EFFECT OF FOUNDATION AND DAM DISCONTINUITIES ON THE RESPONSE OF CONCRETE GRAVITY DAM

Abstract

Studies conducted on gravity dams have indicated that foundation structure interaction effects play an important part in the behavior of dams; however, this aspect had been ignored in the past designs but appropriately included in the last few years. The instance of dam failures like Shih-Kang dam, Koyna dam, Malpasset dam, St. Francis dam and others designed in accordance with the state of the art were very striking. These dams failed not due to deficiency in analysis and design but due to insufficient knowledge of the local geology on which the dam was founded. Therefore, it is necessary to analyze dams modelled as realistically as possible, since its failure may prove to be catastrophic. Analyses of dam foundation systems are usually based on continuum models (FEM studies). Elaborate models have been developed to include the interaction of the structure with the foundation, represented as an elastic or visco-elastic continuum, as well as including coupled hydrodynamic effects. However, limited studies have accounted for the nonlinear behavior that can be induced on discontinuities in the dam body or foundation due to static or dynamic loads. The phenomena of slip or separation along the joints in the foundation or structure can affect the behavior of the system significantly. To analyze the behavior of concrete gravity dams, including foundation, rationally the numerical method adopted should have the capability to include factors that significantly affect the behavior due to discontinuity in the dam structure and jointed rock in the foundations. A numerical technique to represent the structure as an assembly of discrete blocks where joints are viewed as interfaces between distinct bodies is provided by the Distinct Element Method (DEM). This method uses an explicit time stepping algorithm which allows an efficient treatment of the nonlinear phenomena occurring in the rock joints, such as sliding and separation. in DEM codes UDEC 4.0 (Universal Distinct Element Code, 2004), a two-dimensional (2D) and 3DEC 3.0 (Three Dimensional Distinct Element Code, 2003) have been used in this study. The objective is to study the behavior of a concrete gravity dam taking into account the effect of dam and foundation discontinuities on the stability and design of dam. For the analysis of any structure, first and foremost point to be taken into account is to model the structure close to the actual conditions prevailing in the field. A comprehensive modeling for the dam-foundation system taking into account all the discontinuities has been developed. Two-dimensional seismic stability analysis of a cracked Koyna dam has been carried out using UDEC to investigate the stability of the top block above the crack. Influence of the dam discontinuities with discontinuity in the form of crack which runs through the concrete section has a complex phenomenon taking place at the interface of crack. The behavior of cracked dam at the cracked surface is sensitive to interface properties and varying seismic input. Crack profile and its orientation plays an important role in the stability analysis, where toppling of the top block relative to lower block takes place as has been observed in d/s sloping crack. This can be hazardous leading to disaster downstream of the dam by causing flooding and inundation. Seismic stability of dam with horizontal crack is more susceptible to sliding followed by rotation. Thus, sliding and rotation of the top block above the crack contribute to the overall stability of a cracked dam. Modeling of a dam in 3-D involves a number of complexities with regards to dam and foundation portion. Three-Dimensional Distinct Element Code is an efficient tool which has been used for modeling of a dam. As a case study, the modeling and analysis of a concrete gravity dam upcoming in the lesser Himalayas has been taken. Different models IV have been formulated considering the complexities involved in the modeling of the damfoundation system and the results have been compared. Discontinuities in the dam monoliths proves to be more reliable in the analysis of dam than single dam monoliths which under-estimate the response of the dam. Large increase in the relative displacement between the monoliths and permanent displacement at the end of seismic input has been observed at the discontinuities. Model with rock interaction i.e. inclusion of discontinuities or condition prevailing in the field taken for the analysis represent actual events taking place in the field, as in the present study wedge failure on the left abutment has been indicated. A parametric study on the strength of the rock mass clearly indicates that improvement in the overall strength of the rock mass can help in preventing wedge failures on the left abutment. Changing the strength of the rock joints alone cannot prevent the failure. It is also observed that most of the permanent displacement in the dam occurs due to the plastic movement at the dam and foundation discontinuities. The study presents a comprehensive modeling and response of the dam-foundation system taking into account all the discontinuities in the form of dam monolith interfaces and the discontinuities in foundations using DEM. Study with DEM modeling approach provides adequate information for controlling the dam and rock foundation stability at the local and global scales. It is clearly indicated from the study that the interaction between dam monoliths and discontinuities in the foundation rock mass should be a necessary part ofthe gravity dam analysis.