The Keepit Dam Safety Upgrade Project is being implemented to bring the 54m high concrete gravity dam in line with current guidelines for flood and earthquake loading. Stage 2A of the project involves the installation of two vertical 91 strand post-tensioned anchors on each monolith of the spillway section.
During coring of the anchor head blocks for the vertical anchors, deep cracks were observed across some monoliths, dipping diagonally in an upstream direction. In two of the monoliths the cracks were found to be continuous enough to possibly daylight at the upstream face and form freestanding blocks. If the freestanding blocks postulate is correct, the block stability could be currently relying on the friction of the cracked surface and on the engagement with shear keys of adjacent monoliths, which are provided in the vertical contraction joints.
This paper will explain the complex 3-D nonlinear Finite Element Analysis (FEA) conducted to replicate the conditions of the cracked spillway monoliths during the post-tensioned anchor installation. The nonlinearity captured the expected opening, closing and sliding of the crack, as well as its potential pressurisation, and the residual shear strength retention due to asperities of the crack surface. For the shear keys of the vertical contraction joints, the nonlinearity captured the force-deformation relationship of the plain concrete, up to a brittle failure condition if the shear strength threshold was reached.
The 3-D nonlinear FEA was also used to design the optimum number of Macalloy post-tensioned bars required to stitch the freestanding block to the monolith, so that the vertical anchors can be safely installed. In addition, the remedial design accounted for future extreme design flood and extreme earthquake loading conditions, the latter modelled with a time-history analysis.
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The majority of Australian tailings dams over the last 100 years have been successfully built using upstream construction. However, recent major tailings dam failures in some countries have led to a global industry wide review of the design and management of tailings storage facilities, with a focus on the upstream raise method as a common factor for some failures. As a reaction to the recent failures, there is the potential for regulations to become more restrictive and the potential for unjustified pressure on existing and new mines to rule out upstream raising due to possible safety and failure risks.
This paper looks at whether it is the upstream construction method or other more fundamental issues that have led to these failures and examines whether such issues are equally relevant in Australia. Does Australia have a specific advantage in being able to successfully use upstream tailings dam construction or are we fooling ourselves?
The topic of upstream tailings storage is a subject of broad and current interest and the lessons learned from historic failures are rightfully leading to improvements. Implementation of good practice starts with the overall management structure that guides how tailings dams are designed, constructed, operated and closed.
Critical design practice involves understanding the unique site conditions, properties of the tailings and management of tailings placement, as the tailings form part of the overall retaining structure. Good practice during operation of upstream tailings dams is key to reducing the risk of tailings dam failures and the success of safe and sustainable closure.
This paper presents key features of both good and bad practice for the upstream raising of tailings dams and discusses how the design and operation can be made more resilient to ensure the safety of the community and infrastructure. It concludes that upstream raising can be a safe and economical method of tailings disposal if designed, constructed and operated correctly.
Sedimentation of reservoirs is acknowledged as a global issue and likely impacts water storage capacity in Australia. This major challenge to our future water supply is a highly complex process with deposition leading to infilling of the reservoir of course sediments in headwaters following major inflows, progressively to finer fractions towards dam walls. Wave action and catchment inflows during drawdown conditions will further transport and redistribute sediments into the main body of the reservoir.
Managing reservoir sedimentation requires an understanding of the sediment types and deposition patterns across the reservoir. Once the location and type of sediment is known, strategies to mitigate the effects on the reservoir can be determined. Methods typically used for determining sedimentation of a reservoir are empirical or modeling techniques that rely on detailed data from inflow events, suspended solids loads and flow rates. In the absence of this data, more direct measurements to quantify the amount of sediment present can be used. Direct measurements are more robust than modelling approaches that utilise rating curves that can result in over estimations of the sediment present. This study combined several measurement techniques to produce high spatial coverage of the reservoir floor. Detailed validation of this approach was undertaken in one representative reservoir prior to adopting this approach across multiple reservoirs.
Loss of life estimates in dam breach circumstances are a key determining input in establishing the appropriate risk profile for these assets. They can also be useful in identifying the most effective emergency management responses. While there are a range of approaches described in the literature for assessing loss of life for concentrated population centres, there is little specific guidance on approaches to be taken when there is only a small number of properties or where itinerant loss of life has the potential to be the dominant risk element. Itinerants are most commonly considered to be road users, although, they can alternatively be any temporary users of the floodplain. The literature on flood fatalities indicates that the largest number of deaths occurs at vehicle crossings or otherwise when individuals voluntarily enter waterways. An approach has been developed for identifying the cases where itinerant loss of life has the potential to be the dominant vector for flood fatalities. In addition, the available flood fatality literature and associated databases have been reviewed to establish the precursors to fatalities.
A simple stepped procedure is presented which allows the user to identify cases where itinerant risk to life on roads should be considered with a separate procedure and a method presented by which itinerant life loss may be identified.
K.A. Crawford-Flett, J.J.Eldridge, E.T. Bowman, C. Gordon
This paper provides an interpretation of factors governing the manifestation of internal erosion in a New Zealand canal that was constructed during the 1970s. Liner and subgrade soils were sampled during de- watering of Tekapo Canal in 2013, following the surveillance of erosion events over the preceding decades. This paper focuses on the interpretation of erosion susceptibility of liner and subgrade soil gradations sampled at four locations. Of the four locations, Sites 2, 3, and 4 were associated with internal erosion defects. A single location (Site 1) was selected to provide benchmark “intact” (un-eroded) samples.
Interpretation of susceptibility of the widely-graded soils to internal erosion mechanisms was achieved through the application of established empirical techniques for internal stability, filter compatibility, and segregation. Analysis of gradations, which are believed representative of some – but likely not all – canal soils, showed that Sites associated with erosion defects had liner-subgrade interfaces that permitted “some erosion” (NE < D15F < EE), while the Site showing no sign of erosion possessed an interface that met modern filter retention criteria for No Erosion. Based on gradation analysis, internal instability is considered a possibility for subgrade materials in particular. It is possible that subgrade materials that fail No Erosion criteria for liner retention may not represent as-built material and may instead have lost finer fractions in situ due to seepage-induced instability, leaving a coarser-than-placed and filter-incompatible subgrade.
This case study demonstrates the use of gradation-based empirical methods as initial screening tools to assess the susceptibility of soils to internal instability, filter compatibility, and segregation. The relationship between the internal stability of a filter and the filter’s particle retention performance (compatibility) is emphasised. As well as gradation susceptibility, the assessment of other factors such as segregation and hydraulic loads must be considered in order to better-understand susceptibility to erosion mechanisms.
Fault displacement can occur due to primary faulting on a main fault intersecting a dam foundation or rim, as well as by secondary faulting. This secondary faulting may be triggered locally by the occurrence of primary faulting on a main fault; its occurrence is conditional on the occurrence of an earthquake on the main fault. A probabilistic approach is most viable for fault displacement hazard analysis. Unlike the case of probabilistic ground motion hazard, which is nonzero even for short return periods due to the occurrence of a broad range of earthquake magnitudes in a wide region around the site, probabilistic fault displacement hazard is zero for return periods less than the recurrence interval of surface faulting earthquakes on the fault. In Australia, these recurrence intervals typically lie in the range of 10,000 to 100,000 years.
Consequently, the fault displacement hazard due to primary faulting may be zero or negligible for return periods shorter than 10,000 or 100,000 years. For longer return periods, the hazard is best evaluated using a risk-based approach, as recommended by ANCOLD (2018); the alternative of using a deterministic approach, which disregards return period, could potentially yield a large fault displacement. The probability of triggered secondary faulting, conditional on the occurrence of a large earthquake on the main fault, is typically one or two orders of magnitude lower than that on the main fault, and so is even more likely to be zero or negligible for return periods shorter than 10,000 to 100,000 years