To improve the sewer system near Wilmington, Delaware, New Castle County set out to replace the deteriorating Governor Printz Interceptor, which is located near several sensitive utility lines. Working for the contractor selected for the project, Schnabel provided geotechnical and geostructural engineering, dewatering design, and instrumentation and monitoring on this 2,200 foot major 78” sewer line extension.

Support of excavation (SOE) was critical on this 10–30-ft deep excavation due to its immediate proximity to a high-pressure gas transmission line on one side and the existing 42” sanitary sewer on the other. The county placed severe restrictions on the allowable movement of the SOE to limit the impact on the gas and sewer lines. Shutdown of the gas line due to excessive soil movement would also shut down a major highway and rail line through the northeast corridor. The highly variable top of rock also complicated the final SOE design.

We evaluated the geotechnical aspects of the alignment from existing borehole information, modeled the impact of high groundwater elevations in the area, and then designed the dewatering system as well as the SOE for the variable depths to rock. SOE included internally braced sheet piles, as well as internally braced sheet piles with micropile toe pins.

To monitor any possible lateral movement into the excavation that could negatively impact nearby utilities and infrastructure, we also designed and installed a comprehensive instrumentation and monitoring system that, once installed, could be continuously monitored remotely. This system included groundwater monitoring wells with vibrating wire piezometers, borehole extensometers, and shape arrays. The remote system allowed for collection of data at a much higher frequency than manual data, allowing the contractor to make proactive decisions rather than reactive decisions.