The engineering of large dams

Dam building is a challenge. It was a challenge 4000 years ago when 'The, Sadd-el-Kafara' was built near Cairo; it was a challenge 2000 years ago when the Romans built what we believe to be the first arch dam; it is still a challenge today! The twentieth century has brought sophistication: the computer to make possible our present designs, and modern machines to translate these designs into structures. However, the overall harnessing of the forces of Nature is still an Art; the final decision will be a personal one for someone. It will be his responsibility, and it will live with him and after him. It is fundamental for the Engineer to realise that no matter how strong he may build a dam there will always remain some risk-however small-that unforeseen forces may cause it to fail. He must recognize the various undesirable events that may occur, and assess the probability of their simultaneous occurrence. To accept the possibility of some damage under the worst combination of adverse conditions may be both acceptable and economical, but the Engineer must understand the risk that he takes. It is obligatory for an Engineer to know his subject; it is a legal responsibility that his work should at all times conform to the 'present state of the Art', a state that is continually changing and always advancing. Safety is of paramount importance, and to ensure safety the Engineer must combine the latest techniques of investigation and design with the best available methods of construction. He must investigate every detail but never become so involved in the intricacies of design that he fails to visualize the project as a whole. The effects of a new reservoir on the lives and habits of people, fauna and flora deserve the same attention that is given to the mathematical analysis. The materials to be used must be understood. If our objective is to store water, then we must follow through to the effects of such storage-pore pressures, uplift, piping, corrosion and perhaps induced seismicity. if we are to build on a foundation, we must look beyond the rock itself to the properties in situ, particularly when saturated and loaded. If we are to use concrete, then we must comprehend its properties. What will be its strength in the particular dam? It is certainly not the stress at failure of a small, laboratory prepared, uniaxially loaded cylinder. If we are to minimize settlement and creep in rockfill, then we must satisfy ourselves on the properties of the rock, the voids ratio and modulus required in the embankment, and the construction procedure necessary to obtain the required consolidation. A dam must be understood in its three dimensions. What is the true significance of an isolated tensile stress? It will probably be relaxed by creep, by the opening of joints or even a crack that can be acceptable in certain circumstances. On the other hand, a zone of tension will usually be a matter of grave concern that may warrant a complete redesign. Weather modification by Man may well change the whole subject of Hydrology, impact on the environment will certainly be a major factor in project planning, and the possibility of seismic activity being induce.d by the filling of large reservoirs may be vital in some localities. Modern methods of analysis will make it mandatory to consider the dam and its foundation as a three-dimensional problem, even including the dynamic behaviour of the water in the reservoir during an earthquake. It is clear that the engineering of a dam is a complex undertaking. The International Commission on Large Dams has recently published a report on accidents to dams. Some of these are discussed in Chapter 3. It is disturbing that so many failures have occurred in countries that are credited with wide experience. Errors of omission rather than of commission have led to many of these disasters. The failure of a dam is usually a catastrophe-the responsibility of the Engineer is therefore great. The world is at present seeing a proliferation of technical papers and the subject of dams is no exception-certainly more than 1000 articles appearing each year. Improved systems of information retrieval may provide the Engineer with thousands of references but most Engineers will not have time to read so many articles even if they were available to them. Based upon a career in dam engineering and supported by many good friends throughout the world, I have therefore endeavoured to gather together in one book many of the facets of this subject. Some unique structures are reported, some novel designs are described, some warnings issued and some innovations suggested. My purpose is to guide Engineers along safe paths but at the same time to provide stimulation so that they will treat each problem as a special one to be solved with initiative and ingenuity. It has been my aim to present the subject matter in simple language. Adequate references have been provided so that each topic may be studied to the depth desired for the specific project in hand, or beyond into the realms of research. My purpose will have been served if I have encouraged Engineers to use initiative, to think creatively, and above all to apply common sense to all their work. It has been said that the Universities may give degrees but it is God who gives us common sense. Let us never neglect that gift.