Hayward Baker

Gatlinburg SkyBridge Behind the Scenes: MICROPILES

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.

 
Some steel micropile casings in the lay down area in Gatlinburg, waiting to be put to use.

Some steel micropile casings in the lay down area in Gatlinburg, waiting to be put to use.

 

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.

Drill rig ready to install micropiles for the Gatlinburg SkyBridge.

Drill rig ready to install micropiles for the Gatlinburg SkyBridge.

Our micropiles for the SkyBridge use both steel casings and threaded bar. Specifically, we used over 900’ of steel casing, and over 1000’ of bar. The micropile in this picture doesn’t have any grout in it yet.

Our micropiles for the SkyBridge use both steel casings and threaded bar. Specifically, we used over 900’ of steel casing, and over 1000’ of bar. The micropile in this picture doesn’t have any grout in it yet.

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.

Our Special Inspector was onsite for the testing - and it was a beautiful day to be up on Crockett Mountain!

Our Special Inspector was onsite for the testing - and it was a beautiful day to be up on Crockett Mountain!

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.

On the East Abutment of the SkyBridge, getting ready to drill compression piles.

On the East Abutment of the SkyBridge, getting ready to drill compression piles.

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.

 
Our Special Inspector collecting samples of micropile grout for testing.

Our Special Inspector collecting samples of micropile grout for testing.

 

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.

 
East Abutment micropiles getting the finishing touches.

East Abutment micropiles getting the finishing touches.

 

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!

 
This hole would eventually be completely filled with steel and concrete. You can see the concrete forms waiting to be placed at the top of the photo.

This hole would eventually be completely filled with steel and concrete. You can see the concrete forms waiting to be placed at the top of the photo.

 

Here are some of our micropile stats for you:

East Tower: 

  • 8 total micropiles

  • Shortest micropile: 80’

  • Longest micropile 94’ 

West Tower: 

  • 8 total micropiles

  • Shortest micropile: 64’

  • Longest micropile 75’ 

Wind Guys:

  • 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