Tower foundations made easy
Tower erection in marshland environments can be performed quicker, more efficiently and less expensively with modified construction techniques and alternative methods for anchoring the site.
Leger is a field service technician and Calais is general manager for Two-Way Communications, Lafayette, LA.
Sometimes the marriage of two apparently different technologies leads to thoughts such as “I wonder why no one thought of this sooner.”
This was an answer to a question of how to put up a tower in a marshland in South Louisiana. Conventional anchor methods could not be used because of soil conditions, cost and wetland preservation guidelines set by the U.S. Corps of Engineers.
The Corps would not allow heavy equipment off the allowed roadway. Trees could be felled by hand and allowed to decompose naturally. Any soil removal prior to pumping concrete anchors would have to be done by light equipment using board roads. The excess dirt would have to be hauled away, and the top three feet of soil would have to be layered back in the exact order of removal. We had to come up with a way to anchor the tower without disturbing the natural habitat and without getting a 30-ton cement truck to where the anchors had to be set. A major road had to be built just to get the cement trucks to where the tower and building foundation had to be poured.
Solving these problems was easy. We have years of experience in site preparation. Being well-versed in many different fields, we realized that “marsh anchors” have been used for many years by the power delivery industry and would be a viable alternative.
This technology uses 11/2″-square shanked rods with screw helices at three- to four-foot intervals on the initial 10 -20 feet. These rods are then screwed into the ground with three-, five-, seven-, or 10-foot extensions being added until the proper depth and torque are reached. The torque and depth are determined by the soil and by the pulling strength required. Each anchor is then topped with an eye to attach one guy wire. Using screw anchors requires only the availability of an auger machine to screw the anchors into the ground. No digging of holes, forming, and pouring concrete for the guy anchors is required. This technology is cost-effective for both labor and materials. Foundation and guy anchors can be completed with a crew of five in less than a week.
Boning up on theory and techniques was also easy. Finding an engineer to stamp the required drawings was an entirely different matter. After getting over the hump of convincing engineers that a “dumb Cajun” with a heavy accent might actually know what he was talking about, we convinced them that one anchor per guy wire would work. The problem that the engineers had was two-fold. First, they were accustomed to building towers with inner and outer guy points. Second, most of their computer design programs will not allow for individual guy wire loading and individual guy anchor points. Individual guy wire loading is necessary to design the anchor for a 2:1 safety factor (generally in the range of 50,000 pounds of tension per guy). Because tension-holding capability is directly related to installation torque, the anchors are simply screwed into the ground until a layer of earth is encountered that is resistant enough to achieve the required installation torque. Once installed, an anchor can be pull-tested to verify holding power under tension. Usually, an on-site demonstration of pull testing a single anchor will convince any skeptic. It is impressive to see a hydraulic jack exerting 50,000-60,000 pounds of pull on an anchor with no sign of anchor creep even after a half-hour of sustained pressure.
A quick word about soil bores. We don’t believe in soil bores, but they do have their place in initial design. Most soil bores are 50 to 60 feet deep. We have had to screw anchors in as far as 268 feet into the ground! The depth could have been shortened considerably by using multiple anchors with load-distributing linkages. Such linkages have been designed for a retrofit, but we wanted to prove the single-anchor-per-guy concept. Another unique advantage of screw anchors is the ease of adding extensions or additional anchors at a later date, should guy wire capacity need to be increased for additional load requirements or for the addition of torque arms. Experimentation with elevated anchors is currently in progress.
Once everyone agreed about the concept, installation was attempted. The first anchor crew came in with a truck- mounted unit, looked around and left. They returned later with a huge, high-flotation, rubber-tired rig. They bogged it down, packed up and left. The second crew came in, laid planks down and completed the job in 20 working days. We realized that with the proper equipment and trained personnel the completion time could be greatly reduced.
The communications industry has traditionally used concrete for tower anchors. This technology requires a concrete block measuring from as small as 4′ 3 6′ 3 2′ to as large as 6′ 3 12′ 3 30′ against undisturbed earth to hold the guys. Using this method for anchors involves digging, forming and pouring the concrete, then backfilling and tamping in nine-inch lifts. The equipment required for this method includes backhoes, forming equipment and heavy concrete trucks. This method is labor- and equipment-intensive, not to mention time-consuming. The average time to complete the foundation and guy anchors is three to five weeks. Digging, forming, pouring and curing concrete is very time-consuming.
In most parts of the country, concrete and its delivery are no problem. South of I-10 in Louisiana, and all along the Gulf Coast, it is another story. Here, trench shoring, layering the sides of concrete foundation holes, slip forming and slurry pours in a drilled foundation are the norm. Using these techniques presents numerous problems, not the least of which is building an adequate base for a road bed to get equipment where it needs to go. The cost of the road can easily exceed the cost of the foundation. Such a road is only needed during construction of the tower. Normal pickup truck and van traffic for routine maintenance at a tower site doesn’t require the type of road needed to get a 15- to 30-ton rig onto a virgin site.
Would you like to be able to go onto a freshly bulldozed site right after a rain, install all guy anchors and tower foundation, and be ready to stack a tower in less than a week? Down at the mouth of the Mississippi, at Pointe a la Hache and Venice, two such scenarios were completed last year in 12 days. Both towers were stacked within three weeks of the initial clearing of the sites. Both jobs employed screw anchor technology. The use of these anchors as pilings with a pile cap containing the anchor head has been done before, but we have done away with the pile cap by using individual anchors for each guy. The only concrete needed is for the tower base foundation. Here, foundation anchors made of 31/2″ pipe with the same 8″ to 14″ helices are used instead of the solid rod anchor system. This provides better lateral stability and lateral shear strength. The resulting pile cap could be as little as eight or 10 yards of concrete. That small amount of concrete is not a difficult haul for a front-end loader. If you have a dump truck on tracks, as we acquired, this is “no hill for a stepper.”
The estimated cost of one of the aforementioned jobs using conventional methods was nearly $250,000. That covered only the cost of the road, the anchors and the foundation for the tower. Quite a large pill to swallow, but some feel it’s the cost of doing business in South Louisiana. The cost of the completed foundations using screw anchors was much easier to take. The Pointe a la Hache job, including pilings to set a 12′ 3 28′ building six feet above ground level and all guy anchors, was $90,000-quite a savings compared to $250,000. The Venice job, including pilings to set a 12′ 3 28′ building 10 feet above ground and all guy anchors, was $70,000. Again, a savings of $180,000. You can imagine how the tower company felt about the gentleman who saved them that much money?
In the final analysis, screw anchors can be a useful tool for the communications industry. The ability to move onto a cleared site and start stacking tower within a week can be the difference between having customers on your tower and having them go somewhere else. The cost savings and efficiency make the screw anchor method attractive for marshland and wetland projects. As in most things, this technology is not the answer to all tower foundations. There are break-even points where it is just as easy and cost-effective to use either method.
However, in certain areas where logistics and government regulations can hamper tower construction and cause lost revenues, the screw anchor method can be invaluable.