ICMM - International Council on Mining & Metals

CASE STUDY

Community engagement on potential for seismic failure of tailings pond

The Kennecott Utah Copper Tailings Impoundments are located approximately 10 miles west of Salt Lake City, Utah, in the United States, along a major interstate highway and north of the town of Magna. These impoundments store the tailings from the Bingham Canyon Mine about 15 miles to the south. The impoundments include the original south impoundment, which encompasses approximately 5700 acres, and a newer north impoundment, which encompasses about 3400 acres.

The inactive south facility was operated from about 1906 to 2002. It is bounded by a state road to the south and situated immediately to the north of the town of Magna. The perimeter of this impoundment was constructed using a variety of diking methods followed by upstream construction. While upstream construction ashistorically been used in mining, it is generally considered to exhibit poor seismic performance. The impoundment was also constructed prior to the implementation of regulatory and engineering standards in the design of tailings ponds. In comparison, the active north pond has been constructed using modern, state-of-the-art design and construction practice to be stable under the maximum credible earthquake (MCE) event. The MCE is the maximum earthquake ground motion that could be generated from local fault sources.

In 1987, a geotechnical study of the then-active south pond identified a seismic vulnerability particularly at the southeast corner. The study identified a risk of flow failure that could affect the state roadway, a nearby small housing estate, a neighbouring house, a golf course and some Kennecott facilities. Although the annualized risk of an occurrence was small, the consequences of such a tailings flow failure were considerable. As a result, Kennecott implemented a programme of drainage measures to improve stability and commissioned a number of engineering studies to evaluate methods to upgrade the facility to current design standards.

Technical mitigation measures

Due to the size of the tailings impoundment (the perimeter is 12 miles), it became clear that standardized approaches to upgrading the seismic stability would be costly and, in some cases, technically prohibitive. A number of mitigation methods were tested and employed, along with studies to characterize the tailings embankment. These measures included:

• horizontal drains along the toe of the southeast corner;
• a detailed characterization of the tailings to evaluate the drainage properties and distribution of fine materials;
• a large number of geotechnical instruments, including piezometers and accelerometers;
• vertical dewatering wells;
• wick drains to improve the effectiveness of the horizontal drains and dewatering wells;
• a series of step-back dikes to flatten the crest of the embankment;
• the design and construction of the north tailings facility to include the north embankment of the south pond; and
• plans for the safe closure of the south facility.

After the dewatering methods were implemented on a large scale, additional studies were done to determine when and whether the impoundment would meet current Utah State Engineer design criteria. Although dewatering had upgraded the facility to an OBE standard (Operating Basis Earthquake, an earthquake with a return period of about 200 years), it was found that approximately 20 years would be required to meet the more stringent MCE design criteria. During this interim period while the seismic stability was improving but a risk of failure was still present, several other measures were implemented.

An accelerometer-based warning system was installed. This would divert traffic away from state roads in the event of a seismic shake being measured by the accelerometers. The system consists of seven strong ground motion accelerometers set to activate at an acceleration of approximately one fifth of the anticipated ground shaking caused by a MCE event. Three of the accelerometers are linked to a system of message boards and flashing signs located along all intersections of the state roads in the vicinity of the tailings impoundment.

Berms were constructed around residential buildings at risk in the case of a tailings failure. These were intended to deflect or limit any run-out that occurred. Buffer zones were also established on the east and west sides of the facility. An emergency action plan (EAP) and standard operating procedures (SOP) were developed to help Kennecott operations in their planning and notification of public agencies.

Community outreach

The EAP/SOP became the document used to engage with the public on the seismic vulnerability issues associated with the south facility. This document contains:

• notification procedures should an earthquake occur;
• a schedule of responsibilities of Kennecott and various public agencies during a seismic event;
• a catalogue of conditions of potential failure;
• the identification of preventative actions and equipment / resources available for emergency response; and
• predictions of the areas that would be subject to impact should a seismic event occur.

As part of the engagement process, the EAP/SOP document was distributed to public agencies with community responsibilities, such as the Fire Department, Emergency Services, Transportation Services, the State office of comprehensive emergency management and Kennecott supervisory and security personnel.

Public meetings were held to co-ordinate the potential emergency response, inform the public of the seismic vulnerability and seek public input. The public involvement programme included articles in local newspapers, a television news report that was later incorporated into a television programme and an educational video regarding seismic preparedness measures. Kennecott also conducted public tours of the tailings facilities to allow interested parties to observe the remedial measures that were being planned, to monitor the progress of reclamation effort and to familiarize themselves with the tailings impoundment. Kennecott representatives also routinely made and continue to make presentations at the local community council meetings to keep the public informed. Internally, Kennecott has independent consultants periodically review the effectiveness of the dewatering/stabilization efforts. As conditions change, the EAP/SOP is updated and redistributed.

Closure planning

The combination of remedial measures to upgrade the south facility, emergency action planning and public involvement demonstrated Kennecott’s commitment to long-term sustainability and closure of the facility. Reclamation of the south tailings facility had been initiated in 1991 with the construction of step-back dikes at the southeast corner of the facility. Subsequently, additional dikes were constructed near the west abutment. These dikes were gradually built from west to east as deposition on the south tailings facility was transitioned to the north facility.

The current closure planning for the embankment and impoundment seeks to stabilize the surface against fugitive dust and erosion by establishment of vegetation or maintenance of a wet surface and to stabilize the perimeter of the embankment under seismic (MCE) conditions by combinations of berms, envelopment, dewatering or creation of buffer zones in areas that cannot economically or technically be stabilized using other approaches.

Constructing reclamation dikes across the impoundment and gradually seeding the tailings surface have accomplished revegetation of the embankment surface. Revegetation efforts have been assisted in some areas using lime and organic wastes. Planning is now ongoing to provide long-term usage of the closed tailings facility.

In summary, seismic failure of the Kennecott tailings facilities was identified as a low probability/high consequence event. A series of mitigation efforts were implemented along with public and regulatory information programmes. This approach allowed the entire facility to be upgraded while operations continued. In addition, these efforts led to the transition of operations to the north impoundment and to the long-term closure planning of the facility.

Analysis

Given that Kennecott had become aware of the instability of the southeast corner of the tailings pond in a major seismic event, it was imperative that they first seek to improve the stability of the facility. It was equally imperative to engage with the local community about what would happen if such a large seismic event occurred so that they were aware of the processes and procedures being put in place as safeguards. Over more than a decade, Kennecott has persisted with this engagement through public meetings, newsletters and the media, and there is now a better degree of comfort with the situation in the local community