INVESTIGATION OF TENSILE STRESSES AROUND THE HEEL IN CONCRETE GRAVITY DAMS

Abstract

In the past, little attention was paid in the design criteria of concrete dams to tensile behaviour.' because the dam are normally designed to mainly resist compressive stresses. Recent experience has demonstrated however that, contrary to this usual assumption, concrete dams designed in accordance with traditional procedures may be vulnerable to damage from excessive tensile stresses. For concrete gravity dams two types of problems of tensile stresses around the heel and in the main body, mainly occur in relation to cracks caused by applied loads. The determination of the risk level for the tensile stresses around the heel necessitated the development of new numerical procedures, and the use of fracture Mechanics Approach. Review of various methods of analysis and design of concrete dams has been made to indicate the limitations of the existing methods and applicability of criteria based on fracture mechanics of concrete used in dams and foundation rocks. To determine whether the dam is safe, it is necessary to depart from the conventional 'no tensile stress criteria'. A suitable alternative is to consider the development of most probable crack and to find whether such a crack eliminates all tensile stresses at some stage of its progress by entering into foundation zone with compressive insitu stresses. Basic Material characteristics of concrete based on the results of fracture mechanics characterisation tests have been studied. Further to suggest suitable guidelines for dam designers indicating various effects of material parameters and their selections, a detailed study has been carried out for the present finite element study of concrete dams. An empirical approach has been evolved to determine the fracture mechanics parameters KJC and GQ. The recommended approach takes care of both compressive/tensile strength of concrete and its maximum aggregate size. This allows mass concrete mix design accordingly. The difference between modulus of rupture and direct tensile strength of concrete has been explained by the inclusion of softening zone in tension and the question of selection of tensile strength ascribed to mass concrete stressed in tension under seismic load has also been included. Parametric studies of solid and hollow gravity dam sections by using linear elastic finite element analysis have been carried out to select the best section on the basis of minimum tensile stress. Several sections of solid and hollow gravity dam are analysed for a 192.0 m Proposed Lakhwar dam. The effect of various parameters namely, effect of moduli! ratios of foundation rock to concrete, upstream downstream slopes, modifications near the heel and flange width web width ratio, have been studied in details for both the load cases (normal and normal plus earthquake load cases). Parametric studies made by linear elastic finite element indicate that the E /E and base width both have most effective role in the stress R C distribution in a dam body and foundation. The area of tension zone and magnitude of tensile stresses are the indicators for the selection of proper geometrical shape. Numerical modelling of crack in concrete structures is an important aspect. There are two school of thoughts for the modelling of cracks in concrete structures viz. Smeared crack Approach and Discrete Crack Approach. Discrete crack approach is preferred for the modelling of cracks in dam studies because of the basic requirement of a line crack for the application of linear elastic fracture mechanics. Interactive graphical approach have been used to study the behaviour of crack propagation and corresponding stress distributions, deflected shapes and mode of cracking on the screen itself. Frontal solution has been used to avoid renumbering and an interactive mesh generation program is developed to facilitate the data preparation and its checking. Parametric and case studies of solid and hollow gravity dams based on linear elastic fracture mechanics (LEEM) criterion by using the new finite element program of LEEM have been carried out. The most important parameters affecting the zone of tensile stresses and their magnitude and Stress Intensity Factors (SIF) are ER/EC> B/H and type of singularity at the heel. The best location of preformed cracks can be ascertained by LEFM and they certainly relieve the tensile zone from the dam body as well foundation rock . It is also important to notice that the section optimised by linear elastic finite analysis may not be suitable from LEFM point of view.