A brief overview of dam surveillance is given from a South African perspective and more specifically the perspective of the Department of Water Affairs and Forestry (DWAF). DWAF’s Ten Commandments for the design of dam monitoring systems serve as introduction and this is followed by a summary of the design steps. The various parameters that can be measured and the South African preferences are discussed briefly followed by a synoptic description of crack and joint monitoring in South Africa. This provides the background for DWAF’s recent developments in 3-D Crack-Tilt gauges. Some of DWAF’s achievements as well as some of the blunders made by the author during the past 30 years are illustrated by means of a few case histories.
Peter J Burgess, Delfa Sarabia, John Small, H. G. Poulos and Jayanta Sinha
The assessment of settlement behaviour of clay core rock fill dams has always been a challenge for dam designers and geotechnical engineers. The method of construction and the material properties of the clay and rock fill materials used in the dam construction have a significant influence on the inter-zonal interaction and the load transfer that occurs within the dam. At times this load transfer can lead to excessive differential and total settlements. The paper presents a case study of a major dam that experienced large settlements during and after construction. An elaborate analysis has been carried out by modelling the sequences of construction by using a finite element program (PLAXIS).
The paper describes the influence of the degree of compaction and moisture control on non-linear deformation characteristics of clay core. High vertical strains in the wet placed region of the core and low strains in the dry placed regions were analysed for possible shear development between the core and shell. The rock fill for the dam embankment consists of quartzite, metasiltstone and phyllite material. These materials have apparently undergone deformation with increasing height of the dam due to softening and crushing as saturation of the embankment took place. The effect of soil consolidation and strength gains have been considered in the analysis and are discussed. The settlement behaviour of the dam including these effects has been analysed, and compared with the historical post-construction settlements.
This paper is intended to provide valuable information for dam engineers handling clay core rock fill dams – especially where there is excessive settlement of the core.
John Bosler and Francisco Lopez
The ANCOLD “Guidelines for the Design of Dams for Earthquake” were published in August 1998. The guidelines contain a brief outline of the performance requirements and recommend, in general terms, a method of analysis for intake towers.
Over the last three decades there has been considerable research on the seismic performance of intake towers as they move into their inelastic range. In the years following the publication of the ANCOLD guidelines, some of the findings from this research have been incorporated into revised design procedures issued by the US Army Corps of Engineers. These procedures, if embraced by ANCOLD and the local dam engineering community, are likely to have a significant impact on how the structural adequacy of existing towers under seismic loading are assessed.
Rocking behaviour in which the tower becomes unstable as a transient condition has long been recognised as acceptable under certain conditions. Attempts to prevent tower rocking by measures such as retrofitting tensioned ground anchors may, in some situations, be of limited value in improving the seismic performance of a tower and could result in an increase in bending moments in the tower stem. Guidance is now available on the amount of rocking behaviour that is tolerable.
For seismic events greater than the Operating Basis Earthquake most towers will start to exhibit inelastic behaviour. Specific guidance is also now available on the length of time during an earthquake that bending moments in excess of the elastic capacity can be tolerated, the amount by which these moments can exceed the nominal bending moment capacity and the vertical extent of the tower stem that can be stressed beyond its elastic limit.
The paper discusses the different approaches taken by ANCOLD and the Corps of Engineers. Key differences in outcomes are highlighted using a worked example for a typical reinforced concrete tower and the ANCOLD approach is found to be generally, but not always, more conservative. The paper concludes with recommendations for dealing with these differences.
Peter Allen, Malcolm Barker, Shane McGrath and Chris Topham
Are we there Yet? The question we all ask in Tolerability of Risk. The answer is in the journey, which we are all on as owners, regulators or designers.
A number of authorities in Australia are applying risk assessment for the evaluation of dam safety upgrades in accordance with the October 2003 ANCOLD Guidelines on Risk Assessment. A fundamental requirement for the evaluation of risk below the limit of tolerability is the use of the As Low As Reasonably Practicable(ALARP) principle. In making a judgement as to whether an ALARP position may have been reached, ANCOLD suggest the evaluation of a Cost to Save a Statistical Life, good practice, level of existing risk, social concerns, affordability and duration of risk. ANCOLD also suggests consideration of the USBR Criteria for evaluating risk. Recent guidelines on the Acceptable Flood Capacity for Dams developed by the Queensland Dam Safety Regulator provide further insight into the application of ALARP.
The objective of the paper is to make dam owners, regulators and designers aware of some current practice regarding the evaluation of ALARP in Australia, highlight the challenges of applying this principle and to encourage further discussion.
A. Uromeihy, P.G. Ranjith
In response to increasing potable water need and in order to control and collect precipitations, many dams have been constructed and many more are under construction in Iran. Due to the complex geology of the country, many of the dam sites face serious geological problems both during construction and in operation phases. The most predominant types of problems are water leakage and sediment deposition in the reservoirs. In order to define and classify the type of problem with regards to geological condition, the country is divided into eight zonesin whicheach zone demonstrates similar problem on the dam site location. It is found that the water leakage is related directly to either the presence of soluble carbonate rocks in the abutment or the presence of thick permeable material in the foundation. It is also shown that the sediment deposition in the reservoir is related to many factors but the geology of the watershed area has a major effect. Therefore it can be concluded that the geology has a great role in the construction of dams.
C Lake and J Walker
Meridian Energy is the owner and operator of a chain of hydro dams on the Waitaki River in the
South Island of NZ. It operates a Dam Safety Assurance Programme which reflects current best
practice; consequently it has focused primarily on managing civil dam assets. Advances in plant control technology have allowed de-manning of our power stations, dams and canals through centralised control. The safety of our hydraulic structures is increasingly reliant on the performance of Dam Safety Critical Plant (DSCP) – those items of plant (eg water level monitoring, gates, their power and control systems, and sump pumps) which are required to operate automatically, or under operator control, to assure safety of the hydraulic structures in all reasonably foreseeable circumstances.
Recent dam safety reviews have highlighted that the specification and testing of our DSCP is based on the application of ‘rules of thumb’ which have been established through engineering practice (eg. “monthly tests”, “third level of protection”, “backup power sources”, “triple voted floats”). The
adequacy of these engineering practices is difficult to defend as they are not based on published
criteria. The realisation that such rules may not be relevant to the increased demand on, and complexity of, DSCP led us to ask “Which belts and braces do we really need?” The current NZSOLD (2000) and ANCOLD (2003) Dam Safety guidelines give little guidance regarding specific criteria for the design and operation of DSCP.
Meridian has identified the use of Functional Safety standards (from the Process industry, defined in IEC 61511) as a tool which can be applied to the dams industry to review the risks to the hydraulic structures, the demands on the DSCP, and utilise corporate “tolerable risk” definitions to establish the reliability requirements (Safety Integrity Levels) of each protection, and determine lifecycle criteria for the design, operation, testing, maintenance, and review of those protections.
This paper outlines the background to identifying Functional Safety as a suitable tool for this purpose, and the practical application of Functional Safety Analysis to Meridian’s DSCP.