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.
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The As Low As Reasonably Practicable (ALARP) principle was established in the Australian Dams
community in the ANCOLD Guidelines on Risk Assessment in 1994. Since that time, dam owners have been focused on reducing their societal risk to below the ANCOLD Limit of Tolerability (LoT) through dam safety upgrades and are now considering how to justify an ALARP position. This paper presents a framework that provides a systematic approach to assembling the inputs, applying a process and documenting the outcomes of an ALARP assessment. It is a pragmatic approach that aligns with the safety case, which is a legislated requirement for Major Hazard Facilities in Victoria.
The framework has been applied to two dams in Melbourne Water’s portfolio with differing societal risk, size, uses and criticality to the water supply system. It has highlighted the importance of dam safety governance, documentation of procedures, defensible technical analysis and an ongoing engagement with leading industry practice, in demonstrating risks are ALARP.
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.
Two tailings storage cells were raised by constructing new embankments upstream of the existing
embankment walls. The performance of the new embankments was mainly dictated by the underlying tailings that consisted of a thick layer of very soft to soft fine tailings. The fine tailings in one cell was capped by a layer of sand for more than 30 years hence the tailings had mostly consolidated under the load of the capping. The fine tailings in the other cell was under consolidated because the cell had only been capped for about 18 months before the construction of the new embankment. The capping material was sand extracted from the tailings.
Stratification of the tailings was determined by CPT. Undisturbed samples of fine tailings were obtained by a piston sampler for CIU and oedometer testing to obtain parameters required for advanced soil models SHANSEP and Soft Soil (SS) models. These models were incorporated in full 2-D FE models to analyse the stability and settlement of the new embankments at various locations.
The application of advanced soil models such as SHANSEP and Soft Soil by hand calculation and
conventional slope stability analysis is considered cumbersome and labour intensive. This paper
demonstrates that with the help of FE software (PLAXIS in this case), it is practical to implement such advanced soil models to simulate the behaviours of soft fine tailings with reasonable accuracy. A similar approach could be used to model other fine tailings and soft clays. One should be reminded that the reliability of any analysis method relies on validation of the analysis model and parameters adopted.
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.
Lake Buffalo located on the Buffalo River near Myrtleford in Victoria was constructed in the 1960s as a cofferdam for the then proposed Big Buffalo dam. Consequently, the dam was designed for a short life (<10 years) and design features and criteria for a permanent dam were not implemented.
Critical features include a primary spillway with three vertical lift gates, two outlet conduits located
through the spillway piers, a single upstream valve on each outlet conduit for regulation and isolation, and a multi-part bulkhead which is installed in front of the valves for inspection and maintenance.
With the continued operation of the dam beyond 60 years, upgrades appropriate to a permanent dam have been implemented, including addressing deficiencies with spillway gate hoists lifting equipment and redundancy of the outlet conduit vales. This proved challenging, as the operation of spillway structures does not readily align with industry or Australian Standards. This paper will outline the issues encountered, their resolution and the lessons learnt during this upgrade work.