Liquefaction Resistance of Sandy Soils from Undrained Cyclic Triaxial Tests

Recent earthquakes frequently showed liquefaction of sandy soils. In these case one of the major challenges is the assessment of the residual strength of liquefied soil. Laboratory researches have clearly indicated that liquefaction susceptibility depend on many factors including soil grain size distribution, effective confining stresses and applied stress path in terms of cyclic shear stress amplitude. The complex cyclic shear stress path experienced by the soil during an earthquake can be reproduced in laboratory only by using sophisticated testing apparatuses. Cyclic triaxial tests have been widely used to asses soil liquefaction potential especially for coarse-grained soils, as in this study. In the framework of the design for the seismic retrofitting of the Viadotto Ritiro foundations along the A20 motorway connecting Messina with Palermo, a soil liquefaction study has been carried out. With this aim, a detailed geological and geotechnical characterization of the area has been performed by in situ and laboratory tests including the combined resonant column and torsional shear test and undrained cyclic triaxial tests (CTX). In particular, the paper presents the results of CTX, carried out on isotropically consolidated specimens carried out on specimens of a sandy soil to asses liquefaction strength. The results are plotted in terms of deviator stress q versus axial strain epsilon(a), cyclic stress ratio CSR and axial strain epsilon(a) versus number of cycles N-cyc, effective mean stress p versus deviator stress q, cyclic stress ratio CSR and pore pressure ratio R-u versus number of cycles N-cyc. The results show that the cyclic resistance increases with the decrease in the initial confining stress and decreases as the silt content increases and confirme that the coarsest material has a lower tendency to liquefy.