Heavy rainfall and an abundance of impervious surfaces contribute to recurring flooding in the City of Linden. Helping to address the issue, the Linden Stormwater Flood Control project involved the installation of dual, 320-ft long, 66-inch concrete pipes under the Amtrak and Conrail 13-track Northeast Railroad Corridor. Schnabel performed geotechnical design and construction engineering services for the project.

At the site, jacking and receiving pits were located in extremely tight easement spaces, with the jacking pit located 10 feet from a 48-ft high, 500,000-gallon firewater tank in an operating industrial plant. Soil conditions included silts and fine sands below the groundwater table, and the soil cover along the microtunneling alignment ranged from 6–12 feet.

Cruz Contractors retained us to design temporary shoring for the microtunnel tunnel boring machine (MTBM), jacking and receiving pits, and for an open-cut installation of 6-foot-diameter Hobas pipe passing within 5 feet of the firewater tank. We also designed the break-in/break-out walls for the MTBM, and determined jacking loads and suitable grouting pressures for microtunneling.

We modeled the sequencing of jacking pit construction and subsequent MTBM jacking force against the thrust wall using the geotechnical finite element code PLAXIS. Key features of the resulting design involved the requirement to preload cross-bracing struts to reduce deformations during pit excavation, and installation of H-piles connected to the sheeting and cast into the thrust wall to facilitate transfer of thrust wall loads deeper into the soil, thereby reducing potential movement of the tank foundation. We also modeled the open-cut excavation for Hobas pipe installation, and based on results of the analysis specified preloading of struts to ensure against settlement of the tank. For all shoring, measured deformations during construction were negligible and microtunneling was completed successfully.