Screw in guy anchors, guy lines

I have installed many towers and masts over the years, and I'm proud to say I have never had a tower fail. I have seen dozens of towers installed by others fail, however.  Most failures are due to guy line failures including guy anchor failures. Installers, sometimes those who profess to be professionals, occasionally do stupid things.

One of the most common failures is caused by using the wrong clamps or installing clamps backwards. The photos below are from guylines removed from a  Rohn 45G tower: This isn't the only terrible installation I've see.

I always wonder how this stuff stays up. Imagine $10,000 worth of tower and antennas guyed this way, and it really is very common. In the 80's my business did a lot of tower work. After storms we would go out to remove fallen towers, and every single tower I recall failing failed because of improper guying. It was either too few saddle clamps  (especially over over a coated wire), clamps on backwards, wrong size clamps,  loose clamps, or turnbuckles or eye bolts without forged closed eyes.

One antenna tower I removed in Mississippi (Rohn 45G) had the anchors in the wrong direction! They were not even pointed towards the tower.

Guy Anchors

Guy anchors mostly get their pull-out resistance from dirt they have to displace rather than anchor weight. When anchor holes are drilled or bored all the old well-settled or compacted dirt is removed in a line towards the guy line pull direction. Backfilled dirt is never as resistant to pulling something up through it as the original dirt. Remember the load on the soil has to spread out to include undisturbed dirt. The last thing you want is a solid walled uniform diameter plug like concrete filling the hole in the direction of pull. Filling the anchor hole with rock or concrete is a major problem. If you bore a hole (such as for a bust-open anchor), don't backfill the hole with rock or concrete. This actually makes it easy to pull the plug out of the earth. Backfill with dirt and pack the dirt tight every few inches of fill.

If you must use bored holes, use a "bust open anchor." This type of anchor opens by driving a pipe down over the rod. The anchor end then expands out into the soil. It expands and locks in to the fresh undisturbed soil. 

Remember it is all about the soil the anchor is displacing, not the weight. My larger tower uses commercial anchors pulling a 6x4 foot (or larger) rebar re-enforced wall through the ground. We bored the anchor hole down into the wall so the wall pulls against undisturbed soil. To the right (your view) of the 100-pound dog is the trench for the wall. It was dug with a back-hoe at right angles to the anchor direction. You can imagine trying to pull that wall through undisturbed earth!

The anchor is hot-dipped galvanized channels  5/16 inch thick and 4 inches wide. TWO pieces are back-to-back. The other end (eight feet down) has three rebars through holes. The rebar runs the length of the wall.

On lighter towers or smaller lines (under 12,000 pounds breaking) I use screw in anchors. I use an attachment with my tractors that will screw the anchor in. A good-biting anchor will almost twist the shaft while screwing in. If it turns  in easy that is a sign holding power might not be optimum. I use a minimum of a single six inch blade 66 inches deep. I have a jacking fixture we test the load with, and can pull at full pressure (ten tons) against a screw anchor without having it creep upwards. There is generally a small upward shift of a half inch or so that stops as the soil packs, but it won't creep out past that point.

Here is how we solve creep in wet holes:

When we hit a weak hole we back the anchor out a few feet. 

We bore a 12 inch hole at right angles to the anchor rod.

We screw the anchor back just past the hole into the wall edge.

We load the hole with a few sticks of heavy rebar making sure the rebar is against or near the anchor shaft.

We backfill with properly mixed bagged concrete.

Now the anchor blade has to drag a 12" diameter plug of concrete up through the undisturbed earth. I have a Rohn 55G loading two anchors that are in a very wet area with a few thousand pounds, and those anchors have not crept since being installed. They also did not jack out with the 10 ton test fixture.  Prior to the concrete I could pull the anchors up with 1000 pounds of guy tension!

I don't hesitate to use screw anchors except:

1.) I am leery of anything that screws in easily. It has been my experience what threads in easy pulls back up easy.

2.) I am mindful of saturated soils. Even sand has not been a problem, but wet clay seems to slide around the anchor.

3.) I over-pull to test anchors and look for creep. I even built a jacking fixture.

4.) I mark anchors with paint at ground level so I can monitor for any movement, and I watch them the first few months.

Various Anchor Types

Above, a pile of assorted anchors.

Above right, an expandable (bust-open) anchor. This type requires a deep drilled hole. It is "busted open" or expanded with a pipe slid down over over the rod. This type of anchor is well-suited to problematic soils. A screw anchor like the one on the left works great in my dense clay soil, not budging when jacked with a 10-ton jack.

The Rohn anchor on the right requires a backhoe for installing the concrete and rebar.  They also require an equalizer plate.

Screw-in anchor.