Recent tailings dam failures have led to worldwide alarm that we are still getting an average of two
significant tailings dam incidents a year. This is despite the efforts of various industry organisations aroundthe world to raise the standards of tailings dam management. Clearly, a significant number of mining dams are not re silient enough to ensure the required level of safety for sustainable mining operations in a modern world in which there is increasing concern for the environment. This paper updates ANCOLD with international developments in attempting to address shortcomings in the mining industry that is allowing these failures to continue to occur.
In Australia, ANCOLD have released an addendum to the 2012 ANCOLD Guidelines on Tailings Dams, Planning, Design, Construction, Operation and Closure, to coincide with the new ANCOLD Guidelines for Design of Dams and Appurtenant Structures for Earthquake. This addendum also addresses issues of governance of tailings dams and provides additional guidance on the serious issue of static-liquefaction, a critical factor in recent failures.
On the international scene, ICOLD is progressing a Tailings Dam Safety Bulletin that is hoped will set
minimum standards for Tailings Dams for all member countries. In addition, the International Council of Mining and Metallurgy (ICMM) similarly wants to establish an international standard. It is likely that these international bodies will cooperate to ensure a consistent set of guidelines and that countries will accept and implement these.
This paper updates the ANCOLD position regarding guidelines and describes the state of various
international guidelines following the June ICOLD meeting in Ottawa.
The U.S. Army Corps of Engineers (USACE) has a robust Dam Safety Program (DSP) that utilizes risk- informed decision-making to prioritize its portfolio of dams in need of further study and modifications. USACE also utilizes a two-tiered governance structure in which one body makes portfolio recommendations around risk management while the other body oversees the execution of the agency’s routine DSP and makes policy recommendations. The routine program consists of the activities required for interim risk reduction measures, inspections, instrumentation, monitoring, assessments, operations and maintenance, emergency action planning, training, and other dam safety activities. An internal program management tool exists to monitor and track all these activities and generate metrics around execution of the routine DSP, however, it does not include metrics around other aspects of the DSP like governance, asset management, public safety and security, flow controls, or audits/reviews. USACE hopes to identify gaps in its DSP that can be used to correct shortcomings, continuously improve, and to increase the resilience of its DSP, which will enable each project to deliver benefits to the Nation. The Centre for Energy Advancement through Technological Innovation (CEATI), through its Dam Safety Interest Group (DSIG), collaboratively developed a spreadsheet tool known as the Dam Safety Maturity Matrix (DSMM). The DSMM is a facilitated exercise used to help evaluate how well-developed a program is across 12 elements considered to be typical and important of most dam safety programs. Each of the elements is then deaggregated into sub-elements, each of which can be evaluated by the team. The maturity ranges across 5 levels from Needing Improvement to Leading Edge. After all sub-elements are evaluated, an aggregate maturity level is computed that gives an estimation of the overall maturity level of the program. USACE will present the results of its pilot project using the DSMM and share lessons learned regarding its implementation. The short-term goal is to identify program strengths and areas for improvement, while the long-term goal of USACE using the DSMM is to participate in bench- marking across multiple agencies and international dam owners regarding their dam safety programs, for which has never been done to the knowledge of this author.
Many quantified risk assessments finish the failure mode event tree at the estimated occurrence of an embankment breach leading to dam failure outflows and downstream consequences. In some situations, for dams with multiple embankments with potentially different consequences downstream of each embankment, the possibility for further breaches may be pertinent if there may potentially be higher consequences for a multiple breach scenario. The location of an initial breach and sequence of subsequent breaches could also result in different contributions to total risk.
This paper discusses a method applied to investigate the conditional probability of flood overtopping breaches for multiple earth-fill embankments with grass covered downstream slopes.
For the subject dam, preliminary modelling identified that for a flood overtopping breach of an embankment the breach’s development may not be sufficient to reduce the lake level and sustained overtopping flow over the remaining embankment crests could lead to further embankment breaches.
A Monte Carlo dam breach simulation modelling approach was used with a large number of flood events. The simulation modelling considered erosion initiation for a grass slope due to the combination of velocity and duration of flow, and erosion continuing to breach based on duration of flow after erosion initiation. Potential uncertainty of erosion initiation and erosion continuing to breach were represented with probability distributions in the Monte Carlo modelling.
The results from the large number of dam breach simulations were then analysed with post processing to derive conditional probabilities for single or multiple breaches and breach sequence.
Flood inundation consequence and emergency evacuation assessment using advanced numerical modelling tools such as HEC-LifeSim is progressively emerging as accepted best practice, due in part to the growing ease in obtaining the necessary datasets and hydraulic numerical modelling results and the increasing computational power readily available to perform analyses. In turn, these tools are being applied to assess dam failure consequence and the effectiveness of emergency response procedures.
An essential resource is an approved Emergency Action Plan (EAP, also known as a Dam Safety Emergency Plan), which describes how dam owners and disaster management groups notify and warn persons at risk of harm during an emergency event. There have been progressive improvements in the effectiveness of EAPs through a series of reviews and lessons learnt from emergency events, legislative and regulatory amendments and general improvements in communications, monitoring, alerts and public awareness. Effectiveness is measured through feedback from training exercises and expert reviews, however a more quantitative measure is not presently available. This limitation can challenge decision makers who need to balance costs associated with emergency preparedness with anticipated reductions in life safety risks.
The paper explores the feasibility of providing a quantitative assessment of the effectiveness of an EAP using advanced consequence modelling (HEC-LifeSim). Using consequence models for two dams in Queensland, EAP effectiveness is assessed for a range of emergency response measures. The accuracy and reliability of the model parameters applied to each simulation and their impact upon the reliability of predictions of potential loss of life (PLL) are analysed and discussed. The feasibility of the approach is discussed and recommendations to be considered for future applications made.
The Waimea Community Dam will be the largest multipurpose concrete face rockfill dam (CFRD) to be constructed in New Zealand. This 53 m high CFRD will impound a reservoir of 13 Mm3 and is essential to securing the future water needs of the community and environment of the Waimea Plains and wider Tasman/Nelson region.
The design of this unique large dam in the New Zealand context was a long-term collaboration of local dam design expertise and international experience that took the ‘historic precedent based design approach’ for CFRD’s and supplemented this with modern embankment design techniques for the highly seismic environment at the dam site. Design of this High Potential Impact Category dam presented a range of technical challenges for the designers and wider project team, which required new and innovative design solutions and approaches.
The dam features a number of unique arrangements in the New Zealand context including:
The project had its origins in the early 2000’s. Detailed design commenced in 2010, and was externally peer reviewed. The detailed design stage was undertaken in an Early Contractor Involvement (ECI) process which was completed in February 2019.
This paper covers the important seismic design aspects for this large dam, including understanding and designing for the potential range of displacements and embankment deformations to inform the crest parapet wall and diversion culvert designs, and understand how differing rockfill properties might affect the dam performance. Quantifying the range of potential dam performance enabled a more resilient dam design.
Dam spillway gate collapse may have fatal consequences and cause severe structural damage due to flooding, additionally the dam owner will suffer substantial business losses. The repair work required to put a gate back in service can be time consuming, challenging, dangerous and costly. To ensure the reliability of radial gate operation, and depending on the type of trunnion bearing and the structural capacity of the gate arms, the bearing friction should be carefully monitored and gate performance evaluated to confirm the gate’s ability to withstand increases in friction over time. The frequency of monitoring requires careful consideration.
Radial gate arms are normally designed to withstand bending moments from nominal bearing friction. An inappropriate bearing, or a bearing in poor condition, might have friction sufficiently high to cause a gate arm to fail due to the excessive bending moment during gate operation.
An easy and non-invasive way of analysing the condition of the bearing, to ensure safe operation of radial gates where the arms might be prone to increased bending moment, is through friction measurement with the use of strain gauges. This paper briefly presents common radial gate design and some failure modes as a consequence of increased bearing friction, and a method of determining the bearing friction coefficient through strain gauge measurements and experience from the field is presented.