Two tailings storage cells were raised by constructing new embankments upstream of the existing
embankment walls. The performance of the new embankments was mainly dictated by the underlying tailings that consisted of a thick layer of very soft to soft fine tailings. The fine tailings in one cell was capped by a layer of sand for more than 30 years hence the tailings had mostly consolidated under the load of the capping. The fine tailings in the other cell was under consolidated because the cell had only been capped for about 18 months before the construction of the new embankment. The capping material was sand extracted from the tailings.
Stratification of the tailings was determined by CPT. Undisturbed samples of fine tailings were obtained by a piston sampler for CIU and oedometer testing to obtain parameters required for advanced soil models SHANSEP and Soft Soil (SS) models. These models were incorporated in full 2-D FE models to analyse the stability and settlement of the new embankments at various locations.
The application of advanced soil models such as SHANSEP and Soft Soil by hand calculation and
conventional slope stability analysis is considered cumbersome and labour intensive. This paper
demonstrates that with the help of FE software (PLAXIS in this case), it is practical to implement such advanced soil models to simulate the behaviours of soft fine tailings with reasonable accuracy. A similar approach could be used to model other fine tailings and soft clays. One should be reminded that the reliability of any analysis method relies on validation of the analysis model and parameters adopted.
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Following the catastrophic failure of the bottom outlet conduits of the Massingir Dam, a rehabilitation project was launched involving the installation of steel liners and the rehabilitation of the hydromechanical equipment. This paper describes the testing of an emergency gates for possible use as a control gate to maintain supply to downstream water users. It further describes the innovative use of alternative access for concreting and other services, the use and benefits of self-compacting concrete for infill concreting between the steel liner and existing concrete and the programme and cost benefits of pressurising the steel conduit prior to concrete encasement.
The U.S. Army Corps of Engineers (USACE) is responsible for flood risk management across the United States. USACE has more than 710 dams and is responsible for more than 24,000 kilometres of levees. Since 2008, USACE projects have prevented more than AU$1.2 Trillion (in 2017 dollars) in damages from flooding. Although some of this came as a result of dozens of smaller floods, much of that protection came during three events within the last five years. From 2010 through 2017, the U.S. has had three major inland floods and two coastal events where federal flood protection exists: in 2010 on the Cumberland River, in 2011 on the Missouri, Ohio, White, and Mississippi Rivers, in 2015 on several rivers in Texas and Oklahoma, and in 2017 along the Gulf Coast of the U.S. and its territories in the Caribbean. For many of these locations, these events produced record rainfall and the flood of record. USACE operated many large facilities on these systems and those systems overall performed as expected. However, USACE also experienced some operational issues, did a substantial amount of flood fighting, had several incidents, and several failures. This paper will describe the flooding experienced in those events, the operations of the flood protection systems, the performance overall, and some of the lessons learned.
The evaluation of the maximum instantaneous uplift force produced by turbulent pressure fluctuations plays a key role in designing concrete slab protection in spillway chutes and stilling basins. Recent incidents involving damage to chute linings have highlighted the significance of this issue. To evaluate the stability of spillway stilling basin slabs, it is necessary to determine the statistical structure of the turbulent pressure fluctuations in the spillway chute and stilling basin. This can be defined by an extensive experimental work with a scale Physical Hydraulic Model (PHM). This exercise can be prohibitively expensive in terms of time and cost and it is proposed that the use of Computational Fluid Dynamics (CFD) in this application could become a cost effective alternative. A new approach using Detached Eddy Simulation (DES) was applied to the case of a scale physical hydraulic model representing a real-world prototype and the results of the simulation were compared with the direct laboratory measurements. Here the forces and pressures acting on the slabs are evaluated using both CFD and physical hydraulic modelling results. In conclusion, some considerations on the design of slabs with unsealed joints are reported and discussed.
HEC-LifeSim modelling has been emerging in the industry over the last few years and is increasingly becoming the preferred method for detailed consequence and failure impact assessments. The increased adoption rate of HEC-LifeSim modelling is a result of advancements to computation power and hydraulic modelling techniques and allows dam owners to obtain more robust and consistent estimates of the potential loss of life (PLL) compared to the traditional Graham (1999) and RCEM (USBR, 2014) approaches.
This paper will demonstrate, through the use of three examples, how the inputs and outputs from HEC- LifeSim have been used to identify potential ways to better understand the consequences as a result of dambreak.
Since publication in 2003, the ANCOLD Guidelines for Risk Assessment have reached broad acceptance and use in Australia. In practice, dam owners use the principles of risk assessment to drive business investment decisions. As the guidelines undergo revision, it is timely to assess whether our practices need to evolve to more holistically consider all types of consequences, rather than our current focus on loss of life, in decision-making. This paper aims to prompt dam owners and consultants alike to re-assess our focus on loss of life in risk assessment decision-making, and whether we should more meaningfully consider alternative or broader indicators.
An industry survey was undertaken which found that large dam owners are generally happy with the current system of dam safety decision making. However, the survey responses did identify difficulties in relation to justifying investment below the limit of tolerability that are subject to ALARP principles. In a small number of cases, dam owners found it difficult to justify investment when life safety was not important.
Building on the industry survey and subsequent discussions with practitioners, this paper discusses how the current approach to risk based decision making may result in sub optimal decision making. Further it is discussed how there is an important role that economics should play in providing a universally accepted framework for assessing trade-offs and providing consistent evidence to support decision making.