Liquefaction more damaging to pipes - study

NEW RESEARCH: Civil engineering students Kate Brooks and Emily Craigie studied wastewater pipe performance in the Christchurch earthquakes.
NEW RESEARCH: Civil engineering students Kate Brooks and Emily Craigie studied wastewater pipe performance in the Christchurch earthquakes.

Liquefaction caused far more damage to underground pipes than ground movement in the Christchurch earthquakes, a study has found.

Third-year civil engineering students Kate Brooks and Emily Craigie carried out a statistical analysis of wastewater pipe performance in the Christchurch quakes in a year-long study.

The 2010 and 2011 quakes left Christchurch's 1700-kilometre pipe network severely damaged.

Canterbury Earthquake Recovery Minister Gerry Brownlee said yesterday that the Stronger Christchurch Infrastructure Rebuild Team (Scirt) had laid 19km of freshwater pipes, 70km of wastewater piping and 5km of stormwater piping, but was only 12 per cent through the rebuild and repair of the city's horizontal infrastructure.

About 1020km of roads will need rebuilding - about half of the city's roads.

Brooks and Craigie, who are studying at the University of Canterbury, recognised the opportunity for research to be carried out on the network damage in the interests of international academia and the Christchurch rebuild.

They found large amounts of lateral and vertical ground movement because of liquefaction caused the most significant damage.

"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."

The students' research involved more than 35,000 pipes, quake data and photographic evidence.

They were able to find out 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 caused.

Craigie said they calculated repair rates for pipes of different materials, ground velocity and liquefaction.

"One day this data may be used to improve these models and the international understanding of pipe performance in earthquakes,'' she said.

"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."

Brooks hoped the research would be used in Christchurch when decisions were being made on what pipe materials should be used in network reconstruction and maintenance.

"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," she said.

Scirt has been replacing the old pipes with longer-lasting and more robust polyethylene or PVC piping. Other quake-resistant methods include:

- Not burying pipes as deeply to allow easier repairs.

- Pump stations for liquefaction-prone areas.

- Relining less-damaged pipes with a resin-impregnated fabric, similar to fibreglass.

The students recommended a system be established to classify repairs by pipe type.

They thought steps should be taken to reduce the impact of liquefaction on pipes in areas identified as being of "high susceptibility to liquefaction", and further investigation into the performance of certain pipes in liquefied soils should be carried out.

Both students will work for engineering firms in Christchurch next year.

The Press