East Tennessee Grading, Inc. of Chattanooga, Tenn., recently installed a new precast lift station for the city of Chattanooga as part of an overall upgrade to the sewer system. The installation was sited in some challenging terrain, prompting the contractor to look at new approaches to handling the installation.
East Tennessee Grading was founded in 1989. The company specializes in site grading and utility work on a variety of commercial, residential, highways, airports, railroads, landfills, and water and sanitary sewer projects.
On the Chattanooga sewer upgrade project, East Tennessee Grading's estimator and project manager Mike Brock looked at the details of the project and determined that using traditional trench boxes would not be the most cost-effective solution.
"The wet well needed to be set much deeper than what we normally use trench boxes to shore," Brock said. "I know other people who had used slide rail systems before, and they advised that it might be an option."
Brock worked with Greg Ross, Efficiency Production's Slide Rail installer and sales manager, to work up a slide rail system for use on the project.
Efficiency's Universal Slide Rail is a component shoring system comprised of steel panels (similar to trench shield sidewalls) and vertical steel posts. The system can be used in a variety of configurations, such as small four-sided pits; large unobstructed working pits as big as 50 feet by 50 feet with Efficiency's ClearSpan System; or in a linear multi-bay configuration to install length of pipe more than 40 feet.
Slide Rail is installed simultaneously as the trench or pit is excavated by sliding the panels into integrated rails on the posts — either double or triple rails depending on needed depth — then pushing the panels and posts incrementally down to grade as the pit is dug; a process commonly referred to as a "dig and push" system. The system is installed and removed incrementally, which allows the trench to be properly shored throughout the entire installation or removal process, always protecting workers from a trench wall collapse.
"Slide rail works because it keeps excavations tight and vertical with no loss of dirt or fill," explains Ross.
The project was located in hilly terrain, and the lift station, gravity connector and force-main were to be installed more than 30 feet deep in rocky Type C-60 soil. In order to reach that depth, East Tennessee Grading benched down part way into the side of the hill before excavating to grade. To handle the project, Brock rented from Efficiency a four-sided 20-foot-by-20-foot pit system which allowed crews to excavate the pit 24 feet deep to reach grade. The entire excavation took up only a small area and did not require as much backfill material.
"The slide rail system was installed in a just one afternoon and removed in just a half a day," explains Brock, adding that Efficiency's Greg Ross also worked on the site and helped troubleshoot spots that might have been troublesome.
"He literally jumped into the hole and did not leave the site that evening until the system was in," Brock says.
To complete the project, East Tennessee Grading used Cat 345 and Cat 325 excavators to install the slide rail system and the 10-foot-diameter round, pre-cast lift station. A Cat D6 dozer was also used on the project.
Brock and his crew were able to install the entire wet well in one day, aside from the cap which was subcontracted. All told, East Tennessee Grading completed the 28-foot-deep wet well portion of the project from beginning to end in just three days, and Brock notes that the system "went in so much faster and a lot easier than I was expecting."
The new lift station was part of upgrade to the area's sewer system in anticipation of more commerce in that area. The upgrade also included elimination of an elevated sewer line that crossed a nearby creek.