Liquefaction resistance of unsaturated sands: Experimental evidence and theoretical interpretation

Cyclic undrained triaxial tests were carried out on saturated and unsaturated loose specimens of three fine-grained materials. The results confirm that, as the saturation degree decreases, liquefaction resistance sharply increases. In the interpretation of results, the first step was the assessment of the reliability of the use of the 5% strain amplitude criterion to define the attainment of liquefaction. With this aim, the tests were analysed assuming that the soil behaves as an equivalent viscous material. It is shown that the characteristic values of viscosity defining the start and completion of liquefaction depend on soil grading, increasing as the coefficient of uniformity increases. The equivalent viscosity corresponding to a condition of double strain axial amplitude of 5% is fairly constant for all soils. Then, starting from the observation of the relationship between the final volumetric strain and the initial degree of saturation, a theoretical interpretation of the tests is reported. It is shown that the behaviour of the tested sands can be interpreted quantifying the thermodynamic specific energy spent to reach liquefaction, considering the contribution of all phases (soil, water and air). The position of the liquefaction resistance curve depends on the volumetric component of such a specific energy