Some communities in Illinois have gravity sewers still in service that are over 100 years old, and these sewers were built to convey sewage in anticipation of development. Gravity sewers now follow underground paths that must cross over and under other utilities and infrastructure, in addition to public right-of-way (ROW). Crossings may involve traversing state routes, railroads, airports and under structures. It begs the question: why was this facility built over the sewer?
The use of Cured-in-Place-Pipe (CIPP) to rehabilitate gravity sewers in the United States has become common due to the cost being comparable to open cut excavation replacement. To rehabilitate gravity sewers and extend their service life by 50 years or more with minimal or no excavation whatsoever is a huge benefit. It just makes sense to take advantage of CIPP technology to rehabilitate hand built sewers, but how should design thickness be calculated when there is more than just earthen overburden above the pipe?
Design of CIPP liner thickness is governed by ASTM Standard F1216 – Standard Practice for Rehabilitation of Existing Pipelines and Conduits by the Inversion and Curing of Resin-Impregnated Tube. Several variables are considered in designing the liner thickness, such as groundwater level, soil type and depth and live load. When the sewer crosses public ROW and state routes, live load is typically based on the HS-20 axle loading as dictated by the American Association State Highway and Transportation Officials (AASHTO). How does the design change when the loading over the sewer main is not the trench overburden and AASHTO requirements, but is instead, live load being imposed by a railroad, aircraft runway, building or some combination? This presentation will elaborate on the challenges of specifying CIPP design parameters for different live load scenarios. Two examples from the Town of Normal, IL will be presented, discussing how CIPP liner thickness was designed for multiple loading scenarios and how these challenges were addressed during design and construction.