ABSTRACT
Cyclic liquefaction resistance of sand deposits can increase due to microstructure that results from several factors, such as aging and bonding. Available empirical correlations, using either SPT, CPT or shear wave velocity (VS) data, were derived from cyclic liquefaction case histories that considered very young Holocene-age, essentially normally consolidated, unbonded, silica-based soils that may not apply to soils with significant microstructure. Seismic CPT (SCPT) data can be used to identify soils with significant microstructure, since both aging and bonding tend to increase the small-strain stiffness (reflected in the measured VS) significantly more than they increase the large-strain strength of a soil (reflected in the CPT penetration resistance). Hence, for a given soil, both age and bonding tend to increase VS more than the larger-strain cone resistance, all other factors (such as, in situ stress state, density, etc.) being constant. The normalized rigidity index (KG) has been proposed (Robertson, 2016) as a parameter, that combines VS and normalized CPT tip resistance (Qtn), to detect and quantify the presence of microstructure. This paper presents and discusses a suggested method to quantify the increased resistance to cyclic liquefaction due to microstructure by utilizing KG. Results will be presented from older soil deposits in New Zealand well as compare the results with existing correlations that account for “aging”.
