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
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The 2011 Tohoku Earthquake of magnitude 9.0 shook the east Japan and caused enormous damage. As of September 22, The Japanese National Police Agency has confirmed 15,805 deaths, and 4,040 people missing, as well as over 295,047 buildings completely or partially destroyed. About 8,700,000 homes lost power, and about 2,290,000 homes were shut down from water supply. The transportation lifelines such as highways and railways including Shinkansen (high speed train) were disrupted. The earthquake triggered extremely destructive tsunami waves of the height of 15 metres, in the east coast of the Pacific Ocean. Fukushima No.1 nuclear power plant had accidents.
2011 – Perspectives of the 2011 Tohoku Earthquake and Tsunami
The enlargement of the Cotter Dam is being undertaken by ACTEW to provide a greater security of water supply to Canberra. The project involves constructing a larger, higher new dam wall immediately downstream of the existing Cotter Dam, to allow the present dam to continue functioning and supplying water while construction is underway. The project raised a number of environmental issues partly because the Cotter Dam currently supports a self-sustaining population of (endangered) Macquarie Perch, and because the Bendora Dam, upstream of Cotter Dam, contains a breeding population of (endangered) Trout Cod. Bendora Dam will not be physically affected by the works on Cotter Dam, but its operations may be altered. An ecological risk analysis was conducted to identify critical environmental risks that would need to be investigated and managed or ameliorated and management strategies were put in place to reduce risks. ACTEW have adopted an adaptive management approach to the project, but in order to implement that approach it is necessary to conduct effective monitoring of the fish populations of concern. These potentially include the two endangered species, as well as potential predators (such as cormorants) and competitors (such as trout). Power analysis has been used as a tool to evaluate whether it is feasible to monitor key populations sufficiently rigorously to be able to confidently detect a change (either an increase or decrease in a population). For Macquarie Perch and trout it should be possible to detect population changes statistically with a logistically feasible monitoring program.
2011 – Using risk analysis, power analysis and adaptive management to minimise ecological impacts of the Cotter Dam enlargement
Robert Keogh, Rob Ayre, Peter Richardson, Barry Jeppesen, Olga Kakourakis
SunWater owns 23 referable dams and operates a further two dams for other owners. The dams are located across Queensland from Texas and St George in the South to the Atherton Tablelands in the north to Mt Isa in the west.
During the period December 2010 to February 2011 there were several significant rainfall events across Queensland. The first occurred in late December 2010, the second in mid January 2011 and third in early February 2011. Generally it was the most significant rainfall event in Queensland since the 1970’s. 22 Emergency Action Plans were activated simultaneously by SunWater. Eleven dams experienced a flood of record during the events.
This paper will discuss what has been learnt from these events including the optimisation of management structures for a dam owner with a large portfolio of dams: review of O&M Manuals including the adequacy of backup systems: relationships with the State disaster management framework: the value of rigorous communication protocols: managing fear and a general lack of understanding in the community: and the value of being prepared.
Workshop paper – Robert
Mojtaba E. Kan and Hossein A. Taiebat
Abstract: The simplified procedures for evaluation of earthquake induced displacement in earth and rockfill dams are widely used in practice. These methods are simple, inexpensive, and substantially less time consuming as compared to complicated numerical approaches. They are especially recommended to be used as a screening tool, to identify embankments with marginal factor of safety, assuming that these methods always give conservative estimates of deformation. However recent studies show that application of these methods may not be conservative in some cases, especially when the tuning ratio of a dam is within a certain range. In this paper the fundamental theory behind the simplified methods is critically reviewed and practical guidelines are presented that can be used to identify cases where the simplified procedures may not be conservative.
2011 – Reliability of simplified methods for evaluation of earthquake-induced displacement in earth and rockfill dams
Mark Locke and Scott Kindred
The Bulk Water Alliance (BWA) consisting of ACTEW and ACTEW-AGL, GHD, and John Holland / Abigroup, are delivering the Enlarged Cotter Dam project in Canberra, ACT. The greatly enlarged reservoir will require two central core rockfill saddle dams on a ridge adjacent to the main dam site. Construction of these two dams was completed in early 2011. The challenges of the site and the Alliance delivery model have provided opportunities for innovation in both use of materials and construction.
The dam foundations were variably weathered and fractured with some highly weathered seams extending below the cutoff trench foundation. The foundation was grouted effectively using GIN grouting and the entire cutoff trench was shotcreted to reduce the risk of piping of the dispersive core material.
The steep topography provided very limited sources of material suitable for a dam core. Potential contingency plans considered included bentonite enrichment of the low plasticity materials or a change to a concrete faced rockfill dam. The high cost of these options drove the decision to use the available residual soils from small gullies by selectively winning material with a higher fines content for use below full supply level. The lack of room on the ridge for stockpiling and conditioning of clays lead to trialling of a continuous mixer for mixing and conditioning the core which was found to be highly successful.
Filter materials were crushed sands and gravels produced from nearby commercial quarries. The materials and grading were generally high quality, with some challenges producing coarser filter materials by blending available aggregate products. A range of options were effectively adopted for placement of the filters including loader placement, trench boxes and spreading from a modified ejector dump truck.
Enlarged Cotter Dam Saddle Dams – Materials and Construction