Cosas

CASE STUDIES OF DAM FAILURES: A GEOLOGICAL INSIGHT

ABSTRACT: Case studies of past dam failures reveal the lack of dam-foundation-rock interaction study.
Role played by the connection between the contact surfaces of rocks during loading history is still questionable. Parameters like deformation modulus, normal and shear stiffness, cohesion, friction at joints between rocks play a veryimportant role, for which geological features like faults, folds, etc must be considered in the study for the static and dynamic analysis of dams. Special techniques which incorporates discontinuum should be used for analysis, as complex behavior of rock mass is predominantly affected by the presence of structural discontinuities.

1. INTRODUCTION Today the face of the earth is dotted with small andlarge dams and reservoirs contributing in a variety of ways to complex requirements of an expanding technologically advancing civilization. In the total history of dam construction certainly many hundreds, and, if small embankment dams are included in the count several thousand dams have failed. There are no accurate records of most of the failures and extent of damage that resulted from thetorrential inundation of lower areas as a consequence of their failure. Dams and reservoirs and the foundations on which they rest inevitably undergo changes with time. Some of these changes are slow and subtle and don’t reveal their existence unless precisely and constantly monitored. Others, such as those caused by earthquakes, landslides and unexpected floods are of short duration and usually cannot bepredicted, in spite of the fact that the quantity of knowledge conciliating their prediction is constantly undergoing improvement. The responsibility for the construction of a dam with maximum provisions for the safety and constant critical surveillance of the dam, reservoir and foundation during its lifetime no longer is confined to the engineers who built the dams, but is shared by those whohave special knowledge of hydrology, geophysics, geology, and rock and soil mechanics although every precaution may be taken to design and construct a dam with provision for generous margins of safety during excavation and treatment of the foundation. Within the boundaries of the reservoir, it must be recognized that the dam and reservoir behind it create dead load and water pressures that did notexist previously. Accordingly, the material in the foundation, abutment and the reservoir site requires constant monitoring after construction and reservoir filling. 2. DESIGN AND ANALYSIS Two distinct stages in the design of dam are well recognized. The first stage is that of preliminary design when the overall hydrologic, topographic, economic and even political factors involved are examined inrelation to costs and benefits. The geologic, topographic and hydrologic factors as well as the availability of materials of construction and labor cost decide the location and type of the dam. Though, the overall economy of the project, to a large extent, depends on the decisions taken at this stage, the analysis is essentially based on the judgment of experienced engineers based on the availabledata. In the second stage more thorough investigations of the selected site are carried out and detailed analysis of the structural stability of the dam is conducted. It is at this stage and at later stages, when special problems connected with foundation and super structure crop up, where numerical methods of analysis play a very important role in assisting design decisions. Dams are structuresof great importance. It is therefore, necessary to analyze them as accurately as possible, for the failures of a dam may prove to be catastrophic. Also, considering the large amount

involved in the construction of dams it is worthwhile investing a small fraction of it, analyzing them with the best available techniques, even if it means more effort and higher cost of analysis. An accurate...