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Trenchless and sliplined: what a way to go
Knoll, H.
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Trenchless and sliplined: what a way to go
Knoll, H.
A version of this paper is published in / Une version de ce document se trouve dans : Ottawa Construction News, v. 10, no. 12, Dec. 2000, pp. 12, 15
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Trenchless and Sliplined: What a Way to Go
By Henry Knoll
Two summers ago, Robinson Consultants and Dufferin Construction were busily giving new life to an 85-year-old watermain under the full length of Gloucester St. in Ottawa. Yet the people of the city were never fully aware of it. That’s because a trenchless sliplining technique was used for this award-winning project.
Says Mr. Hugh MacDonald, construction supervision engineer of Robinson on the project, “Workers literally slipped an 840-mm plastic pipe into the 915-mm cast-iron watermain, while motorists and pedestrians went about their business undisturbed. I should stress that the firm, Trenchless Design Engineering, helped us make that possible.”
Where bends and valves were involved, construction required the conventional – open-cut – method used to place pipes. The plastic pipe to which MacDonald refers is actually one made of high-density polyethylene (HDPE) which is the same material used for making yogurt containers.
Not having to dig up the street above the watermain not only saved the Region of Ottawa-Carleton and its citizens the frustrations of enormous traffic congestion but also about $1 million in capital costs, compared to conventional techniques according to Mr. Michael Willmets, the Project Manager of the Region. “We not only saved frayed nerves and money, we also saved time, because with conventional techniques, this project would have taken nearly twice as long,” says MacDonald.
All of this so impressed the jurors of a competition sponsored by the Association of Consulting Engineers of Canada that the project won for Robinson Consultants of Kanata an award of merit this fall.
Today, two years after the successful project, water is flowing nicely through the rehabilitated watermain. However, the researchers at the National Research Council’s Institute for Research in Construction (IRC) and the engineers at the Region still show unrelenting interest in the project. In a long-term monitoring program, the researchers are collecting data that is being faithfully transmitted directly from the watermain to their laboratories via data loggers over a telephone line.
Dr. Jack Zhao, the manager for the monitoring program, explains its purpose. “These data provide us with insights into the internal and external conditions of this rehabilitated feeder watermain,” he says.
Trenchless construction means that there is a minimum of excavation from the ground surface. Among this minimum of excavation is usually several large pits, where a section is cut out of the old watermain. Through this opening, sections of a new pipe are pushed or pulled, so as to finally line the entire old watermain, which has now become what is
called the host pipe. A new pipe can be inserted either as one continuous pipe for pressurized pipe or as a string of discrete pipes for gravity pipes. In continuous
sliplining, lengths of pipe are heat welded together before installation in the host pipe. In this project, sections of pipe ranging in length from 130 to 270 metres were inserted. While the HDPE pipe is inserted and even after the construction, the host pipe continues to carry the weight of all the soil above and beside it. Therefore the condition of the host pipe and that of its environment can considerably affect the performance of trenchless sliplining rehabilitation. It is common practice to design the liner pipe as though it alone has to withstand all the expected stresses during its service life. However, the liner pipe, once it has been installed, interacts with the host pipe, even more so where the annular space between the two pipes has been filled with grout, which was the case with this project.
“Just because a host pipe is being rehabilitated does not necessarily mean that it will soon fail,” says Zhao. “In many cases, the host pipe continues to be structurally sound for many years, and thus prolongs the life of the new pipe.”
MacDonald explains, however, that the old watermain was not in good shape. “It had corroded to a third of its original thickness and, besides, it also had casting flaws,” he says.
Just what the factors are that give this new lease on service life is what the monitoring system devised by the IRC researchers is to determine. The monitoring system consists of numerous strain gauges on the inner surface of the cast-iron pipe and even more on the outside of the HDPE pipe.
“The measures of strain are used to calculate the stress and deformation of the pipe structure,” says Zhao. “This information together with measures of other stresses
indicates how well the pipe structure is performing and is likely to perform in the future.” Even these gauges and other instruments were installed without street excavation. Of all these sensors, strain gauges are the most delicate and time-consuming to install, yet their performance and life expectancy depend heavily on the quality of installation. To install them on the HDPE pipe, two sections were cut from an HDPE pipe at the construction site and taken to IRC’s laboratories, so that the gauges could be glued on there. Several precautions against damage during their later insertion were taken. “Strain gauges are so delicate, you just can’t be careful enough with them,” says Ms. Lyne Daigle, one of Zhao’s colleagues.
The strain gauges on the cast-iron pipe were installed on site. The challenge with them was to find a quick and suitable technique that could be used by persons entering into the confined space of a pipe only 915 mm in diameter.
and as receiving pit for the east portion. For the west portion, the instrumented section was fused onto the regular pipe length in the street and inserted by pushing. Four
bearings at the opening of the host pipe helped centre the HDPE pipe during its insertion to avoid damaging the sensors and cables already installed inside the cast-iron pipe. The insertion of the HDPE pipe on the east side was carried out somewhat differently, because of concerns over the long distance through which the sensors would have to travel inside the host pipe.
Zhao credits teamwork for the success of the project. “Collaboration between all the participants made the implementation of the performance monitoring system a great success,” he says. “The readings from it since installation indicate that they are bound to contribute to a better understanding of composite pipe behaviour and pipe-soil
interaction.”
The greater insights are, in turn, expected to contribute to the development of the best ways for predicting the service life of rehabilitated watermains installed using a trenchless method.
Henry Knoll is an Ottawa freelance writer and editor whose firm Scientech Documentation serves Ottawa-area clients.
