Innovative Inspection of Pump Station No. 1 Wet Well

Purpose This abstract presents an innovative approach to the inspection of a large sewage wet well with an isolation procedure that allows for continued availability of the pump station to provide continuous service under all flow conditions. Benefits of Presentation This presentation will provide a benefit to the industry by sharing insights into the development of a plan that manages flows, selection of isolation equipment, describes installation techniques, summarizes elements of a successful safety program, and lists inspection procedures that used multiple inspection technologies to collect high quality data. Background The Great Lakes Water Authority (GLWA) is implementing an array of rehabilitation measures at its Pump Station 1 (PS-1) raw sewage pumping station in Detroit. Built in the 1940s, Pump Station 1 has eight influent pumps and a wet weather design capacity of 1,444 million gallons per day. It is one of the largest pump stations of its kind in Michigan. Planned improvements at the facility include the rebuilding of existing pumping units and various process/mechanical, architectural, and electrical upgrades. Wade Trim is leading a team undertaking the study and engineering design services. The study phase included a visual condition assessment of the concrete and process mechanical elements of the lower wet well areas which are normally under water at depths of 20 to 30 ft of sewage. The facility serves a combined sewer service area and is required to be ready for continuous operation to convey high peak flows during wet weather conditions. Shut down of more than one pump at a time and/or bypass pumping is not an option to allow for isolation and inspection of the wet well areas. Wet Well Inspection Plan Historically, the split wet well located nearly 50-ft below grade has been inspected after installation of individual timber stop logs placed by divers to take half of the pump station (nearly 4 of the 8 pumps) out of service for months at a time. Figure 1 highlights this approach. However, with changing regulatory requirements, the facility needed to be at the ready to provide for wet weather capacity at any time during a storm event. This required a change in the wet well isolation approach by the engineering team. To support a new inspection plan, a hydraulic assessment of various alternative configurations was conducted to see how flow could be delivered to the pumps during storm conditions while maximizing isolation measures set in place. Working with GLWA, Wade Trim identified a plan that included the following components: 1. Purchase and installation of a new monorail system to assist with installation and rapid stop log installation and removal 2. Purchase and installation of new stop logs that could be assembled in sections to allow for ease of removal of a top 10-ft section to allow flow to spill into the isolated wet well section only during storm events 3. Development of an operational sequence to be conducted during storm conditions to allow for removal of staff, inspection equipment, and raising of the stop log segments 4. Design of a rigging and control system that could remove the top sections of stop log walls 5. Preparation of a specialized safety plan to address potential hazards of work in the live wet well areas 6. Development of revised pump operational protocols 7. Selection of survey scanning, photographic and video equipment to document the dimensions and conditions 8. Extensive coordination with the owner, specialty subcontractor, and engineering inspection staff. Figure 2 shows a representation of the revised isolation approach. Not only did the new stop log system provide for isolation of the wet well, but if a significant rain event occurs during the isolation/inspection, the top ten feet of stop logs can be raised allowing the wet well to be put back into service. Wet Well Condition Assessment Techniques Among multiple innovations during this inspection was the use of 3D model capture and scanning equipment. Using mobile laser scanning technology, wet well inspection data was scanned and placed into an immersive cloud-based platform to assist with study alternatives and future design collaboration. Nearly every square foot of the wet well was captured in a model through which the owner and engineering team can navigate. Data from the survey was also used by a Hydraulics Lab to build a scale model of the wet well for extensive hydraulic testing of the performance of the facility. Figures 3, through 6 provide examples of the data collected during the inspection process. Status of Completion and Conclusions The study phase and condition assessment has been completed successfully as of Spring 2021. Preparations, procurement, and installation of the monorail systems took approximately 5 months to complete. Stop log isolation began in early January 2021 and inspections were initiated in late January over a two-day period in the north wet well. Flows were fully isolated and high-quality data was collected efficiently and successfully. Conclusions of the work include: - A dynamic stop log system can be designed that provides adequate isolation for inspection and features that allow for passage of flows during wet weather operation - An extensive safety program for confined space entry procedures is needed in tough environments - Reality capture technologies allow for rapid photographic and interactive video capture of hard to access areas for ease of review later