John Grimston, David Leong, Robin Dawson
The Angat Multipurpose Project, originally constructed in the 1960’s, is located 60 km north-east of Manila, and provides power, irrigation and domestic water supply and flood mitigation. The major water-retaining structures of the scheme are a 131 m high main rockfill dam and a 55 m high rockfill saddle dam.
Previous seismology studies have identified the presence of a possible branch of the West Valley Fault crossing under the saddle dam. If the fault dislocated, the branch under the saddle dam could produce horizontal and vertical shear displacements. Further, earthquake shaking poses a risk outside the fault zone. If the main dam/saddle dam were to fail in such an event, there would be major consequences in respect to both the water supply (serves a population of approximately 10 million) and the large population living below the dams. The dams are thus in the highest hazard category under any internationally accepted standard.
A study to investigate the dam safety aspects and identify remediation works which would bring the seismic performance of the main dam/saddle dam system up to an acceptable level was undertaken and included:
The main conclusions were:
Keywords: Dam, Remedial, Seismic, Fault, Spillway.
Barton Maher, Michel Raymond, Mike Philips
The Queensland Bulk Water Supply Authority (trading as Seqwater) owns and operates North Pine Dam, situated on the North Pine River in the Northern Suburbs of Brisbane. North Pine Dam is an Extreme Hazard Dam consisting of a concrete gravity dam with earthfill embankments at both abutments and three earthfill saddle dams. The spillway consists of five radial gates which are manually operated. Flood operations at the dam are controlled in real time by the Seqwater Flood Operations Centre.
In January 2011, North Pine Dam experienced the flood of record at the dam site with a peak inflow of approximately 3,500 m3/s and a corresponding outflow of approximately 2,850 m3/s. This inflow was more than double the previously recorded flood of record. The inflow was generated by high intensity rainfall both at the dam and in the upper catchment resulting in a rapid rise of the storage. The system which caused this rainfall was also contributing to the major flooding occurring in the adjacent Wivenhoe – Somerset catchment, also being managed by the Seqwater Flood Operations Centre. The rapid rise and fall of the storage presented difficulties for both the Seqwater Flood Operations Centre and the operators at the dam site.
Following the flood event, an analysis of the rainfall and the resulting inflows indicated a significant difference between the Annual Exceedance Probability (AEP) of the rainfall in the catchment and the estimated AEP of the inflow and peak water levels from previous hydrology studies. A detailed review of the flood event was commissioned by Seqwater and undertaken by URS Australia Pty Ltd.
This paper presents details of the flood event, lessons learned for the operation of the dam, upgrade works undertaken to date, results of the hydrology review and the conclusions of the Acceptable Flood Capacity (AFC) study. A key implication for dam owners was the increase in the estimate of the Probable Maximum Flood (PMF) by over 30% due to changes in calibration of the hydrologic model for the catchment.
Keywords: Probable Maximum Flood, Flood Operations, North Pine Dam, Flood Estimation
Richard R. Davidson, Nate Snorteland , Doug Boyer, John France
The US Army Corps of Engineers (USACE) has embarked upon a monumental journey in applying risk-informed decision making in the management of the safety of the 650 major dams for which it is responsible. This process has shifted safety criteria from fully deterministic to a probabilistic basis. There has also been a shift from de-centralized district-based decision-making to centralized management of resources through the new Risk Management Center (RMC) and the Senior Oversight Group (SOG), a group of senior engineers and managers from across the USACE organization. The risk process began about five years ago with a portfolio prioritisation using screening-level risk assessments of the entire dam inventory, culminating in Dam Safety Action Classifications (DSAC) for each of the dams. Based on this risk prioritisation, Issue Evaluation Studies (IES) were initiated for the highest risk DSAC I and II dams, with each study including detailed failure mode and risk analyses for each dam. Because the Corps was relatively new to dam safety risk analyses, and their dam design history was one of following codified manuals of practice, various risk tools were prepared to provide guidance when assessing the risk of potential static, seismic and flood failure modes, as well as life loss and economic consequences of dam failure. Although these tools provided useful guidance to a relative large population of inexperienced risk estimators, many of these early risk assessments were flawed; they provided unrealistically high estimates of failure probabilities and the tools did not help estimators understand or explain each failure mode. To assist the RMC in bringing more defensible risk estimates to the table and improve consistency of the evaluations, the Quality Control and Consistency (QCC) review process was initiated about two years ago. The QCC process provides high level review of IES activities, including detailed reviews of risk analyses, by a small group of experienced dam safety risk estimators. Not only has this brought risk estimates into a more reasonable range, it has provided valuable training for risk estimators, and important checks and balances on the risk-informed decision making process for moving dam safety upgrade projects forward. The justification for a number of very expensive projects has been challenged and, in some cases, re-prioritised, and other projects have risen to the prominence they deserve.
Karen Riddette, Chee Wei Tan, Alan Collins, David Ho
Due to a number of historical stilling basin slab failures around the world, modern basin slab stability assessment approaches now require allowance for hydrodynamic pressure fluctuations. Extreme fluctuations in uplift pressures have been found to occur in hydraulic jumps and plunge pools resulting in high-pressure pulses being transmitted via joints and drainage openings to the underside of the slab. If, peak uplift forces beneath the slab coincide with minimum pressure fluctuations on the top of the slab, the resulting pressure differential can be sufficient to lift a slab. As a result, simple static design based on tailwater depth and mean floor pressures is now considered highly non-conservative.
Through a case study on the Waipapa Dam spillway stilling basin, this paper examines the use of CFD modelling to compute mean hydrodynamic slab pressures taking into account the location of the hydraulic jump and the effect of the impact blocks on the pressure distribution over the slab. By combining the CFD results with empirically-derived pressure fluctuations, uplift scenarios are applied in a FEA model to compute the maximum load in the slab anchors and examine the sensitivity of the stilling basin slabs to uplift failure.
Keywords: Stilling basin, hydrodynamic modelling, CFD, pressure fluctuation, slab stability.
Zhenhe Song, Arjuna Dissanayake, Shunqin Luo
One of the potential tailing dam failure modes that is commonly evaluated is for prediction of earthquake induced crest displacement in relation to available freeboard. The prediction of seismic induced displacement for tailing dams can be evaluated using simplified approaches, i.e. analytical methods by Newmark (1965), Makdisi and Seed (1978), Bray and Travasarou (2007) and empirical method by Swaisgood (2003) and Pells and Fell (2003).
Seismic induced displacements have been estimated using these simplified methods and numerical methods by FLAC and PLAXIS. The results from the numerical modelling were compared with results derived from the simpler analytical and empirical methods. The results indicate the numerical analysis results agrees reasonably well with empirical methods by Swaisgood (2003) and Pells and Fell (2003) and can be used to provide additional confidence in the seismic stability of tailings embankments. However, simplified analytical methods by Newmark (1965), Makdisi and Seed (1978), Bray and Travasarou (2007) could underestimate the seismic induced displacements.
Keywords: Tailing dam, Seismic analysis, numerical analysis, simplified analysis, liquefaction.
The Enlarged Cotter Dam project was selected as a key component in securing the future water supply for Canberra and the ACT region. The RCC gravity dam, when completed, will stand 84m high and will be the largest of its kind in Australia.
The dam was designed, and is currently being constructed, under the Alliance contract model. The collaboration this model brings between the owner and the design and construction teams facilitated a drive in innovation from the design through to the construction stages of the project. The focus of this paper is on some of the key innovative aspects of the project, for consideration on future RCC and dam projects.
Investigation was made into the placement of RCC in 400mm layers, compared to the industry adopted standard of placement in 300mm thick layers. Whilst full scale trials demonstrated that placement in 400mm thick layers was not detrimental to the quality of the RCC, the benefits in terms of increased production were never fully realised due to adverse weather and the geometry of the dam placement area. Some issues were also encountered with regards to the compaction of the GERCC on the dam faces. The results do however suggest that the method warrants consideration on future RCC projects.
The construction of the dam’s secondary spillway included a waterstop installation in a constrained RCC placement zone. By developing an arrangement that could hold the waterstop in place and induce the movement joint in the correct location, this arrangement simplified what could have been a complicated procedure in an already time consuming placement area.
The start of RCC placement was at risk of further delay on account of the extensive mass concrete pours required to level the dam foundation. A conventionally vibrated concrete mix, made from the existing site won RCC materials, was designed so that it could be produced from the RCC batch plant. This method of concrete production, combined with an efficient means of delivering the concrete to the pour area, accelerated the placement process and reduced the cost of construction.
Keywords: RCC, dam, construction.