Determination of cyclic resistance of clean sands from cone penetration test based on state parameter

Abstract

Cyclic liquefaction is a phenomenon during which granular uncemented saturated soils (gravel, sand and low plasticity silt) lose much of its strength and stiffness for a short interval of time, but long enough to cause significant failures. The occurrence of liquefaction depends on the cyclic shear loading induced by an earthquake and on the cyclic resistance of the soil; the latter, due to the difficulties in obtaining undisturbed samples of most liquefiable soils, is deduced from field test results interpreted via empirical correlations which provide the link between cyclic resistance and various test indices. CPT based methods of liquefaction assessment are the most used in practice engineering The cone penetration resistance qc and the cyclic resistance CRR of a soil depend on the material properties and the state of the soil (stress level and density). The latter two quantities can be expressed by the state parameter ψ, which is an indicator of the direction of volumetric strains (dilation or contraction) during shearing; the amount of the volumetric strains govern the stress variation respect to the initial level of stress. In these thesis a link between the cone resistance and the cyclic resistance has been defined using the results of centrifuge CPT tests and cyclic undrained triaxial tests carried out using two well none Italian and Japanese sands: Ticino (TS4) and Toyoura (TOS). The results of cone penetration tests evidenced the existence of a simple relationship between the normalised cone resistance, Qp and the state parameter ψ (Jefferies and Been, 2006). The cyclic triaxial test results have been interpreted to define a correlation between the state parameter and the cyclic resistance ratio CRR at a given number of cycles N, for the two studied sands. Finally results of centrifuge and triaxial tests have been combined to deduce a direct relationship between the normalised cone resistance and the cyclic resistance ratio.