Two aspects of seismically-induced liquefaction are discussed which are of vital engineering significance: the triggering condition and the consequences of liquefaction. The triggering condition is examined with respect to liquefaction analysis, note being taken of the onset condition which is governed by cyclic strength. Consequences of liquefaction are discussed with respect to post-seismic stability analysis, in which the residual strength plays a major role. Procedures used for liquefaction analysis based on the results of in situ sounding tests are introduced, and the applicability of this method for estimating associated ground settlements is discussed. The evaluation of residual strength requires a better understanding of undrained sand behaviour. Results of extensive laboratory rests on Japanese standard sand are examined and new index parameters are proposed to quantify undrained sand behaviour better. The results of laboratory tests on silty sands are examined in the same way. All the results are presented in terms of whether sand behaviour is contractive or diltative. The laboratory-established criterion for contractive or diltative behaviour is expressed in terms of field parameters such as N value from the SPT or q(c) value from the CPT. This correlation permits in situ deposits to be classified as being either able or not able to develop flow slide. The laboratory-established relationships between the normalized residual strength and the field parameters are presented. These correlations are shown to be consistent with many cases of flow failure observed in recent large earthquakes. The relationship between cyclic strength and residual strength is clarified.
