Perils of Interceptor Surcharging

Utilities are more and more frequently resorting to real-time control systems that intentionally block flows in interceptors to take advantage of available in-line storage during storm surges to combat overflows from sewers. Many utilities are also more and more frequently extending their pump station's wet wells into the sewers feeding the wet well to raise NSPSH and to effectively enlarge their wet wells, improving pump performance. Both practices have significant operational end environmental value. Overlooked in these efforts are the impact of often quite significant internal pressures on pipes with joints not capable of containing these pressures. Surcharged sewers with leaking joints, when pressurized even a few feet, pass massive volumes of water outside the sewer into the surrounding pipe bedding and pipe trench backfill. When hydraulic grade lines subside to normal (open channel) operating levels, the sewage pushed outside the pipes pours back into the pipe, bringing bedding fines that eventually result in pipe bedding envelope failure and pipe collapse. Using the case study of the Howard Roosa Combined Sewer Interceptor in Evansville, Indiana to illustrate these points, this paper identifies the causes, mechanisms of failure, evidence of likely failure of surcharges, inspection techniques to identify pipes at risk if surcharged, and rehabilitation methods once a pipe is thus compromised. This paper is relevant to any utility considering or using in-line storage. Relevance, Usefulness, and Takeaways Surcharging interceptors can result in catastrophic and hugely expensive repairs if the pipes are not prepared to contain the pressures. Determining if a pipe can handle the pressures sometimes requires more than CCTV inspection. This paper provides suggested assessment measures utilities can use protect themselves from these failures as well as identify the rather involved corrective measures needed if the wrong pipes are surcharged. Demonstrated Results The project used to demonstrate these condition assessment techniques and remedial measures is largely complete. This presentation will contain meaningful photographs and video clips to convey the lessons learned. Key Lessons Learned 1. Surcharging will remove fines and sink the pipe if joints leak. a.) Most interceptor joints leak. b.) Surcharging induces a geometric increase in joint leakage/pipe failure 2. Before pressurizing a sewer, check soundness of joints. 3. CCTV, even MSI HD inspections, do not provide needed information. 4. Get inside the pipe and get your hands on/in the joints. 5. Inspections in dry weather are very different than inspections following rains. a.) Time your walk to maximize understanding. 6. Understand the pipe bedding envelope. 7. Don't ignore sinkholes - they are your canaries - but don't assume all the subsidence is over the pipe. 8. Repairs that don't address the compromised bedding lead to failure of the repair.