Written by Structural Anchor Supply on September 1, 2022
Challenge Accepted: A pedestrian bridge abutment that could handle the load of a 20,000 lb. vehicle. The project specifications required the abutment to be built 24 feet above existing railroad tracks behind a precast large modular block wall; with utility lines overhead. Was our team discouraged by these conditions? No. With CHANCE helical piles, we could take matters into our own hands, utilizing an abundance of resources to get the job done.
Here are some of the CHANCE-exclusive resources that we used to bring this challenging project to its successful conclusion:
Soil borings on this jobsite showed soft fill in the top 12 feet and cobble layers that were highly variable in density. Armed with this data, the engineering team input the site soil data into HeliCAP v3.0 Helical Capacity Design Software.
HeliCAP is a free resource for anyone designing foundations using CHANCE helical piles or anchors. The software performs powerful calculations using site soil parameters to aid engineers designing foundations, tiebacks, soil nails, and heavy guyed loads.
Using HeliCAP, the engineers determined the depth and torque required for pile installation, estimating an installation depth of 40 feet to develop an axial compression capacity of 137 kip.
Because this project had to consider compression loads mentioned earlier, plus lateral and wind loads that bridges encounter, the engineers chose CHANCE Combo Piles, which consist of a square shaft lead section followed by pipe shaft extensions which are connected with a cast transition. Square shaft lead sections were the ideal choice to muscle through the cobbles and boulders without the need for predrilling. The pipe shaft extensions provide shaft stability in the soft fill soils. The low displacement shaft and higher helix-to-shaft diameter ratio increases the torque efficiency resulting in comparable capacity to a larger diameter round shaft pile but is easier to install when obstructions are present.
In this case, the square shaft lead section was installed with a 6-inch diameter grout column around the shaft for additional capacity, both lateral and in compression. A CHANCE helical pile installed with a grout column is known as a Helical Pulldown Micropile.
After the lead section was installed, the cast transition was added before the pipe shaft extensions were installed. These extensions make the foundation less susceptible to buckling, provide greater lateral capacity and greater moment capacity than the square shaft material.
Because of the wide range of product options offered with CHANCE helical piles, the engineers were able to solve their installation challenges using the best technology for the application.
“Due to the lateral loads in the fill area in the abutment and a cobble layer in the soils below that required a SS200 Helical Pulldown Micropile, it was a perfect spot for a transition from SS200 2” square shaft material to RS4500.337 4-½" OD pipe shaft material,” said Brad Sullivan of Structural Anchor Supply, the CHANCE distributor who provided the helical pile material for this project.
Atlas Foundation Company was the CHANCE Certified Installer for the bridge’s foundation. Teams who install CHANCE products undergo both online and in-person training to ensure they understand the tools, methods, and troubleshooting for any installation situation. Atlas specializes in deep foundations and has been serving the Midwest since 1968. Learn how you can become a CHANCE Certified Installer, giving you the advantage of quality training and the ability to offer a product warranty, resulting in peace of mind for every customer.
The installation took 14 days, including testing and welding. The 14 SS220/RS4500.337 grouted Combo Piles were installed using an 18 K torque head with a Cat 311 Excavator. The helical piles provided solid abutments in the tight, restricted access installation area. The precision with which helical piles are installed allowed the crew to avoid both the overhead utility lines and the underground sewer lines running through the site. With trains passing through at least twice daily, the installation methods satisfied the railroad safety requirements.
A CHANCE Torque Indicator was included in the tooling string. Installation torque reached 12,700 ft-lb, which the crew monitored live from their phones during installation, confirming that the required capacity was reached.
A pre-cast block modular geo grid went around the piers after installation to finish the project.
If you need a wide range of product options to satisfy foundation design challenges, look to CHANCE for a nationwide network of experts who will help you get the job done right.
Take matters into your own hands. Find your local distributor to get started today.
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