The development of geological, engineering geological and geotechnical models is essential for all dams. These models provide the basis for understanding the engineering characteristics of foundation materials and geological structures that are critical to the safe design, construction and operation of the dam.
The use of digital three dimensional (3D) engineering geological modelling techniques is becoming more common for civil infrastructure projects. In addition to established design applications, 3D engineering geological models can be utilised by dam owners, operators and stakeholders for ongoing management of the dam.
The recent option studies at North Pine Dam in Brisbane, Australia, provides an example of collaboration between the owner (Seqwater) and the designer (GHD) to maximise the use of existing information and to enable future information to be efficiently integrated and utilised.
The initial North Pine Dam 3D engineering geological model was developed using historical records dating from the design and construction of the dam in the 1950’s and 1960’s. These records had been carefully stored, collated and digitised by the owner, so that they could be easily georeferenced and incorporated into the 3D engineering geological model.
The initial model was interrogated to identify data gaps and to plan targeted and cost-effective investigations that addressed critical geotechnical issues. The 3D engineering geological model was further refined using the newly acquired data, to develop a comprehensive “3D database” that can be used to visualise and interrogate all existing records as high- resolution georeferenced images and embedded data.
This provides an asset for the dam owner to maximise the use of existing information and reduce the cost of future safety reviews or design.
Fault displacement can occur due to primary faulting on a main fault intersecting a dam foundation or rim, as well as by secondary faulting. This secondary faulting may be triggered locally by the occurrence of primary faulting on a main fault; its occurrence is conditional on the occurrence of an earthquake on the main fault. A probabilistic approach is most viable for fault displacement hazard analysis. Unlike the case of probabilistic ground motion hazard, which is nonzero even for short return periods due to the occurrence of a broad range of earthquake magnitudes in a wide region around the site, probabilistic fault displacement hazard is zero for return periods less than the recurrence interval of surface faulting earthquakes on the fault. In Australia, these recurrence intervals typically lie in the range of 10,000 to 100,000 years.
Consequently, the fault displacement hazard due to primary faulting may be zero or negligible for return periods shorter than 10,000 or 100,000 years. For longer return periods, the hazard is best evaluated using a risk-based approach, as recommended by ANCOLD (2018); the alternative of using a deterministic approach, which disregards return period, could potentially yield a large fault displacement. The probability of triggered secondary faulting, conditional on the occurrence of a large earthquake on the main fault, is typically one or two orders of magnitude lower than that on the main fault, and so is even more likely to be zero or negligible for return periods shorter than 10,000 to 100,000 years
Following the failure of Paloona Dam’s intake trashrack during the 2016 floods in northern Tasmania, a replacement trashrack and support structure was designed, manufactured and installed (by diver) within five months. This was a remarkable feat and hailed as a success at the time.
The euphoria, however, was short lived. A routine dive inspection in January 2018 revealed cracked
trashrack bars on one of the panels and this was after less than twelve months’ operation. This prompted a rigorous investigation where it was determined that the bars suffered fatigue due to flow induced vibration. Indeed it is possible that the bars cracked within a few weeks of returning to service.
The science of flow induced vibration is relatively mature, having been extensively researched over several decades. Its application to trashracks is well documented. However, this experience has shown that the common design approach overly simplifies the fluid-structure interaction. For Paloona, the result was a trashrack design which has proven to be inadequate, not having the resilience required for a dam outlet works component.
This paper revisits flow induced vibration theory as it pertains to trashracks, outlines the findings of vibration testing at Paloona, and suggests a design approach which will avoid similar issues. It is hoped that similar failures can be prevented and the design life expected of trashracks achieved.
Computational Fluid Dynamics (CFD) is the science of predicting momentum, mass and heat transport, and can aid in design and safety issues for dam resilience in modern settings. Applications of CFD have historically been in the aerospace, automotive and chemical process industries with limited application in the hydraulic engineering field; possibly due to the associated computational intensity that is typically required. However, over the past two decades the cost of computing power has decreased substantially while the processing speed has increased exponentially. These developments have now made the application of CFD in the commercial environment feasible. CFD is particularly valuable in complex flow situations where the outputs required cannot be provided by a traditional hydraulic assessment approach and where there are stakeholder drivers such as service life, insurance cover and safety implications of infrastructure. The need for CFD when these drivers and complex flow situations arise, are demonstrated by means of a case study.
In the case study, CFD was used to investigate the flow patterns and the predicted performance of the outlet pipework from Massingir Dam in Mozambique. Three flow scenarios with appropriate pressure and flow boundary conditions were analysed for the outlet pipework, which included bifurcations for power generation from the main discharge conduits. Specific concerns addressed were, firstly, the possible excessive negative pressure in the region of the offtake for power generation and the potential for cavitation effects and, secondly, unacceptable velocity gradients in the power offtake pipework. Results showed that although some negative pressures were possible in one flow scenario, mitigation measures based on the CFD outputs could be considered and designed before construction.
The implementation of CFD in the above case study displays how risk in design can be reduced to ensure safety issues are addressed effectively.
Dam owners manage many complex activities to maintain and operate their dams safely and resiliently. Identifying, and continually improving, the key elements of an effective dam safety program and associated practices can be challenging but are essential to support resilient dams and resilient communities; using the Dam Safety Maturity Matrices (DSMM) is an efficient and thorough way to do this. A maturity matrix is a tool to evaluate how well-developed and effective a process or program is. The matrices were developed within CEATI’s Dam Safety Interest Group (DSIG) for owners to assess the effectiveness of their dam safety program against industry practice, and to assist with identifying improvement initiatives.
This paper will present the matrices and demonstrate how they are used to evaluate the effectiveness (or maturity) of a dam safety program. It will also highlight the benefits associated with using the matrices as an assessment tool, including the identification of improvements that can be made to a dam safety program, and the prioritization of efforts across multiple facets of a dam safety program.
User case studies from dam owners in both New Zealand and overseas will be presented to elaborate on the tool and the process.
Many numerical simulations have tried to model the failure-induced displacements of earth structures due to liquefaction. In this paper, the challenges in modelling such as the large displacement and non-immediate failure of earth structures due to liquefaction are discussed. An advanced bounding surface plasticity model is used to simulate the dynamic behaviour of saturated porous media. A series of benchmark welldocumented seismic events are analysed, and the results are compared to the reported laboratory and field observations. These analyses consist of one centrifuge test on liquefiable sand (Model #12 of the VELACS project) and one earthfill dam (Lower San Fernando Dam in California) subjected to seismic loading that leads to liquefaction. The capability of the model to capture the flow failure due to liquefaction is demonstrated and results are compared with other attempts in the literature to capture similar responses.