Extreme Excavations
Coble Shores Challenges
By Steve Barnhardt
Pipeline Utilities Inc. of Raleigh, N.C., bid for a storm water repair and upgrade project at the University of North Carolina’s Boshamer Stadium that involved removing the existing 60-in. pipe and installing a new 72- and 84-in. pipe. In order to successfully complete the project, Pipeline Utilities would have to excavate to a depth of up to 35 ft at the deepest point in poor soil conditions, and they would then be required to lay 300 ft of the new pipe and set three new 14-ft diameter precast manholes.
 In addition to the complexity of the actual construction, the project would need a fairly complex shoring system. The jobsite conditions that would affect the shoring system began with the soil conditions. Soil is often the first variable that directs the use of a protective system. Generally, as soils worsen with depth, they gain additional weight and exert a greater pressure against a protective system. As the soils become more granular, they also become prone to raveling. Excessive soil raveling can prove problematic as a contractor attempts to shore an excavation. While trying to advance the cut of the excavation, the soil will often just fall in, never allowing you to move your shoring forward. In these types of conditions, your only option is to shore the excavation continually as you remove the soil.
In addition to the poor soil conditions, the depth further complicated the issue. Thirty-five ft is a very deep excavation and at that depth, thousands of pounds per square foot of soil pressure can be exerted upon the shoring system.
The final complicating issue was the large pipe that would need to be installed. For comparative purposes, a standard trench shield may yield about 65 in. of vertical pipe clearance under the pipe spreaders. The larger of the new pipe being installed would require up to 102 in. of vertical clearance, clearly in excess of the conventional pipe clearances for most systems.
The Solution
Coble Trench Safety Shoring Specialists Brett Sondergard and John Knighten met with Pipeline Utilities early in the bid phase for this project and looked at all parameters surrounding the project. A plan was put in place with accurate costs that would be needed before the bid. As always, the plan could be subject to change, but it provided a reasonable approximation of the shoring cost and other costs such as an appropriately sized excavator to work with the shoring system.
“We felt immediately that the slide rail system would easily accommodate the poor soil conditions we would encounter on this project,” says Sondergard. “The slide rail system’s efficiency becomes pronounced in bad soils because you can advance the panels along the rails ahead of the excavation’s cut, which helps to prevent soil raveling. In addition to helping prevent soil raveling, the system has the benefit of being positive shoring. Positive shoring is a system that acts to prevent a soil collapse, as opposed to just protecting employees in the event of a soil collapse, which is typically the role of a standard trench shield.”
In addition, the poor soil conditions exert a greater soil pressure against the system. This effect is similar to that of the depth and together they both increase the stress on a slide rail system. The SBH slide rail system is built to handle these intense soil pressures and can comfortably achieve the 35 ft of excavated depth needed on this project. Large triple slide rails are dug into the ground, and slide rail panels are advanced along tracks in the rail to provide soil protection to the full length of the excavation. During installation, the linear rolling strut is advanced along the rail to prevent “toe in” of the system. Once the system is installed to the full depth, the rolling strut can be rolled up to provide additional vertical clearance as needed. Using site specific engineering, it was possible to achieve the necessary 102 in. of vertical clearance with the site conditions present and ultimately provide the optimal method of shoring the excavation.
The steel sheeting guides were needed due to the presence of the existing lines and are an added benefit to the SBH slide rail system. Steel sheeting guides act as slide rail panels and can be inserted along the tracks in the slide rail. A sheeting guide is inserted along the base of the excavation and at the top of the excavation. The area between the sheeting guides remains open and is shored once steel sheets are inserted along grooves in the sheeting guides. The benefit of this design is that the sheets can be inserted around existing lines, which helps to mitigate the effect of crossing existing utility lines.
Thanks to a thorough planning phase and attention to detail, the slide rail system was installed without any incidents. The system performed and the contractor was satisfied with the system, as well as Coble Trench Safety’s ability to make such a complex shoring project flow smoothly from installation through removal.
Steve Barnhardt is based in Greensboro, N.C.
Operation: Slide Rail
Icon Equipment Distributors Inc. Shores in Close Quarters
Compiled by UC Staff
The shoring mission Icon Equipment Distributors Inc. accepted involved the installation of several utilities in five individual 16-in. steel casings under Whitney Ave. at Yale University. Giordano Construction of Branford, Conn., decided to go with pilot tube-guided auger boring, but were still looking for a shoring solution.
 Before Giordano Construction broke ground, it had several pre-bid and pre-construction meetings. Working closely with the contractor, Icon provided a slide rail shoring system, specifically engineered for the tight conditions of the project. Icon supplied two slide rail systems for the jacking and receiving pits required for the trenchless crossing under Whitney Ave. The jacking pit was 32 ft long, 16.4 ft wide and 16 ft deep, while the receiving pit was 16.4 ft wide, 11.48 ft long and 16 ft deep on the other side of the road.
Once onsite, Icon provided installation training for the slide rail system, as well as all operator training and expertise for the pilot tube boring system being rented. In addition to the steel casing being installed, which needed utility access with its internal waler frames and utility panels, Icon also supplied additional frames and panels to sheet around perpendicular utilities encountered approximately 5 ft under ground.
“Icon provided all the site specific engineering design with a CT. P.E. stamp on all drawings and calculation sheets, as well as safety and assembly guides that are given directly to the crew for reference and material on site,” says David Crandall, Vice President of Sales and Marketing at Icon Equipment Distributors Inc. “Our field technicians were onsite to provide training, assistance and ensure the safe operation and clean installation of the slide rail systems for the project.”
With Icon professionals on the job lending a helping hand, the slide rail system was successfully installed and allowed the job to be completed safely.
Bridge to Shore
Mabey Calms Troubled Jobsite Seas
Compiled by UC Staff
Adaptability is the key. One trench shoring solution doesn’t fit all, and Mabey Bridge and Shore knows it. When the Orlando, Fla.-based Central Florida Underground was contracted to install a concrete intake structure — supplying cooling water for a local power plant — 30 ft into a small lake, interlocking sheet pile from Mabey Bridge and Shore was tailored to get the job done.
 “The project was especially difficult due to the large amount of water (and subsequent de-watering) involved and the distance from shore,” says Greg Richards, Marketing Representative for Mabey Bridge and Shore Inc. “Routine excavations in Florida may involve greater depths than this, but only a minimal amount of groundwater. This project involved holding back a significant amount of above-ground water, which was a great test for Mabey’s water-resistant interlocking sheets.”
Mabey sheet pile is available in a wide range of sizes for various applications and can be used together with Mabey’s hydraulic framing systems when needed. For Central Florida Underground, 40-ft interlocking sheet pile was chosen because of its strength and ability to resist water intrusion. Mabey engineers worked with Central Florida Underground reps to develop a scheme utilizing these sheets, along with Mabey’s Super PowerBrace frames and corner struts to produce a water resistant, yet easily installed cofferdam system.
“Minimal water intrusion created some delay initially, due to the tremendous pressure exerted on our sheets by the lake water,” says Richards. “Mabey salesperson Dave Howard worked with Mabey engineers to minimize water intrusion and ensure dewatering took place consistently to eliminate standing water. Howard was onsite regularly, and CFU teams were in contact with Mabey engineers throughout the duration of the project, to ensure success.”
Once completed, the shored excavation measured 52 ft wide, 32 ft long and 22 ft deep. The system held back 9 ft of water and 8 ft of soil. After thoroughly dewatering the excavation, the installation was completed in less than 12 weeks.
The Power of Shoring
Pro-Tec’s Massive Safe Trenching Solution
By Dan Stupek
“For sheer size and complexity, this was by far the biggest and most interesting trench shielding job I have ever been a part of, and I’ve been working with trench shield systems for over 25 years,” says Gary Carlson, President of Gary Carlson Equipment of Blaine, Minn., the Pro-Tec shielding and shoring distributor for the state of Minnesota.
 The trench shielding job Gary Carlson talked about was a slide rail system he and his staff designed and installed for an excavation and installation of twin 900-ft long pipelines during construction of a new power plant in St. Paul, Minn. The dual 84-in. concrete encased steel pipes came in 25-ft lengths, which were welded together inside and outside.
Project owner XCEL Energy initially envisioned the 16-ft deep, 25-ft wide excavation to be completely walled off by tight steel sheeting supported by huge waler beams and spreaders, a time- and labor-intensive method, not to mention costly.
Carlson’s crew had a better idea. Working with XCEL’s Mike Bergquist and Scott Eddy, the Carlson firm submitted an alternative that met all criteria: worker safety; ease and speed of installation; maximum pipe length allowance; and enough work space to allow pipe welding and grouting for both lines side by side. The slide rail system was an integral facet in the installation of the twin 900-ft long, 84-in. diameter water intake and outflow pipes. Room to work was a problem. Outside diameter of each pipe was 88 in. with 5 ft of space between the twin lines. The excavation for the twin pipes had to be installed in tight quarters, a right of way barely 30 feet wide sandwiched between a river floodwall and a concrete paved street.
Carlson’s design involved a slide rail system consisting of 24-ft long by 8-ft high panels that were stacked to achieve the 16-ft depth and 25-ft spreaders positioned between custom-designed 27-ft long vertical spreader posts. The slide rail shoring system was 130 ft long, 25 ft wide and 16 ft deep. This system eliminated the need for huge walers or sacrificial beams (suggested by other plans), that alone saved an estimated $115,000.
With the slide rail system, as each 25-ft pipe section is welded, grouted and tested, that portion is backfilled and compacted and the panels and spreaders are moved by the CAT 345 excavator to the front of the trench and reinstalled by a small crew. Because the system worked so efficiently, installation rates have exceeded initial estimates. The crew averaged 25 ft of twin pipe per day. Instead of the six-month timeframe originally estimated, the job was completed in just over four months.
The power plant was built along St. Paul’s riverfront by XCEL Energy, a Denver-based electric power utility. The new plant is natural gas-fueled and replaced the 83-year-old High Bridge coal-fired plant. The combined cycle plant produces electricity from two sources of energy. Natural gas is used as a fuel in a combustion turbine, similar to a jet engine. Then the exhaust gas from the turbine is used to make steam in a heat recovery steam generator. The combined cycle is about 30-percent more efficient than a traditional steam plant. The MERP plant is one of the largest construction projects in the upper Midwest
this year.
Dan Stupek is the President of Fensholt Advertising, West Allis, Wisc.
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