In-Place Emergency Rehabilitation of Northwest Interceptor In Urban Conditions

The purpose of this paper will be to present recent advances and techniques along with proper application of proven methodology to rehabilitate two structurally compromised reaches of the Northwest Interceptor Sewer (NWI) in the City of Detroit, Michigan. The NWI is one of the major interceptor sewers in the Great Lakes Water Authority (GLWA) system and serves hundreds of thousands of residents and businesses in Detroit, Redford, Livonia, and Westland, Michigan. As part of the asset management initiative GLWA inspected the NWI using remote operated vehicle (ROV) video techniques in absence of flow control, preventing the lower half of the NWI to be inspected or observed. The ROV inspection revealed several reaches of the NWI along Trinity Avenue and Pierson Street at Warren Avenue that appeared significantly distressed and required emergency rehabilitation. Further investigation, evaluations, and designs were required to perform the required rehabilitation. Manned inspections of these NWI reaches were attempted but were extremely limited due to the high flow conditions. At that time, GLWA activated their Emergency Repair Contract CON-149 with Inland Waters Pollution Control (IWPC) to implement flow control devices and perform the required rehabilitation under a design build approach. An extensive flow control plan was developed and implemented to gain entry. This plan included new flow control gate construction, diversions to other nearby interceptors, and wet weather flow-through designs. Coordination with environmental agencies and knowledge of the GLWA system were important aspects to develop this plan. A geotechnical investigation conducted as part of the initial effort revealed silts and sands surrounding the pipe, with a static groundwater level high enough to drive soil material into the pipe through major cracking, causing concern for further degradation of the pipe. Following implementation of flow control, a manned entry was performed, which revealed more extensive deterioration than was initially apparent, including several sections on the verge of collapse. Immediately following the inspection, rehabilitation options were developed and reviewed based on the variable condition of each reach. Cured-in-Place rehabilitation was not selected due to requirements for long term wet weather flow control that would have been extremely expensive and that would have presented unacceptable risk to the environment. Slip lining methods were not selected due to the large entrance shafts that would be required and the unacceptable reduced lining diameter that would have resulted. Ultimately, a rehabilitation system was selected consisting of: 1) Stabilizing the existing host pipe by emergency support, followed by 2) restoring ground support around the host pipe, and then 3) Installing a reinforced sprayed geopolymer liner to restore the structural lining. This approach allowed for restoration resulting in maximum pipe diameter, with the least risk to the environment. Initial emergency stabilization consisted of first installing steel ribs in the most degraded reaches, followed by groundwater dewatering to minimize further loss of ground through the lining into the sewer. Geotechnical instrumentation consisting of piezometers and surface settlement points were installed to monitor the ground surface above the interceptor, followed by cement and chemical grouting to stabilize the ground around the pipe and further minimize loss of ground into the pipe. Major cracking was repaired in some reaches by installing reinforcing steel rods and anchors followed by over-patching with Eco-Cast lining materials. Following stabilization of the host pipe, the interceptor reaches considered compromised were re-lined with a steel reinforced Eco-Cast lining system. The system consisted of installing horizontal and circumferential steel to limit future cracking of the Eco-Cast liner. This presentation will discuss the emergency approach to the inspection, design, flow control, and rehabilitation methods of the NWI, during which time continuous sanitary sewer service was maintained without interruption, and no sanitary overflow occurred to the environment as a result of the project. Unique and creative methods for accomplishing this repair will be discussed, including the use of a dry weather diversions and bypass; and developing a fully structural finished and crack resistant spray-on lining with thickness of only 3 inches. The rehabilitation of the Trinity and Pierson reaches are complete and flow has been restored to the NWI in this area, with minimal surface disruption and no negative impacts to the environment. The project leaves behind a permanent flow diversion structure that will allow inspections and maintenance for years to come. As a side note, ongoing follow-on tasks for this project will provide two additional permanent flow control structures. This rehabilitation project illustrates several conclusive lessons to the industry described as follows:
Full dewatered inspection of Interceptor sewers is essential, as the initial ROV inspection of this sewer was not adequate due to high flows. Where practical, sewer owners should preemptively install flow controls within their major interceptors that will facilitate regular full-diameter inspections.
Repair methods must be balanced against available and feasible flow control options.
Distressed Interceptor reaches are often distressed due to poor soil conditions such as wet silt/fine sand conditions.
Grouting these reaches to re-establish host pipe confinement is essential usually requiring an exterior dewatering system.
Use of a reinforced sprayed liner system is an effective method to be employed for limited flow control conditions while providing a structural and flexible complete repair, with minimal diameter reduction. Reinforcing is essential for spray-lining systems where future movement or reflective cracking of the host pipe is a possibility.