Monique de Moel, Mark Arnold, Gamini Adikari
Monbulk Saddle Dam, built in 1929, is one of two saddle dams located at the southern end of Silvan Reservoir, near the township of Monbulk, Victoria. The saddle dam is a 5.3m high earthfill embankment with a 230mm wide, centrally located, concrete core wall. The reservoir retained is located in the valley of Stonyford Creek, and impounds approximately 40,500 ML of water at FSL.
Excessive seepage at the right abutment of Monbulk Saddle Dam has been an issue since the early 1970’s. The reservoir has been operating with a level restriction since then to reduce the seepage flows. However; this restriction limits the operational flexibility of the storage. Early investigations concluded that the most likely mechanism for these excessive seepage flows was a defect in the concrete core wall.
Melbourne Water Corporation, (the owner and the operator of the reservoir), undertook a risk assessment for Silvan Reservoir as part of a review of its dams asset portfolio. Based on the information then available, the risk assessment was undertaken using the criteria and guidelines developed by ANCOLD. The result was that the piping risks associated with the seepage from the west abutment at Monbulk Saddle Dam was unacceptable. The risk assessment Panel also cast doubt on the likelihood of the seepage being caused by a defect in the concrete core wall. Melbourne Water therefore engaged SMEC Australia to investigate the likely causes and mechanisms for this seepage and to develop suitable remedial measures for the dam.
The investigations have included a desktop review of historical information, test pit investigations, Sonic borehole drilling, dynamic cone penetration tests, an infrared thermal imaging investigation and an electromagnetic groundwater seepage flow mapping investigation.
These investigations have shown that the most likely cause of the seepage is the presence of permeable foundation layers located beneath and around the existing core wall as the core wall does not extend over the full length of the embankment and becomes shallower towards the abutments.
To satisfy the ALARP principle; risk reduction remedial works Concept Designs are being developed and reviewed.
2011 – Investigating the Piping Risk Associated with Seepage at Monbulk Saddle Dam of Silvan Reservoir, Victoria
B. Ghahreman Nejad, H. Taiebat, M. Dillon and K. Seddon
One of the causes of tailings dam failure has been seismically induced liquefaction during earthquakes. Liquefaction, if mobilised, significantly reduces the stiffness and strength of affected soils in the embankment dam or its foundation and may lead to large deformations and dam failure. This paper reports the results of seismic liquefaction assessment and deformation analyses of Bobadil tailings dam located in Tasmania. The tailings dam consists of a perimeter rockfill starter dam which has been raised in stages using the “upstream” construction method. The embankment raises (formed by clay or coarse tailings) are constructed over a foundation of previously deposited tailings in the impoundment which is potentially susceptible to liquefaction. Extensive field and laboratory tests were carried out to assess the tailings liquefaction potential and also to determine the material properties required for seismic stability and deformation analyses. Numerical modelling of seismic liquefaction and deformation analyses were carried out to predict the magnitude and pattern of deformations that may lead to uncontrolled release of tailings. The results of these analyses are presented and compared with literature report of those observed during past earthquakes.
2011 – Numerical Modelling of Seismic Liquefaction for Bobadil Tailings Dam
Dan Forster, Murray Gillon
A robust and defensible dam surveillance process is considered to be the ‘front-line of defence’ in ensuring dams do not present an unacceptable risk to people, property and the environment. The concept of a ‘Quality Chain of Dam Surveillance’ describes the surveillance process as a multi-linked chain where each step in the process forms a critical link. Without rigorous attention given to quality assurance links in the chain can become tenuous or broken and thus compromise the integrity of the whole chain. Hydro Tasmania is currently re-engineering its existing surveillance process using the Quality Chain of Dam Surveillance as a basis.
This paper presents the concept of the quality chain and uses the Hydro Tasmania improvement initiative as an example application of the concept. The paper is intended to provide a fresh perspective on what is sometimes considered a stale topic and reinforces the need for a considered approach to dam surveillance.
2011 – The Quality Chain of Dam Surveillance
Nanda Nandakumar, Janice Green, Rory Nathan, Kristen Sih& Robert Wilson
A detailed assessment of hydrologic risk was undertaken for Hume Dam. Data available and relevant to the hydrologic risk assessment were collated and assessed. The catchment was divided into 35 different sub-catchments, each with its own set of parameters that characterised the local hydrologic response. Recorded streamflow was used to calibrate the flood response of selected gauged sub-catchments, and a combination of historic and synthetically-derived data was used to validate the model and loss parameters. The 35 models were combined into a single catchment-wide model. A Monte Carlo approach was adopted for the validation of the models and the derivation of Hume Dam inflow and outflow frequency curves. A range of PMFs which satisfy ANCOLD’s definition of the PMF were also estimated. The PMPDF outflow was estimated to be 7,600 m3/s which can be passed by the dam. Depending upon the assumptions made, the peak PMF outflow was estimated to be in the range from 10,300 m3/s to 14,900 m3/s
2011 – Assessment of Hydrologic Risk for Hume Dam
T. Mortimer, J. McNicol, P. Keefer, W. Ludlow
CS Energy’s Kogan Creek Coal Mine located in the Surat Basin in Queensland, services the 750MW coal fired, Kogan Creek Power Station. Strip mining generates large volumes of mine waste which is typically used to construct waste dumps. Recent work at the mine has focused on using mine waste to construct an ash storage facility to store ash that is piped over 5 km from the power station as a dense phase slurry. The use of mine waste to construct the ash storage facility provides significant cost and time savings, however a range of design, construction and operation issues needed to be addressed to operate a facility of this type.
This paper describes some of the key design, construction and operation considerations for the ash storage facility. Design considerations include pipeline transport through environmentally sensitive areas, addressing the stability of the embankment and the use of a partial LLDPE geomembrane lining system to reduce the risk of seepage from the storage. Construction considerations include post construction (pre ash deposition) floor treatment to reduce potential settlement. Operational considerations include ash slurry deposition, water management of the decant pond and progressive rehabilitation of the final landform.
2011 – Design, Construction and Operation of a Partially Lined, Ash Storage Facility Constructed from Mine Waste
Ben Hanslow and David Brett
The Blackman Dam is a 27 m high, zoned earthfill dam located upstream of the township of Tunbridge in the Tasmanian Midlands. The dam has an estimated storage capacity of 7300 ML and an assigned Hazard Rating of High C.
The Blackman Dam was constructed over the period November 2003 to September 2004. The dam supplies water for irrigation to farms in the area and potentially to the local towns of Tunbridge and Oatlands.
Filling of the Blackman Dam commenced in 2005. After substantial filling of the dam and following a heavy rain event, an area of seepage was noted on the far left abutment of the main embankment mid morning of Thursday 13th October 2005. The seepage was reported by the dam operators as being “garden hose flow”. By mid afternoon of that day, this had increased to “100 mm pipe flow” and discoloured. The Dam Safety Emergency Plan was activated.
This paper discusses lessons learnt and provides details on the implementation of the Dam Safety Emergency Plan and emergency actions taken to successfully avoid a breach of the dam wall. The paper also provides details on the geotechnical investigations that were carried out and factors contributing to the piping failure. Embankment repairs were successfully completed by mid 2010 and first filling of the Blackman Dam occurred in 2011.