Liquefaction: Christchurch (Darfield) Earthquake 2010, My Experience and Some Issues

Dr Mizanur Rahman

“Liquefaction” is a potentially catastrophic phenomenon in which saturated/near-saturated cohesionless soil loses strength due to increase in pore water pressure under rapid loading (cyclic/static). Liquefied soil may acquire a degree of mobility sufficient to permit movement from meters to kilometres. Although liquefaction has been long recognized, it was more thoroughly brought to the attention of engineers and seismologists by several natural hazards around the world: Niigata and Alaska (1964), San Fernando Dam failure (1971), Loma Prieta (1989), Kobe (1995) and Chi-Chi (1999) earthquakes. The speaker will discuss his experience with the Christchurch (Darfield) earthquake (2010) and wide spread liquefaction in the City of Christchurch

The presentation will also address the relatively little known phenomenon of ‘static liquefaction’ due to static loading, which has direct correspondence with cyclic instability due to cyclic loading, and can trigger liquefaction due to even a moderate earthquake. The concept of critical state soil mechanics will be used to explain why liquefaction occurred in Christchurch even during relatively small aftershocks.

The effect of fines (particle size ≤ 75µm) in sand on liquefaction will be addressed. Laboratory studies have shown that liquefaction resistance decreases with an increase in percentage of fine particles; however field tests based on a liquefaction screening chart (Youd et al. 2001) shows the opposite trend. The speaker will explain that an inconsistent comparison basis is one of the main reasons for this apparent anomaly.

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