Stage IV of the Christchurch liquefaction study updated the Stage II liquefaction hazard and ground damage maps with further data collected from other organisations, and included two additional maps indicating liquefaction sensitivity to groundwater levels. Stage IVa of the Christchurch liquefaction study used revised groundwater levels and adjustments to the liquefaction prediction algorithm. The outputs of the report were liquefaction hazard and ground damage maps for both average summer (low) and average winter (high) groundwater levels. The maps produced as part of Stage IVa of the report were subsequently included in an Environment Canterbury public education poster The Solid Facts on Christchurch Liquefaction which also contained information on how liquefaction occurs and what can be done to mitigate the liquefaction hazard. Stage IV of the Christchurch liquefaction study contained a number of recommendations to improve the liquefaction potential and ground damage maps for Christchurch. See Object Overview for background and usage information.
Liquefaction is a phenomenon that results in a loss of strength and stability of a saturated soil mass due to dynamic excitation such as that imposed by an earthquake. The granular nature of New Zealand soils and the location of many of our cities and towns on fluvial foundations are such that the effects of liquefaction can be very important. Research was undertaken to build on the past work undertaken at the University of Canterbury studying the effects of the 1929 Murchison earthquake, the 1968 Inangahua earthquake and the 1991 Hawks Crag earthquakes on the West Coast. Additional archival information has been gathered from newspapers and reports and from discussions with people who experienced one or all of these large earthquakes that occurred on the West Coast during the 20th Century. Further, some twenty Cone Penetrometer Tests were carried out, with varying success, in Greymouth and Karamea using the Department of Civil Engineering's Drilling Rig. These, combined with the basic site investigation information, consolidate and add to the liquefaction case history data bank at the University of Canterbury. Many of the sites have liquefied in some but not all of the three earthquakes and thus provide both upper and lower bounds for the calibration of empirical models. While a lack of knowledge of the 1929 source location reduces the value of information from that event, the data form a useful set of liquefaction case histories and will become more so as further earthquakes occur. A list of critical sites for checking of the future earthquakes is provided and recommendations are made for the installation of downhole arrays of accelerometers and pore water pressure transducers at a number of sites.
