Yet we went ahead. It didn't work out well. In this article I'll let you in to the full story, warts and all. Aside from the many problems encountered, and overcome, much went right. Regardless it is a story worth telling. I haven't seen many articles on the internet that described the full procedure of planting a large guyed tower in amateur stations. Just small snapshots.
Yet it's a major project that can provide many lessons to those who want to do the same. I hope you enjoy it, and perhaps learn a few things. That includes the things you should not do.
A local tower construction and service company, Ontower. They are happy to offer their services and expertise to local hams who have turned to them for assistance. This has become common as the ham population ages and fewer are able to help out their friends as they have in the past. In any case it is only a minority of hams with big tower experience. I have put up high guyed towers but I do not consider myself an expert.
I got to know them over several months. They were free with their advice and sensitive to the needs of non-commercial projects. It also helped that they have the equipment and parts on hand to put up towers far bigger than my own. So we came to an understanding.
Act 1 - Clearing a path
The day before the big day the weather unexpectedly turned. The ground was covered in snow. They were unable to drive their truck and backhoe float across the hay fields to reach the tower site. The backhoe operator used its small blade to plow a 250 meter path to the tower base, and then to each of the anchors. Wherever there was a turn a clearing was made to allow for the large turning radius of the truck and trailer, and also for the arrival concrete truck.
|Tower work in Canada, eh?
By the time all was done the fields were crisscrossed with paths made by the backhoe and by people walking from point to point. So far so good. It wasn't to remain that way. The day warmed and the snow began to melt.
I surveyed and staked the field in advance. The crew of two brought a small backhoe. I could have hired a large backhoe but there is an advantage when the equipment operator and crew don't just have experience digging holes but digging holes for towers. Their backhoe had just enough reach to excavate the 6' depth of the anchor holes.
Other than exchanging a few ideas at key points they knew exactly what to look out for and what to do. There were no mysteries to be explained. Our only serious concern was whether we'd hit bedrock since their machine could not cope with that. My luck held and other than some large rocks the excavation for the base and three anchors proceeding without drama.
We did have contingencies in mind in the case we did hit bedrock. For the base it is possible to anchor the pier to the bedrock, at some trouble and expense. Or if the level was sufficiently below the frost line to modify the base to suit. For the anchors they and another expert I consulted suggested sitting the anchor on the bedrock and adding overburden on top and ahead of the anchor (building a mound) to compensate. An anchor's tension limit is not set by its depth but by the mass and solidity of the soil it is pushing up against when the guys are under load.
The excavations were greater than the specification for this size of LR20 tower since the anchors they supplied and the squares and hoops for the rebar cages were for a 350' tower. This requires more concrete, which is an acceptable trade off.
Rebar cages and concrete forms
While one person operated the backhoe the other built the rebar cages. I did the cutting and bending, leaning on my experience and tools from building the Trylon base.
They supplied the square wooden case for the base and round tube for the pillar. For the anchors we dispensed with cases; what they call "mud holes". The downside of the latter is that the holes are inevitably larger than required and require more concrete to fill. It's a trade off: more concrete or more time and expense to build and work with casing. It doesn't have to look pretty.
|Left: anchor and rebar in mud hole; Centre: anchor cage almost ready to go; Right: forms and rebar for base
Notice that the anchor has two large buttressed vertical plates. When under tension the load is distributed over the reinforced concrete, preserving its rigidity. The reinforced base platform distributes the load over a wide area of undisturbed soil. Numerous J-rods bound with circular ties run the height of the pillar to preserve pier integrity under vertical load.
But are these reinforced concrete foundations up to spec? There is no clear answer to this question. Tower manufacturers have become increasingly coy on this point. No one prescription or set of options is sufficient for all soil conditions. And that's what counts. Commercial towers require soil tests and the services of an engineer to specify the foundations for the prevailing soil and environmental conditions, including seismic activity and ground water.
What I have done instead is to oversize and overbuild the base and anchor foundations. I also know that the soil in this area is very stable and the water table is deep. There is an extended period of saturation in heavy rains and spring thaw. The top soil is less than 2' (60 cm) deep, below which is a dense mix of clays and rock. It is so hard that shovels barely dent it, even when soaked in water.
Aligning the anchors
Overnight most of the snow melted, turning the fields into a muddy mess. Half a day was spent finishing the forms, setting the anchors, levelling everything and aligning the anchors to the base. The alignment was done by sight to ensure the anchor rods pointed directly at the base. An inclinometer was used to set the vertical angle of the rods according to the tower spec.
They recommended against using the anchor plate as an equalizer plate since it adds a potential point of failure. Instead the plates were welded square to the rods. With the vertical angle correctly set the pre-load tension on the guys will net to zero vertical force on the anchor rod.
Of course with maximum antenna wind load at the top of the tower there will be an upward force when the wind blows. However that is true whether or not the plate pivots. As they explained, with a round bar for the anchor this is well within the load spec for the material. Alternatives that some use, such as angle iron for the rods, overbuilding is required to maintain anchor integrity.
Concrete - Uh oh!
Early afternoon of the second day we were ready for concrete delivery. Or so we thought. The plan was to have the concrete trucks -- we needed two in sequence -- drive across the hay field and pour directly into each hole in turn. Two time-separated pours were planned for the base, a point which I'll return to later in the article.
Unfortunately by the time the first truck arrived the surface was softened by melting snow and stayed wet due to the heavy overcast. The driver was dubious but trusted the crew since the companies regularly worked together on tower sites. But the final decision was mine. Hoping for the best I gave the go ahead signal. That was a mistake. As I've said before, hope is a 4-letter word that it is best to avoid.
The truck didn't get far. No matter how many tires the load is spread over or how many axles are driven (they have this option) 60,000 lb needs a strong surface for support. I found myself in a difficult spot since I was responsible for getting the truck out of there.
Happily I was dealing with professionals. Ontower got a local construction firm to send out a front end loader. It had two tasks: quickly unload the concrete and pull the unloaded truck out of the mire. While we waited for it the concrete truck driver told me dark stories about the cost of freeing a stuck concrete truck or a ruined truck due to the concrete setting. I reciprocated with how stories of how technology would enable driverless trucks and put him out of a job.
A little laughter goes a long way when you are in a tight spot waiting for rescue to arrive. Every so often he'd interrupt the conversation to add water to the concrete to keep it from setting.
Eventually the front end loader arrived. I have to say I was very impressed at how well it all went from that point. That scoop you see holds a lot of concrete and had no problem going over 200 meters of muck to dump the concrete into the excavations. It took fewer than 20 trips to transfer over 7 cubic yards of concrete. An hour later the concrete truck was empty and towed back to the road. Two anchors were complete. But it was late in the day and we had already cancelled the second truck. The remainder of the concrete work would have to be rescheduled.
There was the unanticipated effect on the anchors of pouring tons of concrete from a height. Front end loaders are not gentle machines. The anchors shifted slightly and were no longer directly pointed at the base. The error was approximately 2°. None of us caught the mistake until it was too late. We discussed it the next day and decided that it would not be a serious problem. The anchor was oversize and round and the deflection would be modest when the guys were tensioned. I still worried about it, but nothing could be done. At least not yet.
Intermission - Winter doldrums & spring floods
We tried to reschedule the concrete work several times, included the depths of winter. The weather did not cooperate. Not only was this job delayed so was the concrete work for my garage. The early end to the season inconvenienced many in the construction trade.
Concrete work can be done in frigid weather if you are willing to pay up. After several weeks we decided it would be best to put the work off until the spring. Although a sensible decision there were a couple of factors not taken into account. First, the excavations and forms were stable when the ground was frozen and ice filled the holes. Spring thaw changed that. Several cubic yards of topsoil slumped into the holes, covering the forms and rebar and allowing the water level to rise higher. Second, we had a record breaking wet spring.
So the delay continued. It continued so long that the work had to be fit into their summer high season. I did what I could by periodically pumping the water and removing some of the slumped material. Saturated topsoil is a gummy soup that is pretty much impossible to shovel. There was also the danger of additional slumping while working below ground. I had to give it up and wait. Besides which I was getting tired of removing ticks that leapt onto me with glee from the head high hay. Protective clothing was little help.
Act 2 - Getting it done
Finally the great day arrived. A new crew arrived and so did a large backhoe with front end loader the same size as the one we used in the fall. Although this machine was a significant expense it meant we could clean the holes, deliver the concrete and back fill all four holes in one day. This time everything went according to plan. It was also an opportunity to renew my respect for heavy machine operators. He demonstrated a care for the work and fields and an artistry in how he went about the job.
The first job was to clean the holes. This involved carefully scraping off the overburden to expose enough of the forms and rebar that they could be lifted without breakage. Mission accomplished. But it was tense and dirty work. The bucket you see is ~3' (1 meter) wide and required gentle manoeuvering. Those machines pack a big punch.
I took on the job of cleaning the rebar so that the crew could do more important work. I ended the day covered in muck, as was the crew. Something died inside the base hole rebar cage which added to the unpleasantness. As if that wasn't enough didn't I already mention the ticks? What I go through to boost my contest scores!
While marking the lines to the two misaligned anchors the senior crew member noticed that if the base was moved 2' (60 cm) he could get all three anchors pointed at the base. That's why the base excavation seen above was extended. Many problems can be solved when you have the right tool at hand.
The backhoe bucket is so large that the anchor hole had to be widened even more than before. A form was prepared in advance with scrap lumber they had on hand. This saved some money on concrete, but require a lot of shovelling to back fill around the form. Without it the form would burst or float on top of the concrete.
The base is done differently. First the forms were rebuilt. Concrete blocks are used as rebar chairs. They must be concrete to ensure a good seal that will repel water infiltration. We used 20" tube for the pillar which is the minimum to sufficient concrete around the rebar. Previously we used 24". It is higher than required so the vertical rebar isn't visible. It'll be dealt with later.
The second frame shows concrete delivery using the "small" scoop. Once the concrete for the platform is poured it is enclosed with plywood. The hole is then back filled halfway and the concrete for the pillar is poured with a shovel. Only a few feet of unsupported tube is exposed so that the concrete does not burst the form.
Afterwards the hole was fully back filled, then compressed and levelled. Extra soil for all excavations (displaced by the concrete) was dumped off the edge of the hay field. Under the crew's direction the areas near the anchor plates were untouched. This will be filled by shovel to avoid damage by the backhoe.
The vertical rebar in the pillar is not centred while the concrete is poured. That's difficult to do. Instead the rebar is manually centred when the pillar is almost full. The concrete holds the rebar in position as it sets. The same procedure was use for my garage pad. That crew lifted the mesh as the concrete was poured to, again, avoid the problems of using chairs.
The final act was to push in the tower 1" round pier pin into the centre of the pillar and level it. It is a myth that concrete is self levelling. That only works when the concrete has a lot of added water, which slows curing. One of crew dribbled some water over it and used a trowel and a small level.
Three days later I drove out on the tractor, mowing tick-bearing hay along the way. I cut the form from the pillar and inspected the result. It looks good! You can see the top 4' of the 10' ground rod that the backhoe operator helpfully pushed in with the bucket until it hit an obstacle. Later I'll try to pound it in further with a sledgehammer.
Looking back, or, to err is human
Many things went right and many things went wrong. My first mistake was being overly ambitious about getting this tower in before winter. It cost me some money and a bit of grief but did not in the end cost me any time. If I'd moved more cautiously it would not have been done sooner.
Another thing I'd do is start with the big backhoe rather than try it with the less expensive small one. I was tentative in the beginning since I wanted to reduce the expense if a smaller machine could do the job and also limit the loss if we hit bedrock and had to resort to an excavator with a rock breaker. Should there ever be a second big tower on this property I now know what to do.
Relying entirely on the judgment of professionals is not always recommended. An important lesson I've learned is that although they are good at their jobs and give good advice they are perhaps too sensitive to the customer's costs and wishes. That is, they have learned in their careers that they must work quickly and get approval for anything that entails extra money or time. Risk doesn't always get the attention it deserves.
I had a few "Gimli Glider" moments with the concrete. Although Canada is a metric country there is a lot of building material that is measured the old way. Thus I am used to ordering concrete in cubic yards. But the concrete company works in cubic meters. They will do the conversion but you must be careful that they in fact do so. For the final stage I needed 4 yards, and I added a half yard margin. The order was relayed through the crew to the operations manager to the concrete company. They assumed and delivered 4.5 meters. The unused concrete was delivered elsewhere. We're working on getting a refund.
The ground in this region becomes saturated quickly in late fall and doesn't percolate downward until late spring. For all the pumping I did to get the work going again the holes completely dried on their own by the time the work recommenced. Fighting with water is a losing game. There are more powerful pumps around than the sump pump I used, but they're expensive even if only rented for a day. I spent an hour or more yesterday disassembling and cleaning the sump pump so that it will again be ready for emergency backup use in the house.
Looking forward - plans to raise the tower
I have not yet finalized how I will raise the tower. I could call in the professionals, which would be quick if more costly (and less fun) than calling in friends. Going the professional route would also require having everything prepared in advance, including all the guys, hardware, tower section repairs, prop pitch motor assembly and more.
Alternatively I can call in a small crane to lift the first 40' so that the bottom guys can be attached. I would then continue upward by gin pole. The method that involves the least equipment is to drop the base section on the pin and temporarily guy it until the tower is high enough that the first permanent set of guys can be installed.
I have decisions to make. Since the concrete will cure faster than I make my choices I can proceed whenever I'm ready. It would be nice to wait until the ticks are gone or the hay is harvested.