When predicting the strength and stability of high-head embankment dams, it is necessary to investigate comprehensively the effect of time on the behavior of the structure during its construction and operation over the long term. In order to determine the design parameters, account must be taken of the variation with time of the strength and deformability of the clay soils under different regimes and loading trajectories. An urgent task in this connection is the solution of the problem of determining the limiting stresses of clay soils* in a constant as well as a variable stress state.In investigations carried out during 1970-1973, the writers studied the limiting stress of natural dense clay in the upper Permian stage of the Tatar deposits in the foundation of the Cheboksary hydroelectric plant, and of Upper Quaternary disturbed loam in the dam core at the Charvak hydroelectric plant. The laws governing the stress-strain state in the prelimiting and limiting state were investigated in the loading regimes prior to failure under stresses which varied, in each test, at a constant rate of increase of the stress bi (in the range 0.006-845 kg/cm z) or of increase of the strain ei (in the range 0.002-30090 per hour). In these two regimes, the entire deformation and failure process takes place under a ~/ariable stress state. In the tests the sample was compressed by an all-around isotropic load applied in increments, with the volumetric strains caused by each increment. The undisturbed clay was investigated under an isotropie pressure of 5 kg/em z and the disturbed loam was investigated under 7 kg/cm 2. The next deviator stage of the loading trajectory was carried out under an equal loading trajectory in all tests, characterized by a coefficient K = o/oi---0.33.Using a third regime, investigations were conducted on the limiting stress under the action of a constant stress state characterized by different stress levels (oi= const). Initially. the soil sample was subjected to an allaround isotropic compression, equal to 5 kg/cm z. The load was applied in the same way as in the tests carried out under the regimes of action of the variable stress state by increments with stabilization of the resulting volumetric strain. Subsequently, the constant stress-state level was ensured by increasing the stress o i at the given constant rate d i. This investigation of Xahe limiting stress was carried out for different rates bi of loading of the soil at the given level o i. This made it possible to investigate the effect of the regime of constant stress level attainment on the limiting stress.All tests described in this article were performed on a triaxial compression apparatus, with the condition o i _>oz=o s applied at the boundary surfaces. Special attention was focused on the dilatancy and its effect on strength and deformability. For all three regimes the relations between the dilatancy index )~=dOd/de i and ei, and between k and el, for the prelimiting state, were investigated.The test data for the two soils investigated under d...
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