J.B Coxwell Contracting originally planned on using sheet piling to shore the microtunneling launch and receiving pits, but instead went with an Efficiency Production manufactured slide rail system supplied by distributor Professional Shoring & Supply.
The Jacksonville (Fla.) Electrical Authority recently let a project to install new infrastructure for expansion and development in the Royal Lakes housing community in Jacksonville. Specifically, the JEA let a project to install more than 11,000 linear ft. (3,353 m) of new force main utility pipe. A portion of which was microtunneled 275 linear ft. (84 m). It was the first microtunneling project in Jacksonville.
Taking on the challenge of installing the new underground utility was J. B. Coxwell Contracting Inc., based in Jacksonville.
Microtunneling is a slightly different process than more traditional jack and bore. However, both processes require the excavation of a launch pit and a receiving pit. This means that Coxwell needed to dig and safety shore at least two excavations that would stay open for an extended duration. And of course, all excavations for underground utility work pose their own set of unique challenges for keeping people safe as they work underground.
Shoring Challenges, Shoring Options
Whenever personnel are working in a trench or pit deeper than 5 ft. (1.5 m), OSHA mandates that the contractor install a protection system of sloping, shielding, or shoring. Because the microtunneling launch pit needed to be excavated in close proximity to a busy road, sloping the excavation really wasn’t an option. And soil conditions were not ideal to install and maintain trench shields. That left the option of shoring the excavation, which is typically done by driving tight sheeting.
“We were originally planning on using sheet piling to shore the launch pit,” said Coxwell Project Superintendent Dean Jones. “But then a representative from Professional Shoring and Supply came over and told us that slide rail might work better. I looked at the brochures and pictures of an Efficiency Production slide rail system, and decided I just wanted to try it.”
Coxwell Chooses Slide
Rail Over Tight Sheeting
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 by 50 ft. (15 by 15 m) with Efficiency’s ClearSpan System; or in a linear Multi-Bay configuration to install length of pipe more than 40 ft. (12 m).
“I’m not a big sheeting guy,” Jones continued. “It’s time consuming and requires a lot of special equipment. It’s old school.”
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. Coxwell installed the system with a Komatsu 400 excavator. It also had an IT38 Cat loader onsite.
Coxwell rented the slide rail components from the Jacksonville branch of Professional Shoring and Supply, an Efficiency Production equipment distributor. And because this was the first time Coxwell had used Efficiency’s slide rail system, Efficiency sent out Shoring Specialist Tim Hurst to assist with the initial installation.
Tight Space No
Problem for Coxwell
“It was a very difficult spot to be digging, with access from only two sides to install the slide rail,” said Hurst. “But the Coxwell guys did an amazing job of installing the system, despite all the challenges.”
For the microtunneling launch pit, Coxwell installed a two-bay Multi-Bay configured slide rail system; 27 ft. long, 14 ft. wide, and 20 ft. (8 by 4 by 6 m) deep. Coxwell maintained an open excavation, free of any cross trench support by utilizing integrated “tie-back” external wales on the outside of the system, and sacrificial members between linear posts below grade. The receiving pit was a four-sided system; 14 ft. long, 10 ft. wide and 16 ft. (4 by 3 by 4.8 m) deep.
Microtunneling Under Busy Jacksonville Highway Saves Time and Money
Huxted Tunneling was subcontracted to perform the microtunneling. It used an Iseki Unclemole TCC Microtunneling machine to push 42 in. (106 cm) steel casing 275 ft. (84 m) under heavily-travelled Southside Boulevard (SR-115). Utilizing the same hydraulic jacking rig, Huxted then pushed 20 ft. (6 m) joint lengths of 24 in. (61 cm) diameter DR18 PE force main transmission line through the casing, with Coxwell personnel making the connections.
The Coxwell subcontracted project was not the first time Huxted has set up its microtunneling rig inside a slide rail system. It worked in a slide rail shored pit once before in Sarasota, Fla.; and recently worked within the same slide rail system from the Coxwell job to microtunnel 240 ft. (73 m) under I-75 in Tampa, Fla.
“My experience with slide rail has been very good so far,” said Huxted Tunneling’s Steve Pollack. “[The Slide Rail] went in very fast, faster and easier than sheet piling in my opinion. We were able to build a good solid back wall to brace the hydraulic jacking rig,” continued Pollack.
“I’ve suggested to other excavation contractors that they might want to consider slide rail instead of sheet piling if they’re bidding on a project that requires microtunneling,” Pollack concluded.
Initially established in 1983 as a firm specializing in clearing and earthwork, J.B. Coxwell Contracting Inc. has expanded its scope of operations to encompass complete site development; including underground utilities, as well as heavy highway and civil construction in the public and private sectors.
For more information, visit www.jbcoxwell.com.
Huxted Tunneling was established in Florida in 1977, originally focusing on underground utility contracting. Soon after its inception, the company began gaining experience in trenchless procedures such as auger boring and installation of steel casing for utility projects and pump stations. Huxted’s capabilities expanded in 1991 to include pipe jacking, hand mined tunnels, and microtunneling.
For more information, visit www.huxtedtunneling.com.
Efficiency Production, “America’s Trench Box Builder,” manufactures the widest selection of standard and custom trench shielding and shoring systems. Efficiency’s products are designed specifically for safe and cost effective installation of utility systems and infrastructure improvements. All products are P.E. certified to meet OSHA standards. For more information, visit www.efficiencyproduction.com.
(James McRay is the director of marketing and media of Efficiency Production Inc. He can be reached at 800/552-8800; firstname.lastname@example.org.)