ABSTRACT
The major causes of earth dam failure are sliding, overtopping, seepage, human intervention and earthquake. Sliding can occur to the reservoir banks, embankment or foundation when the shear stress due to external loads along a plane in the soil mass exceeds the shear strength that can be sustained in that plane. In such a situation the failure along the plane is imminent. Periodic cycle of the saturation of the porous material and the increased pore water pressure reduce the shear strength which leads to loss of stability and produce major landslides. Due to this sudden arrival of soil mass in the reservoir, generally produces a high solitary wave propagating against the earth dam. An initial breach can form immediately in the dam and progressive erosion caused by flowing water will lead to a partial or total failure of the dam. Improvements in the analysis of probable maximum storm have caused significant increase in the predicted probable maximum flood (PMF). Many earlier dams have not been designed on PMF values. As a result many dams once considered safe are now considered unsafe due to inadequate spillway capacity. In the year 1982, United States Army Corps of Engineers inspected 8,639 high hazard dams under Federal Dam Safety Programme. Of these 2,884 dams were found to be potentially unsafe due to inadequate spillway capacity. Seepage of water can be through the embankment, foundation or abutment of the dam. Uncontrolled or controlled seepage through the body of the dam or foundation may lead to piping or sloughing and the subsequent failure of the dam. Due to continuous seepage breach will be formed in the embankment resulting in the outflow through the breach. The breach size will continuously grow as material is removed by outflow from the storage and storm water runoff. The size, shape and time required for the development of breach are dependent on the embankment material and the characteristics of the flow forming the breach. Breaches of this type can occur fairly rapidly or can take several hours to develop. Earthquake at the dam site can produce waves or landslides and consequently leads to a partial or total failure of dam. Depending on the severity of the earthquake the entire dam may be washed off or only a part of it may be removed due to breach failure. Such failure is sudden in nature. Other causes are due to differential settlement of the foundation of the dam. Londe has reported the findings of Middlebrooks who provided a statistical analysis of earth dam failure using 200 case histories. The record covers a period of hundred years. Figure 15.1 provides the result in terms of percentages of each category of failures. From the diagram it is obvious that only 15 per cent of the total failure numbers is by sliding which could be evaluated by usual concept of factor of safety. The remaining 85 per cent do not come under conventional stability analysis and are relevant to design, construction and operation procedures, which can hardly be computed in the conventional manner. To properly describe the gradual breaching of an earth dam, the geometry of the breach must be related to the hydraulics of the flow and bed material properties, i.e., the energy of the flow through the breach at any instant of time must be compatible with work required to be expended in the erosion of the bank material and its transport. It is to be noted that rate of erosion and mode of failure of dam determine the shape and duration of the flood wave to a large extent. Fread assumes the rate of growth of the breach to be time-dependent, with either rectangular, triangular or trapezoidal shape [66].
