Deformability, strength, and creep of clayey-soil core walls of high dams

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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...

“…[1][2][3][4][5][6]. The test data for determining the limiting stress for the same soils in the regime of action of a constant stress oi= const are presented in Fig.…”

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confidence: 99%

mentioning

confidence: 99%

Limiting stress of clay soils in cores of high-head dams and foundations of hydraulic structures

Lomize¹,

Muzafarov²,

Novikov³

1975

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“…

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confidence: 99%

“…Experimental investigations of soils under conditions of a general state of stress [1,2,3] showed that their deformation properties are described by" quite complex nonlinear relations of the form eav= ea~av,=i, =),…”

mentioning

confidence: 99%

Calculation of strains and stresses in earth dams and their foundations

Bugrova¹,

Grebnev²

1976

Experimental investigations of soils under conditions of a general state of stress [1,2,3] showed that their deformation properties are described by" quite complex nonlinear relations of the form eav= ea~av,=i, =),where Oar and eav are the average stress and strain; oi and e i are the intensities of stresses and strains; a and m e are Nadai-Lode parameters of the type of state of stress and strain. Below in solving physical nonlinear problems we will use as the deformation characteristics of the soil the generalized shear modulus G and the volume strain K determined with the use of the relations of Eq. (1) for condition c~ e and c~.At present there are several methods of solving physical nonlinear problems: iterative, step (incremental), mixed step-iterative[4], etc. In the iterative method the secant moduli Gs and K s are used; the calculation is made for the complete load and is reduced to an iteration process continuing unffl the difference in the values of certain controlled quantities (e.g., displacements) for successive iterations becomes permissibly small. In the step method the load isdivided into partsand the calculaffon is carried out stepwise, by load increments, on the assumption that the deformation characteristics of the soil within a given n-th step do not change and correspond to the state of stress and strain after n-1 steps. The step method of loading permits using both the secant Gs, Ks and the tangential (differential) Gt, Kt moduli, in the mixed method incremental loading is combined with iteration processes at individual steps. The last two methods permit better control of the solution, since with their use the development ofthestress-strain state with increase of load is followed.Nonlinear problems are solved on the basis of the finite-element (FEM) and finite-difference (FDM) approximations (methods), methods of local variations, and others. The FDM with the iterative method was used, e.g., in calculating earth foundations and dams [5,6,7]. Problems for earth dams were solved by the iterative method with a finite element approximation [8]. The FEM is used below for solving physically nonlinear problems. A great virtue of the FEM is its indifference with respect to the character of the boundaries and stabilization of the region, laws of variation of the rigidity parameters and external load.To solve two-dimensional physically linear and nonlinear problems by the FEM, a program ('FLINZ-GP') was written in ALGOL-60 language for the BESM-6 computer (authors A. K. Bugrov and K. K. Grebnev) at the Departrnent of Underground Structures, Bases, and Foundationsof the Leningrad Pol}~echnic Institute. The program provided for the use of triangular (TFE) and rectangular (RFE) finite elements, and also their combined use. For certain outlines of the boundaries of the region its division into RFE is done in the computer, and in other cases preliminary division is used. Calculation of the mesh-point (node) forces as a function of the soil's weight is provided for in the computer. Analytic expressions or ...

“…The experiments made it possible to investigate three-dimenstonaI stress for different vaIues of ~o [3,9]. 1 (hollow cylindrical specimen, o.d.…”

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confidence: 99%

Effect of the initial state of skeletal-clay soil on its deformability and strength

Lomize¹,

Fedorov²

1975

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Experimental investigations show the effect of the iuidal state of ard~eially compacted clay soil on the operation of a core or other watertight element in a high-head earth dam. The problem of designing the initial state is considered using as the example the experience of cons~cting the Nurek dam core and is solved on the basis of a comprehensive study of the mechanical properties of the soil for the case of a three-dimensionalstressed state of triaxial compression. As a result of layer-by-layer mechanical compaction of the soil it is necessary to obtain the required artificial structure and state of the soil for the design grain-size distribution. Its mechanical and seepage properties, which determine the regularities of development of the stress-strain state in the core during construcrion, introduction into operation, and subsequent long-term operation, depend to a considerable extent on this structure. A special feature of this article is the comprehensive study of the mechanical properties of clay at different stages of load resistance in a state of triaxial compression. The results of long-term investigations of soils in the prelimiting stage of loading and during flow, carried out at the V. V. Kuibyshev Moscow institute ofC ivil Engineering, are used. In this lies the theoretical interest of the presented solution of the applied problem of constructing impervious elements in high-head earth dams.