The Use of Critical State Soil Mechanics to Characterise Christchurch Soil in Relation to Liquefaction Susceptibility

Abstract

In the recent Darfield and Christchurch Earthquakes in 2010/2011, widespread liquefaction occurred throughout Christchurch which resulted in a subsequent increase in interest in the region’s geology. As Christchurch is now undergoing a rebuilding stage, it is imperative that the soil is studied and if required, strengthened. Much information is still lacking with regards to Christchurch’s unique soil characteristics and behaviour. Currently, the majority of procedures used to deduce liquefaction potential of soils rely on empirical methods. These methods have been proven to work in the past, but it is unsure whether these methods can be used to be adapted for Christchurch’s unique soil and the theoretical basis behind these methods cannot be explained scientifically. Critical state soil mechanics was used to provide an explanation for the soil’s behaviour during the undrained shearing, and a hypoplastic model was also used to obtain some of the soil’s inherent properties in-situ. Soils from the St George’s Hospital site and Pumpstation 15 site in Christchurch were characterised at regular intervals for the critical layers and consequently tested for the critical state lines (CSL) via consolidated undrained triaxial tests. Various models and relationships were then used to predict the CSL using the soil characterisation data or via other methods and compared with the actual CSL. However none of the methods used managed to predict the CSL accurately, and a separate Christchurch exclusive relationship was proposed. The data showed that a correlation between soil properties and the critical state line could be obtained for both sites. A plot of in-situ void ratios and state parameters was also obtained via conversion of the shear-wave velocity plots which depicts the critical layers within both sites. The resultant state parameter values were then developed into cyclic resistance ratios (CRR) which were then compared with cyclic stress ratios (CSR) from recent earthquakes to obtain the factor of safety. This new critical state soil mechanics based method was compared with other empirical methods and was also shown to yield a favourable relationship with visual observations from the soils’ locations.