Further developments in the theory of Soil Mechanics will never be attained without establishing a basic law of deformation of soils which will connect elasticity, plasticity, and criterion of failure.
In this paper the Author presents a basic theory of plasticity which is a full revision of the fundamental theory published in a paper (Ie 10) submitted to the Second International Conference on Soil Mechanics and Foundation Engineering at Rotterdam in 1948, by referring to the experimental results of the triaxial tests carried out since then in his laboratory.
Based on the same principle as in the former thoery that the deformations and failure of such a plastic material as soils may be governed by the amount of evergy or the work done by external forces, this new basic theory is perfectly free from the defects in the former theory, having clear physical meanings and showing definite relationships to the elasticity. The criterion of failure was discussed with proposal on a novel definition of angle of internal friction and cohesion.
By solving some fundamental problems have been derived the formulae for determining the volume change, shearing strains, and yield stress of a soil body under such a stress condition as pure compression (hydrostatic pressure), pure shear, triaxial compression, unconfined compression, or such a strain condition as consolidation with lateral confinement. Formulae were also given for expressing the change of the amount of energy during the application of stresses. Those formulae will be able to give clear and rational interpretations of the test results in Soil Mechanics.
The analysis of the experimental data of triaxial tests has resulted in satisfactorily good agreements with the theoretical derivations, determining reasonable values for the thoeretical constants from the observed data. It may be concluded that the validity of the thoery has been duly proven.

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