Trench boxes are the tried-and-true method to protect workers from cave-ins while working inside trenches, but there are times when their use is not the best or most practical choices.
One instance where an alternative needs to be considered is when excavators are working with unpredictable soils.
This was the case for a contractor working on a major overhaul of an Iowa interstate. His solution was to turn to a slide-rail system.
Keo Way Interchange Project
The Iowa Department of Transportation (IDOT) is giving I-235 in Des Moines a complete $429-million multi-year makeover including: utility relocation, bridge replacement and rehabilitation, interchange reconstruction, and mainline paving. The project began in 2002, and will not be completed until 2007.
The six-year project is phased to allow the maximum amount of work per construction season, with minimal delays and inconvenience.
There were nine projects ongoing in 2005, including the Keo Way Interchange.
In a joint venture, Herberger and United Contractors Inc. were the primary contractors working on building a new westbound ramp and bridge over Keo (Keosauqua) Way in 2005.
Part of the Keo Way Interchange project is the $1.8-million installation of a new storm sewer pipe, which the joint venture has subcontracted to Peterson Contractors Inc. (PCI), Heavy and Highway Contractors headquartered in Reinbeck, IA.
PCI is tapping into an existing 54-in. (137 cm) storm sewer line and diverting flow to another storm sewer with excess capacity.
They are installing 600 ft. (182.9 m) of new 48 in. (122 cm) concrete pipe at a grade of 30 ft. (9.1 m) deep.
They also are installing two RA49 modified junction boxes for utility access. The new pipeline is running west along I-235, approximately 21 ft. (6.4 m) from the highway’s shoulder.
Deep Grade Poses Challenges for Shoring Trench
The deep grade and limited space in the highway’s right of way posed some challenges for shoring the trench.
“Normally, we would use double-stacked trench boxes, but after evaluating the soil conditions and determining that it is very wet C-60 type soil, we quickly realized that trench boxes would not be the best option,” said Doug DeSchamp, PCI’s supervisor on the project. “We also considered open cut, but again, the soil conditions wouldn’t allow that either. We actually tried a test bench, but it quickly collapsed, and we lost part of the road’s shoulder.”
Doug Clark, project engineer of PCI, then contacted Eric Juhl of United Rentals-Trench Safety in Grimes, IA, for shoring options.
“We talked about the possibility of driving sheeting with tie-backs, but the very wet soil condition and pipe depth made that option very expensive,” explained Juhl. “I then suggested an Efficiency Production Slide Rail System, which would be much more cost effective, and could easily handle the depth and soil-conditions with a triple-rail post system.”
Efficiency Slide Rail is a component system comprised of steel panels and posts.
The universal system can be used in a variety of configurations such as a four-sided pit or linear-bay application.
Slide Rail is installed simultaneously as the trench or pit is excavated by sliding the panels into rails on the posts, and 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.
Slide Rail posts have either a double or triple rail which telescope panels to required depth.
In the linear multi-bay configuration, which PCI is utilizing, Efficiency-specified spreader pipes are pinned-in-place to a set of parallel beams with rollers that slide down slotted rails on the inside of two linear posts, providing the cross-trench support.
Efficiency Production Inc. Director of Engineering Mike West noted that, “Efficiency’s Slide Rail System is well-suited for a dig 30 ft. or more because the triple-rail post system keeps side panels to a manageable weight. Contractor’s can still install and remove the system with trackhoes and equipment they typically have in their fleet.”
PCI rented a four-bay linear application slide rail system from United Rentals. Three bays are 16 ft. (4.9 m) long, and the fourth 14 ft. (4.3 m) long. They are using 6 ft. (1.8 m) spreader pipes on the 1 ft. by 11 ft. tall (.3 by 3.4 m) parallel beam spreader assembly for a total trench width of 8 ft. (2.4 m).
The linear-bay configuration allowed PCI to install a 16 ft. length of 48 in. pipe in the front bay of the four-bay system while simultaneously backfilling and removing previously installed bays at the back of the trench. The slide rail then could be leap-frogged for repeated use.
Efficiency Production’s Slide Rail 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.
Because DeSchamp and his crew had never used Slide Rail before, Efficiency Production Inc.’s Slide Rail Installer, Greg Ross, assisted with the initial installation and trained PCI’s crew on how to properly install the Slide Rail System.
“Our first couple days we went a little slower as we learned how to use the system, but now we are going much quicker,” commented DeSchamp. “We’re still on a learning curve, but we are meeting our schedule and have doubled our production since the beginning of the project.” CEG