Learning in Las Vegas4 July 2002
Held in Las Vegas, US, the Tailings Dams 2002 conference offered delegates a valuable opportunity to discuss engineering, dam safety and tailing structures with experts from the tailings dam industry. Carrieann Davies reports
I'LL ADMIT that when my esteemed Editor told me I'd be going to Las Vegas to attend a conference on tailings dams, I had visions of winning my fortune on the slot machines and being able to live the good life for years to come. Unsurprisingly, when the conference was over, I didn't come back to the UK a rich woman, but I did come away with a better understanding of the tailings dams industry.
Sponsored by the Association of State Dam Safety Officials (ASDSO) and the US Society on Dams (USSD), the Tailings Dams 2002 conference took place from 29 April to 1 May 2002 at the Orleans Hotel and Casino in Las Vegas, Nevada, US. Offering more than eight hours of educational instruction conducted by experts in several technical fields related to engineering, dam safety and tailing structures, the conference allowed delegates the opportunity to network with dam professionals from the US and worldwide.
One of the main issues discussed during the conference was the construction and design of tailings dams. In a paper by Donald R East of Knight Piesold and Joseph Giraudo of Barrick Goldstrike Mines, for example, the design, construction and operation of a large gold tailings storage facility in North America was presented to an enthusiastic audience. Delegates were told how a second mill tailings storage facility for Barrick Gold Company's Goldstrike mine, known as the North Block Tailing Storage Facility, was constructed in 1993 to replace the first tailings facility, the AA tailing storage facility, which was nearing capacity.
The North Block facility is a permitted zero-discharge facility and was designed as a state-of the-art facility with regard to containment systems. Components of the facility include a composite liner system consisting of a 60-mil HDPE basin liner and compacted soil liner, underdrain system, embankment drains and a piezometer monitoring system to monitor the hydraulic head on the liner system.
Confinement of the tailing within the North Block facility is provided by embankments around the north, west and south sides with the eastern side butting up to the natural hillside. The embankment is designed and constructed as a zoned earthfill/rockfill structure comprising a low permeability seal zone on the inside face, followed by a sloping drain and then a wide rockfill zone placed by mine equipment.
The authors point out that during construction, run-of-mine rockfill is dozed into the adjacent downstream zone. This achieves a zoned configuration with finer material on the upstream side and coarser material downstream. This serves the dual purpose of achieving a dense, low-permeability fill near the tailing and a coarser erosion preventative fill on the downstream face.
The drainage and tailings deposition systems used at the site were developed by Knight Piesold in Nevada during the late 1980s. Over nine years of operations, the facility has met its design performance targets and has provided a convenient haul distance for construction of those embankment sections using suitable mine waste placed directly by the mine haul fleet.
Another paper on construction methods came from Christopher N Hatton of URS Corporation. Entitled 'Don't sleep with a drip, use horizontal drains to control your dam seepage problems', Hatton's presentation gave an overview of horizontal drain construction. Hatton told delegates that construction of tailing dams using the upstream deposition method requires good stewardship and proper tailing management policies and procedures, as inadequate foundation drainage, careless tailing management and other factors can result in poor drainage and cause elevated pore pressures, thereby reducing dam stability.
Hatton explains that a horizontal drain in simplest terms is a lateral well that flows by gravity, drilled at an oblique angle into an earthen or rock slope. A drain will typically consist of a continuously slotted pipe designed to intercept high permeable earth materials, fractures, or other subsurface water conveyances, thereby providing a preferential drainage path and reducing excess pore water pressures.
Horizontal drains were initially used to stabilise highway cuts and landslides in the late 1930s by the California Division of Highways. These drains have also been used over the last 20 years to provide supplemental drainage for both water retention and tailings dams.
In the paper, Hatton says that success of horizontal drains is dependent on proper material selection. Horizontal drains drilled in sulphide tailings, places the materials used to construct the drain (typically stainless steel surface casing, plastic drainpipe and sulphate-resistant cement for grout) in direct contact with acidic pore fluids; high in reduced iron and sulphate. These conditions are corrosive and pore fluids can produce metal and metalloid precipitants that coat and plug the drain screen.
Hatton claims that good horizontal drain flow depends on placing the slotted drainpipe in contact with a host tailing that can readily convey water. Experience installing horizontal drains has shown that as the drill hole advances, the bit tends to fall and move to the left, facing the drill hole, he says. This results in a drain that is non-linear and has a long sweeping arc shape. The drill hole is initiated on an upward slope to improve drainage and offset the drill steel drop.
An interesting point bought up in the conference was the importance of an observational approach, as discussed in a paper by Ivan A Contreras and Philip Solseng of Barr Engineering and Terrell K Johnson of OCI Wyoming. Entitled 'Monitoring and Operation of a Tailings Dam', the paper discusses a dam which was built in 1974 as part of a fluid retention system. Upon dam completion, the reservoir was filled and significant seepage breakout and very soft conditions downstream of the dam were observed, indicating unsatisfactory performance. Since then a monitoring programme based on the observational approach was implemented to maintain dam operation.
The authors state that several geotechnical issues have been of concern at the dam, including failure by piping due to the dispersive nature of the underlying deposits, excessive seepage resulting in loss of reservoir water, and artesian conditions resulting in very soft ground conditions downstream of the dam. Different corrective measures have been taken over the years to maintain dam operation.
According to the authors, an expected increase in reservoir level required a complete assessment of dam performance and stability. The assessment involved a geotechnical investigation including cone penetration testing, extensive laboratory testing and engineering analysis.
The dam is 487.7m wide and 29m high with 2.5:1 (H:V) slopes. The dam is homogeneous compacted fill consisting of sandy clayey silt. It has a vertical chimney drain and a horizontal toe drain consisting of sand and gravel.
The shells of the dam were built on top of the insitu alluvium, which consists of a mixture of sandy clay and clayey sand. The designed capacity of the dam resulted in a maximum pond elevation of 1934m.
Construction of the dam was completed during the summer of 1974. During the summer of 1975 seepage broke out downstream, even though the quantity of water stored in the reservoir had reached only about half of the designed capacity (elevation 1926m).
To assess dam integrity concerns, the observational approach was used. In this approach, a monitoring programme is established and actions in steps are taken. The result of each action is evaluated with the monitoring programme and action levels are set based on the results.
A monitoring programme was also initiated to allow operation of the dam. This included installation of 48 piezometers and surface monitoring devices. The piezometers indicated that most of the flow was taken flow was taken place along the weathered and facture zone. The artesian conditions were consistent with the field observation of soft ground conditions downstream of the dam. It was decided that to dissipate the high pressures downstream of the dam relief wells should be installed. In 1980, a total of 12 relief wells were installed downstream of the dam to eliminate the artesian conditions observed. The relief wells are connected to a sump. The flow at the sump is measured on a regular basis to monitor well performance.
After the installation of the relief wells, the pressures at the piezometers decreased. As a result, the relief wells were effective in reducing the pressures downstream of the dam. Additionally, a toe berm about 0.76m thick was placed at the downstream toe to improve access and increase the effective stress in that area.
Another action that was taken to enhance the deficiency of filters was the installation of a filter system on the abutments of the dam to improve seepage control and minimise potential piping. After the installation of the filter system, the pond level was increased to elevation 1929m.
Delegates at Tailings Dams 2002 were also presented with papers on reclamation of tailings dam facilities. Charles S Bishop of Bowser-Morner Associates and Gene D Campbell of Abot Engineering described how Czar Coal Corporation proposes to construct the Middle Fork slurry impoundment and lake as part of its ongoing coal processing operation. The impoundment is to be located in southwestern Martin County, Kentucky, in the headwaters of Rockcastle Creek.
To reclaim the slurry impoundment, several design and construction features need to be incorporated into the dam embankment and spillway systems and regulations of several agencies need to be complied with.
The dam embankment will be constructed of coarse refuse from the preparation plant. Embankments constructed from this material are not totally watertight structures. The permeability of compacted coarse refuse at 95% standard proctor maximum dry density is typically around 10-5cm/sec. For a permanent lake, the seepage through the dam embankment must be minimised. To accomplish this objective using on-site materials, a zoned internal embankment configuration was utilised.
Dam embankment construction and slurry disposal operations are designed to be accomplished in three stages. The middle fork impoundment and lake project has been conceived and designed to provide for an additional 25 years of coal processing operations at the Pevler Preparation Plant. According to the authors, it minimises disturbance of additional land and streams by siting the structure at the extreme headwaters of the stream in an area where extensive mountaintop removal mining has already occurred. The project proposes to leave a permanent lake which will be a recreational and wildlife resource, and could be utilised as a water supply.
When I asked a fellow delegate what he had gained from the conference, he answered readily that he 'had found some great contacts and learned more about the various aspects of tailings dams'. With this answer, it seemed the conference organisers had achieved their aim of providing an informative and interesting conference.
As for my aim of winning my fortune, maybe next time I'll be lucky…
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