2017 – Factor of Safety? – Do we use it correctly?
Jiri Herza, Michael Ashley, James Thorp
The principle of minimum acceptable factors of safety has been used to assess the stability of embankment dams for decades. The commonly applied minimum acceptable factors of safety remain very similar to those recommended in the early 1970’s, despite the development of new design tools and better understanding of material behaviour. The purpose of factors of safety is to ensure reliability of the dam design and to account for uncertainties and variability of dam and foundation material parameters, uncertainties of design loads and limitations of the analysis method used. The impact of uncertainties and reliability of input values into stability analyses was recognised many decades ago, and the factor of safety was recommended depending on the loading conditions and the consequences of failure or unacceptable performance. Interestingly, the minimum recommended factors of safety used today do not take into account the potential consequences of dam failure or the uncertainties in input values, and are based on the loading conditions only. Yet, several authors have demonstrated that a higher factor of safety does not necessarily result in a lower probability of failure, as the analysis also depends on the quality of investigations, testing, design and construction. This paper summarises the history of the factor of safety principle in dam engineering, discusses the calculation of the factor of safety using commonly used analytical tools, demonstrates the impact of uncertainties using a case study and provides recommendations for potential improvements.
$15.00
Related products
-
Papers 2017
2017 – Certainty in Uncertainty: Navigating Environmental Impact Assessment Regulatory Processes
Learn moreGeraldine Squires
There is increased pressure from stakeholders for projects to include evaluation of emerging broader development issues within the environmental assessment process. These emerging issues are not well documented or understood and at the forefront of untested preliminary government policy positions.
Agencies expect proponents to invest in evaluating these matters outside of typical assessment practices. Requests are made late in the evaluation and approval process.Assessmen involves matters not directly related to the project or within the proponent’s control and occurs late in the project development cycle.
The Lower Fitzroy River Infrastructure Project (LFRIP) was identified through the Central Queensland Regional Water Supply Study in 2006, as a solution to secure future water supplies for the Rockhampton, Capricorn Coast and Gladstone regions. The Gladstone Area Water Board and SunWater Limited, as proponents, propose to raise the existing Eden Bann Weir and construct a new weir at Rookwood on the Fitzroy River in Central Queensland.
The LFRIP environmental impact statement (EIS) was approved, subject to conditions, by the Queensland Coordinator-General in December 2016 and the Commonwealth Minister for the Environment and Energy in February 2017. Achieving conditions that will realise positive environmental outcomes while simultaneously achieving project objectives, particularly with regard to timeframes and costs, was not without its challenges.
The EIS was developed in accordance with the requirements of the State Development Public Works Organisation Act 1971 (Qld) and the Commonwealth’s Environment Protection and Biodiversity Conservation Act 1999, including an extensive stakeholder consultation programme. These regulatory requirements are well understood and applied to projects as normal accepted practice. They ensured that potential project impacts and benefits were identified, that appropriate levels of effort were applied to investigations to establish baseline conditions and that risks to and impacts on environmental (including social and cultural) matters were adequately mitigated and managed.
The environment is not static. Emerging issues and perceptions results in regulation and policy changes in response to political and social drivers. During the development of the EIS both new legislation and new policies were imposed on the project.New legislation resulted in additional assessment around matters previously considered mitigated and managed (fish passage). New legislation introduced new matters for assessment (connectivity). Collaboration and engagement with stakeholders were key to understanding the applicability of these elements to the project and for developing an approach to address the legislative requirements late in the project’s development and assessment process.
In Queensland,policy is emerging to mitigate and manage impacts of development on the Great Barrier Reef World Heritage Area’s universal values. The EIS was required to address the direct project impacts on water quality and the impacts arising because of the LFRIP (facilitated development). Water secured by the LFRIP is for urban, industrial and agricultural purposes. Urban and industrial developments are well regulated and subject to specific environmental approvals processes. Use of water for agricultural purposes, intensive irrigated agriculture in particular,is less regulated. Policies developed are reactive and require individual projects to address these impacts.In the absence of regulatory guidelines for assessment of consequential impacts, the project adopted a collaborative approach. The proponents established a working group, including State and Commonwealth technical agencies. This allowed for robust and scientifically defendable methodologies to be developed and agreed upfront. Streamlining the approach by including key decision makers assisted in managing expectations and focused the assessment on realistic and achievable outcomes relative to the project. The result was defendable outcomes allowing timely decision making and avoided rework as much as possible.
This paper describes developments in environmental assessment relating to new and augmented weirs.
Learn more -
$15.00
Papers 2017
2017 – Impact of Detailed Consequence Assessment on Leslie Harrison Dam Upgrade Works
Learn morePeyman Andaroodi, Barton Maher
Seqwater is a statutory authority of the Government of Queensland that provides bulk water storage, transport and treatment, water grid management and planning, catchment management and flood mitigation services to the South East Queensland region of Australia. Seqwater also provides irrigation services to about 1,200 rural customers in the region that are not connected to the grid and provides recreation facilities. Seqwater owns and operates 26 referable dams regulated under Queensland dam safety legislation.
Leslie Harrison Dam is an Extreme Hazard category dam located in the Redland Bay area of Brisbane.A significant portion of Population at Risk is located within a short distance downstream of the dam, reducing the available warning time in the event of a dam safety issue and impacting on the estimated loss of life used to assess risk. Following the Portfolio Risk Assessment undertaken by Seqwater in 2013, a series of detailed investigations were undertaken to confirm the assessed risk and the scope and urgency of the upgrade works.
Before a final decision on the scope and timing of the dam upgrade is made, Seqwater has completed a detailed review of the downstream consequences. This review was intended to update the Population at Risk(PAR) and Potential Loss of Life(PLL) estimates using the latest estimation methods for a range of scenarios. Three life loss estimation methods were used including empirical and dynamic simulation models and the results were compared.
This paper discusses the updated consequences assessment and the impact on the assessed risks, for Leslie Harrison Dam for both the current dam and the proposed upgrade scenarios using the revised Potential Loss of Life estimates.
Learn more -
$15.00
Papers 2017
2017 – Nagmati Dam – A Project of Environmental and Cultural Significance
Learn moreRichard Herweynen, Suraj Neupane, Paul Southcott and Ashish B. Khanal
Kathmandu, the capital city of Nepal, is home to more than five million people. Three major rivers including the Bagmati run through the city of Kathmandu, providing the environmental and cultural lifelines for the civilisation and local people. High population growth in Kathmandu over the past 30years has put a serious environmental strain on the Bagmati River. Water is drawn from the Bagmati River for drinking, farming, industries and construction. Due to the lack of capacity in the current sewerage systems, untreated sewage is entering the river system, along with high quantities of rubbish. Although a holy river, the Bagmati River is highly degraded, with reduced flows, high pollution, and a fresh water ecosystem that is now destroyed.To revive the Bagmati River, the Government of Nepal with funding from the Asian Development Bank (ADB), is undertaking the Bagmati River Basin Improvement Project (BRBIP). One of the sub-projects is the construction of a dam on the Nagmati River to store water during the monsoon period for environmental release during dry season.Since November 2015, Entura have been involved in the investigation and detailed design of the Nagmati Dam. Through a simple storage model, it was determined that 8.2Mm 3 of live storage was required to meet the environmental flow objectives. To achieve this storage a 95m high dam was required at the Nagmati site, with a concrete faced rockfill dam (CFRD) determined to be the best option.This paper will present the development of this unique project, highlighting how a number of the challenges were addressed, leading to a sustainable project.
Learn more -
$15.00
Papers 2017
2017 – Tailing Dams – How Do We Make Them Safer?
Learn moreDr Andy Hughes
Tailings dams continue to undergo failures at an unacceptable rate compared to water storage dams, including failures at operations owned by high profile mining companies.Tailings dams have often a different form and method of construction than water storage dams in that tailings dams continue to be raised over time as part of the mine operations and rise to considerable heights. These failures are often the result of a combination of design, construction and operations actions that are controlled by humans and must be better coordinated and managed in the future. The consequence of failure can be widespread flows of tailings and water over the landscape and water courses. This can have extreme consequences in terms of life loss, environmental damage, social license to operate, company value, and mining industry sustainability. Therefore,it is necessary that the mining industry strive for zero failures of tailings facilities. Any additional technology and information that enables an owner of a tailings dam to be more certain of its condition and thereby reduce the risk of failure is of tremendous value to reliable tailings and mine water management.The Willowstick method uses low voltage, low amperage, and alternating electrical current to directly energise the groundwater by way of electrodes placed in wells or in contact with seepage or leaks. This approach has been successfully used to identify water flow paths through, under and around tailings dam in plan and elevation.The Willowstick technology provides additional information to supplement the geological, geotechnical and hydrological, evaluations analyses and designs, and to further improve tailings dam safety by more robust designs if necessary. This paper, using several tailings dam case studies, illustrates the procedure, findings, and the benefits of the Willowstick methodology. The findings of many Willowstick surveys range from tailings dams where the methodology has confirmed the design evaluations, to tailings dams where new groundwater and leakage flow paths were identified. In the latter case, the dam designers were able to update the designs, based on the new information,to mitigate the identified risks and to improve the overall safety of the tailings dams in accordance with the goal of zero failure.
Learn more -
$15.00
Papers 2017
2017 – Conditions Under Which Identified Faults Contribute Significantly to Seismic Hazard in Australia
Learn morePaul Somerville, Andreas Skarlatoudis and Don Macfarlane
The 2017 draft ANCOLD Guidelines for Design of Dams and Appurtenant Structures for Earthquake specify that active faults (with movement in the last 11,000 to 35,000 years) and neotectonic faults (with movement in the current crustal stress regime, in the past 5 to 10 million years) which could significantly contribute to the ground motion for the dam should be identified, and be accounted for in the seismic hazard assessment. The purpose of this paper is to provide guidance on the conditions under which these contributions could be significant in a probabilistic seismic hazard analysis (PSHA)and a deterministic seismic hazard analysis (DSHA).We consider five primary conditions under which identified faults can contribute significantly to the hazard: proximity, probability of activity, rate of activity, magnitude distribution, and return period under consideration
Learn more