Vicki-Ann Dimas, Wayne Peck, Gary Gibson and Russell Cuthbertson
Globally, reservoir triggered seismicity (RTS) is a phenomenon sometimes observed in newly constructed large dams worldwide, for over 50 years now. Over 95 sites have been identified to have caused RTS by the infilling of water reservoirs upon completion of their constructions worldwide. In Australia, there are seven confirmed sites with observed RTS phenomenon that are summarized by temporal and spatial means.
With almost 40 years of seismic monitoring, primarily within eastern Australia, several of Australia’s largest dams have monitored and recorded many RTS events. At present, twelve dams are 100 metres and above in height as possible candidates, with seven of these actually causing RTS and a disputed possible eighth dam.
Important factors of RTS are reservoir characteristics (depth of the water column and reservoir volume), geological and tectonic features (how active nearby faults are and how close to the next cycle of stress release they are temporally) and ground water pore pressure (decrease in pore volume under compaction of weight of reservoir and diffusion of reservoir water through porous rock beneath). RTS is an adjustment process often delayed for several years after infilling of reservoir before eventually subsiding within 10 to 30 years, when seismic activity then returns to its prior state of stress.
Generally there are two type of RTS events, either a major fault near the reservoir most likely leading to an earthquake exceeding magnitude 5.0 to 6.0, or more commonly, a series of small shallow earthquakes.
Seismic monitoring of all dams (except for Ord River) are presented with spatial and temporal series of maps and cross sections, showing the largest earthquake, build-up and decay of RTS events.
Keywords: Seismic monitoring, reservoir triggered seismicity (RTS), earthquake cycle
Chris Topham, Andrew Pattle, David Tanner, Oliver Giudici
Many owners around the world have dams that rely on grouted, post-tensioned rock anchors for stability. The anchors were installed during the original construction of the dams or retrofitted to improve stability during their operational life. The use of fully grouted post-tensioned anchors spanned the period of the 1960’s to 1980’s. The main issue with these un-sheathed grouted rock anchors is the question of integrity of the grout column protecting the anchor and concerns about possible corrosion of the high tensile wires from which the cables are constructed. While some of these anchors have corrosion monitoring systems installed, it is difficult to validate such data and there is considerable uncertainty over the condition of such anchors. To compound the problem, replacement of the anchors is technically complex, extremely costly and difficult to justify in the absence of known condition. For example, Hydro Tasmania has recent experience of work to cease reliance on such anchors at Catagunya Dam that cost $41m in 2009. With fifteen dams relying on some form of post-tensioned anchors, Hydro Tasmania has recently taken the unusual step of over-coring and extracting three post-tensioned rock anchors from operating dams in order to assess their condition. In what is believed to be a world first, a 42m long 70 strand high tensile anchor was overcored and removed from Meadowbank Dam in 2014. A further two anchors were successfully extracted from Repulse Dam in 2015, in conjunction with a group of international sponsors with similar anchors. This paper uses the 2015 work to illustrate the methodology used to extract the anchors, outlines the information gained from this unusual work, and presents the results of the condition of the extracted anchors. The paper concludes with some inferences for other owners with similar anchors and suggestions for further work.
Keywords: Grouted, post-tensioned rock anchor, ground anchor, corrosion, over-coring, extraction, dam safety.
Richard R. Davidson, P.E., CPEng Kenneth B. Hansen, P.E.
Early in the twentieth century, placing concrete core walls within embankment dams was a popular construction technique for small to medium height dams. It became in vogue as a replacement for the popular British dam construction technology of puddle clay core dams which were used between the 1860’s and 1920’s. It avoided the many problems with semi-hydraulic / manned placement methods of the puddle clay cores within narrow trenches. However, after the mid 1930’s this concrete core wall construction fell out of favour because of the improvements made in embankment compaction methods and the difficulties in building reinforced concrete core walls to more significant heights.
Today concrete core wall embankment dams are now reaching an age where their continued performance is being questioned. This dam building technology has become extinct and is unknown to the last few generations of dam engineers. Therefore, it is relevant to re-examine this dam building technology in a modern context and work on answering the following questions. How have these dams performed after almost a century of service? Are there unanticipated performance features that have produced positive results when subjected to extreme flood and seismic events? Does the concrete provide enhanced performance over time? What role does steel reinforcement play in the performance of the core wall? Are there lessons here that can be applied to the more common concrete cutoff wall solutions being applied to embankment dams with seepage problems? This paper examines these questions with a number of illustrative case histories to provide a retrospective illumination of this forgotten dam building technology.
Keywords: Embankment dams, Concrete core walls, Dam construction history.
Since their development, rock mass classification systems have used and manipulated various populations of geomechanical data to allow a rock mass to be divided into different domains or engineering ‘masses’ with the aim of assisting in the geotechnical design of underground openings, excavations, foundations and ground support systems.
Each of these methods consider different characteristics to generate a material classification; including rock strength, joint weathering, defect spacing, in-situ stress and groundwater. However, none of these systems cater for classification of the rock mass based on whole rock weathering, whole rock strength and incipient defect spacing along a borehole.
This new classification system, the Rock Condition Number (RCN), has been developed to reduce the human factor of variability in interpretation when collecting data to classify the rock mass, as other methods, such as Rock Quality Designation (RQD), are prone to significant variability based on the experience of the person logging the core. RQD provides an indication of rock quality over the length of the cored interval, which varies depending on the drilling equipment and ground conditions. This value may typically be calculated over an interval of 1.0, 1.5 or 3.0 metres. The RQD system does not allow for the rapid identification of thin, though important features in the subsurface.
Using data captured electronically in the field, the RCN calculates an instantaneous classification of the rock mass at any point along the borehole, highlighting variations within the rock mass by assessing a combination of characteristics, allowing rapid identification of potential hazardous zones within the rock mass. This allows for significant improvements in efficiency during the assessment and design process/es. Resolution is greatly improved over RQD, with thin, though important, zones of weak material highlighted using this new process.
Comparison between existing classifications and the RCN using real field data indicates the RCN provides greater resolution when identifying deficient zones within the rock mass.
Keywords: Rock mass characterisation, RQD, Rock Condition Number, rock quality, dam foundations.
R. Nathan, P. Jordan, M. Scorah, S. Lang, G. Kuczera, M. Schaefer, E. Weinmann
This paper describes the development and application of two largely independent methods to estimate the annual exceedance probability (AEP) of Probable Maximum Precipitation (PMP). One method is based on the Stochastic Storm Transposition (SST) approach, which combines the “arrival” and “transposition” probabilities of an extreme storm using the total probability theorem. The second method – termed “Stochastic Storm Regression”(SSR) – combines frequency curves of point rainfalls with regression estimates of areal rainfalls; the regression relationship is derived using local and transposed storms, and the final exceedance probabilities are derived using the total probability theorem. The methods are used to derive at-site estimates for two large catchments (with areas of 3550 km2 and 15280 km2) located in inland southern Australia. In addition, the SST approach is used to derive regional estimates for standardised catchments within the Inland GSAM region. Careful attention is given to the uncertainty and sensitivity of the estimates to underlying assumptions, and the results are compared to existing AR&R recommendations.
Keywords: Annual exceedance probability, Probable Maximum Precipitation.
Phillip Kennedy, Robert Murphy, Pat Russell, Chi Fai Wan
Central Highlands Water (CHW) owns thirty four dams varying significantly in size, age, and condition. Thirty of the dams are used for water supply purposes with the remainder providing storage for wastewater reuse schemes. Out of the thirty-four dams, eighteen are more than one hundred years old. They are zoned earthfill embankments, some with a puddle clay core. Fourteen of the dams have been assessed as having potentially high to extreme consequences if the dam fails. The key safety issues among these high consequence dams are inadequate flood capacity, slope instability, and high potential for piping.
CHW’s management policy includes a commitment to identify, assess, prioritise improvements to, and periodically review the safety of its dams, and implement a dam safety upgrade works program. CHW’s Water Plan 3 (2013 – 2018 economic regulatory period) includes nine dam safety upgrade projects, which were identified from risk assessments and investigations carried out over several years.
In 2013, CHW and MWH formed a Delivery and Operational Efficiency Review (DOER) Group to refine and confirm priorities for the proposed dam safety upgrades. The main objectives of the DOER Group were to identify solutions to meet current ANCOLD guidelines and any opportunities to achieve 10% – 20% reduction in capital expenditure costs during planning or delivery of the works for Water Plan 3, while achieving the intended risk reduction. The key elements of the DOER were to (1) form a working group to cover operational, planning and executive management considerations together with dam safety consultants and Victorian dam management experience; (2) closely scrutinise previous assessments; (3) challenge the justification for the project; (4) understand the priorities whilst aiming to deliver a major works program; and (5) identify additional investigations.
Initial investigations of the DOER Group developed a revised program of works allowing confirmed capital works to proceed while investigations into other projects were carried out. The follow-up investigations have identified optimal outcomes through a program of cost-effective solutions for CHW.
This paper aims to share the experience from planning the DOER, and the further investigations that resulted in the development of an optimised delivery strategy for the upgrade projects.
Keywords: Delivery and Operational Efficiency Review, Risk.