Richard M Robinson, Siraj Perera, Gaye Francis
Due diligence has become endemic in Australian legislation and in case law, to the point that it has become, in the philosopher Immanuel Kant’s terms, a categorical imperative. That is, our lawmakers seem to have decided that due diligence is universal in its application and creates a moral justification for action. This also means the converse, that failure to act demands sanction against the failed decision maker.
This applies to dam safety management which represents the archetypical high consequence – low likelihood event. It is now essential to have positively demonstrated safety due diligence in a way that can withstand post-event judicial scrutiny. Presently the only way this can be done is by using the notion of criticality and precaution, not hazard and risk. The test is not that of risk acceptability (as low as reasonably practicable or ALARP), rather it is that no further reasonably practicable precautions (so far as is reasonably practicable or SFAIRP) are available, and that what results is not prohibitively dangerous.
This paper will document the difference between the two approaches and how to positively demonstrate safety due diligence. It also discusses the definition of ALARP as stated in ANCOLD’s Guidelines on Risk Assessment 2003 and the relevance of the safety case principle for dam safety management.
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.
Gideon Steyl, Ralph Holding, Lis Boczek
A Monte Carlo method for assessing liner systems is applied with outcomes demonstrating the range of discharge that could occur over the liner interface. The Monte Carlo approach allows for variation of fill material over the liner system and includes the assessment of a second compacted zone either above or below the liner zone. In this paper clay liners were evaluated due to regulatory guidelines and it could be demonstrated that similar performance to a 1 m clay liner could be attained using compacted material to reduce discharge over the liner interface. The approach applied in this paper allows for at least a worst-case quantification of seepage risk which could be included in liner selection criteria or presenting liner options to regulators.
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.
James Thorp, Ryan Singh, Jiri Herza
Responsible management and operation of tailings and water storage facilities comprises a series of activities and projects that must be delivered within the commercial realities of the organisation and operation context of the facility owner. All projects are constrained by several variables, which are commonly represented by the Project Management Triangle of Scope, Time, and Cost. These variables are often finite and mutually exclusive, and delivery of the required outcome is accomplished by successfully managing each variable. The activities (variables) associated with the long-term dam safety are sometimes omitted to meet the immediate project requirements. In addition, the commercial realities, such as a selected project delivery model, can have a significant impact on dam safety risks through the allocation of risk, ability of the key decision makers, and the undue commercial pressures applied by each project delivery model. This paper presents several case studies where the project and commercial realities have led to decision making that impacted dam safety and increased the risk presented by the storage facility. While the immediate impact of these decisions may appear to be minimal, all stages of a tailings or water storage facility’s life span are impacted. This paper presents learnt lessons with the aim to prompt both owners and consultants to reconsider their commercial processes and project delivery strategies and limit unforeseen risks to the safety of tailings or water dams.
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.