For intraplate regions such as Australia, identifying and quantifying activity on tectonic faults for inclusion in probabilistic seismic hazard assessments can be challenging due to the typically long return period for ground-rupturing earthquakes associated with these structures. Return periods of 10,000’s to 1,000,000’s of years mean that surface displacement evidence is prone to degradation through erosion and burial, and paleoseismological ‘trench’ excavations may not uncover geology old enough to reveal previous events. As a consequence, there is often little or no preserved evidence of past ground rupturing events on these structures. Rather than ignoring faults which show no evidence of neotectonic displacement, we present an alternative approach; in addition to considering active faults (movement in the last 35,000 years) and neotectonic faults (movement in the last 10 Myr) in seismic hazard assessments, we also consider faults which otherwise show no evidence of neotectonic activity but which are aligned favourably with the current stress regime and are therefore potential sources of earthquakes and accompanying strong ground motion.
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While structures such as a dam walls, pipelines, gas storage tanks, and nuclear facilities are vulnerable to the shaking from earthquakes, they are even more susceptible to differential movement on faults passing beneath their foundations.
In the past, the probability of surface rupture of a fault was calculated by making some simplistic assumptions about the distribution of earthquake magnitudes. Improved databases of earthquake ground faulting now allow the probability of surface rupture to be estimated in a more realistic fashion. Computing software that uses a Monte Carlo approach has been developed to allow the effect of various scenario choices on rupture probability to be investigated.
Using this software, it is found that the most significant influence on rupture probability is the long-term fault slip-rate. Other assumptions about the faulting style, maximum magnitude and conversion parameters have only a moderate influence on the results.
There have been several instances in recent history in Australia of surface faulting due to earthquakes, but there has been only limited damage to infrastructure due to the remoteness of these earthquakes. The software that has been developed will allow a considered assessment and comparison of the hazard and risk due to both ground shaking from earthquakes and from surface rupture.
Two-dimensional hydraulic modelling technology has advanced significantly in recent years, providing powerful and flexible tools that are now routinely used for a wide variety of flood risk assessments. Assessing the downstream impacts of catastrophic dam failure represents an extreme test for the accuracy and stability of hydraulic models. Catastrophic dam failure can present an extreme risk to downstream infrastructure and public safety. Hence, it is important to have confidence in the estimated magnitude of potential impacts to design suitable, costeffective mitigation measures. The highly visual output of two-dimensional models adds credibility to their results. However, validation data for extreme hydraulic conditions is rarely available, resulting in uncertainty in the accuracy of model predictions and in the risks associated with dam failure. By validating numerical model results against analytical solutions for cases of simple geometry and also against realworld data, an improved level of confidence can be obtained in the accuracy of the model representation of these extreme hydraulic conditions. In this paper, we assessed the capability of the TUFLOW hydraulic modelling software package to accurately simulate an idealised dam break scenario by comparing the model results to analytical solutions. We also compared the model results for coastal inundation by a tsunami to real-world data from the 2004 Banda Ache (Indonesia) tsunami. The results showed that the HPC solver version of TUFLOW correctly captures the dam break flood fronts and the flood wave propagation and TUFLOW HPC is well suited for dam break flood modelling.
HEC-LifeSim modelling has been emerging in the industry over the last few years and is increasingly becoming the preferred method for detailed consequence and failure impact assessments. The increased adoption rate of HEC-LifeSim modelling is a result of advancements to computation power and hydraulic modelling techniques and allows dam owners to obtain more robust and consistent estimates of the potential loss of life (PLL) compared to the traditional Graham (1999) and RCEM (USBR, 2014) approaches.
This paper will demonstrate, through the use of three examples, how the inputs and outputs from HEC- LifeSim have been used to identify potential ways to better understand the consequences as a result of dambreak.
Investigations into the core material of earth fill dams are undertaken reluctantly due to the potential to cause damage to the embankment. Where investigations are required, Cone Penetration Testing (CPT) is increasingly used to assist with the geotechnical assessment of dam embankments. The risk of hydraulic fracture within embankment core material is well known and procedures are typically adopted to minimise the risk of hydraulic fracture during remediation of the holes. Backfilling is typically done in stages allowing for an initial set of the cement/bentonite grout mixture prior to subsequent lifts.
While the risk of hydraulic fracture is well understood, the lesser known risk of pneumatic fracture is a possibility where certain conditions exist. This paper discusses CPT investigations at Fairbairn Dam, operated by Sunwater in Central Queensland, and the challenges faced in undertaking the remediation of the CPT holes. The potential for pneumatic fracture of the embankment core was highlighted during the investigations and details of alternative techniques adopted for reinstatement of the holes are presented. Recommendations are made to appropriately manage the risk of pneumatic fracture when undertaking CPT’s through embankment core.
The revised magnitudes of the Geoscience Australia’s NSHA18 earthquake catalogue approximately halve the rate of occurrence of earthquakes of a given Mw magnitude in Australia. This yields probabilistic ground motion levels that are significantly lower than the present design levels at dam sites in Australia that are not near faults, and is expected to result in a general reduction in ground motion levels at dams not near faults estimated for all Risk Assessments, and for Deterministic Assessments for all consequence levels except Extreme Consequence. For the latter, the ANCOLD (2018) guidelines will tend to increase existing SEE ground motion estimates for both of the methods used to estimate the safety evaluation earthquake (SEE). By requiring the use of the Deterministic SEE if it is larger than the probabilistic SEE, and by requiring use of the 85th fractile of the Probabilistic SEE if it is larger than the Deterministic SEE, the ANCOLD (2018) guidelines for Deterministic Assessments are much more conservative than the ICOLD and NZSOLD guidelines for Extreme Consequence dams, especially at those located near faults.