The City of Riverside, Calif., recently faced the need to upsize existing sewer lines in a very tight, densely populated area of its central city. City planners answered the design challenge by capitalizing on the benefits provided by a traditional material combined with new installation techniques.
Riverside?s wastewater collection system comprises 870 miles of pipelines and serves a population of 300,000, transporting approximately 31 million gallons of flow daily. A 6-inch sewer line, originally installed in the 1930s, was now undersized for the area and population it served.
The city needed to find a way to overcome several design obstacles. The alignment of the sewer to be replaced ran through narrow alleyways between residential buildings. A traditional open-cut installation would have blocked traffic access for residents and impacted adjacent streets. The easements were very narrow and prohibitive for open excavation.
Additionally, most of the buildings were multi-unit residences and disruption to their wastewater services needed to be kept to a minimum, so timing and service coordination were major factors. The existing pipe was mostly 6 inches in diameter and needed to be upsized to 8 inches. These conditions made a trenchless installation preferable to minimize disruption.
Going Trenchless
Pipe bursting was selected as the desired method due to the potential for speed of installation, competitive cost in comparison to other available trenchless methods and the ability to upsize. The city chose to replace the existing 80-year old vitrified clay pipe (VCP) with new VCP with a projected service life of 200 years.
VCP?s abrasion and corrosion resistance, its rigidity and long lifespan make it a preferred choice of material for gravity sewers for many agencies. Until recently however it was not considered an option for pipe bursting. Machine manufacturer TT Technologies Inc., coordinating with VCP manufacturer Mission Clay Products LLC, designed a new static pull pipe bursting machine capable of installing segmented clay pipes. TT Technologies and the National Clay Pipe Institute worked closely with the City of Riverside?s engineering division in the design phase of the project, offering advice and technical expertise on both method and material.
?Partnership and communication between the city, equipment manufacturer, material supplier and contractor is important,? said Bryan Vansell of Mission Clay Products. ?That was something that was really a positive on this project. We all learned a lot from each other.?
The ?Downtown Sewer Main Replacement by Pipe Bursting Method? project included approximately 2,300 linear feet of sewer, specified VCP, to be installed by static pipe bursting. The engineer?s estimate was $693,000, and Arizona Pipeline Co. of Corona, Calif., was awarded the contract with a bid of $630,866. Construction began in early 2009. Mission Clay Products supplied No-Dig VCP jacking pipe in 5-foot lengths, with 316 stainless collars and EPDM rubber compression gaskets. Arizona Pipeline used TT Technologies? Grundoburst 800G static burst machine.
The hydraulically operated machine pushes interlocking steel rods through the existing pipe, backward from the receiving pit to the launching pit. In the launching pit, the bursting head and expander are attached to the bursting rods. The rods are fitted through each new No-Dig clay pipe joint to follow, and the segments of pipe are held in compression behind the expander by a hydraulic end plate that fits behind the last piece of the pipe train. Efficiencies were realized by loading two segments of pipe at a time. The expander and new pipe are pulled back toward the receiving pit, bursting the old pipe and displacing the fragments into the surrounding soil. The equipment?s jobsite footprint above ground is minimal, and was further reduced by the use of the stacked segments of clay pipe. This method eliminated the need for a long laying area as required when using a fused pipe.
The soils in this area were expandable and well suited to pipe bursting. The main challenge was the number of sewer laterals. The multi-unit residences lining the alleyways meant a high concentration of laterals. In order to keep laterals in service for residents throughout the duration of the project, it was required that the contractor reconnect all laterals for each newly burst and installed section at the end of each business day. The contractor pre-excavated the lateral connections, completed a pipe bursting section and then reconnected all laterals to the new mainline in the same day.
In another example of the collaborative effort between city and manufacturers, a more rapid way of accomplishing this was developed using a core drill and Mission Clay?s TwisTee tapping saddle. Once holes were cored in the mainline pipe, the EPDM rubber saddle and ABS tee were installed by hand in seconds. (The grooved tee twists into a grooved saddle, expanding the saddle and locking it in as it goes.)
The depth of the sewer was minimal overall (generally 8 feet), which allowed the laterals to be excavated more easily or their extremely high number (80) would have posed too great an obstacle.
?The number of laterals was a problem. One real positive was that after the contractor?s learning curve with the new method, I would say it was 30 percent faster, or more, than open trench,? said Lonny Young, principal engineer for the City of Riverside.
Vansell was pleased with the outcome of the project. ?I am excited by the opportunities presented by both the pipe bursting method and the TwisTee tapping saddle,? he said. ?We expect to see many more applications for them in the future.?
Garrett Richardson is Regional Sales Manager for VCP manufacturer Mission Clay Products. He serves on the Executive Board of the Engineering Contractor?s Association in Los Angeles, and has worked in the pipeline industry for ten years with training in trenchless construction methods through the Colorado School of Mines and Louisiana Tech University.
