This article was originally published in Underground Construction Magazine in December 2010.
Projects requiring excavation are recognized as among the most dangerous in construction, and protecting personnel who must work in trenches and surrounding areas from cave-ins is a priority with life and death implications.
For small jobs, trench protection may be as simple as dropping a trench box into the excavation. Larger jobs are more complex. A major project often requires extensive engineering and the use of specialized shielding and shoring.
North America’s largest rental supplier of trench safety systems and products is United Rentals. The company also is a provider of pre-construction planning and turnkey installation and removal services. United Rentals operates more than 50 trench safety rental branches that offer the latest technologies in steel and aluminum trench shields, aluminum trench shores, steel sheet pile with modular waling systems, steel crossing plates and custom slide rail systems.
Recently, United Rentals provided a Mega Brace shoring system for an Xcel Energy project in Colorado to pull water from the South Platte River downstream from Denver at the discharge point of the Denver Metro Wastewater Treatment Plant, and divert it to the company’s Cherokee Power Plant approximately half a mile away.
General Contractor was Garney Construction, Littleton, CO. Working as a subcontractor to Garney, Territory Unlimited, Berthoud, CO, built a grouted boulder drop structure in the South Platte River that was integrated with a Garney-installed concrete diversion structure and 170 linear feet of 24-inch ductile iron pipe to bypass water to a pump station wet well structure.
Territory Unlimited installed the Mega Brace system with teams of United Rentals personnel providing expertise on site throughout the project.
The Mega Brace system was engineered by D. H. Charles Engineering, Santa Rosa, CA, and was used in a 30-foot deep, four-sided 35-by-30-foot excavation for construction of a 25-foot high, 25-by-20 foot concrete wet well structure, said Kris Graham, United Rentals outside sales representative, Denver.
Graham said a sloping system was ruled out because it would have required removal of more than 4,666 cubic yards of soil, compared to the 1,166 cubic yards for the Mega Brace.
Other bracing systems were evaluated and rejected.
“Beam and plate,” said Graham, “would require counter sinking the beams below the bottom grade, but the soil reports indicated extremely hard materials that would have been very slow for the required caisson drilling. We had the same issues with the sheet pile options, but were able to engineer it without the toe in.”
Graham said slide rail was an option that could have been made to work, but would have required multiple sets of cross struts running both north/south and east/west. Not only would the installation have been longer due to the excavator needing to work around the cross struts, it would have required the concrete crew to work around these cross struts and then have an excavator on stand-by to pull them up as the concrete structure progressed.
Sheet pile with conventional I-beams welded to sheet pile would have achieved the same results, Graham continued, but would have required certified welders to cut and weld the rings/levels of I-beam and cut and grind them from the sheet pile at the time of removal. This would have required an estimated one-day per ring/level and downtime of crew and equipment to swing the I-beam to welders.
“Instead, the Mega Brace system was installed with the same size crew and equipment in an average of two hours per ring/level, saving close to one week on the installation and the cost of the certified welders,” Graham said.
Manufactured by Ground Force Engineering, Harrogate, North Yorkshire, England, the Mega Brace takes the place of conventional I-beams for use as a waler system in a sheet pile/beam and plate installation, said Graham.
“Mega Brace,” he said, “has pre-manufactured lengths of I-beam and cross struts that have the ability to quickly pin together to fit the length or width required for the construction project. What makes the Mega Brace system unique compared to I-beam walers is that a six-inch telescoping hydraulic ram is pinned to the ends of the I-beam extensions or struts. This allows the system to pump in or out to fit the excavation and hold the system in place without the required welding to the sheet pile required of a conventional application.”
When the project was bid, said Graham, United Rentals had 90 percent of the Mega Brace equipment required on hand. By the time work was ready to begin, equipment was committed to other projects, and United Rentals transferred equipment from Florida and Georgia for the Colorado project.
Graham said that to install the bracing, Garney Construction first pre-drilled a series of 30-foot deep dewatering well points around the excavation site. In accordance with the engineer’s design, five feet of soil was excavated from the top and sloped back.
“One of the Mega Brace ring/levels was assembled with the hydraulic rams pulled in smaller than the finished pit size,” he continued. “This was used as a template for the sheet pile being driven. After all the sheeting had been driven to grade/point of refusal, Territory Unlimited crew members excavated the pit seven feet with the Mega Brace ring/level going down at the same time. At this point the Mega Brace hydraulic rams were pumped out one at a time in all four corners.”
Safety chains were then secured from the Mega Brace to the top of the sheet pile. Territory Unlimited excavated another seven feet and the four legs of the Mega Brace were lowered into the pit and pinned together and the four hydraulic rams pumped out to lock the system into place.
“Safety chains were attached again,” Graham continued. “We repeated the process with a double stacked ring/level of Mega Brace seven feet below the middle ring. At this point, Territory Unlimited completed the excavation to final grade.”
Three days were required to drive the sheet pile, and it took five days to install the four rings/levels of Mega Brace to the 25-foot depth. Excavation of the remaining five feet and completing the finish grade took another two days.
Soil conditions were sand and gravel. “The soil type made for a relatively simple installation,” said Graham. “The challenges of the project were the depth of the excavation and dewatering necessary because of the close proximity to the river.”
United Rentals provided two teams on this project to make sure things went as smoothly as possible.
Graham said Mega Brace system has been used in the United Kingdom and Europe for 30 years and was brought to the Unites States about five years ago.
“United Rentals Trench Safety Branches have been installing Mega Brace systems for five years,” Graham said. “This project would be considered moderately-sized compared to some of the projects United Rentals Trench Safety has done on the east coast. During the field and classroom training we did with Ground Force Engineering personnel from England, we saw projects with entire city blocks excavated 50 feet deep in London using the hydraulic Mega Brace and cross strut systems. Garney and Territory Unlimited are the first to use the system in Colorado. Both companies were great to work with.”
United Rentals is the largest equipment rental company in the world, with an integrated network of 549 rental locations in 48 states and 10 Canadian provinces. The company’s approximately 7,400 employees serve construction and industrial customers, utilities, municipalities, homeowners and others. The company offers for rent approximately 2,900 classes of equipment with a total original cost of $3.8 billion.
In addition to shielding and shoring equipment, United Rentals’ trench safety branches also carry confined space ventilators, detectors and other equipment and offer 24-hour emergency response and professional safety training.