2016 – Root Cause Analysis (RCA) in Dam Design
Malcolm Barker
This paper will present the use of Root Cause Analysis (RCA) as a means of evaluating the causes for failure modes and is based on work completed for an upstream tailings storage facility (TSF) raise where significant transverse and longitudinal cracking was observed.
The design of the TSF was based on the use of a starter wall with perimeter discharge from spigots spaced at about 25 m centres along the upstream crest. The TSF was raised using an upstream design and during routine inspections two years after completion of the raise, transverse cracks of up to 30 mm were noted on the crest and longitudinal cracks up to 40 mm width were noted on the downstream slope of the raised embankments. Concerns were raised over the extent and depth of the transverse cracking and the risks they pose to piping, seepage and containment.
Field investigations including test pitting and material testing were completed to evaluate the depth and extent of the cracking. The findings from field investigations, together with a review of the historical aerial photographs and superposition of the cracks and the locations of the spigots were then used in a Root Cause Analysis workshop.
Discussions on all causes for the cracking, asking the question “why did the problem occur?”, and then continuing to ask “why that happened?” until the fundamental process element that failed was reached”.
During the workshop, the most significant contributors for the transverse and longitudinal cracking and the likely location, extent and size of the cracks were evaluated. This identified the potential for traditional structural hog and sag bending moments causing the transverse crest cracking with the potential for transverse cracking at the interface of the raise and the original tailings. This was not previously identified as a potential piping location. The longitudinal cracking was considered to be mainly owing to settlement of the upstream tailings.
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2016 Papers
2016 – Probabilistic Slope Failure Risk Estimation of a Dam with Zones of Weak Foundation Material
Learn moreRyan Singh, Bob Wark
For existing dams built before modern theories and understanding of soil mechanics were fully developed, it was often the case that comprehensive investigations into the properties of the embankment and foundation material were not carried out. With more stringent dam safety requirements and engineering criteria, and a better understanding of soil mechanics, it is necessary to undertake embankment and foundation investigations on such dams, with the view to gain a better understanding of the embankment and foundation conditions.
This paper details the method used for a risk-based assessment of a dam’s stability against slope failure for steady-state seepage conditions, based on a probabilistic assessment of differing interpretations of the material properties for the foundation. To achieve this, several separate interpretations of material strength models were developed for a foundation, using various subsets of available tri-axial data. The mean strengths of these models were used to assess the stability, and to account for the variation in strength properties of each model, the sampling distribution of the mean was used to assess the likelihood of failure.
Finally, an event-tree type risk analysis was used to calculate a value for the probability of slope failure.
A case study has been presented using this method.
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2016 Papers
2016 – Kanmantoo Tailings Storage Facility (TSF) – Optimising the Design, Construction and Operation of a Fully HDPE Lined Tailings Storage Facility
Learn moreDavid Piccolo, Gareth Swarbrick, Garry Mostyn, Bruce Hutchison, Rodd Brinkmann
Hillgrove Resources owns and operates Kanmantoo copper mine some 44 km southeast of Adelaide.
An important feature of the mine is its tailings storage facility (TSF) which is fully lined with HDPE, and double lined at the base, fully under drained, has a secondary underdrainage system for leak detection and a multi-staged centralised decant system. This onerous design of the TSF was developed in consultation with DMITRE between 2007 and 2010 amid concerns of groundwater protection and effective water management.
The Authors were approached in 2010, following construction of the initial stage of the TSF, and charged with developing the design to increase storage from 13 to 20 million tonnes, as well as optimising the design and construction of future stages.
This paper presents the more interesting aspects of the design and construction optimisation between 2010 and 2016 including:
- Optimisation of the:
o extent of the double liner and underdrainage system,
o protection layer below the liner, and
o quality control process adopted during construction. - Design, construction and operational responses to the challenges arising from the raising the TSF including the effect of increased tailings height on the decant structure, buried pipes, and the double liner.
The design and construction approaches have been scrutinised and accepted by regulatory authorities, and implemented by the mine operator over a period of 6 years. The paper includes lessons learnt during the implementation process.
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2016 Papers
2016 – Tullaroop Dam: Dealing with Filter Breakdown and Compaction During Construction
Learn moreJamie Cowan, Chris Kelly and Gavan Hunter
Dam safety upgrade works were undertaken at Tullaroop Dam in 2015/16 to reduce the risk of piping through the main embankment. Unexpected cracking and elevated pore water pressures were observed within this earthfill embankment over a period of 10 to 15 years. In 2005/06 a filter and rockfill buttress local to the embankment was constructed on the left abutment after a 60 mm wide diagonal crack opened up on the downstream shoulder from crest to toe.
Similar to the 2005/06 upgrade works, the 2015/16 embankment works were direct managed by Goulburn-Murray Water. Filter and rockfill materials were sourced from commercial quarries previously used for dam upgrade projects and for which significant testing of materials had been undertaken, especially on the fine filter.
Mid project it became clear that the fine filter was breaking down under handling and compaction such that several in-bank gradings fell outside the specified fine limit. Further testing of quarry surge piles, site stockpiles and in-bank placed filters was undertaken to understand the extent of the breakdown. It was assessed that the breakdown was occurring on the 0.5 to 2.0 mm fraction, generating finer sizes in the 0.1 to 0.6 mm fraction. The increase in fines content (minus 75 micron) was less than 1% and met specification. The in-bank material was accepted as placed and the specified filter envelope adjusted to allow for the observed breakdown.Difficulties were experienced with compaction of the fine filter in the inclined chimney filter to achieve the target density in the range 65% to 80% Density Index when the layer width reduced to 0.75 m for a 0.5 m compacted lift thickness. No difficulties were experienced when the layer width was 1.5 m or in trenches. Further trials were undertaken on the embankment to better understand the compaction issues and used different roller types. It was assessed that an important factor was the arching effect of the adjacent coarse filter. Going forward thinner lifts were used and smaller width rollers to achieve the specified minimum density.
The paper provides details on the embankment construction works, focusing on the fine filter breakdown and compaction issues. Details of the testing undertaken, the actions to resolve the issues and interactions with the supply quarry and construction team are provided.
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2016 Papers
2016 – Spillway gates, CFD analysis, Uplift forces, Hydrodynamic forces, Flow Modification, Gate Locking Mechanism.
Learn moreChris Kuenne
Paradise Dam is located on the Burnett River 20 km northwest of the town of Biggenden in Queensland. It is a gravity dam with a height of 37 metres and a total capacity of 300,000 ML. It was primarily constructed to service local agriculture.
The dam features a complex outlet works contained within a tightly constrained footprint. It provides for irrigation releases, fish passage and power generation. Additionally, the outlet is required to pass very high environmental flows of up to 270 m3/s.
The dam was subjected to major flooding in 2013 resulting in significant damage to the mechanical equipment associated with the outlet works, and severe scour downstream of the spillway.Since construction, the operating range for the environmental outlet has been restricted. A rough operating zone has been identified through which the gates are quickly moved through. It is believed to be caused by the dynamics of the gates and the upstream conduit arrangement. Failure of the downstream stainless steel liner associated with the conduit has also occurred. The environmental outlet lacks the ability to be isolated from the storage, complicating the maintenance / modification of the gates. At the time of design, it was agreed by the alliance partners that major maintenance of the gate would be planned for when the reservoir was low, being below the intake bellmouth.
The irrigation release valves suffer from high vibration levels during operation. Component failure and severe corrosion have also been experienced.
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This paper details:
Operational and maintenance experiences and restrictions since commencing operation including the impact of flooding;
Investigation and testing of environmental gate dynamics and the impact of these on the intake tower;
Failure of the environmental conduit liner, investigation and proposed rectification;
Proposed method to enable servicing of environmental gates without the use of a bulkhead and without draining the storage;
Proposed enhancements to irrigation valves to reduce vibration and extend service life. -
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2016 Papers
2016 – Characteristics of Near Fault Ground Motions for Seismic Design
Learn morePaul Somerville
This paper describes the unique characteristics of near-fault ground motions for use in developing ground motions for the design and evaluation of dams that are located close to identified active faults. These characteristics include near-fault rupture directivity effects, permanent ground displacements, and hanging wall effects. In Australia, active faults make a significant contribution to the Maximum Credible Earthquake (MCE) only at near-fault sites when Probabilistic Seismic Hazard Analysis (PSHA) is used. However, some sites may be close enough to nearby or even more distant identified active faults that a Deterministic Seismic Hazard Analysis (DSHA) produces MCE ground motions that are for larger than those obtained using a probabilistic approach even for very long return periods. Knowledge of the unique characteristics of near-fault ground motions should be applied to the development of ground motions for the design and evaluation of dams that are located close to identified active faults.
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