Cyclic and monotonic simple shear testing of native Christchurch silty soil

The first cyclic simple shear tests on high-quality retrieved specimens of Christchurch silty and sandy soil were performed to evaluate their liquefaction potential. The test specimens originate from stratified silty soil deposits in southwestern Christchurch which did not typically have surface manifestations of liquefaction after the intense shaking of the 2011 Christchurch earthquake. The soil's undrained cyclic stress-strain responses indicate cyclic mobility. The differences in the cyclic responses of the soil with different amounts of non-plastic silt are subtle. Specimens with greater cyclic stiffness degradation typically had lower post-cyclic stiffness. The soil stiffness was significantly lower in post-cyclic tests than in monotonic tests due to the large differences in the initial effective stresses between the two types of tests, loss of strength, age, and fabric effects. The field-adjusted laboratory-based cyclic resistances agree with CPT-based cyclic resistances from simplified liquefaction triggering procedures. Both assessments indicate the silty soil deposits generated high excess pore water pressures and liquefied during the Christchurch earthquake even though surface manifestations of liquefaction were not typically observed at the considered sites. The absence of liquefaction manifestation at the stratified silty soil sites highlights the importance of their system response.