Technical Tips:   Bolting in Soft Sandstone

Placing solid expansion bolts in soft sandstone is difficult, requiring technique and practice. And TIME – it takes 20-30 minutes to put one bolt in, and 40-60 minutes to put in two, generally the minimum for a permanent anchor. Here’s one way to do it:

Consider whether it is really necessary.

Okay, now that we’ve done that:

The biggest difficulty drilling in soft sandstone is in getting the bolts to “tighten up”. There are two kinds of bolts in common usage. Both are expansion bolts made for concrete and manufactured by the Powers Company, formerly known as Rawl. The ones with blue plastic parts are called Power Bolts or “blue tips”, and are the good ones. The ones with red plastic parts are called Power Studs or “red tips”, and are of considerably lower quality. Though the configuration of the cone makes the red tips easier to set in difficult situations, the steel used in the red tips is of very low quality, so the bolts are not very strong AND are vulnerable to crack propagation. Don’t use the red tips!

There are Stud-type bolts made for climbing that are good. Most of them are designed for limestone or granite, where the rock is hard, and bolts are relatively easy to get to set! The steel in these is quite good, but the cone/expander configuration may or may not work in soft rock.

Powers Blue tips have a cone on the end of the bolt. Once the bolt is inserted into the hole, the cone must stick against the side of the hole (at the back) while the bolt turns and pushes the sleeve into it. If the hole is too loose, or the rock at the back sandy or crumbly, the cone will not stick and will spin with the bolt. This is called a “spinner” – a bolt that is not properly set and therefore very, very weak, but that cannot be extracted. The goal is to not leave spinners.

Powers Power Bolts: 1/2" on left, 3/8" on right

In fairly hard stone, a hole is drilled with a hand drill, then blown out with a small piece of tubing. The bolt is loaded with a bolt hanger, then tapped into the hole and spun up with a wrench until it goes tight. The hand drill has a tendency to make a slightly oversized hole, but the blue tip bolts are large for their designated size, and the hole sometimes needs to be enlarged slightly (especially at the outside) to get the bolt in. Tapping in the bolt should not require a lot of force (thus the verb “tapped”).

In soft rock, we first drill a 3/8” hole, then we ream it out with a ½” drill bit, to get a really, really good ½” hole. Blow it out, tap in a ½” bolt, spin it up – Deluxe!

Hard Rock Directions

Drill a hole.

Carefully select the position of the bolt. Select a place of solid rock that is in a good place for the rappel and rope pull, minimizes grooving in the rock, is convenient to start the rappel, and provides a comfortable (or at least reasonable) stance to drill from. Hold the drill firmly against the rock, and smite it firmly with the hammer. Turn the drill ¼ turn after each strike. The tip of the drill acts as a chisel. Turning the drill is important to making a round hole, and keeping the drill bit from binding in the hole. Rock dust will accumulate and exit the flutes, but will tend to bind the drill bit. Pull the drill out and blow out the hole every 20 or 30 hits, or when turning the drill becomes difficult.

Hold the drill perpendicular to the surface of the rock. Concentrate on holding the top of the drill (where you hit it) in one location, and your hole will come out straight. Drilling good holes requires practice – fun to do while sitting around the campfire!

Drill the hole to the working length of the bolt, plus ¼”. The blow-tube can be used to measure the length of the hole.

Once the hole is deep enough, it should be cleaned. In normal rock, blowing it out really well is good enough. In softer rock, use a test-tube brush to clean the inside of the hole, then blow it out again until nothing comes out. Dust or grit in the back of the hole will prevent the bolt from setting so it must be avoided.

Load the bolt with a bolt hanger. This is usually done ahead of time, and the bolts are carried with hangers in place. Leave the small plastic spacer – it acts like a lock-washer to help hold the bolt tight. Bolts tend to fall apart in the bolt bag – be careful not to drop the parts.

Tap the bolt into the hole. Remove the little round sticker that tapes the cone to the sleeve before inserting. If the hole is really good, you will have difficulty getting the bolt started in. Use the drill bit to flare out the top of the hole a bit. In hard rock, don’t worry about making the hole too big. The tip of the bolt should fit into the hole, and the bolt should slide in with light taps. Tap the bolt into the hole until tight to the surface.

Moment of truth. Tighten up the bolt. Put your wrench on the bolt and spin it up. Lefty loosey, righty tighty. After one or two turns, the bolt should start resisting as the cone starts pushing the sleeve against the wall. If it does not tighten, you’re screwed! Tighten the bolt, and the resistance increases smoothly. Using a 6” wrench, the proper torque for a 3/8” bolt is about the most you can apply with your hand using a 6” wrench. Smaller size bolts are easily over-tightened, causing a loss of strength. When you get close to tight, be sure to position the bolt hanger in the finished direction.

We got a Spinner! If you have a spinner, the bolt not tightening up, then we should try to save it. The problem is the cone is spinning with the bolt, rather than sticking to side of the hole. The “Save” is made by applying sheer force to the bolt hanger, which pushes the back of the bolt against the hole and hopefully makes the cone stick long enough to get it started. This works maybe 25% of the time. You can apply sheer force by clipping a sling to the bolt hanger and standing or hanging on it. Don’t pull out!

Sometimes the bolt can be saved by dismantling it. Try un-screwing the bolt. You might need to apply sheer force to get this to work. If you can unscrew from the cone, you may be able to pull the bolt, then the sleeve pieces out, then screw the bolt back into the cone and pull that out. This is a low-percentage play. Then clean the hole and drill out wider or deeper, or both.

Bolting in Soft Sandstone

Bolting in the soft stuff is much more difficult. The rock tends to crumble as it is drilled, and the gritty surface makes getting the cone to stick more difficult. To do a good job in soft sandstone, use ½” blue tip bolts and a two-step drilling process.

First, drill a 3/8” hole to the correct depth, bolt length plus ¼”. Switch to a ½” drill bit and ream out the hole, using the twist and force from your hands, but not the hammer. This will almost always produce a very straight, very clean ½” hole. Drill to the correct depth. Clean the hole really well.

If the rock is of any quality at all, the ½” bolt will not fit into this hole. Use the drill bit to enlarge the outside of the hole to accommodate the tip of the bolt. Clean the hole again. Tap the bolt into the hole and tighten up as above.

In pretty good sandstone, it is possible to use 3/8” bolts. I have not used the drill-and-ream process with a ¼” / 3/8” drill combo, but it would probably work. The outside surface of sandstone tends to be harder than the interior rock – so longer bolts are actually harder to get to set and not necessarily stronger than shorter bolts.

Building an Anchor

A single bolt does not usually represent a solid, reliable anchor. Because bolts are un-inspectable, two bolts are usually considered the minimum for a true “anchor”. So place another bolt…

Distance – bolts should be placed at least 2X the length to the bolt apart. They should also be placed at least 1X the length of the bolt from a crack, edge or other defect in the rock.

Position – Bolts used to be placed side-by-side about 12” apart. This works, but it works better to place one high and one low (ie, stacked vertically). These can be tied together with webbing more-easily and with an easier equalization.

Do a good job. Your work will be used by many people without much thought for many years. Be responsible.