Steven Fox, Garry Fyfe
This paper describes some key details of the construction of the Lake Eppalock Main Embankment Remedial Works Project. This $8.25 million earthworks project was completed on a “live” storage to an accelerated program. As the dam owner Goulburn-Murray Water took the decision to directly manage the construction of these works with resultant benefits in timing, risk management and project management costs.
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This paper describes the use of a high strength woven geotextile and preloading to stabilise the surface of a very low strength tailings pond, and the incorporation of a geosynthetic clay liner (GCL) within the final capping design to complete closure. The pond, which contains tin and copper tailings, formed the lower tailings containment area of a three-tiered tailings storage, located directly above the Wild River in North Queensland. Stabilising the lower pond (area 2,500 m2), which contained tailings of “zero strength” in the central area involved the placement of a woven geotextile over the surface, which was anchored around the perimeter. The placement of finger berms (preloading fill) on the geotextile was successful without exceeding the bearing capacity of the tailings overall. Settlements of the berms were closely monitored to allow the system to support construction plant. After the finger berms were joined, they were widened until the area was covered. A sand layer was then placed over the area followed by a GCL to form an impermeable barrier prior to the placement of clay and topsoil.
Murray Thompson and Geoff Chenhall
The Hastings District Water Supply Augmentation Scheme [HDWS] includes a 10GL off-creek storage dam, which is currently under construction and due for completion in October 2001. The Cowarra off-creek storage dam is required to meet predicted long-term urban growth demands for water supply and to ensure protection of environmental flows in the Hastings River.
Since 1985 the Hastings Council has progressively developed a strategy for the augmentation of the water supply scheme. A very successful ongoing consultation process with both the local community and key government agencies during the planning and implementation phases of this project has highlighted a number of key issues including:
“That the impact upon aquatic flora and fauna in the Hastings River should be minimised and appropriate safeguards developed by maintaining minimum river flows to ensure that the river habitat is not adversely affected”
The subsequent HDWS Environmental Impact Statement, 1995 was one of the first in NSW to recognise the importance of environmental river flows in the assessment of the aquatic ecological effects of water supply schemes. This paper to be presented to the ANCOLD Conference on Dams will detail the investigation, planning, implementation and current construction activities associated with the Cowarra Off-Creek Storage Dam.
David Dole and Brian Haisman
The $75 million remedial works at Hume Dam on the Murray River near Albury have been of national significance. The rehabilitation program associated with the structure itself and with its appurtenant works is now in the final steps of construction. The authors summarise this program with an emphasis not on technical details, but on decision processes. Equity in this dam is, in effect, held in equal parts by three State governments and by the Commonwealth government. At the same time, in response to the national water reform agenda, the governments have agreed upon new cost sharing arrangements that more nearly reflect the value of services to each government. The particular problems of decision-making within this evolving inter-State environment are discussed.
Lessons from experiments with application of risk analysis are discussed. Finally, the matter of adequacy of the structure for extreme floods is still under consideration. Hume Dam will presently pass the Design Flood developed in accord with Book VI (1999) of Australian Rainfall and Runoff, and the Dam Crest Flood has been estimated to have an annual exceedance probability of 1:110,000. _ Retrofitting a spillway to pass the estimated Probable Maximum Flood will double the cost of remedial works and is estimated at 10 times the cost of similar capacity built into original construction. The authors discuss the public policy elements of this pending decision.
P.J. Ritchie and N.A. Currey
Kidston Gold Mines commenced operations in 1984 and built a dam to safely store the tailing waste from the ore processing. The dam was progressively raised 5 times (3 downstream and 2 centreline lifts) and has an active surface area of 310 hectares; stores 66 Mt of tailing and is 32 metres high at its maximum height. The dam was decommissioned in September 1997.
Rehabilitation planning for the tailing dam commenced in 1994 with an 11 hectare direct revegetation trial established in March of that year. A 40 ha trial was established in 1998. Both sites have been the subject of intensive scientific research by the (University of Queensland) Centre for Mined Lands Research group. This research assisted in understanding the issues of revegetation stability and sustainability, biological cycling, soil chemistry and surface erosion.
The aims of rehabilitation is to meet the Queensland Department of Mines and Energy (DME) key closure criteria. These include; creating a stable landform, not only for the dam wall structure but also of low surface erodibility, maintenance of acceptable downstream water quality by controlling poor quality seepage and runoff and by meeting an acceptable final end land use criteria for the structure.
Ongoing research is addressing the long term hydrology of the tailing dam with an aim towards understanding the overall water balance. Three consulting groups are involved in what is considered to be a novel approach. Evapotranspiration rates from pasture and tree species have been measured during the 1999 wet and dry season. This information, along with climatic and soil suction data is then used as one of the key parameters for the unsaturated zone modeling. One output from the “Soilcover” model is seepage into the saturated zone in the tailing dam. Water movements in the saturated zone are being modelled using Modflow. The acid oxidation potential for the dam is also being evaluated in light of the long term water movements in the saturated and unsaturated zones of the dam. This process will allow short and long term prediction of dam seepage quality and quantities.
The geotechnical stability of the final dam wall structure as defined by the Factor of Safety, ranged from 2.0 to 2.3, which meets the long term DME recommended stability target FOS of 1.5 for slopes.
In order to evaluate the impact of metal toxicities in grazing cattle, a grazing trial has been established on the pasture covering the surface sediments of the tailing dam. This work is being supported by the Qld EPA, Qld DME, Qld Health and the NRCET, and will assist in understanding metal uptake in grazing animals on rehabilitated mined lands.
Pieter van Breda, Peter Walton, Kate Lenertz and Tim Sheridan
The Warragamba Dam Auxiliary Spillway Project, designed to manage floodwaters up to a Probable Maximum Flood event, was approved by the NSW Minister for Urban Affairs and Planning on February 12, 1998. An Environmental Impact Statement prepared for this project predicted that noise, dust (suspended and deposited), blasting, vibration, water quality and revegetation would be the significant environmental issues requiring management throughout the construction phase.
The closest residents are approximately 200m from the construction activity. The works must not interfere with the operation of the Dam, which stores 80% of Sydney’s drinking water and the integrity of the existing infrastructure must be maintained at all times. The approved proposal was to emplace the 2.2Mm3 of spoil excavated to create the spillway in an area 25 ha by 20m high on top of a ridge on the left bank adjoining the Blue Mountains National Park. This created visual impact and rehabilitation challenges.
Although the contract for this project was primarily performance based, strict environmental clauses were incorporated to manage these priority issues. Noise and dust modelling were required from each pre-qualified Tenderer, to demonstrate capability of compliance with NSW Environment Protection Authority requirements. This formed part of the tender assessment. Criteria were also developed for revegetation, specifying numbers of endemic trees, shrubs and grasses per 400m2 of spoil emplacement in order to create a floral community similar to the existing adjacent National Park.
The implementation of these requirements and the development of a site Environmental Management Plan by the Sydney Catchment Authority, Australian Water Technologies and Abigroup Contractors, whilst maintaining productivity, has proven to be a working example of the benefits of Partnering.