JEC Group have brought together the international community of composites leaders and executives in our Composites Circle as an unique networking opportunity to meet with both peers and future partners.
Elmsford, N.Y., is located in Westchester County midway between White Plains and Tarrytown, approximately 40km from the eight million residents of New York City. Incorporated in 1910, the town has a colourful history dating back to the early 1700s. As with many older communities, maintaining its charm also requires significant maintenance of an aging infrastructure system.
(Published on October-November 2005 – JEC Magazine #20)
Westchester County’s main line, the Saw Mill Valley trunk sewer, was originally c o n s t ructed in the 1940s and was showing signs of aging. Many preventive repairs had been made over the years to prolong its life. The line traverses the town adjacent to the Saw Mill Parkway, collecting and carrying sewage fromthe town to the Yonkers joint waste-water treatment plant.
Hydrogen sulphide takes toll
However, in December 2002, a major collapse of the pipeline occurred at the intersection of Route 119 and the Saw Mill Parkway. Years of attack from hydrogen sulphide gas had finally taken their toll. The gas was generated in the slower-flowing sewerage, then released at more turbulent areas. The H2S gas reacted through a microbial-induced corrosion process with the existing bacteria in the sewer to create sulphuric acid that eventually deteriorated the roof of the concrete pipe. The brunt of the damage occurred at the point where a force main discharged into the interceptor sewer.
To avoid interrupting service, the County quickly stabilized the collapse so that flow could continue through the pipe. They engaged the engineering firm Camp Dresser & McKee (CDM) of Cambridge, Massachusetts to assist them in a further evaluation of the pipe’s condition. The goal was to find a solution that would extend the life of the interceptor sewer. Field investigations included closed-circuit video inspection of the downstream pipe, which revealed significant deterioration that affected several pipe sections totalling approximately 335.3 metres.
Three possible solutions were evaluated, including open-cut replacement and two rehabilitation methods. The first rehab option would have required bypass pumping, in order to work on the interceptor while it was dry. The second rehab possibility was to slipline the pipe without interrupting the flow.
The ultimate decision was affected by such factors as the risk and hassle of bypass pumping, the resulting disruption to the local traffic, and the capital cost. Sliplining “live” was selected.
Corrosion resistance needed
Contract documents were developed around the chosen alternative. The only product that was specified for this sliplining method was Hobas CC-GRP (Centrifugally Cast Glassfibre Reinforced Plastics) pipe. The reasons included its high resistance to sulphuric acid.
Hobas pipes are stringently tested in accordance with international standards in a sulphuric acid environment.
When CDM designed the liner pipe, they gave no consideration to the existing concrete host pipe because of its deteriorated condition. The County had faith in Hobas since they had previous experience with the product.
Years earlier, the lower end of the 76-cmdiameter force main that discharged into the interceptor sewer had been relined with Hobas 61cm diameter sliplining pipe. This line, which was rehabbed in 1997 as part of the Tarrytown Extension project, is still performing perfectly.
Greatest flow recovery
The existing sewer segments were of diameters 1500 and 1600mm, so Hobas SewerLine of DN 1300 and 1500 diameter were chosen. This downsizing would allow for a design clearance of 25.4mm between the host inner diameter and the liner outer diameter.
A bell-spigot joint that is flush on both the exterior and interior was used to provide the greatest clearance between the existing concrete host sewer and the outside diameter of the joint, which is typically the area with the tightest fit.
CDM completed a hydraulic analysis to determine if the reduced diameter would also diminish the interc eptor’s flow capacity. “We perf o rmed a hydraulic analysis of the sewer, comparing present conditions to the new relined sewer and, even with the decreased diameter, found negligible decreases in flow capacity,” said Chris Patackis, e n v i ronmental engineer with the New York office of the engineering firm.
The extremely smooth, Manning’s 0.009 interior of the Hobas CC-GRP pipe allows for substantial flow recovery in most sliplining applications.
Northeast Remsco Construction, Inc. of Toms River, N.J., has been installing Hobas pipes for over a decade and was aware of its benefits from previous installations. “We sliplined about 305m of existing 2000-diameter sewer with DN 1800 under active flow conditions in Massachusetts. Also, we pushed about 91.4 m of SewerLine DN 1300, in the dry, through a casing we installed undera flood control dike in Connecticut.
On the same project, we installed about 152.4m of 1.45m pipe via microtunnelling under the Connecticut Department of Transportation Route 2. All our prior experience with Hobas pipe systems has been positive,” said Richard Palmer, p roject engineer with the installation contractor.
- Year of construction: 2003-2004. - Duration of construction: 3 months. - Length of pipes laid: approximately 285m of 1.5m-long pipes and app. 37.8m of 1.37m-long pipes. - Pressure class: PN 0. - Diameter: DN 1300 & DN 1600. - Stiffness class: SN 6000. - Method of installation: sliplining. - Application: sewage collection. - Client: Westchester Department of Public Works. - Contractor: Northeast Remsco . - Construction, Inc., Toms River, N.Y.
Features: - high corrosion resistance and negligible decreases in flow capacity despite the decreased diameter. - extremely smooth interior.
The pipe can be inserted in high or low flow and pushed up stream or down. In this case, the segmented, gasketsealed pipes allowed for live insertion. “ T h e re was no option to bypass flow from this interceptor sewer, and at times the flow was within six inches of the crown of the pipe,” Palmer explained.
The Hobas pipe also gave the contractor flexibility in the pushing directions and pit locations. This was a great benefit, since the existing 1.68m line made several direction changes and varied in diameter along the project alignment. The line was located 4.57m deep along several highly travelled roadways, requiring that the installation pit locations be chosen carefully.
Field service available
The project was rather complex even though it was fairly small, consisting of 285.3m of DN 1500 and 37.8m of DN 1300 pipe, plus a variety of closures and fittings. Because of the complexity and other reasons, the Westchester Department of Public Works required that a Hobas representative be available on site during the entire project.
Hobas field service personnel were there at the beginning of the job, and representatives were in constant contact with the contractor to provide assistance when it was needed.
Once installation was complete, the line was grouted and inspected. The annulus at each end of each pipe run was bullheaded. Then, cellular g rout was pumped into the annular space between the host pipe and the sliplined pipe. A post-construction video inspection and videotape were re q u i red to verify the complete and p roper installation of the pipe.
The project was started in December 2003 and, despite the winter weather, it was substantially complete by Febru a ry 2004. With this successful project behind them, Westchester County is looking forw a rd to years of reliable service from their new pipeline.