Liquefaction cause of far more damage than the shaking

UC students’ study finds liquefaction caused far more damage than the shaking

November 2, 2012

A year-long University of Canterbury (UC) students’ study has found liquefaction caused far more damage than ground movement in the Christchurch earthquakes.

Third-year 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.

Previous studies indicated a strong correlation between peak ground velocity, ground strain and hence pipe damage in earthquakes.

The two 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.

``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. 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,’’ Brooks said today.

``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. Brooks and Craigie recognised the opportunity and need for research to be carried out on the network damage in the interests of both international academia and the Christchurch rebuild.

The Stronger Christchurch Infrastucture Rebuild Team (SCIRT) has been established to carry out the rebuild of roading, wastewater, water supply and storm water infrastructure.

``After talking with them we decided to focus our attention on the wastewater network which suffered major damage after the earthquakes, leaving thousands of residents without access to a working sewerage system.

``SCIRT has recognised the need to quickly repair and replace damaged components within the system, while simultaneously improving resilience to future seismic events. To improve the resilience of the network it is necessary to understand the current faults and areas needing improvement within the network.

``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.

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.

Craigie said they calculated repair rates (repairs per km) for pipes of different materials, ground velocity and liquefaction. Such conventions are used internationally in establishing fragility models for buried pipes and establishing design standards for pipes in earthquake prone areas.

``One day this data may be used to improve these models and the international understanding of pipe performance in earthquakes.

``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.

``Study in this area will be continued next year by PhD student Melanie Liu, who is working on a related subject - seismic resilience for sewerage systems. This is an exciting area of research with plenty more work to be done,’’ Craigie said.

Brooks and Craigie recommended a system should be established to classify repairs by pipe type. 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.

The pair carried out their research under the supervision UC's Dr Sonia Giovinazzi and Associate Professor Mark Milke.

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