Soil liquefaction caused more damage than violent ground shaking during the Christchurch earthquakes, a new study has found.
The thousands of quakes during 2010 and 2011 caused tens of millions of dollars worth of damage to Christchurch city's 1700km underground pipe network.
University of Canterbury civil engineering students Kate Brooks and Emily Craigie said large amounts of lateral and vertical ground movement due to liquefaction was shown to cause the most significant damage.
Christchurch's sandy soil was shaken so violently that it caused water to rise through its pores in a previously largely unheard of process called liquefaction.
"Liquefaction was a major consequence of the Canterbury earthquakes and little is known about buried pipe performance during this phenomena. Our collation of data provides a useful snapshot of pipe failures in liquefied soils," Ms Craigie said.
The third-year students carried out a statistical analysis of wastewater pipe performance in the Christchurch earthquakes and they looked at 1488km of pipe which had sustained 2078 break damages.
Their 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.
"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," Ms 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."
The pair hope their research will be used in Christchurch when decisions are being made about what materials should be used in pipe network reconstruction and maintenance.
Brooks and Craigie have recommended that a system should be established to classify repairs by pipe type and say steps should be taken to reduce the impact of liquefaction on pipes in areas identified as being of high susceptibility to liquefaction.
