A well-established national contractor was awarded a storm sewer project in a very populated city in Illinois. The project would require the installation of a 42-inch and a 60-inch storm sewer pipe via bore including a pre-cast manhole. The project would require three bore pits with an underground cut of 42-ft in length, a width 16-ft, and a depth of 30-ft. Additionally, the project would require the excavation of a receiving pit that would measure 24-ft in length, with a width of 12-ft, and a depth of 24-ft, that a competent person on site classified the medium to stiff clay as Type-B.
The project presented several challenges that would require experienced underground engineering due to its unique location and soil conditions. The site would be located in a conglomerate area of the city, a few feet away from a high-in-traffic interchange. The underground cuts for the bore and receiving pits would be located adjacent to active ramps, two feeder roads, and a few feet above an overpass. The contractor would need four supporting systems that could provide an open-span area in a tight environment while securing the workers during the underground installation. The protective devices would also need to provide positive shoring against the pressure load originated from the existing active interchange structures and an operational crane located adjacent to one of the excavation cuts. Additionally, the protective systems would have to meet The State of Illinois Department of Transportation’s request of allowing a maximum deflection of 1.5 ft. due to poor soil conditions found in the area. The contractor would also need to complete the underground phase in a timely manner before other contractors could begin the installation of new flyover ramps.
The contractor contacted the NTS in-house team of engineers, who developed an elaborate underground plan that would address all of the challenges. The contractor selected a site-specific system 80 brace for the three bore pits and a slide rail system with a sheeting guided frame for the receiving pit. The system 80 brace would be composed of sheet piles, static walers, and hydraulic braces. The slide rail system would be composed of corner posts; slide rail panels and sheeting guide frames. It was determined that a progressive installation on both protective systems would allow for minimal soil disturbance. The contractor was impressed with the protection and versatility of both protective systems as well as extremely pleased with the service provided by the NTS team of engineering.