Safety of Existing Dams Evaluation and Improvement (1983) / Chapter Skim
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2. The Safety of Dams
2. The Safety of Dams
Pages 4-40
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From page 4... ...
In the last decade, several major dam failures have increased public awareness of the potential hazards caused by dams. In today's technical world, dam failures are rated as one of the major "low-probability, high-loss" events.
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From page 5... ...
The remainder of this chapter presents a discussion of these elements. s CAUSES OF DAM FAILURES Dam Failure Surveys A number of studies have been made of dam failures and accidents.
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From page 6... ...
Table 2-2 pertains only to concrete dams and lists the number of incidents in the USCOLD surveys for each principal type of such dams. Tables 2-1 and 2-2 list incidents by the earliest, or "triggering," principal cause as accurately as could be determined from the survey data.
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From page 7... ...
As indicated, the modes and causes of failure are varied, multiple, and often complex and interrelated, i.e., i Dam failures 1900-1975 (over 15 m height)
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From page 8... ...
, individually and collectively, and should be made periodically by experts in the field of · ~ c tam engineering. Many dam failures could be cited to illustrate complex causes and the difficulty of identifying a simple, single root cause.
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It also explains why different evaluators could arrive at different conclusions regarding prime causes. Certainly any such table should be accompanied by 9 Dams Built tic ( Arch Buttress 8 ~ Gravity Fl LL Failed Dams FILL LL Buttress G ravity LL ~ c: ~ Arch O ; Buttress C' ~ Gravity TOTA L CONCR ETE Fl LL ;~ ~ 6 .
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From page 10... ...
The principal reason for overtopping was inadequate spillway capacity. However, in 2 failure cases overtopping was
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From page 11... ...
11 TABLE 2-1 Causes of Dam Incidents Type of Dam Embank Concrete ment Other* Totals Cause F A F A F A 17 A F & A Overtopping Flow erosion Slope protection damage Embankment leakage, piping Foundation leakage, piping Sliding Deformation Deterioration Earthquake instability Faulty construction 2 Gate failures 1 TOTAL 6 3 3 5 6 18 14 23 11 2 5 2 3 6 2 7 3 17 13 14 43 28 29 3 3 2 1 19 19 77 163 7 27 10 17 17 13 37 34 13 23 14 17 49 7 28 6 31 2 9 3 3 2 3 2 5 7 103 182 285 37 66 35 37 11 *
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From page 12... ...
In the latter case, overtopping and/or poor compaction of the spillway-embankment interface was suspected but not confirmed. With respect to the 3 concrete dam failures, the spillways were destroyed in 2 instances and in the other, a small buttress dam, the entire dam was destroyed.
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From page 13... ...
Six concrete dams, 1 steel dam, and 11 embankment dams were involved in the 18 failures. In at least 11 of the 49 accidents, which involved 6 concrete and 43 embankment dams, the leakage occurred in the abutments.
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From page 17... ...
The 2 failures involved corrosion of outlet pipes, which allowed leakage and piping of embankment material into the outlet. The 9 accidents involved 3 embankment dams and 6 concrete dams.
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From page 18... ...
The failures occurred in concrete gravity dams and in 1 case was attributed to the omission of reinforcing steel. The 3 accidents occurred at embankment dams and in 2 cases were caused by poor bonding between old and new embankment material, leading to seepage and slope failures (in 1 case during drawdown)
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From page 19... ...
Unless continuously wet in a freshwater environment, timber structures such as cribbing will eventually decay from water content cycling and insect infestation and attack by organisms. Low-quality riprap will soften and disintegrate, destroying its effectiveness for erosion and slope protection.
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From page 20... ...
Technical inspections are those involving specialists familiar with the design and construction of dams and include an assessment of the safety of project structures. Maintenance inspections are those performed at a greater frequency than technical inspections in order to detect at an early stage any significant developments in project conditions and involve consideration of operational capability as well as structural stability.
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From page 21... ...
The first inspection should, if at all possible, include interviews with the original designer, the owner, the constructor, and current as well as previous caretakers/operators. Inspection Scope The field examination must be both systematic and comprehensive because very subtle changes or visual indicators can often be important in the evaluation of an existing or potential safety problem.
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From page 22... ...
, and Guidefor Safety Evaluation and Periodic Inspection of Existing Dams (Forest Service and Soil Conservation Service 1980~.
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From page 23... ...
Maintenance Inspections Formal maintenance inspections should be conducted on a semiannual-toannual basis to monitor the behavior and condition of the structure and of all operating equipment. The inspection should be performed by an engineer or experienced supervisor of dam operations, who should note any adverse changes in physical conditions, such as erosion, corrosion, blockages of drains, blockages of spillway channels and other water passages, and subsidence.
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From page 24... ...
MAINTENANCE Normal maintenance activities include the surveillance of the project's physical conditions and the timely correction of any deficiencies that might develop, as well as the preservation of the operating capability of the project. The inspections described in the section Maintenance Inspections should be an integral part of the maintenance program.
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From page 25... ...
Bureau of Reclamation 1980) , Guide for Safety Evaluation and Periodic Inspection of Existing Dams (Forest Service and Soil Conservation Service 1980)
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From page 26... ...
Whatever data are available for review can normally be obtained from the owner's files or from the files of the state agency regulating the safety of dams, if such an agency exists and takes an active role in the particular state in which the facility is located. Design Data The review should reveal whether the original design criteria and assumptions are satisfactory based on the current state of the art and, if not, whether they are acceptable.
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From page 27... ...
All of the above can be invaluable in reviewing the safety of a dam as well as in establishing investigational programs. Operating Records and Maintenance Records of operation and maintenance activities are often more readily available to the reviewer than are design or construction records.
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(See section on Classification of Inundation Areas. ~ In this connection the emergency action plan (discussed
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Evaluation and Conclusions After all available data have been reviewed and the site has been examined in detail, the evaluator should analyze all pertinent information revealed by the record, all conditions observed at the site, and the results of any engineering calculations. One of the most useful techniques to apply in the process of evaluating the safety and stability of an existing dam and its appurtenant works is to compare performance, as indicated by field observations, instrumentation measurements, and the results of any required special investigations performed to evaluate a specific problem, with the assumptions and calculations macle in the original design of the facility.
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From page 30... ...
For example, unless available data on spillway design indicate conclusively that the spillway meets present-day design standards, a new flood estimate should be made, and the existing spillway should be analytically tested for its ability to safely handle the updated flood. Or, as another example, if the stability of an embankment dam appears marginal for any reason (such as apparently over-steep slopes, unusual saturation patterns, low-strength soils, or indications of high foundation pore pressures)
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From page 31... ...
When the perceived problems involve areas of specialized engineering practice and there would be significant losses from failure of the structure, experts in the pertinent specialties should be brought in as consultants. EMERGENCY ACTION PLANNING Current Policies and Practices While the intent of dam design, construction, operation, maintenance, and inspection of dams is to minimize the risk of dam failures, it is recognized that the possibility of dam failures still exists.
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From page 32... ...
Planning should be coordinated with local officials, as necessary, to enable those officials to draw up a workable plan for notifying and evacuating local communities when conditions threatening dam failure arise. Some states and several federal agencies have already developed their own emergency action planning guidelines and have implemented plans at dams consistent with the major elements contained in the Federal Guidelines.
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From page 33... ...
Where there is a series of dams on a stream, analyses should include consideration of the potential for progressive "domino effect" failure of the dams. Appendix A of Chapter 4 provides an example of guidelines on estimating modes of dam failure for formulating emergency action plans by an investigator-owned utility.
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From page 35... ...
Actions to Be Taken to Prevent Failure or to Minimize Effects of Failure Development of an Emergency Action Plan An emergency action plan should be developed for each dam that constitutes a hazard to life and property, incorporating preplanned emergency measures to be taken prior to and following assumed dam failure. The plan should be coordinated with local governmental and other authorities involved in public safety and should be approved by the appropriate toplevel agency or owner management.
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From page 36... ...
Coordination with local government and civil defense officials would determine responsibility for the notification Normally, this would be a local government responsibility. When it is determined that a dam may be in danger of failing, the public officials responsible for the decision to implement the evacuation plan should be kept informed of the developing emergency conditions.
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From page 37... ...
The assistance of local civil defense personnel, if available, should be requested in preparation of the evacuation plan. State and local law enforcement agencies usually will be responsible for the execution of much of the plan and should be represented in the planning effort.
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From page 38... ...
Locating Local Repair Forces Arrangements should be made with, and a current list maintained of, local entities, including contractors, and federal, state, and local construction departments for possible emergency use of equipment and labor. Training Operating Personnel Owners of large impoundments should have technically qualified project personnel who are trained in problem detection, evaluation, and appropriate remedial (emergency and nonemergency)
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From page 39... ...
Initiation of Predetermined Remedial Action At least one technically qualified individual, previously trained in problem detection, evaluation, and remedial action, should be at the project or on call at all times. Depending on the nature and seriousness of the problem and the time available, emergency actions can be initiated, such as lowering the reservoir and holding water in upstream reservoirs.
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From page 40... ...
, Subcommittee on Emergency Action Planning, Dam Safety, Emergency Action Planning. Forest Service and Soil Conservation Service, U.S.
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