At present, considerable attention is being devoted to investigating the deformability and strength characteris'tics of soils in the cam of three-dimensional stress."The following data on the strength and deformability of soils obtained on apparatus with independently controlled principal stresses are considered in the design model.~" Strength. For many soils the Mohr andMises-Botkin strength theories (used in traditional calculations withrespect to the first limiting state) prove to be substantially invariant to the form of three-dimensional stress. Inother words, the parameters of these strength theories depend on the relationship between the principal messes at which the experiment is set up (see, e.g., [5]). Figure In the calculation results presented below we treated the laboratory data by the method proposed by 8otkin [6] and supplemented by consideration of the dependence of strength on I1" 7 -~ 11"~ (I: , where I~ o is the value of II o at soil failure. The specific expression of gq. (1) was described earlier [2]; in addition to effect on strength, /~o reflects the substantially nonlimar relation II~ =II~ (Io)(for vo = const), observed at a high stress level.In soil mechanics the ratio of the sine of the angle of deflection to the sire of the angle of friction is considered for determining the degree of approximation to limit equilibrium [7]. /~ nalogously, with consideration of Eq. (1), we introduce the ratiowhere II o is calculated on the basis of the actual stresses at a point.The stress eondi~tion that is safe from the standpoint of the possibility of failure (condition of hydrostatic pressure) corresponds to the value fl --0; the state of limit equilibrium corresponds to the value ~ = 1. If we calculate