The 22 February 2011, Mw6.2-6.3 Christchurch earthquake is the most costly earthquake to affect New Zealand, causing 181 fatalities and severely damaging thousands of residential and commercial buildings, and most of the city lifelines and infrastructure. This manuscript presents an overview of observed geotechnical aspects of this earthquake as well as some of the completed and on-going research investigations. A unique aspect, which is particularly emphasized, is the severity and spatial extent of liquefaction occurring in native soils. Overall, both the spatial extent and severity of liquefaction in the city was greater than in the preceding 4th September 2010 Darfield earthquake, including numerous areas that liquefied in both events. Liquefaction and lateral spreading, variable over both large and short spatial scales, affected commercial structures in the Central Business District (CBD) in a variety of ways including: total and differential settlements and tilting; punching settlements of structures with shallow foundations; differential movements of components of complex structures; and interaction of adjacent structures via common foundation soils. Liquefaction was most severe in residential areas located to the east of the CBD as a result of stronger ground shaking due to the proximity to the causative fault, a high water table approximately 1m from the surface, and soils with composition and states of high susceptibility and potential for liquefaction. Total and differential settlements, and lateral movements, due to liquefaction and lateral spreading is estimated to have severely compromised 15,000 residential structures, the majority of which otherwise sustained only minor to moderate damage directly due to inertial loading from ground shaking. Liquefaction also had a profound effect on lifelines and other infrastructure, particularly bridge structures, and underground services. Minor damage was also observed at flood stop banks to the north of the city, which were more severely impacted in the 4th September 2010 Darfield earthquake. Due to the large high-frequency ground motion in the Port hills numerous rock falls and landslides also occurred, resulting in several fatalities and rendering some residential areas uninhabitable.
In the 4 September 2010 (M W 07.1) and 22 February 2011 (M W 06.2) earthquakes, widespread liquefaction and lateral spreading occurred throughout Christchurch and the town of Kaiapoi. The severe soil liquefaction and lateral spreading in particular caused extensive and heavy damage to residential buildings, Christchurch business district (CBD) buildings, bridges and water supply and wastewater systems of Christchurch. After the earthquake, comprehensive field investigations and inspections were conducted to document the liquefaction-induced land damage and lateral spreading displacements and their impact on buildings and infrastructure. The results of ground surveying measurements of lateral spreads at approximately 120 locations along the Avon River, Kaiapoi River and streams in the affected area reveal permanent lateral ground displacements at the banks of up to 2Á3 m that progressed inland as far as 200Á250 m from the waterway, causing significant damage to structures located within the spreading zone. Different features and magnitudes of spreading were identified, which were often affected by a complex interplay of ground conditions, topography, meandering river geometry and local depositional environment. The spreading was characterised by very large and highly non-uniform ground deformation causing stretching of building foundations and the buildings themselves. Road bridges suffered a characteristic spreading-induced damage mechanism including back-rotation of the abutments associated with deck pinning and damage at the top of the abutment piles. The wastewater system of Christchurch was hit particularly hard by the liquefaction and lateral spreading, and approximately 60% of the damaged pipes of the potable water system were located in areas of severe liquefaction and lateral spreading.
The Canterbury earthquake sequence provides an exceptional opportunity to investigate the effects of varying degrees of liquefaction on the built environment. Many multistory buildings in the Central Business District were heavily damaged by liquefaction-induced ground movements during the Christchurch earthquake, but not by other earthquakes (e.g., the Darfield and June 2011 events). Cone penetration test (CPT)–based liquefaction triggering evaluations were conservative. The conservatism in the liquefaction triggering assessments led to post-liquefaction ground settlement estimates that were generally similar for the large events in the earthquake sequence, whereas significant ground settlements and building damage in the CBD were only observed for the Christchurch earthquake. Moreover, the liquefaction-induced ground settlement procedures do not capture important shear-induced deformation mechanisms and the effects of ground loss due to sediment ejecta. Performance-based earthquake engineering requires improved procedures to capture the differing levels of performance observed in Christchurch.
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