Sliplining Milwaukee’s 90-Year-Old CSO to Increase Flow Capacity
One concern that almost all municipalities have about the sliplining process is the ability of a line to retain flow conveyance capacity after lining a host pipe with a smaller diameter pipe. Due to the nature of the lining process, any lined pipe will have a smaller diameter than the host pipe once the lining is completed. This project was determined to be an ideal test case to understand the effect of lining the large diameter sewer on the sewer’s maximum flow capacity.
The City of Milwaukee, Wisconsin’s Combined Sewer Overflow (CSO) was designed and constructed in the 1920s as a relief sewer to protect against overflows and flooding of sanitary and storm flow after heavy rain events. Due to advanced age, sections of this system have been losing integrity over the past few years. The CSO located 60 to 70 ft below the existing street grade, is not a part of Milwaukee’s Deep Tunnel System, which is owned and operated by the Milwaukee Metropolitan Sewage District, which was built in the 1990s to augment the CSO.
The CSO line was inspected in 2015, revealing large running cracks at the sewer key (top of pipe) and heavy I/I (infiltration/inflow) issues in a particular section along North 26th Street from Cherry St. to McKinley St. It was decided that this 1,042-ft section would be rehabilitated as part of the City’s 2017 sewer program, according to Wadee Rafati, civil engineer with the City of Milwaukee’s Environmental Engineering Section. The existing host pipe in this section, constructed in 1926, was a 120-in. monolithic concrete sewer (tunnel Section) with a vitrified brick lining at the flow line.
This particular sewer was the ideal candidate for the sliplining rehabilitation method. The sewers size and depth make a full-scale replacement too costly. Also, the condition assessment did not show any significant loss in the host pipe shape or significant sagging, so the sliplining pipe could be installed and would have a positive slope. In addition, this sewer is also a CSO pipe and designed to carry overflow after a significant rain event and do not directly serve any parcels.
The City’s in-house staff engineers performed the hydraulic analysis of the existing CSO and the proposed sliplined CSO based on flow characteristics provided by Hobas.
The City has been using Hobas pipe material for sliplining projects for years, in order to preserve the integrity of original lines. The sliplining method provides the benefits and life of a brand-new pipe while creating minimal construction impact. City of Milwaukee staff engineers designed the sewer rehabilitation by specifying 110-in. inside diameter (ID) Hobas Centrifugally Cast Fiberglass Reinforced Polymer Mortar (CCFRPM) Pipe to be slip lined into the 120-in. existing line.
Host pipes of this age generally exhibit mineral deposits, cracks and deflections; therefore, sometimes significant preparation must be performed. “The host pipe must be in a structurally sound condition (no sagging, or shape loss in pipe) for sliplining to be considered as a pipe rehab option,” said Rafati. “The host pipe must be cleaned, and any protruding attached materials (calcium, etc.) must be cut out before installation.”
Removal of foreign materials that intrude on the host pipes inside diameter, allows the City to maximize the sliplining pipe that can be used. In this case, a 110-in. liner with a 114-in. OD was placed inside a 120-in. host pipe.
The City’s EES in house engineers prepare the plans, specifications, engineer’s estimates and bid documents for all sewer projects in Milwaukee. This project was no different and because of the EES’s familiarity with preparing sliplining projects, they received competitive bids from four regional contractors.
Globe Contractors Inc. of Pewaukee, Wisconsin, was contracted to rehabilitate the line. The firm had experience with Milwaukee’s deep sewer lines that connected to the CSO system. Globe was able to place their work shaft in a very convenient location, minimizing disruption to traffic and the public, according to Rafati. “The contractor worked very fast, digging the work shaft and installing the pipe much quicker than what was anticipated. We were very pleased with their work,” he stated. “They spent about one month digging the work shaft, and about two months installing the liner.”
Monitoring the Improvement
RJN Group Inc. (RJN) conducted before and after flow monitoring to understand the effect of lining the large diameter sewer. RJN installed ADS Flowshark Triton+ flow monitors equipped with a submerged area/velocity (AV) probe and a crown-mounted ultrasonic probe in two locations – one upstream and one downstream of the section to be sliplined.
The submerged AV sensor took two depth measurements via a pressure sensor and an up-looking ultrasonic sensor, while also measuring the peak velocity. The crown-mounted ultrasonic probe measured the depth of flow for redundancy. In order to verify that the flow monitors were collecting accurate data, RJN performed manual calibrations throughout the metering period of three weeks prior to the lining and three weeks after completion of the lining.
The ability to convey flow is not only affected by the size of the pipe but also the slope of the pipe and the roughness of the pipe. In free-flowing (flow by gravity) conditions, Manning’s Equation can be used to predict the roughness and resulting capacity of pipes.
Manning’s Roughness Coefficient can be calculated by using meter data and the as-builts both before and after the lining to obtain a value that symbolizes whether the liner has a rougher or smoother surface than the host pipe, and thus a decreased or increased ability to convey flow. As “n” is in the denominator of the Manning’s Equation, a lower n-value correlates to a smoother pipe and, subsequently, the ability to convey more flow.
Readings from the monitors were plotted on a scattergraph in order to illustrate the flow velocity versus the flow depth for each collected data point, creating a visualization of how the depth and velocity vary with one another.
After the lining, the pipe’s flow surface was the Hobas liner resin, which is known to be a very smooth surface. By using the collected data and a regression analysis, the Manning’s Roughness Coefficient for the pre-lined and post-lined pipe was calculated. In this location, the data indicated that the coefficient went from a pre-lined 0.015 to a post-lined 0.010 and increased the Manning’s maximum flow capacity of the pipe from 575 million gallons per day (MGD) to 650 MGD.
The result was that RJN’s testing and analysis proved that the decrease in roughness was so significant that flow capacity at each location actually increased despite the reduction in diameter from a 120- to 110-in. pipe. The upstream meter location also showed evidence of increased flow conveyance capacity despite being upstream of the actual lined pipe. This can be explained by the fact that consecutive pipes act as one continuous composite, ergo not one location will be affected by the rehabilitation, but the whole “run” of pipe will experience a change.
“The sliplining method allows for minimal construction impact, while still getting the benefits and life of a brand new pipe.
“The increase in flow velocity due to the smooth surface of the HOBAS pipe (lower manning’s roughness “n”) increases the pipes overall capacity. This increase helps to offset the potential decrease in the capacity caused by decrease in pipe size after sliplining. Of course an increase in the sewer flow rate gives the sewer more capacity, which is important,” according to Rafati.
HOBAS is working with the City of Milwaukee to perform flow monitoring to test 10-year old installed HOBAS pipes, to confirm manning’s roughness coefficient “n” value do not deteriorate over time.
To read the complete RJN Group white paper describing the monitoring of this project, email Kimberly Paggioli, P.E. at KPaggioli@HobasPipe.com or contact your Hobas Pipe USA area manager.