Liquefaction hazard in the Wellington region

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Dellow, G.D.; Perrin, N.D.; Ries, W.F. 2018 Liquefaction hazard in the Wellington region. Lower Hutt, N.Z.: GNS Science. GNS Science report 2014/16. 71 p.; doi:10.21420/G28S8J

Abstract: The greater Wellington Region has a higher annual probability of damaging ground shaking than most of the rest of New Zealand. This means that the region is exposed to all of the effects of strong earthquake shaking including fault rupture, ground movement, liquefaction and landslides. This report deals specifically with the liquefaction hazard in the greater Wellington Region. Liquefaction is a process that leads to a soil suddenly losing much of its strength, most commonly as a result of strong ground shaking during a large earthquake. Not all soils, however, can liquefy in an earthquake. The following are particular features of soils that potentially can liquefy: The soils need to be composed of loose sand and/or silt with very little or no clay. Such soils do not stick together the way clayey soils do. The soils need to be saturated (i.e. located below the water table) so all of the space between the grains of sand and silt is filled with water. Dry soils above the water table do not liquefy. This simplifies the identification of sediments (soils) that are vulnerable to liquefaction. The sediments must be relatively young (less than ~10,000 years old) and deposited in a low energy environment (e.g. settle out of suspension). Thus the places most likely to accumulate sediments prone to liquefaction are lagoons and estuaries near the coastline where sand and silt suspended in flood waters can settle out of suspension. Other locations are overbank silt deposits (again silt settling out of suspension from floodwaters), and point bar and channel deposits in meandering river systems. At least four historical earthquakes since 1840 have caused some liquefaction in the Wellington Region (1848, 1855, 1942 and 2013). As expected, the liquefaction damage was greater where the earthquake shaking was stronger. With regards to the likely severity of liquefaction in the Wellington Region, a key observation was made by Edward Roberts, a trained engineer who, after the 1855 earthquake, travelled extensively in the Lower North Island observing ground and building damage due to the earthquake. He observed that the “plains of the Manawatu” (i.e. between Tokomaru and Foxton) were affected by liquefaction “to a much greater degree” than anything in the Hutt. Liquefaction has also shown a tendency to recur in the same areas. In the Wellington Region, the sites most vulnerable to liquefaction have been the lower parts of the Wairarapa Plains from the northern (or inland) end of Lake Wairarapa to the coastline at Lake Onoke, the lower reaches of the Hutt River south of the Waterloo-Melling area, and reclaimed land around the margins of Wellington and Porirua harbours. Liquefaction has occurred elsewhere but has been limited in extent. This report presents two liquefaction maps for each area studied. The first of these shows the assessed range of liquefaction susceptibilities. Five liquefaction susceptibility classes have been used, but not all are present in some areas. The second map shows areas where potentially destructively damaging liquefaction could occur. These maps are based on published geological maps, historical accounts of liquefaction during strong earthquake shaking and subsurface information from boreholes to identify materials, predominantly loose sand and silt that are most susceptible to liquefaction. The map scales range from 1:50,000 to 1:250,000 and reflect the scales of the source data. These maps are not intended to be used at a site specific or property level to describe the liquefaction hazard. A correct use of these maps is to identify areas where further, more detailed investigation of the liquefaction hazard is warranted. The more detailed investigations should include detailed geomorphic mapping at larger scales (1:10,000 to 1:25,000), geotechnical characterisation of the subsurface materials to a depth of at least twenty metres and assessment of the shallow, unconfined groundwater surface and its seasonal and tidal variations. It is recommended that this more detailed work to assess and quantify the liquefaction hazard is undertaken before liquefaction hazard information is included on formal documents such as district plans and LIM (Land Information Memoranda). (auth)