Paul Somerville, Andreas Skarlatoudis, Jeff Bayless, Polly Guan
The 2019 ANCOLD seismic guidelines state that “A hazard assessment should be conducted for earthquake magnitudes Mw 5 and above. However, under certain circumstances, smaller magnitude earthquakes may form the lower limit. With masonry dams, slab and buttress dams, older concrete dams, and structural concrete components of dams, Mw 4 earthquake magnitudes should form the lower limit.” However, when using probabilistic Uniform Hazard Spectra (UHS) with Mmin less than 5.0 per the 2019 ANCOLD Guidelines, the hazard will be overestimated unless Conditional Mean Spectra (CMS) are used to represent the ground motions. As described by Somerville et al. (2015), use of the UHS can significantly overestimate the seismic hazard levels presented by individual earthquake scenarios because the UHS envelopes the ground motions from multiple earthquake scenarios in one spectrum. This overestimation is especially true of the ground motions from small magnitude earthquake scenarios. The probabilistic UHS may have large short period ground motions with contributions from a range of scenario earthquakes, but if the UHS is used as the design spectrum, these ground motions will often be represented by earthquake scenarios having inappropriately large magnitudes, long durations, and high long period ground motion levels. As a result, these design ground motions have the potential to overestimate the response of the structure under consideration. By using CMS spectra and time histories, the large probabilistic peak accelerations, predominantly from small earthquakes, are better represented by earthquakes having appropriately small magnitudes, short durations, and lower long period ground motion levels, yielding more realistic estimates of the response of the structure.
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Now showing 1-12 of 14 3380:
Qian Gu, Joshua Chan
Tailings Storage Facilities (TSF) constructed using upstream methods may have static liquefaction risks due to the strain softening behaviour of contractive tailings. Conventional Limit Equilibrium Analyses (LEA) using either peak strength or residual strength fail to address the stress-strain compatibilities between materials at different stages of softening or hardening, resulting in over or underestimating embankment stabilities. Static numerical analyses (Finite Element or Difference) are unable to identify the threshold stability due to their inability to converge close to or beyond equilibrium conditions.
In this study the failure triggering process is modelled with dynamic Finite Element Analyses (FEA) with the stress-softening behaviour of contractive tailings simulated by Norsand Model. The embankment failures are identified by either non-zero residual velocities along downstream face, or a drop in average shear stress along potential failure surfaces under increasing disturbing surface pressure. Threshold disturbing surface pressure estimated using these two methods are in close agreements. Factor of Safety (FoS) values estimated from peak mobilised shear strength are found to be between those estimated using the peak and residual shear strength in LEA. q-p’ stress paths in tailings clearly show the stress ratio increasing towards and beyond instability ratio during undrained triggering process. The developments of zones of shear softening and p’ reduction with increasing undrained disturbances help visualise the failure triggering process.
Neeta Arora, Prashant Agrawal, Yogendra Deva, Ravi Kumar
The tectono-lithologic complexities and the accompanying extreme mass wasting processes make the Himalaya a difficult terrain for river valley development projects envisaging dams and other diversion structures. Besides exceptionally thick riverbed deposits leading to management of deep foundations, abutting the dams often poses challenges in view of difficult ground conditions. The paper looks at three scenarios where the presence of highly decomposed strata, slumped mass and unconsolidated riverbed material led to serious problems in abutting the dams and invariably delayed the project completion. The design approach to special abutment issues is discussed in the light of investigations, explorations, laboratory and field tests, etc. In conclusion, while dependable engineering geological mapping and assessment is considered the backbone, innovative investigations and engineering play crucial role in successful implementation of projects.
Andrew Northfield, Peter Hill, Muhammad Hameed, Hench Wang, Sam Banzi
In 2018 WaterNSW undertook a Portfolio Risk Assessment (PRA) for 20 dams across the greater Sydney area.
This paper describes the estimation of consequences for this large and diverse portfolio of dams. For some dams the population at risk were greater than 100,000 people whereas for others there were no permanent PAR which required the careful consideration of itinerants. This diversity of the dams required that the approach for estimating the consequences be tailored to the specific characteristics. For example, the approaches for estimating the potential loss of life (PLL) varied from a detailed simulation model (HECLifeSim) to a simpler empirical approach (Reclamation Consequence Estimation Methodology (USBR, 2014) to bespoke consideration of itinerant campers and users of walking tracks. For some dams the economic costs were driven by direct infrastructure costs whereas for other the indirect costs dominated the total economic cost for failure.
Mark Pearse, John Pisaniello, Sam Banzi, Peter Hill
A completely new dam safety regulation framework was introduced into NSW in November 2019. The new framework addresses all aspects of dam safety management. The implications for dam owners in respect of risk reduction measures (RRMs) that will need to be undertaken have been the matter of debate and are the focus of this paper. The Dams Safety Regulation 2019 requires that dam owners eliminate or reduce the risk posed by their dams but “only so far as is reasonably practicable” (SFAIRP). This is a change from the previous Dams Safety Committee requirement that risks should be reduced as low as reasonably practicable (ALARP). The previous guidance around the extent and timing of risk reduction has been removed and dam owners are now required to determine what is ‘reasonably practicable’. These changes were anticipated to save hundreds of millions of dollars from the reduced cost of risk reduction measures across the state of NSW. These savings appear unlikely to materialise given that dam owners are likely to be highly cognisant of the need to meet the common law expectation that RRMs should be implemented unless the costs associated with the RRMs are grossly disproportionate to the benefits gained. The key changes in the new regulatory framework are identified along with the legal and financial implications in regard to RRMs followed by next steps that should be considered by dam owners in NSW. Many of the implications are applicable to other dam owners who operate under common law (including all states of Australia and New Zealand).
The confluence of several technological innovations including drones, photogrammetry, and thermal imaging has enabled the development of a novel approach to defect mapping and monitoring for large dams. A pilot project trialling the methodology was completed at a rockfill embankment dam with a concrete spillway and is presented as a potential means of improving the accuracy and reliability of condition monitoring. The pilot project included two main objectives: digital inspection and mapping of defects within the concrete spillway; and drone-based photogrammetric survey of the rockfill embankment. Defect mapping of the concrete spillway utilised drone-based photography and Structure from Motion (SfM) photogrammetry to develop a high-fidelity 3D model, from which visual defects could be identified and mapped in a virtual environment. Thermal infrared (IR) imagery of the structure provided an indication of potential shallow subsurface defects in the concrete. Photogrammetric survey of the embankment structure utilised drone-based photography, SfM photogrammetry and a network of precisely surveyed ground control and verification points to develop a georeferenced point cloud, digital elevation model and elevation contours. The results of the project were delivered via a web-based digital twin which included georeferenced results from defect mapping, aerial survey and tools for visualisation, measurement, and reporting.