Rick Friedel, Len Murray, Gerrad Suter, James Penman, James Watt, Hendra Jitno
The Hidden Valley tailings storage facility (TSF) has set a new precedent in environmental management of tailings in Papua New Guinea (PNG). Modern mining in PNG arguably began with the development of Bougainville Copper in the late 1960s, and continued through to Ok Tedi, Porgera, Lihir, Misima (and others). These mines have proceeded with deep sea or riverine tailings deposition, rather than construction of a tailings dam to retain the mine waste within an impoundment; as is the practice throughout the majority of the mining industry.
The Hidden Valley TSF is comprised of two large earth and rock fill dams, raised by the downstream method. Starter dam construction was completed in 2009. At final height the Main Dam will be one of the highest tailings dams in the world. The dams are constructed of pit waste and therefore have the dual function of storing tailings and waste rock.
Construction of the starter dams and subsequent raises is complicated by conditions at the site. Water management was, and remains, the dominant issue. High rainfall, weak erosive soils, material availability, dense vegetation and remoteness of the site provide constant challenges to construction. The Observational Approach to construction was recommended by the designers and adopted by the mine operator. This involves a knowledgeable pre-assessment of what is likely to change and having contingency plans to deal with possible major issues. This approach allows changes to the design during construction so the “as-built” product is suited for the site, fit for purpose, and remains consistent with the overall intent of the design.
The TSF has been in operation since August 2009 and monitoring data of the structures has been collected during construction and operation. This data is reviewed to confirm design assumptions and assess dam performance.
Personnel involved with this project combined their experiences working in the PNG environment and dam building from other locations. This process led to close interaction between the mine operators, designers and construction teams. Team work and diligent construction practices were and will continue to be necessary to construct and operate the pioneering TSF in PNG.
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Thomas Vasconi, Mike Gowan
This paper describes the methodology adopted for the design of a 180 m-high stepped chute spillway to be constructed on a mine tailings storage facility (TSF) in Africa. This TSF dam, constructed using the “downstream method”, will be raised progressively via a series of nine lifts as mining proceeds. The first eight will be equipped with an operational spillway sized for the 1in 10,000 AEP whilst the ninth will house the closure spillway sized for the Probable Maximum Flood. The problem, common to all staged tailings dams, is how to design the spillways for such raising sequence? The very steep ridge declivity favored locating them in a unique configuration rather than the more usual separate hillside spillway on each dam abutment. The design of such spillways was challenging since it had to integrate the TSF interdependency parameters (water balance, dam raise sequence) whilst including flood routing, spillway sizing, stepped spillway design components. Challenging aspects of the design also included optimizing the costs associated with the short service life of these spillways. Furthermore, the design was undertaken in a way that the operating stepped chute could be upgraded and reused at mine closure. The design incorporates an innovative solution which allows reduction in the rock armouring quantity of up to 40% with associated cost benefits, and sustainability in terms of material usage. The lessons learnt in applying this innovative and sustainable design are useful for other sites requiring adaptive construction and short service life spillways.
Keywords: Tailings storage facility, stepped chute spillway, hydrology, hydraulics, mine water management.
Aric Torreyson, Krey Price, Bob Hall
In a 2004 feasibility study, the U.S. Army Corps of Engineers (Corps) and Ventura County Watershed Protection District (VCWPD) recommended decommissioning Matilija Dam, a concrete arch dam originally constructed to a 60-metre height in 1948. A decade after its completion, the United States Bureau of Reclamation (USBR) constructed the Ventura River Project, comprising additional facilities designed to meet the growing water demand of Ventura County. Robles Diversion Dam, a 7-metre high by 160-metre long diversion structure located downstream of Matilija Dam, was built under the Ventura River Project to feed Lake Casitas, a water supply reservoir that serves as an integral part of the overall project.
Due to extreme sedimentation, Matilija Dam no longer serves its intended water supply and flood control purposes. In addition to the loss of storage capacity, other issues surround the dam, including adverse environmental impacts from its continued operation, seismic considerations, and structural concerns. These concerns led to the decision to decommission the dam as an essential step in rehabilitating key ecosystems in the Ventura River Catchment and reducing future risks to public safety. According to current estimates, 5 million cubic metres of sediment has accumulated behind the dam and will need to be removed in conjunction with the dam decommissioning; minimising the associated downstream impacts has been the subject of additional government studies.
The USBR determined through detailed hydrologic, hydraulic, and sediment transport analyses, including numerical and physical modelling, that the existing Robles Diversion Dam was not capable of passing the increased sediment load expected to result from the removal of Matilija Dam. To increase the sediment transport capacity across its spillway, the existing diversion dam requires modification. Under contract with the Corps, Tetra Tech and its subcontractors are completing the design plans for the Robles Diversion Dam modifications.
This paper presents unique aspects of the Robles Diversion Dam modifications, including sediment management procedures guided by numerical and physical model results and issues associated with the design of a rock ramp spillway and high-flow fishway, expansion of the existing spillway gate structure, and raising of the dam embankment. The rehabilitation efforts reduce impacts to the migration of endangered fish species and allow for the eventual removal of Matilija Dam, which is the ultimate goal in the effort to balance engineered structures with a natural river setting. When completed, the project will provide fish passage to the upper catchment for the first time in over sixty years.
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
Jim Walker, Jamie Macgregor
The Pukaki Canal Inlet structure is a large gated culvert and stilling basin structure, it is a High PIC appurtenant structure to the Pukaki Dam, located in the Mackenzie Basin area of New Zealand’s South Island.
The 560m3/s capacity inlet structure is founded on glacial moraines. It controls flow from the178 km2 Lake Pukaki storage into the 80m wide, 22km long Pukaki/Ohau canal. It is the owner’s (Meridian Energy) most important valve, as it feeds 1550MW of hydro generation on the Waitaki River.
A risk assessment in late 2009 identified a previously unrecognised trigger for a potential failure mode for the stilling basin. Principally, ongoing erosion of the reinforced concrete base slab could lead to failure of water stops in the slab joints potentially leading to slab uplift, foundation erosion, and ultimately, catastrophic failure of the Pukaki Dam. To better define the risk to the structure, further inspection of the stilling basin was recommended.
A dewatered inspection of the stilling basin was required, as further dive inspections would not improve our understanding of structure condition. Because the stilling basin cannot be isolated from the canal, this requires dewatering the entire Pukaki/Ohau canal, presenting significant risks of damage to the canals from slumping and lining failure. A dewatered outage also has major business revenue impacts.
This paper describes how Meridian were able to take advantage of a transmission network outage, scheduled for just six days after the risk was identified, to plan, safely dewater, inspect, and rewater 22km of hydro canal, and not just to inspect the Pukaki Canal Inlet structure, but also to implement repairs to the stilling basin slab which have successfully mitigated the structure safety and operational risks. This huge undertaking involved mobilising an army of people, plant and materials, and cost over NZ$1.8m. From identifying the risk to the structure, to completing repairs took just 13 (very busy) days.
Lessons learned in the areas of dam safety and asset management are presented. As well as those contributing to the success of the project in seizing an opportunity to mitigate the identified dam safety and operational risks.
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