C+S November 2020 Vol. 6 Issue 11


LiDAR Captures Accurate As-Built Condition of Combined Sewer Overflow Pipes in City of Milwaukee

By Jon Chapman and Robert Seleen, P.E.

The City of Milwaukee Department of Public Works (DPW) wanted to better understand the existing condition of their next stretch of com- bined sewer overflow (CSO) pipe to be slip lined. They were interested in finding a cost-effective and timely solution that would mitigate the construction delays and change orders that they had experienced on prior projects. The City contracted with the civil engineering and sur- veying firm of raSmith (Brookfield, Wis.) to provide advanced survey technologies and create current and very accurate as-built documenta- tion of their CSO pipes. The City’s Combined Sewershed 3016 is comprised of over 4,000 acres and incorporates a large Combined Sewer Overflow (CSO) net- work. Pipes range in size between 66 and 144 inches in diameter and are generally located 35 to 90 feet underground. Installed in the 1880s to 1920s (with additional installations in the 1990s), the aging system requires regular rehabilitation. Since the 1960s the City of Milwaukee DPW has identified and re- paired pipes within the network. Past repairs have involved grout work, formed concrete liners, steel liners, shotcrete, and fiberglass mortar pipe (slip lining). Slip lining is a trenchless technology that has been used since the 1940s to repair leaks and rehabilitate existing pipes and is generally considered to be very cost effective. Slip lining is completed by installing a smaller, carrier pipe into a larger existing host pipe. The particular section of pipe that the DPW was recently looking to slip line was comprised of approximately 3,700 linear feet of pipe, ranging in diameter between 6 and 7.5 feet, at depths ranging between 35 and 50 feet. This stretch contained three radii and four tangent sec- tions of pipe. While there are several different methods used to install a slip line, this particular project utilized what is known as the segmental method. This construction process involves excavating and exposing a section of existing pipe, allowing a rigid slip liner to be lowered into the pipe with a crane. Nominal length sections of 15 to 20 feet of slip liner are lowered one at a time into the pipe. Each segment then rides on a series of rails that are preinstalled in the bottom half of the pipe. After each pipe segment is lowered into the opening and is resting on the rails, it is then pushed horizontally into place by a skid loader that is lowered in before the pipe segments. Once each segment is pushed into the correct horizontal position, the annular space between the two pipes is grouted and the ends are sealed.

One of the challenges with this process presents itself when trying to slip line a curved section of pipe. Because each segment of slip liner is rigid and straight, and is pushed horizontally into place, it is very important that the length of each segment is no longer than what the radius of the existing pipe curve will allow to be pushed through it to avoid being hung up or stuck. Assuming you have confidence in the accuracy of the existing drawings, these maximum lengths can theoretically be calculated. Accuracy issues with old drawings aside, existing drawings are not going to show certain as-built conditions like pipe deflections caused by settlement or shifting, sediment buildup or other deposits that may also impact the ability to slide the slip liners through the pipe. An example of one of the cross section views of the CSO pipe, showing nominal diameter versus actual diameter due to deposits, deflections or other anomalies.

Map showing the location of the 3,700 LF of CSO pipe that was scanned, including three radii and four tangent sections.



november 2020

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