Instability line as a basic characteristic of non-cohesive soils

In the paper, the experimental results of undrained response of non-cohesive soil subjected to either monotonic or cyclic loading, are analysed and discussed. The analysis was based on the results of comprehensive testing programme made of "Skarpa" sand investigated in the triaxial compression. Two main undrained behaviours such as liquefaction and cyclic mobility were experimentally identified and studied. In the analysis of undrained behaviour, the concept of steady state of deformation has been adopted. It was shown that the undrained response of non-cohesive soils is governed by different failure mechanisms than in the case of drained samples. In undrained failure mechanism, an important role is played by an instability line, which may be identified with the initiation of potential liquefaction.