Detroit River Interceptor Flow Control Facilities Construction

This abstract summarizes recent design and construction efforts undertaken by the Great Lakes Water Authority (GLWA) to repair and increase the level of service of the 90- to 100-year-old brick and concrete Detroit River Interceptor Sewer (DRI) in the City of Detroit, Michigan. The sewer is 8 to 16-feet in diameter, approximately 65,000 linear feet long, and serves as one of GLWA's primary interceptors. The work involved a full engineering inspection, including portions that had not been inspected since their construction in the 1930s. Because of the lack of redundancy in the system as well as high flows and lack of flow controls, a major challenge was gaining access for the inspection and repairs. Ultimately, the design-build team selected by GLWA devised a unique flow control system involving in-system storage and diversion structures, that are allowing for detailed inspection and controlled repairs of the system. This paper will describe development of the flow control system, together with the construction that was substantially completed in August 2021. Background As large sewers in major cities across the country begin to exceed their design lives, the need for creative flow control facilities will only increase as these aging assets require repair. These permanent facilities provide long term benefits over the use of temporary bypass pumping that is expensive and require large set up and piping areas. The DRI is one of the major interceptor sewers in the GLWA sewage conveyance system, serving hundreds of thousands of residents and businesses in the northeast Detroit area and suberbs. Between 2012 and 2016, an inspection was completed of the DRI that revealed significant distress in some sections. The main challenge in conducting the rehabilitation of the DRI was the lack of flow control facilities to reduce/eliminate flow within the tunnel to allow for manned entry to conduct the repairs. The upstream reaches of the tunnel could be accessed after draining these sections via a series of pump station shutdowns that gave crews a relatively normal working day during dry weather conditions. However, the downstream three quarters of the DRI did not benefit from this approach enough to allow for manned entry, and certainly not for conducting repairs. In order to accomplish flow control, the winning design build team of Jay Dee Contractors (Jay Dee), design engineer FK Engineering Associates (FKE), and specialized subconsultant's Applied Science (ASI) and Anderson, Eckstein and Westrick (AEW) proposed the construction of two flow control gate shafts, a diversion chamber with a connecting tunnel to the North Interceptor East Arm (NIEA), and two diversion drop shafts on the Morrell Sewer and Livernois Relief Sewer. These facilities, along with temporary pumping at the WRRF Pump Station No. 1 (PS-1) allow for wastewater levels low enough for manned entry and repairs within the entire DRI during dry weather conditions. Flow Controls Development and Design Hydraulic analysis was performed using existing flow meter data, the GDRSS model, and previous studies. The detailed flow control study involved inspecting existing regulators, determining bypass pumping arrangements at pump stations, investigating diversion locations, making flow measurements on numerous trunk sewers and the DRI, confirmation of the storage times with updated flow rate information, and determining the procedures for gradually releasing the stored wastewater. On this basis, a flow control system was devised consisting of two in-system flow control gates, a 1000-foot ling 9'-2' tunnel diverting flow to the adjacent NIEA Interceptor, and four flow diversion structures. A detailed operation protocol was also developed to allow for safe operation of the system, while maintaining sewer service to all upstream customers. Following the hydraulic analysis of the in-system storage gates, flow diversion structures, and the NIEA diversion Tunnel; there were a number of challenges in design of these structures. Each of these structures was located in heavily urban areas, with a myriad of utilities, and critical infrastructure that needed to be protected. Design of each structure had to consider relocation of existing sewers as large as 60 inch diameter, high pressure gas mains, water transmission mains as large as 42 inch, and various other utilities. An addition, geotechnical conditions that involved miscellaneous urban fill required special provisions to safely construct the new underground facilities. And in all cases, structures and utilities immediately adjacent to the new structures and tunnel were protected through incorporation of a robust geotechnical instrumentation program. Construction The in-system storage gate shafts were constructed first, each consisting of a temporary earth retention structure (TERS) that encompassed the existing tunnel, each about 15-feet in diameter and up to about 30 feet deep. Within each shaft, the existing DRI was exposed to below springline and the top half of the tunnel was removed via wire saw. Permanent reinforced concrete gate shafts were constructed on top of the existing tunnels, bearing on the tunnel at springline. Stainless steel gates and rolled angle gate guides were installed to hold flow within the existing tunnel upstream of the gate, allowing for approximately 8 to 12 hours of storage time and man access downstream. Following completion of the flow control shafts, work began on the DRI diversion chamber and crossover tunnel to the NIEA, see Figure 2 below. The diversion chamber consists of a similar set up to the flow control shafts, however, it included the 9-feet 2-inch diameter crossover tunnel that allows for all upstream flow to be diverted to the parallel NIEA tunnel. The tunnel was mined with an open face tunnel boring machine utilizing a rib and lagging primary liner and a glass fiber reinforced polymer mortar pipe (GFRPMP) finished liner to reduce corrosion impacts. Construction of two drop shaft diversions were constructed on the Morrell Sewer (see Figure 3 below) and Livernois Relief Sewer, both tributary to the DRI, to divert flow to the NIEA downstream of the crossover tunnel, thereby reducing flow into the downstream DRI. The drop shafts consisted of TERS constructed over the existing NIEA tunnel to allow for the construction of finished concrete gate structures and hand mined lateral tunnel connections to the existing sewers. With the completion of the described flow control facilities, flow depths and velocities can now be controlled within the entire DRI during dry weather allowing for manned entry inspection and tunnel repairs, which are ongoing.