Here - Stuff

(A)(B)▲ ▲ Figure 14. A) Subsidence of a two-story house adjacent to the stopbank in north Kaiapoi caused by the 2010 earthquake. B) Additionalsubsidence of the same house after the 2011 earthquake.LIQUEFACTION OBSERVED ADJACENT TOHEATHCOTE RIVERHeathcote River, located in the southern boundary ofChristchurch, meanders around the base of the Port Hills fromwest to southeast. It drains into the Avon-Heathcote estuarybefore draining into Pegasus Bay. Earlier studies have indicatedthat aside from parts of the eastern suburbs, the areas aroundthe Heathcote River are underlain by loose saturated sand andsilt, which have high potential to liquefy (Christchurch CityCouncil 2005).Following the September 2010 earthquake, a quick drivethroughinvestigation was conducted along the HeathcoteRiver, specifically targeting areas that were denoted as havinghigh potential for liquefaction-induced damage. However,there was very little evidence of ground distortion and liquefactionin this area, with only a few sand boils found in aperiod of about two hours of drive-through and on-foot surveys(Cubrinovski et al. 2010).On the other hand, after the February 2011 earthquake,significant ground distortions due to liquefactionwere observed adjacent to Heathcote River. In flat areas withshallow ground-water tables (e.g., St. Martins, Opawa, andWoolston), a number of structures such as stopbanks, bridges,and residential properties suffered severe damage due to liquefaction.Figure 15 shows the distribution of liquefaction alongthe Heathcote River observed during a walk-through investigationconducted two weeks after the 2011 earthquake. Again,the circular and square dots in the figure correspond to themaximum distance from the Heathcote River at which lateralspreading was observed. It is clear from the figure that severeliquefaction occurred at limited areas along the HeathcoteRiver—considerably smaller than the ones observed adjacentto Avon River (see Figure 7B).The lower areas along Wilson Road in St. Martins may bethe worst-hit areas near the Heathcote River. Figure 16 showsthe St. Martins library, a brick building whose collapse wascaused by the differential subsidence of the foundation grounddue to liquefaction. Ejected sands, with thickness on the orderof 20 cm, were deposited around the right half of the foundation.Additionally, a tilted power pole can be seen in the rightside of the figure, indicating that liquefaction occurred at shallowdepth in this area.Heathcote River winds along the foot of Port Hills, andtherefore the topography around the river is full of ups anddowns. From geological information, loess deposits are presentin the subsurface at the base of Port Hills (Brown and Weeber1992). No liquefaction was observed at the ground that is consideredto be loess.Figure 17 shows a trench in a residential property underconstruction in Eastern Terrace just beside the river. Thetrench depth was greater than 2 m and yet no groundwater wasobserved. Soil samples were collected from the trench at depthsof 1 and 2 m from the ground surface, and their grain sizes wereanalyzed. Figure 18 shows the grain size distribution curves ofsoils taken from the trench in comparison with those of soilscollected in other parts of Christchurch and Kaiapoi. The opendots correspond to the ejected sand collected from sites adjacentto Avon River and Kaiapoi, while the solid dots representsoils collected at a slope in Port Hills and near the HeathcoteRiver where liquefaction was not observed. It can be seen thatthe sand boils have similar grain size distributions, regardless ofthe location where they were collected (in Kaiapoi or adjacentto the Avon or Heathcote rivers). They have fines content lessthan 25%. On the other hand, the subsurface soil adjacent tothe Heathcote River, which did not liquefy, contained >90%fines with clay content >20%. Therefore, it is possible that thepresence of unliquefiable soil at subsurface is a major reasonSeismological Research Letters Volume 82, Number 6 November/December 2011 915