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Case histories: a worldwide view

pubblicazioni - Libro

Case histories: a worldwide view

Dams are structures tightly connected with the surrounding environment through foundations. The lack of knowledge of the actual site conditions in dam design may strongly affect dam safety.

In addition, natural events and possible construction defects are among causes of dam troubles. ICOLD – International Commission on Large Dams – defines the term “failure”: “Collapse or movement of part of the dam or its foundation, so that the dam cannot retain water” differentiating it by the term “incident” which refers to the troubles which have not caused the discharge of the reservoir water. The National Committees have reported to ICOLD 176 failures in relation to the 17406 registered dams (number not including the roughly 85000 dams existing in China where failures occurred to dams built between 1950 and 1980 are valued to be 3%, but the failure rate has dropped sharply for dams built after 1980). Referring to concrete dams, the failure rate of arch structures is the lowest. Buttress and multiple arch dams are more vulnerable. Reported failures occurred mostly owing to overstressing or instability of the foundation as well as abutments. When looking at concrete dams, irrespective of the particular types, their overall failure rate is considerably lower than that of embankment dams. The failure rate of concrete dams has inversely decreased with their service age, as a result of the steady increase of the technological knowledge and reliability of design and construction methods. Regarding embankment dams, they are the most numerous in the world, especially the lower ones; more than 100000 embankment dams are in operation in the world (10 m < H < 15 m). More than 70% of the dams registered by ICOLD (H > 15 m and/or V > 1 hm 3 ) are embankment dams. The main causes of break for the embankment dam are overtopping, piping through the dam or foundation. Terzaghi’s studies on the internal erosion and the consequent definition of the filter rules have contributed to reduce 10 times the failure rate of embankment dams for piping, together with the development of the earthmoving equipments, especially the ones for material compaction. The case histories reported in this chapter refer to failures happened for different causes and they have been chosen not for the entity of the damage occurred, but for their representative characteristics. These case histories have been a useful lesson for owners, designers and builders. In the last 50 years a very important progress has been made in development of dam design, construction, and operation. As a matter of fact, the number of dam failures has been considerably reduced thanks to the advancement of technology as well as to the lessons learned by the careful analysis of incidents and failures. Most of these lessons have given rise to the improvement of safety criteria and have been taken into account within national legislations devoted to dam safety and into international recommendations that represent a reference to the whole dam engineering community.

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