David Brett, Robert Longey, Jiri Herza
The independent expert review panel for the Mount Polley Tailings Storage Facility failure came out strongly recommending changes to the technology of tailings dams in British Columbia (and by inference, world-wide). The Panel had examined the historical risk profile of tailings dams in British Columbia and recommended, amongst other things, that best available technology (BAT) be adopted for tailings disposal. Examples of BAT, described by the panel, included “dry-stacking” of filtered, unsaturated, compacted tailings and reduction in the use of water covers in a closure setting. The recommended technologies would require a major shift in current practice and raises many questions, such as:
– Are these recommendations appropriate in Australia?
– Does this signal the end of the tailings dams as we know them?
– Do the current Australian National Committee on Large Dams Guidelines (ANCOLD) apply to these new BAT technologies?
– If not, is there a role for ANCOLD in setting standards for the future?
This paper discusses the Mt Polley tailings dam failure and searches for answers to these questions. In particular, this paper reviews the background to “dry-stacking’, to explore the implications for the Australian mining industry.
Keywords: Tailings Dam, Dry Stacking, Best Available Technology
Nihal Vitharana, Nuno Ferreira
The raising and/or stabilising of existing concrete gravity dams by continuous concrete buttressing is a viable solution and, in some cases, it is the only solution available. There are few medium-large dams in Australia currently under consideration for raising with continuous buttressing.
Two of the major issues to be surmounted are: (a) the existing dam should not be subjected to cracking (particularly on the upstream face) due to heat-hydration effects, and (b) the requirement for the two dam bodies to resist the hydrostatic and other loadings as a monolith (unified dam).
However, there is great need for understanding the mechanisms involved in selecting an appropriate heat-of-hydration model and in calculating thermal stresses rationally. Due to such lack of understanding, expensive precautions, mostly with compounding conservatisms, would be adopted in concept and detailed designs eg. shear-keys on the interface, artificial cooling, post-grouted interface, anchor bars at the interface, concrete with high cement contents. On the other hand, unsafe designs could be the result.
The paper discusses these issues highlighting that a rational approach can be adopted to economise the design and construction processes. An example is also presented to demonstrate how the potential for temperature-induced cracking in new and old dam bodies can be evaluated with reduced uncertainty by considering all the mechanisms involved in a holistic way.
Keywords: Heat-of-Hydration modelling, raising concrete dams, thermal stresses, concrete buttressing
Michael Bassett-Foss , David Bouma , Dewi Knappstein
The Wairarapa Water Use Project (WWUP) in the southern North Island, New Zealand, is investigating new water storage schemes involving large dams that will allow the community to make use of the water resources that are currently available, but not necessarily available at the time they are needed. It is estimated that the 12,000 hectares currently irrigated in the Wairarapa could be increased to about 42,000 hectares depending on actual demand. The WWUP provides for a range of possible needs, such as supply of new areas of irrigation, increased reliability for existing irrigation and frost fighting, environmental augmentation of low summer river flows, environmental flushing flows, stock drinking water, power generation, municipal water supply, and recreational use.
WWUP objectives include early engagement of stakeholders, early integration of financial, social, cultural and environmental factors in decision-making, management of uncertainty associated with the preliminary level of investigation and evolving regulatory framework, development of an equitable framework for efficiently comparing options, and balancing long and short-term considerations.
A large number of dam options were identified, storing 3 to 80 million m3 of water, and progressively narrowed to a shortlist of 2 sites through a complex process of concept development, desktop studies, site visits, hydrological analyses, cost estimates and multi-criteria analyses.
The WWUP demonstrates how sustainable new major water storage schemes can be promoted in a highly regulated environment of a developed nation.
Keywords: Dams, water storage, stakeholder engagement, environment, water allocation, multi-criteria analysis
Gavan Hunter, Andrew Pattle and Mark Foster
A piping incident occurred during first filling of Rowallan Dam, Tasmania in 1968. The incident occurred at the interface of the embankment with the spillway wall, a 15 m high near vertical wall, where the contact earthfill eroded into the single stage downstream filter. Repairs were undertaken in 1968/1969 and the reservoir has operated largely without incident since.
A risk assessment in 2009 identified that piping through the embankment at the spillway wall interface remained a significant dam safety risk. Investigations in 2010 encountered cracking within the earthfill core at the spillway wall interface.
Dam safety upgrade works were undertaken in 2014/15 to address the piping failure mode at the spillway walls and also within the upper portion of the embankment. The works required excavation down to a rock foundation at depths up to 18 m adjacent to the spillway walls and this excavation provided an unusual opportunity to closely examine active piping features that had been preserved when interim repairs in 1968/69 had arrested the progression of piping. The repair comprised reconstruction of a significant portion of the embankment at the spillway and the reconstruction of the upper 7 m of the crest, which included dual filters downstream of the earthfill core.
The findings from the forensic investigations of the deep excavations adjacent to the right spillway wall are described in this paper along with a summary of finding from the 1968/69 repair works and a discussion of the piping mechanism at the spillway wall. The paper also covers the design and construction of the repair work. The focus of this paper is on advancements in our understanding of piping risk arising from the Rowallan Dam work.
In conclusion, (i) the upgrade works successfully reduced the dam safety risk of Rowallan Dam; (ii) the findings support the methodologies of the piping toolbox; (iii) the case study provides insight into filtering and crack filling mechanisms that have a broader implication for estimating the risks of internal erosion within existing dams; and (iv) the findings support the assessment of the low residual risks for piping through the embankment away from the upgrade work areas (crest reconstruction and spillway walls).
Keywords: Earth and rockfill embankment, piping incident, piping mechanism, dam safety upgrade.
Steven E Pells, Philip J N Pells, William L. Peirson; Kurt Douglas and Robin Fell
The method of Annandale (1995) is widely used by Australian practitioners for the assessment of erosion in unlined spillways. This method is based on comparison to various case studies, where the geology at each site is characterised using the Kirsten index (a rock mass index previously developed to assess the rippability of rock), and the hydraulic conditions are characterised using the unit stream power dissipation. In this paper, the historical development of this comparative design technique is traced and is critically reviewed against the original geotechnical and hydraulic data, and against a new, independent, dataset gained from unlined spillways in fractured rock in Australia, South Africa and the USA. It is shown that, while erosion can be usefully correlated against rock-mass indices and hydraulic indices, this ‘comparative’ design technique has been promoted beyond its reach – the data do not support the inference of an erosion ‘threshold’ as presented by Annandale (1995). It is argued that this type of analysis should be used only as an initial ‘first indication of erosion potential’, as originally proposed by van Schalkwyk (1994b).
Keywords: scour; erosion; spillways.
Nikifor Petrovic, Sladoljub Pezerovic
Dam rehabilitation works at the Visegrad Hydropower Project on the River Drina in Bosnia and Herzegovina were completed in October 2014 after two years of very challenging and collaborative effort between the client, designer and contractor.
The successfully accomplished remedial works programme was a highly complex geotechnical intervention. The dam was constructed on a karst foundation extending up to 200 m below reservoir floor level. Rates of seepage through the foundation increased over time, from 1.4 m3/s following first impoundment in 1989, to 14.7 m3/s in 2009.
The rehabilitation works comprised:
Preparatory works (site installation, work platforms, conveyer belts, electricity and water supply, drilling and grouting equipment installation);
Site investigation works (drilling of boreholes, measurements of inclination, geo-physical carotage, downhole video, underwater camera recording);
Installation of monitoring equipment and implementation of real time recording system;
Installation of inert material into a sinkhole within the storage area and into the bore holes located upstream of the dam; and
Grouting of the foundation area using different grout mixes and grouting methods.
During rehabilitation works the main achievements were:
A total of about 37,300 m3 of inert material (granular materials with different fractions from 0 to 32 mm) was installed into the foundation cracks and caverns. This was a significant achievement due to very complex geological conditions and resulted in a seepage reduction through the foundation and improvement of the overall safety and stability of the dam.
The total consumption of grouting material was in access of 2,500 tonnes of cement, bentonite, sand and additives.
After completion of the work, seepage of water through the foundation was reduced to about 4.5 m3/s.
Keywords: Seepage, remedial works, dam, grouting, inert material.