NONLINEAR ANALYSES OF A HIGH ROCKFILL DAM WITH EARTH CORE

Abstract

The present study aims at studying the stability of high rockfill dam with particular reference to the proposed 260 m high Tehri Dam, U.P. (India) by a study of stresses and displacements within the dam section for end-of-construction and reservoir-full conditions. Two alternative sections with vertical and inclined cores have been studied in detail. The analyses were carried out assuming plane strain condition, rigid foundation and nonslipping embankment founda tion contact. Sequential construction analyses using nonlinear properties have been carried out. An account of the various techniques of constitutive relationships expressing the stress-strain properties of materials has boon given. The functional form as defined by Kondner's hyperbolic stress-strain model and modified by Duncan and Chang has been used for studies reported in the thesis. Nonlinear joint material behaviour has been considered by using hyperbolic stress-displacement relationship. Preliminary studies indicated that the use of mixed graded elements i.e. linear elements in the shells and parabolic elements in the core and transitions improve the overall accuracy of the solution and hence have been used in the analyses. For the end-of-construction condition, the section was discretized into ten layers to simulate sequential construc tion. In case of parabolic elements in the core and transition, the number of layers was reduced to five and the mixed mesh was so generated as to have interconnected nodes. Subsequently, joint elements were also provided along the material boundaries and the core section was discretized into seven layers. The section with the two cores were then analyzed for reservoir-full loading. The stresses obtained in the analysis for the end-of-construction condition were assumed as initial stresses, and the stresses obtained during water loading were additive. The filling of reservoir was simulated in five stages, and the upstream shell and transition were taken as submerged. A general purpose program with sequential construc tion and nonlinear material behaviour has been developed. The program includes a variety of elements and solution algorithms. A mixed graded combination of two types of elements can also be used. Isoparametric, numerically integrated curved joint elements have been developed and incorporated in the program. Variety of elements and even a combination of any two types of elements, can be considered along with nonlinear material behaviour of the joints. Various solution algorithms have been developed and incorporated in the program to give an option to the worker to use the desired solution algorithm simultaneously or any one at a time to account for the nonlinearity of the materials. The algorithm based on residual force approach has been further modified to give an economical analysis and higher rate of convergence. The studies indicated that the provision of joint elements considerably improves the stress distribution between different materials of the composite dam section as compared to the conventional solution without joint elements in which overstressing occurs due to the requirement of compatibility of displacements at the interfacial nodes. The differences observed in the stress and displacement distributions in the core and transitions in the analyses with mixed graded elements and linear elements, have shown a tendency to even out with the ,:, provision of joint elements. The studies indicated that for the vertical core under end-of-construction condition, there is practically no separation along shell-filter-core interfaces, both upstream and downstream. However, for inclined core separation of the interface is indicated along the core-filter interfaces in the region of high deviatoric stresses. The distribution of shear and normal stresses along the core-transition interfaces is indicative of discontinuous contacts along the material boun daries, even though the nodal differential displacements are small. The finite element analyses of 260 m high Tehri Dam under reservoir full conditions have indicated higher horizontal displacements in the section with vertical core, whereas higher vertical displacements are obtained in the section with inclined core. Separation has been observed on'the upstream core-filter interface in the section with both the cores as aresult of water loading, while no separation is indicated along the downstream material interfaces. ; v. The water loading causes increased horizontal and vertical stresses within the core. The horizontal stresses are found to be higher than the hydrostatic pressure all through the height, thus indicating safety against hydraulic fracturing. The factor of safety obtained by finite element method conforms closely to that obtained by simplified Bishop's method. A review of the important work done in the field of application of finite element technique to analyses of high earth and rockfill dams has also been included, and scope for future work has been outlined.