Liquefaction did more damage than shaking: study

Workers clean up the black gunk which is caused by the liquefaction in Christchurch. Photo / Brett Phibbs

Liquefaction did more damage to underground pipes than ground movement in the Christchurch earthquakes, a University of Canterbury study has found.

In a year-long study of wastewater pipes in the city, third-year civil engineering students Kate Brooks and Emily Craigie found large amounts of lateral and vertical ground movement due to liquefaction was shown to cause the most significant damage.

The students said previous studies had indicated a strong correlation between peak ground velocity, ground strain and pipe damage in earthquakes.

The two students carried out a statistical analysis of wastewater pipe performance in the Christchurch earthquakes and looked at 1488km of pipe which had sustained 2078 break damages.

"We found liquefaction was a significant contributor to pipe failure and much of that was out east of the city which was the worst affected area for pipe damage," Brooks said. "In particular, we found that smaller diameter pipes at greater depths were most susceptible. Materials that performed the worst within the Christchurch wastewater network were earthenware, concrete, reinforced concrete and asbestos cement.

"Primarily it is hoped that our research will be used here in Christchurch when decisions are being made as to what pipe materials should be used in network reconstruction and maintenance. Emily and I are both working for engineering firms here in Christchurch next year so it should be exciting.

"The older more brittle pipes present in the Christchurch wastewater network, asbestos cement, cast iron, earthenware and reinforced concrete suffered higher amounts of damage than the plastic pipe materials, polyvinylchloride and polyethylene," Brooks said.

The 2010 and 2011 Canterbury earthquakes left Christchurch city's 1700km pipe network severely damaged.

"In our study we undertook a detailed investigation into the performance of buried wastewater pipes and investigated how physical parameters (such as material, diameter, depth of burial, length and age), along with seismic forces (permanent ground displacement and peak ground velocity) contribute to a pipe's failure susceptibility," Brooks said.

The research involved processing large databases of more than 35,000 pipes, earthquake data and photographic evidence to quantify earthquake damage and identify key trends in pipe performance.

They were able to draw conclusions on what pipe materials suffered the most and least amount of damage, as well as how other factors such as depth, peak ground velocity and liquefaction affected the amount of damage sustained.

Brooks and Craigie recommended a system should be established to classify repairs by pipe type and steps should be taken to reduce the impact of liquefaction on pipes in areas identified as being of high susceptibility to liquefaction. They said further investigation into the performance of polyvinylchloride and polyethylene pipes in liquefied soils should be carried out.