Continuity is the key16 October 2000
The prolonged construction period of tailings dams can be a direct challenge to quality assurance. As Rod Kostaschuk, Ken Brouwer and Jeremy Haile explain, continuity is an essential part of preventing the failure of these structures
In the majority of cases, water storage and hydro power dams are designed and constructed to their final configuration in a single design and construction sequence. A new engineering firm may be retained part way into construction or it may be necessary to raise or reconstruct the dam many years later. If ownership of the dam changes, or the original design engineer is no longer available for the next phase of design and construction, formal hand-over procedures are required to ensure continuity of design responsibility, quality assurance (QA), dam safety and monitoring systems.
The design and construction of a tailings dam usually takes place over a much longer period of time than for a water storage or hydro power dam of comparable size. A starter dam is designed and constructed to contain the tailings from the initial mining period, typically for one to two years of operations. Thereafter, the crest of the dam is raised in construction stages over the life of the mining development, which can be several decades. Constructing the dam in stages reduces initial capital investment, and distributes ongoing capital expenditure over the life of the mine.
Staged construction allows tailings dam designers to adopt an observational approach so the designs can be regularly adjusted and optimised as development proceeds. With this approach, the results of performance monitoring are used to confirm the initial design predictions, and changes to the design can be implemented as appropriate. However, the prolonged construction period can also present additional challenges for continuity of the design and construction QA when ownership of the mine changes hands, or when different engineers are responsible for the design and QA of the continuing stages of the dam.
Key documentation for the continuing design and construction of a tailings dam includes a detailed design report and a construction report for each stage of construction, together with an annual review of operating performance. These reports should include detailed design assumptions and analysis, as-constructed drawings, and summaries of the results of the field and laboratory QA tests completed during construction. In addition, construction reports or annual inspection reports must also include an interpretation of the results from instrumentation systems. In the observational approach, continuous data collection and interpretation provide essential information for the design of future stages.
The following four case histories demonstrate how QA procedures were implemented during the continuing staged development of large tailings embankments. These examples have been selected to illustrate the particular challenges that arise when there are changes in ownership or when a different design engineer is retained part way through the staged development of the tailings dam.
The Alumbrera mine is located in northwestern Argentina, and started production in late 1997. The first stage (stage 1) of the tailings embankment was 50m high and completed by March 1999. It incorporates a free-draining design with downstream collection of seepage through pervious foundation soils.
During the construction of a grout curtain in stage 1, an unexpected deep, permeable bedrock zone was hypothesised based on the results of core drilling. Sufficient time was available to fill the tailings dam with fresh water to a depth of approximately 10m prior to start-up. Observations of seepage flows in springs and drainage zones, and of groundwater pressures in instruments, facilitated an optimal design for the seepage pump-back system. The design also takes advantage of the relatively low permeability of the hydraulically deposited tailings solids to control seepage recycle rates.
The original tailings dam designer continues to provide full time field services for construction of the dam, now 70m high, with a projected ultimate height of 165m. The original hydro-geologic consultant also remains involved in the project. The site laboratory provides for thorough QA sampling and testing of the fill materials. Instrumentation in the dam, the foundation and the tailings continues to be read weekly, interpreted and reported. Annual reports present as-built drawings, instrumentation records, and interpretation of the monitoring data. This information is combined with detailed in situ testing of the tailings to allow the long term design of the dam to be optimised. The seismic response of the dam is an important consideration and is evaluated using sophisticated finite difference models.
Quality assurance procedures
The QA procedures include periodic peer reviews throughout development, on behalf of the owner and the regulatory authorities, and the designer also retains independent specialist reviewers for critical design aspects. The QA systems are regularly updated and each staged expansion can make efficient use of the experience gained during operations, as well as the extensive construction and monitoring records obtained during previous construction programmes. The continuity of the owner’s team and the design engineering team allows for secure operating performance and cost-effective design of ongoing staged expansions to the dam.
The Montana Tunnels mine is located in the state of Montana, US. The first stage of the tailings dam was completed in 1987. The original design approach envisaged that the subsequent staged expansion of the dam would be completed by the downstream construction method. However, by using the observational approach, it was determined that the modified centreline construction method represented a more cost-effective and environmentally acceptable method for ongoing development of the dam. The modified centreline design requires knowledge of in situ characteristics of the fill materials in the dam and the tailings, and the in situ water pressures.
The parent company of the mine became bankrupt in 1998, resulting in a new ownership structure. However, the engineer for the tailings dam did not change, so continuity was maintained for subsequent ongoing staged expansions. This continuity has facilitated the ongoing development of the dam in a manner that allows the design to be adjusted according to changing conditions and allows for optimisation of the embankment. A total of 14 design and construction stages have been completed since 1999, and the earthfill/rockfill dam is now over 100m in height. Changes in the owner’s operating team have had only a minor impact on the design and operation of the tailings facility. A key aspect of the successful transition is the continuity of support from the design engineer.
Omai gold mine
The Omai gold mine is located in Guyana, South America. The original design engineer completed the feasibility design of the dam, and provided the design and provision of field services for the construction of the starter dam. The starter dam was a 10m high earthfill structure, which was constructed by December 1992.
The original designer ended its involvement with the tailings dam after completion of this initial stage. A new consultant was retained by the owner for ongoing technical support for subsequent staged expansions of the dam, but there was no formal transfer of information and responsibility for the dam. Three years later, the dam height had been raised by over 30m to a maximum height of 44m.
However, both the design and the quality assurance and control systems were inadequate for the staged expansions to the dam, as it was constructed without proper transition materials between fine filter sands and the coarse rockfill shell zone.
The dam failed in August of 1995 as a result of massive piping (internal erosion) of the saprolite core of the dam due to the faulty construction of the gradation rockfill adjacent to the sandfill of the main dam. Contaminated water and tailings flowed out of the impoundment and into the Essequibo river.
Information from instrumentation published after the failure indicated that a dramatic and unexpected rise in pore pressure occurred in the dam months before the failure, but appropriate measures were not taken. Proper application of the observational approach during the design of the ongoing stages of this dam would have prevented this failure. Similarly, routine inspections and QA testing should have demonstrated the susceptibility for internal erosion due to the lack of a filter relationship for the fill materials.
This example illustrates the importance of clearly defining the roles and responsibilities of the various parties when a transfer of services is completed. The need for a formal hand-over of design and QA responsibilities is particularly evident from this case history.
The Kemess mine is a large development in northern British Columbia, Canada that began production in the summer of 1998. The original engineer provided the design services for feasibility design and permitting, and designed the 65m high starter dam (stage 1) and the next 10m high expansion (stage 2) of the dam. The original designer also provided full time QA field services during construction of stages 1 and 2.
A very weak relict failure surface from an ancient landslide is present in the foundation of the dam, and special provisions are required to ensure the stability of the dam during construction and operations. The design of the dam relied on the observational approach and incorporated extensive monitoring systems to evaluate the performance of the embankment.
An independent, specialist technical review panel was also consulted throughout the design and construction programme, and regular peer reviews were completed.
The original engineer withdrew its services for ongoing design and QA services in September 1998 due to concerns about construction procedures and due to the prolonged lack of payment for services rendered. The owner immediately retained a new consultant to continue with design and QA services for the tailings dam.
The original engineer insisted on implementing a formal procedure for transferring the design and QA responsibilities to the new consultant. The owner and the regulatory authorities were included in the process and the independent technical review panel also assisted in the transfer.
During the transfer process the owner went into receivership and the receivers operated the mine for almost a year before the current owners purchased it. Construction and operation of the Kemess tailings dam has continued successfully, despite these changes in engineers and owners.
The continuity of QA systems is critical to the successful development of a tailings dam that relies on an observational approach. Where the design engineer remains on the project, QA is easily maintained even when there is a change in ownership. But when the design engineer changes later on during the development of the dam, it is critical that a formal hand-over of information and responsibility for the dam takes place.
There are occasions in water storage or hydro power dam developments where existing dams are modified, improved or enlarged, but often the owners and the original designers complete these adjustments and appropriate continuity is achieved.
However, with tailings dams there may be instances when new owners or different engineers are required to make modifications to the dams, and in these instances, it is essential that there is a procedure for transferring the responsibility and management of the QA systems. The previous design engineers should be used, if possible, to assist in the transfer, and independent third party peer review should also be included in the new design team.