A prelude to seismic design for water & wastewater pipe
There are between 12,000 and 14,000 earthquakes around the world each year. Only the ones registering at magnitude 7 or magnitude 8 generally become worldwide news. Such was the case in Indonesia this summer when a massive earthquake resulted in a disastrous tsunami, killing more than 800 people in Palu. The first force of the waves and tremble of the event get the most coverage, but there also are ripple effects to these seismic events, such as pipe damage.
In the U.S., Californians deal with earthquakes on a regular basis. Many pay them no mind after years of regularly living through low magnitude quakes. There are building and road codes that address the natural occurrence to ensure public safety, but when it comes to water infrastructure, there are not any hard and fast regulations regarding how pipe should handle these events.
“Given the lack of historical seismic concerns and issues with our storm and sanitary concrete pipe below ground products, there have not been any overarching need for seismic design considerations to be incorporated into our national standards,” said Russell Tripp, P.E. and president of the American Concrete Pipe Assn. (ACPA). “In the major relatively recent California events—Los Angeles and San Fransisco—the only concerns noted were disconnected sanitary services from the street to homes and business. Major feeder and trunk lines were not affected like aboveground structures, many of which collapsed.”
While there are no seismic design requirements for underground pipe, the American Society for Civil Engineers has been working on a Manual for Purpose on the subject. ACPA and the Plastics Pipe Institute (PPI) have been involved in meetings for that publication.
Josh Beakley, vice president of engineering for ACPA, said there has been literature on seismic design for gas pipelines, but when it comes to water, he has not seen much.
“They key to any rigid pipe in a seismic event is having good joints to take the movement,” Beakley said. “You’ve got the movement and then you’ve got the loss of soil support, depending on how much the vibration builds up the water pressure and the soil can weaken around the pipe.”
When it comes to concrete pipe that does not hold pressure, Beakley said it tends to maintain its shape through seismic events because it is a rigid pipe.
But, he noted, pressure from within the pipe combined with the liquefaction of the soil—a weakening of soil during earthquakes—can help pipe maintain shape. Richard Mueller, P.E., president of the American Concrete Pressure Pipe Assn., said concrete pressure pipe has been effective in areas with seismic events. He referenced an article by Henry H. Bardakjian, P.E., Michael McReynolds, S.E., P.E., and Dr. Mehdi Zarghamee, P.E., that reviewed the performance of gasketed joints on buried concrete and steel pipelines after seismic events in California.
According to that article, transient movement and deformation of the soil and permanent ground displacement due to fault movement or large scale soil movement are two phenomena to consider
during seismic events.
“Large-diameter concrete pressure pipe with gasketed bell and spigot joints can resist these movements because they have good compressive strength and beam strength,” Bardakjian et al note in the paper. “The tensile strain in the soil causes tensile strain in the pipeline. The joints allow small relative movements which relieves the tensile forces.”
As for plastic pipe, PPI Director of Engineering Camille Rubeiz, P.E., said high-density polyethylene (HDPE) pipe is well-suited for water and gas lines in areas with regular seismic activity. HDPE pipe installed with horizontal directional drilling (HDD), he added, can be a benefit.
“Directional drilling fluids that come along with the installation put the pipe in a great position to even perform better in earthquakes,” Rubeiz said. “Because of its ductility, high strain resistance, corrosion resistance, monolithic system, because of all these features, polyethylene performed very well.”
Rubeiz referenced a report from the Water Research Foundation in 2012 that mentioned that during earthquakes in Chile in 2010; Christchurch, New Zealand, in 2010; and Japan in 2011, HDPE distribution pipe was the only material that suffered no damage, while damage was reported for other materials. The extent of that damage is not categorized in the report, which notes that damage can sometimes not be realized until much later than the timing of the event taking place.
Due to the soil movement and the phenomenon of liquefaction, Beakley said joints are among the most important things to consider when it comes to reducing damage during seismic events.
Ed Nunes, Hymax product manager for Krausz, said handling the deflection and movement from earthquakes is one of the primary features of the products he manages. The couplings, he said, can deflect by up to 4 degrees, so when the soil moves, the couplings can act as shock absorbers for the pipe between fittings.
“As long as the deflection isn’t greater than 4 degrees, then we’ll move with that activity,” Nunes said. In addition to the compression of the gasket to get a seal, Nunes said the couplings use the pressure within the pipe to maintain that compression.
He said with the age of pipe in the U.S. and particularly in California and other regions with regular seismic activity, the continued stress on pipes likely will cause line breaks and water loss. When developing the couplings, Nunes said the engineers recognized that a coupling that does not flex with the system could put added tension further down the line.
Absorbing that movement has had real world success, he added.
“We’ve done testing all over the world for this type of deflection,” Nunes said. “We have a number of big customers out in California, which every day you see something in the news, a water pipe breaks, so they use our technology to help prevent further breaks.”
As new products are developed, Nunes said he will work with utilities to conduct field trials of the equipment to ensure that what occurs in the lab will occur in the real world.