Jamie 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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Kate Brand, Matthew Ind
Failure impact assessments of tailings dams are largely pre-determined by the input assumptions and, due to lack of supporting data, the results can be highly subjective. Despite numerous guidelines available for undertaking failure impact assessments of water dams, there are very few technical guidelines on how to form the above assumptions and how to undertake dam breach modelling of a tailings storage facility (TSF).
Tailings dam failure databases are limited, with the available information generally not analogous with the TSF under assessment, especially given the rising volume and height of modern tailings dams. ‘Rule of thumb’ methods are often referred to, with a percentage of tailings and water assumed to be discharged along with assumptions of the breach height and width made.
Using a case study, this paper compares a range of potential failure impact assessments generated using typical methods of analysis and runout modelling to demonstrate the reliance on engineering judgement in failure impact assessments. Given the subjectivity observed within the results, consideration should to be given to the level of reliance on tailings dam failure impact assessments in formulating emergency action plans. It is recommended that regulators take an active role in formulating tailings dam impact assessment guidelines.
Mark Arnold, Gavan Hunter and Mark Foster
Following the dam safety risk assessment for Greenvale Dam in 2008, Melbourne Water implemented a 3.0 m reservoir level restriction on the operation of the storage as an interim risk reduction measure. The 3.0 m restriction coincided with the ‘as constructed’ top of the chimney filter in the main embankment. This interim action reduced the dam safety risk to below the ANCOLD limit of tolerability.
Dam safety upgrade works were undertaken in 2014/15 to bring the dam in-line with current risk based guidelines and to enable the removal of the interim reservoir restriction, bringing the storage back to full operating capacity. Greenvale Dam was required to remain operational throughout the works and this required careful consideration of the dam safety risk during construction.
Deep excavations were required within the crest and downstream shoulder of the embankments, that,, without adequate management, had the potential to increase risk to the downstream population. Excavations up to 18 m depth were required into the wing embankments for construction of full height filters from foundation to crest, excavations up to 7 m deep were required in the main embankment to expose and connect into the existing filters and secant filter piles up to 13 m deep were used to connect the new chimney filter of the wing embankments with the original chimney filter of the main embankment.
A key element of the design and construction of the upgrade works was managing dam safety during construction. Dam safety considerations included (i) design based decisions to manage the level of exposure; (ii) implementation of further restrictions on reservoir level by the owner Melbourne Water; (iii) construction methods to manage exposure; (iv) an elevated surveillance regime during the works and (v) emergency preparation measures including emergency stockpiles and 24 hour emergency standby crew. The construction based dam safety requirements were focused on early detection and early intervention, and were managed via the project specific Dam Safety Management Plan.
This paper focuses on dam safety management including the decisions made, actions taken and construction requirements and touches on how these relate to the key project features.
James Penman, Terence Jibiki, Len Murray, and Mark Rynhoud
Two earthfill embankments are being constructed to form an impoundment in a mountainous region with a tropical climate. The embankment abutments are underlain by tropical weathered rock/soil including a significant thickness of residual soil. Previous slope failures within the area, including a 150 m wide failure of a construction access road, have been potentially due to weakness of the residual soil. In order to quantify the potential risk to the embankments, a geotechnical characterisation program consisting of in situ and laboratory testing was completed to determine the shear strengths and loading response within the residual soil material. This paper summarises the geotechnical investigation program and characterisation of this tropical residual soil in the context of the embankment stability.
Results of laboratory direct simple shear testing are presented and compared to common empirical methods for estimating the undrained shear strength of both over-consolidated and normally-consolidated materials using index properties and/or over-consolidation ratios. Methods used for comparison include those proposed by Skempton (1952 & 1957), Bjerrum-Simons (1960), Lambe & Whitman (1969) and Wroth & Houlsby (1985) for normally consolidated material and Ladd (1977) and Jamiolkowski et al. (1985) for over-consolidated material.
The results of in situ testing, including pocket penetrometer data and field shear vane data, are also presented.
David Scriven, Lawrence Fahey
Paradise Dam is located approximately 20 km north-west of Biggenden and 80 km south-west of Bundaberg on the Burnett River in Queensland. The dam was designed and constructed under an alliance agreement with construction completed in mid 2005. It is a concrete gravity structure up to 52 m high, the primary construction material being roller compacted concrete (RCC).
In January 2013 the flood of record was experienced at the dam with a depth of overflow on the primary spillway reaching 8.65 m following heavy rainfall in the catchment from ex-tropical cyclone Oswald. The peak outflow was approximately 17,000 m3/s. This equated to a 1 in 170 AEP flood event. When the flood receded it was discovered that the dam and surrounds had suffered severe damage in a number of locations including: extensive rock scour downstream of the primary dissipator and the left abutment, damage to portions of the primary dissipator apron, and the loss of most of the primary dissipator end sill.
SunWater initiated a staged remediation program to manage the dam safety risks and by November 2013 had completed the initial Phase 1 Emergency and Phase 2 Interim repairs. Phase 3 of the program was to implement a comprehensive Dam Safety Review (DSR) and a Comprehensive Risk Assessment (CRA). The DSR became arguably the largest ever undertaken by SunWater and included: extensive geotechnical investigations, large scale physical modelling, numerical scour analysis, stability analysis, and an extensive design assessment. This paper describes some of the key aspects of the DSR undertaken related to the flood damage.
BJ Rochecouste Collet, PC Blersch, AL Olivier
The paper shows how multidisciplinary engineering can create future-proofed solutions for dam management and how innovation is bringing those advancements to life. It includes the retrofitting of a small hydropower station to an existing dam structure, enhancing the use of the dam. The paper also reveals design, technical and project finance considerations underpinning the multi-disciplinary services.
Water supply in South Africa’s economic hub of Gauteng journeys from the Lesotho Highlands Water Project (LHWP) through three river systems that converge in to the Vaal Dam from which the water is treated and pumped for domestic usage. One of those systems delivers water originating in Lesotho to discharge into the Ash River. The Ash River was, by origin, a small river with an environmental reserve flow of 50 l/s. However, with the LHWP significantly adding flow, the annual average increased to 24,500 l/s (24.5 m3/s) and is set to increase further with future phases of the scheme. To mitigate significant erosion caused by the greatly increased flow, several structures were erected along the river, including the Botterkloof Dam. Whilst the energy was dissipated in the dam’s spillway, a private developer studied if the water could be used for energy generation. The river also offered some rapids some 1.6 km downstream, which also showed potential for hydropower generation. An option to combine the two sites was also considered.
Aurecon conducted the feasibility study in 2010 for both sites, including the combined option, which concluded that there would be significant benefits in the implementation the projects in two separate schemes. This boasted many advantages including reduced capital investment, reduced social impact (canoeist), reduced geotechnical risks, and lesser land acquisition leading to a better return on investment. Aurecon are currently providing engineering, procurement and construction management (EPCM) services for the entire project. Construction of the 4.4 MW hydropower station commenced construction in September 2014 and was commissioned ahead of time and under budget.
The founding conditions under the proposed hydropower station location, comprising interbedded sandstone and mudstone was fairly poor with the mudstone effectively decomposing in less than two days. The Botterkloof dam was build on a thick layer of sandstone which dips quite steeply towards the right bank. The right bank on the other hand comprises an old paleo channel. The Boston A dam, located on the left bank immediately adjacent to the Botterkloof Dam, is founded on the weather mudstone and the spillway is grass lined. The power station construction was constructed in the narrow space in between the two existing dams – the Botterkloof Dam (owned by the Department of Water and Sanitation (DWS), Government of South Africa) and the adjacent privately-owned Boston A Dam. Permission had to be obtained from the respective owners and all regulatory permits approved before the project could be submitted to the South African Renewable Energy Independent Power Producer Programme (REIPPP) implemented by the Department of Energy (DoE) of the South African Government.
Another significant challenge in the construction itself included the need for deep excavations through the left embankment of the Botterkloof Dam and adjacent to the spillway stilling basin whilst such construction needed to be done without affecting operations and stability of either of the two dams.
The solution was a shallow intake, followed by a cut and cover concrete penstock leading to a compact hydropower station housing a single 4.4 MW vertical “Compact Axial Turbine” Kaplan turbine ending in the tailrace, which was rotated at 90 degrees.