A. Scuero, G. Vaschetti, J. Cowland, B. Cai , L. Xuan
Nam Ou VI rockfill dam is part of the Nam Ou VI Hydropower Project under construction in Laos. The scheme includes an 88 metres high rockfill dam, designed as a Geomembrane Face Rockfill Dam (GFRD), which when completed will be the highest GFRD in Laos. The only element providing watertightness to the dam is an exposed composite PVC geomembrane, installed according to an innovative design now being increasingly adopted to construct safe rockfill dams at lower costs. The same system will shortly be installed on a water retaining embankment for a coal mine in NSW, Australia, and has been approved for a tailings dam in Queensland, Australia. At Nam Ou VI the geomembrane system is being installed in three separate stages, following construction of the dam. The first two stages have been completed, and the last stage will start in November 2015. The paper, after a brief discussion of the adopted system’s concept, advantages and precedents, focuses on the construction aspects.
Keywords: GFRD, PVC geomembrane, waterproofing, rockfill dam.
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.
Jiri Herza and John Phillips
The design of dams for mining projects requires processes and technology that are unfamiliar to many mine owners and managers. Dam designers rely on ANCOLD assessments of Consequence Category, commonly leading to a High rating for mining dams due to a combination of potential loss of life, impact on environment and damage to assets such as mine voids, process plants, workshops, offices, roads, railways etc.
From this High Consequence Category the relevant annual exceedance probabilities for design parameters and loading conditions such as earthquakes and floods are selected.
Mining companies have sophisticated methods available for assessing risk, yet for their assets they often adopt an order of magnitude lower security for earthquake and floods even though the consequences in terms of lives at risk and impact on project are similar.
The discrepancies in the design standards lead to situations where extreme dam loads are adopted to prevent damage and loss of life in assets that theoretically would have already collapsed under much lower loads.
One difference may be that some mining dams exist in an environment which is controlled by a single entity. Unlike other dams, failure of these mining dams would therefore impact only individuals and assets which fall under the responsibility of the same entity.
This paper discusses the discrepancies between the design of mining dams and the design of other mine infrastructure. The paper considers the impact of discrepancies on the overall risk to the mine and compares the degree of protection offered by a factor of safety and the influence of reliability of design input parameters, alternate load paths and design redundancy.
Keywords: Dams, tailings dams, mining, acceptable risk, factors of safety
Chriselyn Meneses, Simon Lang, Peter Hill, Mark Arnold
Risk is the product of likelihood and consequences. Much effort is put into the risk assessment process for large dams to ensure there is a consistent approach to estimating failure likelihoods across an owner’s portfolio. For example, the use of common peer review teams and methods like the ‘piping toolbox’ allow the risk assessment team to apply repeatable logic and processes when estimating failure likelihoods. However, the methods for estimating life safety consequences are often not applied consistently. This inconsistency leads to estimates of potential loss of life (PLL) that vary between dams in unexpected ways, because results from the most commonly applied method (Graham, 1999) are sensitive to threshold changes in flood severity and dam failure warning time.
The recently released Reclamation Consequence Estimating Methodology (RCEM) is intended to supersede Graham (1999). RCEM varies fatality rates continuously with DV, and is therefore less sensitive to changes in flood severity. In this paper, estimates of PLL from RCEM are compared with results from Graham (1999) for five dams. Results from the latest US Army Corps of Engineers model for estimating the consequences of dam failure (HEC-FIA 3.0) are also compared with RCEM and Graham (1999) for one dam. Comment is then made about the important considerations for applying RCEM consistently across a portfolio of dams.
Keywords: potential loss of life, dam safety, risk analysis
Maree Dalakis, Dr Saman de Silva, Siraj Perera and Dr Gamini Adikari
This paper describes the results of a statistical and qualitative analysis on historical dam safety incidents in Victoria, the first study of its kind conducted in the State. The study investigates trends arising from qualitative dam safety incident data collected by the Department of Environment, Land, Water and Planning since the year 1996. The reported incidents are categorised based on their severity and statistical trends are identified in relation to the types of incidents common to regulated and unregulated dams, as well as common responses to incidents, including their post-incident operation. The geographical distribution of incidents across the State is also analysed to determine the effects of seismicity on dam safety incident rates. Furthermore, the unique Victorian conditions of sustained drought and subsequent flooding and their impact on incident rates are investigated through the combined analysis of geographical incident distribution and streamflow data. The incident data is further assessed according to the frequency of visual inspection and reporting of the structures in order to gauge the relative influence of these practices, and dam regulation in general, on mitigating incident risk in dams. An understanding of dam safety incident trends and the impact of inspection and reporting practices is increasingly important given the increasing expectation for dam owners to properly operate and maintain their assets with minimal resources and finances.
Keywords: dam, safety, incident, historical, failure.
Paul Southcott, Tim Griggs & Jamie Campbell
Suma Park Dam is the principal water supply dam for the City of Orange in central NSW. The 30m high single curvature concrete arch dam has a High A consequence category and required an upgrade due to an inadequate spillway capacity. To maximise the benefits of this major capital works, the Council also sought to increase the storage capacity and modernise the outlet works to help supply the rapidly growing population of the city.
Challenges that needed to be overcome to develop an affordable and safe solution included: very high flood inflows; limited freeboard; a highly stressed arch with a narrow crest width; poor access to the toe and right abutment; and a saddle dam located on a deeply weathered foundation.
Innovations incorporated into the design of the works included: Monte-Carlo based modelling of the flood hydrology that better estimated the design inflows resulting in a significant reduction in flood upgrade requirements; precast parapet crest units that incorporated crest widening to improve constructability; an anchored toe block to ensure the toe of the arch is stable; an upgrade to the stilling basin; and an auxiliary spillway incorporating Fusegates at the saddle location designed only to operate in floods in excess of the 1;1,000 AEP event with minimal loss of storage.
Construction of the works is now well underway. A number of challenges have been overcome during the construction stage including a re-design of the auxiliary spillway to use Fusegates and discovery of Naturally Occurring Asbestos (NOA) on site. Construction of the upgrade works is expected to be completed by the end of 2015.
Keywords: Concrete arch dam, flood upgrade, pre-cast, fuse gates, anchoring.