malcolm.barker@ghd.com.au Dam Safety During Design, Construction and Operation Malcolm Barker Principal Dams Engineer GHD Pty Ltd PO Box 668, Brisbane 4000, QLD, Australia Introduction In the foreword to the Malaysian Guidelines for Operation, Maintenance and Surveillance of Dams (Malaysian Inter-Departmental Committee on Dam Safety 1989), it is stated “The expanding inventory of dams has brought into focus the subject of dam safety”. At the time of the guideline publication, there had been a considerable increase in the inventory of dams with those constructed during the previous 10 years accounting for 90% of the total storage. The Bakun Dam alone, which is due for completion in 2010, will more than double the current storage capacity. This, together with a number of other dams under design or construction within Sarawak, is a significant structure within the world context of large dams, which again stresses the need for dam safety. The safety of a dam covers all stages in the life cycle commencing with the inception stage through the planning and design stages followed by the construction and commissioning, ongoing operation and maintenance and surveillance and in certain cases decommissioning. 1. Background Throughout history, dams built to store water have occasionally failed with the resulting loss of life, social, economic financial and environmental losses. Failures have involved dams built without application of engineering principles, but have also involved dams built to accepted engineering standards of design and construction at the time. The technology of dams has improved with the increased knowledge of design principles and of the characteristics of foundation and dam materials, and it is generally agreed that safe dams can be built and existing dams can be safely maintained with proper application of current technology. According to FEMA (1979), the goal of making dams “as safe as practical” implies a limit to maximum reasonable effort. It must be recognized that no dam can ever be completely "fail-safe" because of incomplete understanding of or uncertainties associated with natural (earthquakes and floods) and manmade (sabotage) destructive forces; with materials behavior and response to these forces; and in control of the construction process or in human error associated with the operation and maintenance of a dam. Dam safety management must, therefore, ensure that uncertainties are properly balanced with competent technical judgment. The overall purpose of Dam Safety Guidelines and any associated Legislation is, therefore, to enhance national dam safety. The immediate objective is to encourage high safety standards in the practices and procedures used by regulating agencies or designers and owners for dam site investigation, design, construction, operation and maintenance, and emergency preparedness. As safe as practical may be related to As Low as Reasonably Practicable (ALARP) in the emerging risk assessment approach being used for evaluation of upgrade works where risk is defined as “The measure of the probability and severity of an adverse effect to life, health, property, or the environment” (ANCOLD 2003). In consideration of ALARP, the following statements apply; - “Risk is tolerable only if risk reduction is impracticable or if its cost is grossly disproportionate to the improvement gained” and - “Residual risk is tolerable only if further risk reduction is impracticable or requires action that is grossly disproportionate in time, trouble and effort to the reduction in risk achieved”. The level of risk is reduced within the ALARP region by consideration of the effect of the remedial options on the various failure modes with respect to their likelihood of occurrence and the consequence of failure from which the risk is calculated as the product of likelihood and consequences for all failure modes and hazards affecting the structure being evaluated. At the present stage of development, the use of risk assessment in reaching a conclusion on the safety of a dam is seen as an enhancement to the traditional Standards based approach (ANCOLD 2003) and is not an alternative or the sole basis for decision making. A dam safety management program is, therefore, a system that incorporates dam safety values as part of the culture of the organisation and the day-to-day operation of a dam and comprises policies, procedures and investigations which minimise the risk of dam failure. The benefits of a functional dam safety management program (NRM 2002) are as follows: