Following the installation of our micropiles, we turned our focus to getting the remainder of our foundations prepped for concrete. This included the steel concrete reinforcement, our steel subsurface structures that our towers would eventually anchor to, and the creation of the concrete forms themselves. The first thing that needed to get done before the major rebar work could be performed was the placement of the subsurface steel. And that meant we had to assemble it!
Each side’s subsurface assembly would consist of two (2) bases, 44 threaded rods, plus some significant hardware. The placement of the threaded rods was critical (more on this later), so the top of each assembly would also have a template to help ensure that everything was held perfectly in place while the concrete cured.
The steel rods are 2” in diameter and weigh about 50 pounds each. The bottom of each rod needed two nuts, one inside the frame and one outside. These nuts were each about 3 1/2” across, weighed 3 lbs. each, and needed to be torqued to 2,500 lbs. Naturally we just did that by hand…okay, by hand with a super cool specialized hydraulic torque wrench!
After the rods were secured to the frame and torqued down, we did a little Quality Control (QC). Each rod and hardware set was marked at the location it was supposed to stay. We still needed to torque down the top nuts, do a little transport, and then place the assembly into the hole. Even though odds were slim that anything would move, it’s better to know for sure that everything is secured and in place than to assume, right? Right.
The next step was to place the templates at the top of the rods. These templates aligned the rods to the positions they would need to be in when the big towers at each end needed to be placed down on them (a few steps later). If even one rod out of the total 44 rods per assembly was out of the tolerance range, the entire tower would not fit into place. No pressure!
MEANWHILE, AT THE ABUTMENTS…
…the concrete forms were being put into place by our General Contractor Mike Walston of MCW Construction. And these things were beefy! Made from 2x10 lumber and heavily reinforced, these forms would each eventually hold hundreds of thousands of pounds of concrete. We’ll save the exact number for later.
But you didn’t think we would move forward without our QC, did you? Before anything went into the holes after the forms were set, we brought out our surveyor to triple check that the placement of the forms themselves was correct, and that the alignment of the East and West foundations - about 700’ feet apart on two separate hillsides of Crockett Mountain, mind you - was within acceptable design tolerance. Once again we were pleased with the results. After weeks of drilling, digging, tamping, and carpentry on two separate sites on the side of a mountain, it was estimated that the difference in elevation of the two forms was…a little less than one half inch. Boom.
AND NOW, BACK TO THE LAYDOWN AREA!
Once the templates were placed onto the tops of the assemblies, it was time to load them onto the trailer for transport from the laydown area over to the abutments. There, the two main parts of each tower’s subsurface assembly had angle iron placed across the top and secured onto the top of the templates. This had two functions. First, it held the subsurface at the correct position so that the rods would align with the holes in the tower baseplates. Second, the angle supported the subsurface assembly from above so that it would be installed at the appropriate depth within the foundation’s concrete.
Once the angle was on and secured, everything was checked and double checked, and everyone was ready…we lifted the subsurface into place.
So, when all was said and done, each abutment had one of these monster subsurface assemblies placed inside the forms. Each assembly weighs in at about 4,500 lbs. WITHOUT adding in the top template, top angle that tied everything together, and the associated template hardware - all that would come off later for the placement of the towers. In addition to the two main foundations, we also had four small assemblies and forms to complete, one per wind guy foundation. And those went smoothly, too.
SPECIAL THANK YOU: While the project was far from complete at this stage, we just can’t move on to the next part of the story without recognizing our GC, Mr. Mike Walston. Mike’s workmanship, attention to detail, and insistence on constant precision is second to none. This project just wouldn’t have been the same without Mike and his crew, and it was an absolute pleasure to partner with them. Thanks, guys!
NEXT UP: Rebar, Sensors, & Concrete Pours
When you’re anchoring a bridge that weighs over 1 million pounds to the side of a mountain, you’re probably going to spend some time thinking about foundations. The very first element that went into the ground for the Gatlinburg SkyBridge was our micropiles, which literally bond the bridge to the earth. ERi chose one of the best geotechnical engineering firms in the world to work with us on this project, and are very happy that Hayward Baker, Inc. (HBI) agreed to join the project team for our micropile design and installation.
According to HBI: “Micropiles are a deep foundation element constructed using high-strength, small-diameter steel casing and/or threaded bar. Micropiles are also known as minipiles, pin piles, needle piles, and root piles. The micropile casing generally has a diameter in the range of 3 to 10 inches. Typically, the casing is advanced to the design depth using a drilling technique. Reinforcing steel, typically an all-thread bar, is inserted into the micropile casing. High-strength cement grout is then pumped into the casing. The casing may extend to the full depth or end above the bond zone with the reinforcing bar extending to the full depth. The finished micropile resists compressive, uplift/tension, and lateral loads and is typically load tested in accordance with ASTM D 1143 (compressive), ASTM D 3689 (uplift/tension), and ASTM D 3966 (lateral).” Hayward Baker tells us a little more about micropiles at this page on their website if you’re curious.
The Hayward Baker team wasted no time once they had their machines onsite. The first order of business was to install a sacrificial micropile. This micropile’s entire purpose consisted of being tested well beyond the capacity that all of our other micropiles would need to perform.
Once the sacrificial micropile was installed, Hayward Baker brought in a massive steel beam and hydraulic cylinder for the testing. They welded plates onto the sides of the micropile casing, and got their equipment set up to measure even the tiniest bits of deflection that might have occurred during the test. The test was performed according to the ASTM D3689 “Quick Load” method, and tested the micropile to 200% of its design load capacity of 320 kips.
The test pile had a total length of 79 feet. 54 feet of that was free length, and 25 feet was bond length. When the maximum load that the micropile was designed to hold was applied, the micropile deflected only 0.387 inches! When HBI increased the test load to 200% of what the micropile was designed to hold (640 kips), the micropile still deflected less than an inch! The test pile performance was within the acceptable tolerances of HBI’s pile design.
SCIENCE SIDEBAR: “What in the world is a kip” you say? A “kip” is a unit of force that is equal to 1000 pounds-force. The word comes from the shortening of the term “kilo-pound”. So when we say that this micropile was designed to withstand 320 kips in tension, we mean this one little pile is designed to hold 320,000 pounds pulling on it with no problem. And it was ultimately tested with 640,000 pounds (640 kips) and passed!
With everyone very satisfied with the results of testing the micropile design, it was time to get to work installing the real micropiles for the Gatlinburg SkyBridge foundations.
Each end of the SkyBridge would ultimately have ten (10) micropiles installed. Eight (8) micropiles would be in the main bridge foundations, and two (2) micropiles would be installed for the wind guy anchors - one per anchor. Here’s a video to show a little bit of the process for those of you who like to see how things work.
During installation, further testing procedures happened with the grout that went into the casings to make sure that the mix cured as strong as it was designed to. Samples were taken by our Special Inspector throughout the entire pour, and were tested at three (3), seven (7), and 28 days. The grout had a specified design strength of 5,000 PSI minimum. Not only did all samples meet those requirements, most samples tested out at 7,000+ PSI, and again everyone was pleased.
Once the micropiles were drilled and grouted, we excavated down to allow for room for the concrete forms. Then the micropiles needed to get cut down to their final length and have caps installed.
After everything was cut down to the proper elevation and everything was prepped, it was time to get ready for the concrete forms to be placed. But one last check! We had our team take measurements to make sure that everything was installed within tolerance before moving on. Check and double check!
Here are some of our micropile stats for you:
8 total micropiles
Shortest micropile: 80’
Longest micropile 94’
8 total micropiles
Shortest micropile: 64’
Longest micropile 75’
4 total micropiles (1 per anchor)
Shortest micropile: 45’
Longest micropile: 55’
Total feet of micropiles in all foundations: 1,480’ (500’ of which is embedded in rock)
NEXT UP: Concrete Forms & Subsurface Prep