THREE. DIMENSIONAL FINITE ELEMENT ANALYSIS OF CONCRETE GRAVITY DAMS - EFFECT OF VALLEY SLOPES AND VALLEY BASE WIDTH

Abstract

Concrete gravity dams of all height can be built at sites where rock formations are strong enough by feasible corrective measures such as grouting, dental treatments, etc. The failure of dam causes catastrophic damage to life and property, therefore extreme care and precaution must be taken while designing the dam. In earlier days concrete gravity dams are assumed as two dimensional structure for design and plane strain analysis is used in which the stresses normal to horizontal planes are assumed to have a linear trapezoidal distribution. But this assumption is valid when the valley width is wide and transverse joints are not grouted. However, in narrow valleys the dam behaves as a three dimensional structure. The transverse joints grouted and provided with shear keys to make it monolithic structure. Therefore, the load is transferred partly to the foundation and partly to abutments. Since some part of load is transferred to abutments, so the stresses developed in the dams is less than two dimensional analysis. Thus in three dimensional analysis it is possible to adopt a smaller section of the dam thereby, reducing the cost of project. The load transfer to abutments will be negligible in wide valleys and it will depend on the valley width and shape. Therefore it is important to know the limit of valley width beyond which the dam behaves as a two dimensional structure. This forms the objective of the present study. In addition to this the effect of change in valley slopes and valley base widths are also visualized. For this purposes dam of 100 m height is analysed on five valleys of different canyon angle ranging from 30°, 35°, 400, 450 & 50° resulting in a valley width factor (f3) of 2.2, 2.4, 2.6, 3 & 3.4 respectively keeping the base width of the valley as 100m. This will give the effect of change in slope whereas for determining the effect of change in valley base width, the dam is analysed for different valleys of increasing base width keeping the slope of the valley constant resulting in a.valley base width factor equals to 2.8, 3, 3.3, 3.6, 3.9 & 4.2. The valley is assumed to be symmetrical in this study and the result of three dimensional analysis is compared to that of two dimensional analysis. The two parameters studied in the present study are stresses and displacements which is determined and compared two and three dimensionally. In the analysis the dam is considered as a solid monolithic structure having no transverse joints. The analysis is carried out using elastic material constants (Young's modulus E, & poission ratio v) and density of the concrete and rock. Bassalt rock is considered as foundation material for the analysis. The FEM element used in the study are SOLID 95 for three dimensional analysis and PLANE 82 for two dimensional analysis. ANSYS FEM package is used for the analysis of present study. The conclusion of the study is that the value of stresses and displacement in 3 D analysis due to change in valley base width is more closer to two dimensional analysis as compared to the value and stresses obtained due to change in valley slopes. It is also concluded that the pattern of stresses and displacement is same for similar loading condition at all the section analysed in this study.