Brendan Sheehan, Chris Topham, Alan White, Rowenna Lagden
Darwin Dam is a 21m high embankment dam constructed on a geologically complex foundation that includes karst limestone features. The dam retains the top 15m of Lake Burbury on Tasmania’s west coast, and borders the Tasmanian Wilderness World Heritage Area. Defensive design of the dam addressed the key failure modes of piping through the complex foundations of limestone, sandstone, gravels and silts, and guarding against sinkholes forming in the limestone foundations. During construction, a comprehensive range of instruments were installed in the dam and foundation, as a long term means of monitoring this structure. A range of surveillance data has been collected since lake filling and this data, along with historic geological investigation information, was used to develop a three dimensional (3D) geological model of the dam and
foundation with phreatic profiles. The software used was a commercially available geographical information system. This tool has assisted Hydro Tasmania to better understand and manage the dam. The paper outlines the need for a 3D model, the methodology for development of the model, resources required, limitations and lessons learned. The benefits of the model, such as aiding understanding of foundation behaviour, assisting with interpretation of surveillance data, supporting decision making, and potential use during incident response are also discussed.
Keywords: Three dimensional, computer model, karst foundation, geology, hydrogeology ,dam surveillance
Now showing 1-12 of 41 2974:
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
Ben Greentree, David Bamforth, Matthew O’Rourke and James Willey
A series of relatively small floods occurring between end of construction in 1978 and late 1980s caused extensive and dramatic rock erosion to the very steep unlined section of the Googong Dam spillway. Following a review of hydraulic performance at larger floods, the spillway’s future erosion potential was evaluated and it became clear that extensive remedial work was required. A detailed design was developed comprising the retro-fitting of a full concrete-lined chute, the raising and extension of the spillway chute walls, strengthening of the upstream training walls and excavation of a large plunge pool. The Googong Dam has an ANCOLD hazard rating of ‘extreme’ because of its location upstream of Queanbeyan and Canberra.
In early 2008, the Bulk Water Alliance (BWA), comprising ACTEW Corporation Ltd, (in cooperation with ActewAGL) (the Owner), GHD Pty Ltd (the Designer) and Abigroup Contractors Pty Ltd in joint venture with John Holland Pty Ltd (the Constructors) was formed to deliver a package of water security projects for the ACT, one of which is the Googong Dam Spillway Upgrade.
After preparation of a construction methodology and target outturn cost (TOC), the project was approved by the Actew Board and construction commenced in February 2009. Completion is due in late 2010. A number of significant geotechnical, structural and logistical challenges were encountered during construction, resulting in major changes to the construction methodology necessitating design changes. The changes were incorporated within the original TOC, without instigating scope change contractual claims and while still maintaining spillway functionality in line with Owner operational requirements.
This paper presents delivery phase challenges that necessitated construction methodology and design changes to achieve best for project outcomes; how these challenges were overcome through genuine innovation reliant on a collaborative effort by all the Alliance partners; and how the contractual framework of the Alliance was essential for the change management process to be successful.
Tony Harman, Richard Herweynen, Malay Ghosh
Following a number of years of investigation into the condition of the existing 1960’s post tensioned anchors at Catagunya Dam Hydro Tasmania embarked on an options study to determine the best method to restore the dam stability to acceptable limits for the long term. The required solution was intended to not only resolve the issue of anchor deterioration but also to increase the flood capacity of the dam.
Based on preliminary design work a concrete buttress solution was recommended and approved for detailed design. The preliminary design utilised a simplified, 2-dimensional, rigid body model, including crack analysis. As part of the detailed design a finite element model was developed to refine the preliminary design. However, this model did not support the simplified analysis and further non-linear finite element analysis demonstrated that the proposed passive buttress design solution was not technically feasible. The options were reconsidered and the adopted solution was to replace the original anchors with new modern anchors with a high level of corrosion protection.
The new anchors adopted are the largest post tensioned anchor loading currently used for a dam in the world. This along with the existing post-tensioned anchors and the tight geometry of the dam, which has a central spillway with a cantilevered ogee crest, provided significant challenges with the design of this dam upgrade. Some of the key design challenges included:
– Appropriate level of modeling and analysis to be able to make sound design decisions. (Hydraulic modeling and FEA).
– Congestion due to the tight geometry of the original design.
– Anchor head block detail to ensure the loads would be adequately secured and dispersed into the dam body
– Crest cantilever support to ensure that structural integrity was retained during construction and later in service. Innovative installation of carbon fibre reinforcement was used.
– Strain compatibility. It was important to ensure the structural contribution of new and old working together and that the consequences of application of new large stresses was manageable.
– Existing anchor degradation. The design needed to ensure that stability compliance was achieved for complete to zero effectiveness over time.
– Maintaining operability of dam and power station during construction.
– Achieving an effective long term maintainable solution.
This paper will present the risk associated with committing to a solution too early and the design challenges and the solutions finally developed, providing the dam industry with a valuable reference for future similar projects.
Analysis and Design Challenges Associated with the Catagunya Dam Restoration Project
Peter A Ballantine, Christopher V Seddon
Massingir Dam, constructed in the late 1970’s on the Olifants River in Mozambique, is a 48 m high zoned earthfill dam. Due to various safety concerns, the dam was operated at a reduced full supply level of 110 masl, compared to the design full supply level of 125 masl. Between 2004 and 2006 remedial works were undertaken, including the construction of a berm on the downstream face of the dam, grouting and drainage of the foundations and installation of the spillway crest gates. From December 2005 the storage level of the dam was allowed to increase.
On 22 May 2008, with the reservoir storage level at 122.43 masl and the gates on the outlet conduits closed, the reinforced concrete conduits failed at the downstream end, releasing an estimated 1,000 m3 /s of water into the Olifants River.
A 2-D finite element analysis was undertaken in order to establish the safe load bearing capacity of the as-constructed conduits. On the basis of the analysis, it was concluded that the original design did not take proper account of the pressure that would develop within the thick concrete sections of the conduit. In view of assumptions regarding the load paths, the reinforcement was not placed in the most appropriate positions.
This paper describes the events leading up to the failure of the conduit, presents the findings of the investigation into the failure and makes recommendations on the basis of the findings.
Jared Deible, John Osterle, Charles Weatherford, Tom Hollenkamp, Matt Frerking
The original rockfill dike, constructed in 1963 to form the Upper Reservoir at the Taum Sauk Pump Storage Project near Lesterville, MO failed on December 14, 2005. The Upper Reservoir has been completely rebuilt as a 2.83 million cubic yard (2.16 million cubic meters) Roller Compacted Concrete (RCC) Dam in compliance with FERC Regulations. The project is the largest RCC project constructed in the USA and is the first pumped storage project to utilize an RCC water retaining structure. The project is owned and operated by AmerenUE and consists of an Upper Reservoir and a Lower Reservoir connected by a vertical shaft, rock tunnel, and penstock. The Powerhouse has two pump-turbines with a total generation capacity of 450MW.
A refill plan was developed to monitor the performance of the dam during the first refill. Because it is a pumped storage project with no natural inflow, the reservoir level can be raised and lowered with reversible pump turbines. The refill plan includes hold points when the dam s performance will be assessed at eight reservoir levels. Monitoring of the performance of the dam is done through instrumentation readings and visual inspections. Inspections check for alignment changes, leakage, seepage, cracking, or any other unusual or changed conditions. Instrumentation monitored during the refill program includes piezometers, seepage weirs, survey monuments, and joint meters. The level control system for the project was also evaluated during the refill program. This paper summarizes the monitoring and inspections conducted during the refill and the performance of the dam during this period, and the performance of the dam during the initial period after the refill program.