Liquefaction vulnerability of Puketoka formation soil deposits using effective stress analysis

Abstract

Puketoka formation soil deposits predominantly consist of a heterogeneous sequence of interbedded fine-grained sands, silts, and clays derived from volcanic ash and tuff, estuarine peat, and coarse-grained soils. These deposits are present in the southern and western parts of the Auckland Region, New Zealand. The available geological maps and reports indicate that Puketoka Formation contains pumiceous grains, though these are often not picked up or described by engineering geologists. Geology-based liquefaction criteria show that these deposits are unlikely to liquefy. However, the Puketoka formation soils have high pumice contents that may skew the results of traditional liquefaction analyses. Moreover, recent detailed desk studies for liquefaction assessment of this formation showed that the Puketoka Formation loose sands and sandy silts are potentially liquefiable due to ground shaking from earthquakes with a return period as short as 100 years; consequential effects would increase (greater settlements and greater risk of lateral spreading) for earthquakes with a return period greater than 250 years. Thus, more investigation is required to understand the seismic response of the Puketoka Formation and site-specific detailed assessments can help understand the liquefaction vulnerability of these deposits. Toward this end, numerical simulation of a site in South Auckland is performed using the PM4Sand model to investigate the behavior of these soil deposits in terms of stress-strain relation and excess pore water pressure generation. First, the model and input parameters are calibrated using available laboratory undrained cyclic direct simple shear tests on undisturbed Puketoka Formation soils. Next, site-specific 1-D numerical simulations are performed to investigate the liquefaction performance of the soil deposits. The results obtained provide better insights into the liquefaction potential of these deposits.