Engineering generalizations of an investigation of the effect of the time factor on long-term strength and deformability of clays of high-head earth dams

Abstract

The general design of a high earth dam, the comparatively slow required periods of layerby-layer placement of the large volume of earth, and the need to put the reservoir into service before construction of the dam is complete create significant characteristics with time of the effect of these circumstances on the overall construction regime --the main load by its own weight developing in a dynamic regime. The result of soils in a prelimit (elastic) state are undergoing a long-time nonequilibrium state of structural transformations, which radically influence their strength and deformability at various stages of dam construction and subsequent long-term operation.It is required to estimate the effect of the time factor on the strength and deformability of soils by solving the quite complex problem through an experimental study of clay in state of primary creep under the effect of a variable stress and with consideration of the effect of structural transformations. The theory of creep of a continuous soll medium, as is known, does not include analogies with respect to the state equations reflecting the listed characteristics of the problem being considered, and phenomenological knowledge of the necessary regularities is quite limited. Under these conditions it was expedient, relying on certain principles of the theory of limit states of a structure, to use the engineer's usual strength concepts in a study of the long-term strength of soil, but to study it with consideration of nonequilibrium processes in the presence of a variable stress, which are determined by the characteristics of the effect of the time factor for the given engineering problem [1][2][3][4][5][6][7].Numerous soil investigations and, in particular, the investigations we performed on earth dams, have established the significant role of the history of loading in the prelimit region, hereditarily influencing the critical state of the long-term strength and regularities of subsequent flow. Consequently, when solving the problem of long-term strength with consideration of nonequilibrium processes in the presence of a variable stress (i.e., the problem of primary creep) it is necessary to study under the same regimes the prelimit and flow states, by so doing eliminating the method of instantaneous loading. Having accepted these principles, we established the content and method of approximate investigation* of the selected problem of the theory of primary creep by a laboratory study of the prellmit stressed state and of the long-term strength in the critical-llmit state of transition to flow, and flow reactively occurring in a dynamic regime.The essence of the study of the deformability and long-term strength presented herein consists in the following. Clayey soil, from sandy loam to disturbed clays, preliminarily mechanically compacted to a very stiff consistency was investigated. The soil was loaded in various regimes of triaxial compression of the specimens; the time factor was always incorporated by assigned rates of development of principal stresse...