Laboratory investigation of the undrained cyclic behaviour of a natural coarse sand from undisturbed and reconstituted samples

The paper describes the results of an experimental research on the undrained cyclic behaviour of a natural coarse sand and gravel deposit sited in a seismic area near Gioia Tauro a town located on the continental side of Messina Strait (Italy). The study was performed through laboratory undrained cyclic triaxial tests carried out on both undisturbed and reconstituted samples. Undisturbed samples were recovered by ground freezing technique. Tests on reconstituted samples were aimed at analysing the influence of different factors such as fabric of tested material and stress-history (overconsolidation ratio). The influence of soil fabric has been investigated by two different preparation methods namely air pluviation and sedimentation in water. Fur-themore, two different overconsolidation ratios were analysed, namely OCR=1 and 3 corresponding to the ex-pected range of pre-consolidation stresses for the investigated site. All specimens were reconstituted at the same void ratio measured on intact samples and tested under an isotropic effective stress equal to the in situ vertical effective overburden stress. Cyclic liquefaction resistance obtained from water sedimented samples was very close to that exhibited by undisturbed samples, while air pluviation led to a marked underestimation. The agreement between water sedimented and intact samples was better for normally consolidated specimens if liq-uefaction resistance data not corrected for membrane penetration effects were used for the comparison; on the opposite the correspondence was closer for specimens overconsolidated to some OCR between 1 and 3 if the comparison was based on corrected data.