edge distances, or placements creating clipping complication, feature-caused un-clipping, or cross-loading scenarios.
For better or for worse, Northwest Vermont is no the only climbing area with a medium-soft sedimentary rock. The benefit of this fact is that we can draw from the experience of others who have put time, effort, and money into trying to solve this same dilemma. Fortunately for us here in Vermont, we need not look any further than our neighboring state of New Hampshire and similarly formed cliffs of Rumney. With a similar type of schist, Rumney has long since turned to the use of adhesive-based bolts (a.k.a. “glue-ins”) to solve similar problems [1].

Glue-in bolts: A sticky proposition


Epoxy adhesives are designed as a “system” with a specific application process and tools. The biggest distinction here for our purposes is the type of delivery system used for the adhesive; mainly capsule versus cartridge systems.

Capsule systems make use of small glass vials of hardener and epoxy that are inserted into a cleaned hole. The fastener (i.e. the bolt) is then hammered into the hole, breaking the vial. The capsule contents must be mixed in the hole in order to properly mix and attain maximum strength. Most of these systems require the glue to be mixed at a certain RPM for a certain period of time in order to ensure the two-part epoxy mixes properly and the proper temperature for the thermochemical reaction is obtained. This require a special tool to attach a power drill to the fastener and spin it in the hole which is typically impractical for climbing bolts. Additionally, the capsule must be specifically rated for the fixture (bolt) and the hole volume. A change in hole depth or diameter or in bolt size can yield too little or too much glue in the hole.. The former being an especially dangerous situation. In practice these systems are inefficient, impractical, and often produce lower quality results (not as strong, unaesthetic, etc).

Examples of capsule glue systems
Examples of capsule glue systems

Cartridge glue systems contain the epoxy and the hardener in two separate chambers and use a special applicator gun which is similar to a caulk gun. Typically each glue system has a specifically designed applicator gun (though some can be used with a traditional caulk gun) and a special mixing nozzle that mixes the epoxy and hardener as it is dispensed from the cartridge. These systems tend to be more expensive per hole but result in a vastly improved working experience and higher quality results. As such, cartridge systems are the recommend application method.

There are a few “de-facto” standard adhesives considered vetted enough to be safely used in climbing applications. One should read that as “they have been used enough without mass deaths attributed to their use to be considered ‘safe'” as there are no UIAA/CE certifications for these adhesives. The main players in use in the US are the Hilti RE-500, Powers AC100+ Gold and ITW Redhead A7 epoxies. The Hilti offerings are true two-part epoxies and provide high strength results but have a high cost and a longer drying time (8 hour cure time @ 60° F). The Powers and Redhead A7 glues are a vinylester and acrylic respectively and are significantly cheaper than the Hilti epoxies. Additionally these both have cartridges that will fit in regular caulking guns though the OEM guns are specifically designed with the proper thrust ratio for their cartridges and yield better experiences generally. Aside from low cost, vinylesters and acrylics offer incredibly fast gel and cure times (approx. 5 min gel / 30 min cure @ 60° F). Something that can be both a blessing and a curse depending on the time of year.

In the end these glues all work and are viable as a protection system for climbing. Hilti has never formally recognized climbing as an acceptable use case of their products, however Hilti’s glues are a benchmark worldwide. Fixe recommends the Powers AC100+ and that is what they sell for their cartridge glue-in system. The Redhead A7 is what is used in Rumney. One additional note is that ITW Red Head has stated they believe A7 to be particularly suited to the needs in a climbing protection application [2].

Glue-in bolt types

Glue-in bolts for use in climbing protection applications come in a variety of designs. Each design brings advantages and disadvantages. Below are some of the common bolt types covered in more detail below.

  • Bühler style bolt
  • Single-leg eye bolt
  • U-bolts or staples
  • Threaded rod w/ hanger

Complicating the selection of a bolt for use in the US, the domestically available offerings are fairly weak, unlike in Europe where it seems every climbing company has at least one glue-in bolt design available. Currently the domestically available options are from Fixe Hardware, Petzl (prohibitively expensive), and Wave Bolt with other options being available via import and the miraculous Internet.

Single-leg eye bolt

Until recently this design of bolt may have been the most recognizable to the North American rock climber. Due to our historically low use of glue-in bolts many times the few places that used them used only the easily available models from Petzl or Fixe. Though this is not always the case, an example of which being in Rumney where they’ve made use of industrial construction grade threaded eyebolts with much success. While these bolts are not certified for use in climbing, they should meet the UIAA requirements of 25KN (UIAA-123) based upon the published working load limit of 2,200 lbs and assuming WLL is 25% of breaking strength. This is something to keep in mind if selecting a non-certified bolt (CE/EN or UIAA). In the Rumney case while these bolts are not stainless steel and they do not seem to be showing corrosion issues, it would behoove a developer to use a stainless steel bolt, preferably certified to EN 959/UIAA 123 standards.

There are mainly two types of eye bolts; forged and welded. Generally forged eye bolts are expensive to manufacture (but yield high quality results) and most eyebolt models end up being of the welded variety. Petzl’s offerings are both forged and their $15-$30 price range reflects that fact. Regardless of manufacturing process, each model of bolt will have installation instructions specific for that design, so it is imperative that you understand how the manufacturer intended their fixture to be placed. If the fixture is certified for climbing, the installation instructions ensure the bolt meets UIAA specs. Deviation from the installation instructions may compromise the bolt’s strength.

For eye bolts the biggest question regarding properly installation deals with whether the bolt needs to be countersunk or not. Based on my research there seems to be some confusion on this topic. From my reading if the bolt does not have an angled end-tip or shoulders then the eye needs to be countersunk to meet UIAA specs to prevent rotational forces from breaking the glue-bolt bond. It becomes unclear when the bolt has an angled end-tip. Some feel that is enough to counter the rotational effect. Counter to this, Fixe’s installation instructions for their welded eye-bolt specify that the eye should be countersunk. Ultimately a developer should follow the directions for the specific fixture, especially if the bolt is climbing certified. Lacking information the developer should appeal to more knowledgable parties or perhaps choose a more mainstream bolt design, preferably something proven in climbing applications.

Bühler style bolt

Historically this home-grown bolt was developed by the late Frankenjura climber Georg Bühler to be easily manufactured in a home setting. The Bühler bolt is said to be almost universal in the German speaking climbing world and in fact many European climbing manufacturers offer a Bühler design. The Bühler bolt offers an inherent resistance to rotational forces by way of its design and therefore does not need to be countersunk. This gives this design a fairly straight forward installation process. Contemporary Bühler bolt designs are introducing an “interference” fit into their design which adds a small mechanical compression force to the bolts placement (i.e. the bolt must be lightly hammered into its hole). The helps the bolt remain in the hole when placed in overhanging placements, a particular weakness for glue-ins in general. Examples of commercially available Bühler bolts are the quality innovative twisted-leg design by Jim Titt at Bolt Products and the American-designed Wave Bolt.

Staple or U-bolt

Once popular in Europe the staple bolt should be considered passé and an inferior solution to eye and bühler bolts. Various concerns about the staple bolt make it a questionable solution. Testing has shown that failures occurred at only one leg of the staple bolt, indicating load is not properly balanced between the two legs. Additionally, there is concern over the proximity of the two holes as the distance does not meet any proposed guidelines for hole spacing (typically 1.25x-2x hole depth). The principal concern here being that micro-fractures radiate outward from the drilled hole. In some cases these micro-fractures may intersect between the two legs causing a contiguous fracture and weakness. [3, 4, 5]

Threaded rod

A typical construction application of epoxy adhesives is to glue threaded rod into concrete and attach a fixture to the threaded rod. It is possible to use this same paradigm along with a traditional hanger when using glue-in bolts. A threaded rod is inserted into the hole with glue which is allowed to cure. Then a hanger is installed and torqued to the appropriate tension. The costs of materials and the added hassles of this method make it a less desirable option given the existence of glue-in bolts specifically designed for climbing. As such, this option will receive no consideration beyond this description.


While there are variations in the glue system and the bolt chosen, the general installation procedure can be addressed in general terms. The time-sensitive nature of dealing with glues presents some challenges to installation and necessitates a dialed-in installation procedure.

In general glue-in installation can be broken down into the following steps.

  1. Drill/enlarge the hole for your glue-in bolt.
  2. Clean out the initial debris and dust with an air pump, blow tube, compressed air, etc
  3. Ensure your glue-in bolt fits in this hole; if not repeat steps 1-2.
  4. Clean out the hole with a tube brush and blow out the hole with air. Repeating until you no longer have dust exiting the hole when it is being blown out. This step is imperative because if the hole is left dirty the glue will bind to the dust and can cause dramatically reduced failure values for the glue (up to 90%). It is not uncommon to repeat this cycle possibly a dozen or more times in schist.

Repeat steps 1-4 until all your holes are drilled and prepped. At this point you can begin the actual installation process of the glue and bolts. As a rough rule you can fill the hole about 1/2 to 3/4 full with glue. You may choose to run a small bead of glue down the length of the bolt’s leg(s) to help ensure coverage. Insert the bolt and begin twisting it in the hole to coat the hole, bolt, and displacing any air cavities encountered/created while inserting the bolt. You should have some excess glue coming out of the hole. If there isn’t glue coming out of the hole and there is still volume to be filled in the hole, remove the bolt add more glue and repeat the insertion procedure for the bolt. Once the bolt is fully inserted and properly orientated, cosmetically dress the glue around the hole to achieve a sealed and smooth finish, possibly removing excess.

An example of a cosmetically finished glue seal around the bolt and hole.
An example of a cosmetically finished glue seal around the bolt and hole.

Installation isn’t rocket science but the installation process is more complicated than mechanical bolts and more prone to errors that can make or break your plan. Below are some observations on equipment that will help the process and thoughts on why.

  • Holster – Glue will invariably leak from the glue applicator gun so it’s important to have somewhere to store the gun while in between holes. Cheap plastic tubing can be purchsed from your big box hardware store that you drill some holes into to create a sling to strap to your harness. Tape the bottom off with a plastic grocery bag that will act as a drip bag which will catch the leaking glue. This will prevent the glue from getting all over you, the route, and the base of the crag. Your gear, shoes, and future climbers will thank you.
  • Rag – Have some place to wipe off excess glue as you’ll inevitably get glue on you while smoothing out the glue around the edges of the hole. Strapping it on your quad seems to work the best; alternatively use a crappy pair of pants.
  • Gloves – At best, these glues are nasty substances and you may not want it coming into contact with your skin. Some latex gloves might not be the worst investment if you’re uneasy about the hazardous material handling sheet available for your glue of choice. Something that is form fitting to your hand is best as this will allow you to still properly cosmetically dress the glue. Or…
  • Wodden popsicle sticks/coffee stirers – These can be helpful to help smooth out the glue around the edge of the bolt/hole to properly seal the hole and make it aesthetically complete without having to touch the glue.
  • Plastic zip-lock bag – When you start a nozzle it’ll take the glue some time to properly mix. The initial output of glue should progressively change color as it comes out of the nozzle in the first few squeezes that exit the nozzle. Dispense these initial cycles into a plastic zip lock bag so you can ensure that the glue properly mixes and hardens after the cure time. If after the cure time your sample bag doesn’t have properly cured glue, your work is at best suspect and you need to re-evaluate whether the bolts need to be replaced.