The excavation for the tank-set was large but not particularly deep. However just below final grade was a pressurized aquifer.
Jones Covey Group Inc. (JCGI) headquartered in Rancho Cucamonga, Calif., recently installed four large underground storage tanks as part of the construction of a new gas station in Lynnwood, Wash.
The excavation for the tank-set, though rather large — about 65 sq. ft. (6 sq. m) — was only 16 ft. (4.87 m) deep. However, danger was just below the surface.
“The ground conditions were a very dense glacial till,” said Mike Roach, JCGI’s estimator on the project. “But the big problem was the presence of a pressurized aquifer beneath the excavation site, creating a situation where the groundwater level was above rough grade of the site.”
There were more challenges for JCGI.
“The gas station is being built where the old Lynnwood high school was located and there is a retaining wall just a few feet off one side of the excavation,” Roach explained. “Because the gas station development company requires the fill ports for fuel deliveries to be located outside of the main gas pump vehicle queues, there were no options to relocate the tanks. We were forced to place the excavation as drawn, meaning we’d have to dig and install shoring with no direct access for the excavator on one whole side near the retaining wall,” said Roach.
Finding a Shoring System
Because the excavation was deeper than five feet (1.52 m), a trench protection system was required as mandated by OSHA. That means JCGI needed to slope, shore, or shield the jumbo-sized working pit. Shielding was immediately ruled out as impractical. Sloping the pit might have been possible if not for the retaining wall just a few feet from one of the banks. Ninety percent of the time, a large excavation like this would utilize some type of sheeting system or beam and plate; however none of those over-toeing systems would work because of the likelihood of puncturing the aquifer.
JCGI, along with project engineers from D.H. Charles Engineering Inc. and excavation subcontractor OSG Dozing, called D.P. Nicoli.
“We got together to look at all the possible shoring options,” said Kai Zimmerman, shoring specialist of D.P. Nicoli. “But I knew there was really only one option: slide rail.”
Slide Rail System Is the Solution
Slide rail is a component shoring system comprised of steel panels (similar to trench shield sidewalls) and vertical steel posts. It is installed by sliding the panels into integrated 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” shoring system.
“The reason I knew slide rail would work is because it is considered ’positive shoring,’” explained Zimmerman. “There is no over-excavation, no toeing with slide rail. It keeps constant pressure against the surrounding dirt, which was critical in keeping the bank along the retaining wall in place.”
D.P. Nicoli exclusively represents Efficiency Production’s manufactured slide rail system. The universal system can be used in a variety of configurations, such as small four-sided pits, an obstruction-free ClearSpan configuration, or a Multi-Bay configuration to install large tanks and structures or lengths of pipe more than 40 ft. (12.2 m).
ClearSpan Shores Large Excavation
With equipment rented from D.P. Nocoli, subcontractor OSG Dozing excavated the tank-set area and installed a 64 by 66 by 16 ft. (19.5 by 20.1 by 4.9 m) ClearSpan configured slide rail system. ClearSpan’s design is unique to the industry in that it shores a large working pit without any need for cross-braces. The system stays entirely open by utilizing innovative waler I-beams on the outside of the system at the top, and inside the system low in the excavation to prevent deflection. The walers integrate into the slide rail system with the use of custom brackets with rollers that slide along the posts.
Four tanks were set in the slide rail shored excavation: three 54 ft. (16.45 m) long, 30,000 gal. (113,562 L) tanks; and a 37 ft. (11.27 m) long, 20,000 gal. (75,708 L) tank. All four tanks were 10.5 ft. (3.2 m) in diameter. The heavy equipment on the job used to install the slide rail system and set the tanks were a 60-ton (54.42 t) mobile crane from Ness & Campbell, a 460 John Deere excavator, a 644 Deere front loader and a 210 Deere excavator.
High Praise for
Slide Rail System
The excavation contractor, OSG Dozing, had used slide rail before, but this was the largest system they had installed.
“The [slide rail] system is great, everybody likes it including the GC, engineers and project owner,” said OSG Dozing’s foreman, Rod Hudson. “Plus, D.P. Nicoli always offers great support whenever we rent equipment from them.”
“D.P. Nicoli’s representative, Kai Zimmerman, was instrumental to this project with his planning and installation support,” added Roach. “Our collective efforts gave everyone a high level of confidence in the project even before the first shovel was in the ground.”
Jones Covey Group Inc. is a Southern California-based, licensed comprehensive general contractor and construction management firm specializing in project development, design build, construction, construction management, and post-construction services.
With offices in Northern California and the Pacific Northwest, D.P. Nicoli, Inc. has served the piling and shoring needs of the heavy construction industry for 26 years.
Efficiency Production provides 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, according to the company.
For more information about JCGI, visit www.jonescovey.com. For more information on D.P. Nicoli Inc., visit www.dpnicoli.com. For more information on Efficiency Production, visit www.efficiencyproduction.com.
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