Shallow Dead Man Anchor

Our antenna towers need help to stand upright. Gravity alone can't provide stability since it can only pull them against the ground but cannot provide lateral stability. Ground anchors are what give the tower stability to withstand the forces they must face: vertical, horizontal and torque. One big anchor if it's self supported and several if it's guyed.

In most cases the anchor is a solid mass of reinforced concrete. There is one directly under the tower. A guyed tower has several additional anchors. 

Anchor specifications depend on tower load and soil conditions. To get a building permit for a tower you may be required to have a soil test done by a professional. Those without the need to acquire a permit should not skimp on the anchors or a thorough soil inspection or test. Nature and the laws of physics pay no attention to pieces of paper, or hope.

There are articles in this blog that go into some detail of the anchors for my several towers. They require siting, excavation, reinforced concrete and alignment. I used heavy equipment and hired skilled labour when it made sense to do so.

Anchors are not only for towers. Sometimes we use trees or a convenient rock to hold the ends of wire antennas. A house bracket can anchor a light duty tower. The demands on anchors for non-tower use are often less strenuous. 

I ran into a case recently that I believe is worth an article. It wasn't for a tower but for my overhead run of cables from the hay field trenches to the switching system and cable connections at the base of the Trylon tower near the house. 

Looking back in the blog I find that I never really discussed that feature of my station. That's a gap I'd like to fill. However, right now I'll leave that and focus on the ground anchor that supports the overhead messenger cable and the multitude of coax and control cables that hang from it.

The weight of all those cables is substantial. There is also the tension to reduce sag and allow people to walk safely underneath. The existing messenger cable from the tower to a 10' aluminum post at the edge of the hay field is ~60' of ⅛" aircraft cable. Two intermediate posts take up much of the sag without requiring the higher tension required for an uninterrupted span. Tension on the messenger cable is perhaps a few hundred pounds,. That's less demanding than for a tower but should not be treated lightly.

A leg of the Trylon tower base section anchors the cable at one. At the other end is a screw anchor and diagonal guy to the top of the post. A wide wood base and steel peg pounded into the ground keeps the post from leaning sideways. It's ugly but it works. Or at least it has worked until now. 

An accidental collision with the mower several weeks ago exposed a weakness with the screw anchor. Despite no damage from the mower, the screw anchor was dislodged a few inches. Over the past several years I've occasionally had to screw the screw anchor back in a few inches as it pulled from the soil and the turnbuckle bottomed out. It has become too unreliable to ignore. 

The post that supports the messenger cable (a salvaged 10' long Hy-Gain boom section) is creeping and the cables are creeping down the sagging messenger cable. The bend in the post was there all along, due to high impact with the ground when the tower it was on failed. That was over 30 years ago and this is the first use I've found for it!

Although not in immediate danger of collapse, the time has come to rebuild. I had intended the overhead run to be a temporary measure until I could trench the cables to the Trylon. Due to the constant addition of cables and the horizontal roots sprouting from the surrounding birch trees the overhead run will have to be permanent. So I need to do it right. I stabilized the post with a long rope back to the tower and proceeded to design and install a better ground anchor.

Let's review basic facts about ground anchors. The mass of the anchor does not support the tension load. The load is supported by the resistance of the soil on the anchor. The larger the surface area of the anchor and the deeper it is placed the greater the mass of soil that must be displaced by the load. Undisturbed soil can withstand a greater load than top soil. 

The anchors for my large guyed tower are 2' × 2' × 8' blocks of reinforced concrete sitting 6' below grade. That provides ample resistance for the 4000 lb static load and 3× that amount of dynamic load. The attributes are responsible for the "dead man" moniker. Concrete forms for the anchors are often called coffins. Aside from this little joke I'll stick with calling them ground anchors.

The concrete is reinforced with steel rebar so that the anchor behaves as a rigid block. There is tension across the forward and upper faces as the rod is pulled towards the load (red arrows). Tension is greatest at the rod and diminishes as you move outward. Without reinforcing the anchor will eventually crack from the tension. Reinforcing rods across the front and top faces (large black dots on the left) give the anchor strength in tension. Unlike a large tower anchor, there is little shear force in a small anchor so that there is little benefit from constructing a rebar "cage".

The requirements for the anchor to support the overhead cable run are not severe. The tension is unlikely to ever exceed 500 lb. The wind load is not a large factor since the mass of cables are low to the ground and sheltered by trees. A small and relatively shallow reinforced concrete anchor is sufficient, and it will be superior to the screw anchor it replaces.

The main problems with the screw anchor are that screwing it in disturbs the undisturbed soil and it may not penetrate deep enough for the relatively small area of the anchor plate. Proper installation requires a tractor with an auger driving attachment. This is the preferred method since it disturbs the soil far less than doing it manually and you can power through dense subsoil and stones. That is, if the anchor has the strength to withstand the forces involved.

The 2' screw anchor that held the overhead run also shows how the auger end (which doubles as the anchor surface) is bent from screwing past rocks in the soil. The steel grade of these farm oriented anchors is not great. It is cheaper to use more steel than to make the steel higher strength.

The screw anchors I manually installed for the 80 meter yagi's small tower are holding well but they are longer and penetrate deeper than the one above. They were very difficult to install and the newly disturbed soil had to "rest" over the winter and spring thaw before they were safely set in the soil. They, too, will be replaced when I rebuild the 80 meter yagi tower. That job is scheduled for 2023.

The new overhead run will have a stronger messenger cable and the post will be a 10' steel pipe. The pipe is visible above, alongside the aluminum post it will be replacing. The top of the post is guyed to the new anchor. 

The anchor is placed a few feet behind where the screw anchor was located so that there is more headroom underneath. People are less likely to walk into it. The excavation fit neatly between the two trenches to the 150' tower. The roots of the nearby spruce trees go down rather than spread horizontally and were not in the way.

The rod is 4.5' of ⅛" × 1.5" bare angle steel. I have 3 identical lengths set aside for the future work on the 80 meter yagi. I cleaned the loose rust, drilled a hole for the guy attachment and coated the steel with rust paint. The few inches to be encased in concrete are not painted since many coatings don't bond well to concrete and even rusty steel will bond. 

The purpose of the paint is to slow, not prevent, corrosion due to soil, water and moisture. The rod can be uncovered occasionally to check for corrosion. The anchor is not affected by removing soil from around the rod.

The hole was dug with a small shovel and garden trowel to keep it from becoming too large and consuming more concrete than needed. A concrete brick serves as a chair for the rod. Hole depth is 30" (75 cm), which is well into undisturbed soil. If you live in a cold climate you should place the anchor below the frost line to prevent shifting due to frost heave.

The rod is aimed at the tower. You can see a large concrete block being used as an emergency repair to the existing screw anchor. The angle of the rod is low because it is further back of the steel post than the post's height. That lowers the rod's angle so that the rod must be longer than usual to rise above grade. A string pulled tight from the top of the steel post is used to align the rod's direction and vertical angle. This ensures that the rod does not twist or bend under tension.

Mixing concrete is a lot like baking. Make a well in the centre of the dry ingredients, add just the right amount of water and mix it until you have a consistent viscous goo. I used two 30 kg bags of pre-mixed concrete (cement, sand and stone). I've used electric or tractor driven mixers for large jobs where concrete delivery was impractical or expensive. For a small job like this a wheelbarrow and shovel will do the job. The total volume of concrete was slightly more than 1³ ft, which is enough to make an anchor 8" × 8" × 27". There is enough room to encase the rebar by the recommended 3" (7.5 cm) of concrete.

At left is the coated rod sitting on the chair. It is loosely pierced by a few inches of rebar to fix the rod in the anchor. Digging a narrow and deep hole is sloppy work. I used stones to fill unwanted cavities to preserve the anchor's shape and avoid the need for a third bag of concrete.

In the middle panel the anchor is half done. I have just laid an 18" length of 10M rebar to reinforce the upper face of the anchor. Its twin is more forward and buried below the rod to reinforce the front face of the anchor. See the diagram further above. At right all the concrete is in place. A small trowel was used to smooth the surface. Although a buried anchor doesn't need to be pretty it is good practice to eliminate gaps and cavities where water can collect and freeze. Over the years the concrete will crack from repeated freeze and thaw cycles.

Before walking away I rechecked rod alignment. After several days the hole was ready to be back filled.

Notice how the 1ft³ of soil displaced by the concrete has miraculously vanished! Of course it's all there but has been used to level the uneven ground at the edge of the hay field. Small stones were buried in the excavation so they didn't need to be carted away. The sledgehammer was used to compact the loose soil as it was shovelled into the excavation. No further subsidence is expected. There was a lot of displaced soil and rock from the excavations of the big towers. The backhoe moved it out of the hay field.

I will wait at least another week before putting tension of the anchor. Concrete is typically rated by its strength after 28 days of curing. In this case that's 3000 lb, and of course strengthened by the rebar. I am happy to start work on a tower after only a few days since the concrete is strong enough in compression to build upward. It is better to wait for the concrete to cure more completely when it is to be placed in tension.

More on the winter prep work keeping me busy is still to come. But now my focus is shifting to the CQ WW SSB contest this weekend.