By Jean Thilmany
Using a simulated earthquake, Cornell has determined that a new type of earthquake-resilient utility pipe destined for Los Angeles is ready for the next big disaster.
Margaret Stack, an undergraduate at the School of Civil and Environmental Engineering at Cornell University, was part of the team that set up the test of the earthquake-resistant pipeline. The pipeline was instrumented and placed within a device that can simulate a fault rupture, and then buried beneath 80 tons of soil to simulate a real-world scenario. Courtesy of Cornell University
August 16, 2016—Craig Davis, Ph.D., P.E., G.E., M.ASCE, the water system resilience program manager of the Los Angeles Department of Water and Power (LADWP), headed to Ithaca, New York, last month to get an up-close and personal view of a simulated earthquake. While upstate New York may seem a world away from earthquake country, Ithaca is home to Cornell University and its Geotechnical Lifelines Large-Scale Testing Facility, which is used to investigate how underground structures like pipelines or culverts would distort under extreme ground rupture from such natural disasters as earthquakes.
So in July Davis joined two professors in the geotechnical group of the university's School of Civil and Environmental Engineering—Thomas O'Rourke, Ph.D., Hon.D.GE, Dist.M.ASCE, the project's principal investigator, and Harry Stewart, Ph.D., P.E., M.ASCE, the laboratory director—as well as Brad Wham, a postdoctoral associate in the group, and their colleagues. The team simulated an earthquake in an experiment designed to test a new type of resilient pipeline that is being considered by the LADWP as it seeks to improve its aging water distribution system. And the test was a resounding success.
In 2013 Los Angeles began a program of upgrading its 7,000 mi of water pipelines-many of which are a century old-and the effort is now part of the city's "Resilience by Design" plan, formulated by the city last year to guide its agencies in creating more resilient buildings, telecommunications infrastructure, and water systems. (Read "Preparing for the Big One" in the February 2015 issue of
.) By the end of this year, the LADWP plans to have replaced 175,000 ft of main-line and 13,000 ft of large-diameter trunk-line pipes; by 2020, it aims to have replaced 260,000 ft of main-line and 22,000 ft of trunk-line pipes.
The LADWP has a vested interest in choosing pipelines for this replacement project that are as resistant to natural disasters as possible: the city's water utility system—the nation's largest—crosses more than 30 seismic fault lines. After a large temblor or even after lower-level seismic activity, many of Los Angeles's four million residents could be without water, Davis points out, as could firefighters, hospitals, and other civil services. Additionally, Los Angeles is vulnerable to ever-stronger storms, hurricanes, and landslides. Every day sees at least one break in the city's aging and corroding water pipelines network, Davis says.
So Davis flew across the country to watch the simulated earthquake that would test the performance of a continuous steel pipe that its manufacturer, the Tokyo-based company JFE Holdings, markets as being especially resistant to breaks from earthquakes or other strong forces. Traditional water pipes are joined in sections by a rubber gasket, and if large ground movements occur, those joints may slip and pull apart, causing water to flood into the ground or spew into the air. For this reason, many water utilities—especially those in areas prone to strong ground motions—have turned to earthquake-resistant pipelines that feature specially designed joint mechanisms that enable the pipe to move in sync with the ground without rupturing the joint, Davis says. Manufacturers of this type of pipe include IPEX, of Verdun, Quebec; Kubota, of Osaka, Japan; and U.S. Pipe and American, both of Birmingham, Alabama, each of which have conducted experiments with Cornell's lifelines group.
But JFE's system is different. It is intended for use in large-diameter steel transmission pipelines, and it has no individual segments or joints that could be dislocated during an earthquake. Instead, the pipeline features what its manufacturer calls a wave mechanism that absorbs ground deformations. When the ground shifts dramatically, the feature accommodates bending and axial deformations without a rupture of the pipeline, Wham explains.
Wave features are extra material that can be retrofitted into the pipeline in places where rupture is a concern. The system absorbs the ground movement by concentrating any deformation at the locations of the extra material. When the ground shifts, the wave will bend and displace, rather than rupture, reducing stresses along the pipeline. "The concept is similar to the flexible straws that come with soft drinks; extra plastic is folded up in the area where you want the straw to bend, and it continues to carry fluid regardless of its orientation," Wham explains.
While pipeline manufacturers do test their products themselves, utilities considering pipeline replacements don't want to rely entirely on manufacturers' own tests, Davis explains. Cornell researchers have worked with utilities in Los Angeles, Seattle, San Francisco, Portland, Oregon and Vancouver, Canada, to test new robust pipelines. "Our research is focused on making water supplies and other critical lifeline systems resilient to natural hazards and human threats," O'Rourke says.
The Cornell also team develops numerical models to predict pipeline performance under a variety of conditions, including ground deformation caused by earthquakes and erosion, and undermining from floods and hurricanes. Even adjacent construction—deep excavations and tunneling, for example—can induce ground deformations. The models are used to help design experiments to test and further refine the models for extreme such loading conditions.
"There is a strong interest in using computer models to understand and predict pipeline-system performance during natural disasters," Wham says. "It is important to understand how individual components behave and contribute to the larger-network level performance."
To understand how the JFE continuous pipe would perform during an earthquake, the researchers added more than 120 monitoring instruments to a 28 ft long, 8 in. diameter section of pipe with two wave features, placed it so that it straddled a simulated fault-rupture zone, and then buried the pipe under 80 tons of soil, Wham says. They used a hydraulically powered split box—an apparatus used to subject full-scale pipelines to earthquake-level fault disruptions—to displace the pipe by 2 ft along a 50-degree crossing angle, forcing the buried pipeline into a combination of compression and bending.
The pipeline met and even surpassed expectations. It didn't so much as spring a leak despite reaching deformation levels greater than three times the current design recommendations, Wham says. Additionally, the results can be applied to pipelines up to 80 in. in diameter, or perhaps more.
The test also returned other valuable information about the pipeline's performance. For example, after the fault-rupture test, the researchers excavated the pipeline and measured peak pipe deformation, which can help inform and improve the lab's computer simulations. From deformation information returned during the large-scale test and from preliminary compression and bending tests, the researchers can extrapolate how the pipes will perform in various burial conditions and how they'll react to seismic events when they have been placed in the ground at different angles. The results will help officials identify where wave features could be installed relative to the potential rupture hazard to best increase network resiliency, Wham says.
"The 'waves' are about as long as the pipe is wide in diameter," Wham says. "So instead of having to dig out and remove an entire length of pipeline, construction crews can make an excavation the size of several parking spots and replace a shorter section of pipe with one or more wave features."
That's important because replacing pipelines is expensive, and must be prioritized on the basis of a number of factors, including the cost of replacement and the age and condition of the existing pipeline, Wham explains. For example, the LADWP recently spent $10 million to replace relatively short spans in spot locations, including 1,750 ft of 6 in. diameter pipeline in the city's Sherman Oaks neighborhood and 4,540 ft of 8 in. pipeline at Northridge Hospital Medical Center.
Now, Davis says, he can return to Los Angeles confident that if the city chooses to use the wave pipeline in its future replacement projects, residents can rest assured that the water supply that is so critical to city functions will remain intact even if the "big one" strikes.