Jiri Herza, Nihal Vitharana, Alex Gower
The Western Australia Water Corporation plans to increase the storage capacity of Millstream Dam, which is located near Bridgetown in the south west region of WA. The existing dam is an 18 m high zoned earthfill embankment constructed in 1962. The dam suffered a block heave of the foundation at the downstream toe during the first filling, probably attributable to high foundation pore water pressures. The dam upgrade will be challenging due to complex and unfavourable foundation soils coupled with these artesian pressures.
The dam is founded on lateritic soil, which is a common weathering profile throughout the region. These soils formed in a tropical environment of fluctuating water tables, severe leaching and translocation of iron oxides over many millions of years. As a consequence some of the lateritic horizons at Millstream Dam have been modified such that they exhibit behaviours that are not consistent with conventional constitutive models and correlations. These are attributed to a complex structure of the soil microfabric, which comprises clay particles bonded together into larger aggregates. The clayey aggregates are also bonded to each other, forming a porous matrix of silty or sandy appearance characterized by low dry density and high void ratio, which may nevertheless disintegrate on working.
Comprehensive geotechnical investigations and extensive laboratory testing have revealed that the foundation materials display characteristics of clayey and granular soils. Under shearing, these soils demonstrate high initial strength, which gradually reduces as the inter-aggregate bonds are broken and the relative position of the aggregates changes. Several soil samples also exhibited significant contractive behaviour on shearing generating high pore pressures under undrained conditions.
This paper presents the investigation and design methods used in the foundation design of the Millstream Dam upgrade with emphasis on unusual behaviour of the foundation media.
Challenges in dam design on lateritic soils
Cubit T, Swindon A, Tanner D
Catagunya Dam is located on the Derwent River in Tasmania’s south east. During construction of the dam in early 1960’s 412 post-tensioned anchors were installed, however the integrity of the original anchors can no longer be assured. The stability of the dam was restored between 2008 and 2010 using 92 modern, large diameter, load monitorable and corrosion protected post-tensioned anchors. These are the most highly stressed anchors applied to a dam at this time.
Some of the key construction challenges included installing 53 anchors within an operating spillway, utilising a very limited construction window and replacing severed surface reinforcement using carbon fibre rods.
This paper details how these challenges were resolved and presents a number of innovative solutions developed along the way.
Steven Slarke, Martin Mallen-Cooper, Andrew Evans, John Prentice
As part of the Murray-Darling Basin Authority ‘Sea to Hume Dam’ program to restore fish passage along the River Murray, an innovative Denil fishway is being retrofitted into Mildura Weir (Lock 11). Due for completion in the latter half of 2010, the fishway will allow the upstream and downstream passage of medium and large sized fish past Mildura Weir, which has a difference in water levels of 3.5 metres.
Constructed on the sloped concrete apron at the left abutment of the Dethridge weir, the Mildura Weir Denil fishway design is innovative in the River Murray. The Denil fishway is essentially separate from the existing weir, and its superstructure can be fully removed from the river during floods. The fishway can also be progressively removed during periods of rising floodwaters, maintaining operation during periods when fish migrate in particularly large numbers. The fishway represents a cost effective design, reflecting the decision to maintain the current weir structure for a further forty years, but still providing passage to a broad range of fish sizes and species. Innovative fish monitoring and carp separation facilities will be provided, shared with the other River Murray fishways. But, unique to the River Murray, viewing windows are provided to allow the public to observe fish negotiating the fishway, and to enable a better understanding of fish movement.
Richard Herweynen, Colleen Stratford
Assessing the potential for erosion of foundation rock downstream of a spillway is a problem faced on many dams, whether new or existing. The problem is made particularly difficult not only due to the uncertainty in determining the erosion potential of the rock, but also due to the variable hydrologic characteristics of flood events.
The selected spillway option for Wyaralong Dam comprises a centrally located primary spillway with a secondary spillway located on the left abutment. A stilling basin energy dissipater is provided at the toe of the primary spillway. Downstream of the secondary spillway, an apron channel will direct flows back to the stilling basin. However, for flood events larger than the 1 in 2000 AEP event, the capacity of the secondary spillway apron is exceeded and flows spill out across the left abutment of the dam towards the river channel. Erosion of this left abutment was viewed to be a potential dam safety issue, and as such, careful consideration was required during the design stage to determine the acceptability of this spillway arrangement.
In order to provide structure to a problem which often relies solely on engineering judgment, a decision process was developed, taking into consideration some of the more definable aspects of the problem. These aspects included the geological characteristics, the initial hydraulic characteristics, the flood duration, the nature of erosion should it occur and the stability of the dam. This paper describes the decision process and methodology used at Wyaralong Dam to
determine the acceptability of erosion. This paper will present the process in a way that it can be used by others in future dam projects, both new and upgrades.
A Unique and Holistic Approach to the Erodibility Assessment of Dam Foundations
After a period of drought for many years, inflows during May and June 2009 resulted in releases from North Pine Dam. These releases resulted in deaths of fish downstream of the dam wall including lungfish. The Australian Lungfish is a protected species under the Environmental Protection and Biodiversity Conservation Act 1999 (Australian Government). The events of 2009 have shown, however, that a proactive response supported by sound knowledge is required to minimise lungfish losses from flood events and other dam operations activities. A framework has been developed for the management of lungfish populations in Seqwater storages. The framework centres on a Seqwater Fish Management Policy, and four broad strategies that are considered necessary for addressing fish management in Seqwater storages: Fish Management, Storage Operations, Communication, and Research. These strategies are being used as a basis for identifying, planning and managing a range of actions designed to ensure that impacts to lungfish are minimised. Seqwater intends to develop the framework further to include long term management initiatives such as implementing viable technologies for preventing lungfish strandings, habitat protection and restoration activities that support viable lungfish populations, as well as establishing priorities for managing risks to other aquatic vertebrates in Seqwater storages, including other protected species, recreationally and commercially important species; e.g. turtles , carp, mullet, etc.
Jim Walker, Sergio Vallesi, Neil Sutherland, Peter Amos, Tim Mills
The Tekapo Canal is a 26km long hydropower canal owned by Meridian Energy Ltd in New Zealand. Completed in 1976, the canal is 40m wide, 7m deep and has a capacity of 120m3/s. The canal was constructed from compacted local glacial soils with a compacted silt lining sourced from till deposits.
During 2007 and 2008 the canal showed signs of leakage where it crossed over a twin barrel culvert structure. In October 2008 a diver inspection identified depressions and sinkholes on the invert of the canal above the culvert. Approximately 6m3 of silty gravel lining material had settled. Testing showed direct and rapid connections between lining defects and seepage outflows at the culvert outlet headwall. Subsequent ground penetrating radar survey confirmed the presence of voids above the culvert barrels. Diver placed filling of the defects with granular materials was immediately implemented, and a series of remedial actions over the next four months were required to arrest deterioration and enable the canal to remain operational.
The paper describes the initial response to this situation and the immediate measures taken to prevent failure. It also describes the medium term and ongoing measures implemented to maintain the safety of the canal while permanent remediation requirements are assessed. The lessons learned from this event, and their impacts on Meridian’s Dam Safety Assurance Programme (DSAP) are also discussed.
Immediate response measures included ongoing filling of lining defects with filter gravel, intensive land based and diver surveillance of the canal, planning and resourcing for emergency contingency actions in the event that a risk of breach developed. Medium term measures included arresting leakage by placing a low permeability blanket of silty gravel over the damaged area using a concrete pump, and constructing external buttresses capable of safely withstanding large discharges should deterioration of the canal structure occur.
These short and medium term remedial measures were completed with the canal full and in operation and continue to perform well 20 months later. Continuing risk mitigation measures include enhanced surveillance and monitoring (land based and using divers), localised treatment of defects, as well as ongoing monitoring and review of the Dam Safety management regime and sustained Emergency Management preparedness.