When heavy rainfall hits the centuries-old historic center of Alexandria in the state of Virginia, the sewage systems are overloaded. The CSO system integrate sewer lines from buildings and catch basins that collect stormwater during rainfall. These flows travel through a combined sewer line to a water treatment plant, where they are processed before being discharged into natural water bodies such as rivers. However, when heavy rainfall exceeds the treatment plant’s capacity, a sewer regulator diverts excess flow directly to the discharge point, leading to potential environmental contamination.
The inefficiency of these systems during extreme weather events stems from urban expansion. When these systems were originally designed in the 1800s, the volume of sewage was significantly lower. As metropolitan areas grew, the construction of buildings and roads reduced permeable surfaces, increasing runoff and overwhelming the CSO systems.
To prevent wastewater from entering rivers during heavy rainfall, Herrenknecht is contributing to Alexandria’s RiverRenew project. This initiative includes constructing a new tunnel system to connect four existing CSO lines, enhancing capacity and preventing untreated wastewater from being discharged into the Potomac River. The project includes a 3,471-meter-long tunnel, a 4,428-millimeter-diameter open-cut sewer pipeline, multiple diversion structures, a pump station, and a wet-weather treatment facility. Once complete, the system will capture and treat 98% of combined sewer flows, reducing overflows from 70 per year to fewer than four and protecting the Potomac River, Hunting Creek, and Hooffs Run. The central component is the 3,471-meter-long tunnel excavated using the Herrenknecht EPB Shield "Hazel," which will direct combined sewage to the AlexRenew Water Resource Recovery Facility for treatment before being safely released into the river. The tunnelling process proceeded smoothly under 3-4 bar.
The TBM setup incorporates a conveyor belt (H&E Logistik) and Multi-Service Vehicles (MSVs from TMS), enabling continuous excavation without reliance on muck cars. The application of a tunnel belt and MSVs for a project of this scale is a distinctive approach that enhances efficiency and production rates compared to conventional excavation methods. The use of MSVs eliminates the need for rail installations while providing flexible transport, resulting in improved production rates.
Hazel was designed for optimal efficiency, ensuring continuous operation and a seamless excavation process. Its performance has been instrumental in maintaining project timelines and delivering high-quality results.
Alexandria´s RiverRenew has been marked by effective collaboration between Herrenknecht and the Traylor Bros. Inc, fostering a strong working relationship, ending with the breakthrough in March 2024. Upon completion, the tunnel system will have the capacity to carry 680 million liters of water per day, significantly enhancing Alexandria’s ability to manage stormwater and wastewater.
