Pipeline Utilities Inc., Raleigh, NC, had put a bid in for a storm water repair and upgrade project at the University of North Carolina’s Boshamer Stadium. The university took advantage of the ongoing renovation of its baseball stadium to also replace the storm water infrastructure that runs below the stadium and ball field.
Pipeline Utilities was charged with removing the existing 60-inch pipe originally installed in the 1970s and now in a deteriorating state; then installing new 72-inch and 84-inch pipe. In order to successfully complete the project, Pipeline Utilities had to excavate 35-feet at the deepest point in poor soil conditions. They were then required to lay 300-feet of the new pipe and set three 14-foot-diameter precast manholes.
In addition to the complexity of the actual construction, the project required a fairly complex shoring system. OSHA requires a protective system on any project where a trench will be greater than five feet or where job site conditions warrant. This requirement helps to ensure that workers can complete their jobs safely through the use of a protective system to guard against or prevent a soil collapse. The intricacies of a shoring system can often go unappreciated until a project such as this demands a more complex planning phase.
The job site conditions that would affect the shoring system began with the soil which is often the first variable that directs the use of a protective system. In good soil, there are many possible ways to shore a system; however, as the soil conditions become worse, the possible options for a protective system become more limited. Generally, as soils become worse, they gain additional weight and exert a greater pressure against a protective system. In addition to the extra weight, 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 shoring to move 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 overall depth of the excavation further complicated the issue. At 35-feet deep, thousands of pounds per square foot of soil pressure can be exerted upon the shoring system. The deeper the system goes, an increasingly greater pressure is exerted upon the system. This shoring system needs to be engineered to handle the loads at these depths without fail.
The final complicating issue was the large pipe that needed to be installed. For comparative purposes, a standard trench shield may yield about 65 inches of vertical pipe clearance under the pipe spreaders. The larger of this new pipe being installed would require up to 102-inches of vertical clearance, clearly in excess of the conventional pipe clearances for most systems.
All of these factors culminated into a very challenging project and one in which the contractor would have to rely heavily on the fact that the chosen shoring supply firm would have to perform as promised. Pipeline Utilities Construction Manager Chris Acord knew the project would require a demanding shoring system.
Acord said, “we knew early on that with this project’s depth, soil conditions, pipe size and time schedule, the standard trench box shoring was just not going to be an option. We felt that the contractor that came up with the best shoring solution was probably going to get this project. We have relied on Coble Trench Safety for our shoring needs for many years and brought them onto this project early in the bid process. They recommended looking at a slide rail system from the very beginning and that is what we ended up going with.”
Coble Trench Safety Specialists Brett Sondergard and John Knighten met with Pipeline Utilities very 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. However, it provides a reasonable approximation of the shoring cost as well as other costs such as an appropriately-sized excavator to work with the shoring system. During these initial meetings, a wide range of alternatives were discussed. For example, steel trench shields could be modified with steel arch spreaders to achieve the needed pipe clearance of 102 inches; however, this system would require significant engineering to determine the viability of the system at the required depth, as well as the issues that would be created involving the weight of this type of system.
After discussing a number of alternatives, a combination of linear slide rail system and pit style slide rail with steel sheeting guides became apparent as the best way to shore the excavation.
Reflecting upon the planning phase Sondergard said “we felt immediately that the slide rail system would easily accommodate the poor soil conditions we would encounter on this project. Slide rail systems were designed, in part, to accommodate very poor soil conditions, and often shine on these job sites. The slide rail system’s efficiency becomes very pronounced in bad soils because you can advance the panels along the rails, ahead of the excavation’s cut, which helps to prevent a soil raveling. In addition, the system has the benefit of being positive shoring. Positive shoring is a system which acts to prevent a soil collapse, as opposed to just protecting employees in the event of a soil collapse – the role of a standard trench shield. The benefit of positive shoring becomes paramount when dealing with adjacent structures, tight job sites or right-of-way issues.”
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-feet of excavated depth needed on the project. Large triple slide rails were dug into the ground, and slide rail panels advanced along tracks in the rail to provide soil protection to the full length of the excavation.
Coble Trench Safety features the SBH slide rail system, which provides a rigid system of strength while also being highly customizable and adaptable to individual job sites. This customization was needed during the installation phase. Linear rails were driven into the ground and the cross bracing system took the form of a linear rolling strut. During installation, the linear rolling strut advances along the rail to prevent “toe in” of the system. Once the system is fully installed to the full depth, the strut can be rolled up to provide additional vertical clearance as needed. Using site specific engineering, it was possible to achieve the necessary 102-inches 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 is an added benefit to the SBH slide rail system. Steel sheeting guides acts 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 remain open and is shored once steel sheets are inserted along grooves in the sheeting guides. The benefit to this design is that the sheets can be inserted around existing lines, which helps to mitigate the effect of crossing or existing utility lines.
The system helped minimize the time spent installing and removing the shoring system so that as much time as possible on the project could be devoted to performing the actual removal and upgrade of storm water lines.
Through a thorough planning phase and attention to detail, the slide rail system was installed without incidents. Reflecting on the project, Acord said, “In this project scenario, the slide rail system was the best option and worked just as advertised.”
FOR MORE INFORMATION:
Coble Trench Safety, (866) 999-8344, www.cobletrenchsafety.com