The conditions of K₀-consolidation and plane strain compression are most frequent in-situ stress conditions, such as in construction of embankment on natural deposit soft clay grounds. First of all, the development and advantages of a new designed t_riaxial apparatus for the conditions of K₀-consolidation and plane strain compression are presented. Secondly, the undrained shear strength properties of clay under the processes both primary and secondary compressions are discussed from the experimental results at any consolidation time. Some conclusions obtained from this paper indicate the points at issue in some previous methods for evaluating the increase in undrained shear strength of clay during consolidation.